c:aucscaucsc bia edits 2008intermediate toc 2008 intermediate text 031208.pdf · 2012-11-04 ·...

Intermediate Course

Appalachian Underground Corrosion Short CourseWest Virginia UniversityMorgantown, West Virginia

Copyright © 2007

APPALACHIAN UNDERGROUND CORROSION SHORT COURSEINTERMEDIATE COURSE

CHAPTER 1 - CORROSION CELLS IN ACTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

REVIEW OF BASIC CONCEPTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Basic Course Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

A Corrosion Cell Concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Dry Cell Example of a Corrosion Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-2

DEMONSTRATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-3

Dissimilar Metal Corrosion Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-4

Corrosion Cell Showing Effect of Electrolyte pH Characteristics on DissimilarMetal Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

Corrosion Cell Demonstrating Corrosion Caused by Differential OxygenConcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-5

Corrosion Cell Demonstrating Corrosion Caused by Dissimilar SurfaceConditions on Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-6

Demonstration Showing the Corrosion Cell Resulting from Differences inElectrolyte Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

Demonstration to Show the Effect of Anode-Cathode Ratio on CorrosionRate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-7

Demonstration to Show the Corrosive Effect of Stress in Metal . . . . . . . . . . . . 1-9

Demonstration to Show Effect of Cathodic Protection on a Corrosion Cell . . . 1-9

Demonstration Flexibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

Demonstration Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-10

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-11

CHAPTER 2 - INSTALLATION OF GALVANIC ANODES . . . . . . . . . . . . . . . . . . . . . 2-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

REVIEW OF FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

The Galvanic Corrosion Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Galvanic Anodes and Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

Galvanic Anodes Plus Coatings and Electrical Isolation . . . . . . . . . . . . . . . . . 2-2

GALVANIC ANODE APPLICATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

General Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-2

Specific Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-3

Advantages and Disadvantages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-5

GALVANIC ANODE CHARACTERISTICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Galvanic Anode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Magnesium Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Zinc Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-6

Aluminum Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

Use of Special Chemical Backfill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-7

GALVANIC ANODE INSTALLATION PROCEDURES . . . . . . . . . . . . . . . . . . . . . 2-8

Single Anode Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Multiple Anode Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Anode Lead Attachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-8

Test Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-9

CHAPTER 3 - INSTALLATION OF IMPRESSED CURRENT CATHODICPROTECTION SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Impressed Current Cathodic Protection System . . . . . . . . . . . . . . . . . . . . . . . 3-1

Conventional Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Distributed Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

Deep Anode Bed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

POWER SUPPLIES FOR IMPRESSED CURRENT CATHODIC PROTECTIONSYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-2

Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Thermoelectric Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Engine - Generator Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Turbine - Generator Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

Wind Powered Generators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3

ANODE BED MATERIALS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-4

Packaged Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Anode-Lead Connection Insulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Cables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-5

Cable Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

Cable Splices/Repairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-6

SELECTING AN ANODE BED SITE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Soil Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Soil Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Interference with Foreign Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-7

Power Supply Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Accessibility for Maintenance and Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Vandalism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

Purpose of Anode Bed and Site Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-8

INSTALLATION PRACTICES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Anodes - General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Vertical Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Horizontal Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Deep Anode Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-9

Cable Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

Rectifier Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-10

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-11

CHAPTER 4 - CRITERIA FOR CATHODIC PROTECTION . . . . . . . . . . . . . . . . . . . . 4-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

CRITERIA FOR STEEL AND CAST IRON PIPING . . . . . . . . . . . . . . . . . . . . . . . . 4-1

-850 mV With CP Applied Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-2

Polarized Potential of -850 mV Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-4

100 mV of Polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-5

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6

Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-6

Net Protective Current Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-7

Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

Other Criteria For Steel And Cast Iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

300 mV Potential Shift Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-8

E-Log I Curve Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-9

CRITERION FOR ALUMINUM PIPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10

CRITERION FOR COPPER PIPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-10

CRITERION FOR DISSIMILAR METAL PIPING . . . . . . . . . . . . . . . . . . . . . . . . . 4-11

CHAPTER 5 - STATIC STRAY CURRENT INTERFERENCE TESTING . . . . . . . . . . 5-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

DEFINITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

STATIC INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-1

INTERPRETATION OF DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-2

Finding the Source of Static Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-4

Determining the Point of Maximum Exposure . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

MITIGATION OF STATIC INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

Mitigation by Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-5

Mitigation by Addition of Cathodic Protection . . . . . . . . . . . . . . . . . . . . . . . . . . 5-6

Natural Potential Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-7

TYPICAL EXAMPLES OF STATIC INTERFERENCE . . . . . . . . . . . . . . . . . . . . . . 5-7

Summary of Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11

Things to Watch Out For . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5-11

CHAPTER 6 - CORROSION CONTROL FOR PIPELINES . . . . . . . . . . . . . . . . . . . . 6-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-1

COATINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

History of the Use of Coating Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-2

Types of Underground Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Enamels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-3

Fusion Bonded Epoxy - FBE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

Extruded Plastic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-4

Hot Applied Mastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

Cold Liquid Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-5

Hot Applied Waxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Cold Applied Wax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Prefabricated Films and Tapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-6

Heat Shrink Sleeves and Tapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

Directional Drilled Crossings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

Desired Coating System Qualities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-7

Surface Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-9

Coating Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-10

HANDLING COATED STEEL PIPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12

Transportation and Handling of Mill Coated Pipe . . . . . . . . . . . . . . . . . . . . . . 6-12

Pipe Coating Over the Ditch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-12

Holiday Detection and Repair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-13

Procedures for Laying Coated Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

HOLIDAY DETECTION AFTER BACKFILLING . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

SYSTEM MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-14

CASED CROSSINGS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15

What is a Cased Crossing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15

Component Parts of a Cased Crossing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-15

Proper Methods for Installing Cased Crossings . . . . . . . . . . . . . . . . . . . . . . . 6-15

Testing Cased Crossings for Electrical Isolation . . . . . . . . . . . . . . . . . . . . . . 6-16

ISOLATING JOINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17

What Does an Isolating Joint Do? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17

Applications of Isolating Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17

Isolating Flanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-17

Installation of Isolating Flanges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19

Isolating Unions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19

Isolating Couplings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-19

Testing Isolating Joints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-20

Repair of Shorted Isolating Flange . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-20

TEST POINTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21

The Purpose of Test Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21

Types of Test Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-21

Installation of Test Points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-25

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6-26

CHAPTER 7 - RECTIFIERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

CATHODIC PROTECTION SYSTEM POWER SUPPLIES . . . . . . . . . . . . . . . . . . 7-1

REVIEW OF ELECTRICAL FUNDAMENTALS . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-1

THEORY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-2

Rectifying Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-3

Components of a Rectifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-4

Transformers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-5

Rectifying Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

Selenium Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-6

Silicon Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7

Accessory Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-7

TYPES OF RECTIFIERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8

PREVENTIVE MAINTENANCE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-8

RECTIFIER EFFICIENCY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10

RECTIFIER SELECTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-10

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7-11

CHAPTER 8 - CATHODIC PROTECTION SYSTEM MAINTENANCE ANDTROUBLESHOOTING PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

MAINTENANCE PROGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

Periodic Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

Coating Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-1

Rectifier and Anode Bed Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

Galvanic Anode Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-2

Test Station Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3

PERIODIC SURVEYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-3

REPAIRS AND/OR REPLACEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7

Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7

Rectifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-7

Impressed Current Anode Beds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9

Galvanic Anodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9

Test Stations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-9

TESTS USED IN CATHODIC PROTECTION SYSTEM TROUBLESHOOTING . 8-11

Percent Leakage Current Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11

System Current Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-11

Surface Potential Surveys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-13

Testing for Pipelines in Contact with Casings . . . . . . . . . . . . . . . . . . . . . . . . 8-15

CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8-16

2007 Revision

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CORROSION CELLS IN ACTION1-1

CHAPTER 1

CORROSION CELLS IN ACTION

INTRODUCTION

This first chapter of the Intermediate Course isintended to accomplish two things. The first willbe a review of the basic concepts involved in acorrosion cell - what is necessary for corrosionto occur on underground pipeline systems andthe development of a basic corrosion formulafor steel. The second intended accomplishmentis the development of methods for practicalclassroom demonstrations showing, visually,(1), how various corrosion cell conditions canresult in active corrosion and, (2), how cathodicprotection can be used to prevent corrosion.Demonstration tests will be outlined to show avariety of corrosion-causing conditions and toshow the beneficial effect of cathodicprotection.

REVIEW OF BASIC CONCEPTS

A thorough understanding of how a corrosioncell functions is essential for the corrosionworker on underground structures. Fullknowledge in this basic area permits the workerto analyze more readily and understandcorrosion problems encountered in the field.This in turn permits an informed approach tothe development of corrective solutions to thecorrosion problems encountered.

Basic Course Reference

The essential basic concepts of corrosion celloperation and the various field conditions whichcan result in active corrosion cells on pipelinesare covered in Chapter 2, "CorrosionFundamentals", of the Basic Course. Thefollowing material on basics is summarized from

the source.

A Corrosion Cell Concept

Figure 1-1, will serve to illustrate basic points.

First - and important to understand - is thatthere are four essential elements of anycorrosion cell if active corrosion is to occur.These are:

1. There must be an anode

2. There must be a cathode

3. There must be a conducting electrolyte inwhich both anode and cathode areimmersed.

4. There must be a metallic conductorbetween the anode and the cathode.

All of these are present in Figure 1-1. In thefigure, the electrolyte is shown as water. Thiscould just as well be moist earth or an aqueouschemical solution. In any case, the aqueouscomponent is naturally ionized with positivelycharged hydrogen ions (H+) and negativelycharged hydroxyl ions (OH-).

The anode is that metallic element of the cellwhich discharges ions into the electrolyte andcorrodes. The cathode is that metallic elementwhich collects ions from the electrolyte anddoes not corrode. For there to be a currentflowing between the anode and cathode therehas to be a driving voltage between the anodeand cathode. In galvanic corrosion cellsencountered in the field, there will naturally be

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(for a variety of reasons) a voltage between theanode and cathode. In Figure 1-1, both theanode and cathode of the laboratory cellillustrated are iron such that (under the uniformconditions shown) there would not be a voltagedifference between them. For the purpose ofthe illustration, then, the driving voltage isrepresented by a battery inserted in the wireconnecting the anode to the cathode.

Now we can review the electrochemicalreactions that can occur at the anode andcathode under Figure 1-1 conditions. First, asdiscussed in the Basic Course reference, thereis a flow of electrons migrating in the oppositedirection from the ions discussed above.Actually, current flow is a flow of electrons - oneampere flowing for one second in theconventional fashion represents a transfer of6.28 x 1018 electrons. This is basic and directlyrelated to the corrosion process.

At the iron (or steel) anode of Figure 1-1,conventional current discharging from theanode into the electrolyte must be accompaniedby electron migration from the anode throughthe metallic connection (wire) to the cathode.These electrons become available as a result ofthe ionization of iron atoms. Each atom of ironbreaks down (ionizes) into one positivelycharged iron ion (Fe++) and two negativelycharged electrons (e-, e-). As a surplus ofpositively charged iron ions builds up at theanode surface, they are attracted to andcombine with negatively charged hydroxyl ions(OH-) in the ionized electrolytic environment.This results in the reaction (as shown onFigure 1-1):

OH- + OH- + Fe++ = Fe(OH-)2

Fe(OH-)2 is ferrous hydroxide.

As this process continues, the iron (or steeldisappears (corrodes) and is replaced bycorrosion products. The basic reactiondeveloping ferrous hydroxide (typically a whitishor pale greenish corrosion product) as shown isto form ferric hydroxide which is the familiarreddish brown rust.

At the cathode, the negatively chargedelectrons that have migrated there from theanode combine with positively chargedhydrogen ions in the electrolytic environment.This can be represented (as shown on Figure1-1) by:

e- + e- + H+ + H+ = H2

H2 is hydrogen.

The hydrogen so developed forms on thesurface of the cathode as a cathodicpolarization film. There is no attack on the ironcathode during this process.

Earlier, it was indicated that the aqueouscomponent of the electrolyte in which the anodeand cathode are immersed is naturally ionized.The representation of the breakdown of watermolecules into positively charged hydrogen ionsand negatively charged hydroxyl ions is givenas follows:

2H2O = 2H+ + 2(OH-)

Granting the necessary presence of these ionsin the electrolyte, a fundamental equation forthe corrosion of iron at the anode can berepresented by:

Fe + 2H2O = Fe++ + 2H+ + 2(OH-)

Where the 2H+ (after combining with negativelycharged electrons) represents the cathodicpolarization film and where the Fe++ combineswith the 2(OH-) to form Fe(OH-)2 - the ferroushydroxide corrosion product.

Dry Cell Example of a Corrosion Cell

A simple, yet effective, example of a corrosioncell with which everyone is familiar is thecommon zinc-carbon flashlight cell although theaverage user seldom, if ever, thinks of it as acorrosion cell. But it is.

Figure 1-2 illustrates the components of theflashlight cell. In the cell by itself we have onlythree of the four requirements for an active

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corrosion cell: an anode (the zinc can), acathode (the carbon rod central electrode), andan electrolyte (the ammonium chloride). This,then, is an inactive corrosion cell.

When the cell is placed in a flashlight and theswitch is pressed, a metallic circuit is activatedbetween the anode and cathode through theflashlight bulb. This is the fourth necessarycomponent for an active corrosion cell. Currentis now discharged from the zinc can anode intothe electrolyte and the anode then corrodes.The carbon rod cathode is not affected.

In Figure 1-1 discussed earlier, a cell in theconnecting circuit between anode and cathodecaused current to flow in the cell between thesimilar-material electrodes. In the case of thezinc-carbon flashlight cell, however, there is anatural potential (voltage) between thedissimilar materials comprising the anode andcathode. In the case of a new zinc-carbonflashlight cell, this potential is approximately 1.6volts.

The galvanic potential difference between zincand carbon which is used to advantage in theflashlight battery is an example of the use of thepractical galvanic series of metals from Table2-1 of the Basic Course, shown as Table 1-2 inthis chapter. This table lists metals and certainother conducting materials from least noble (ormost likely to be anodic) to most noble (or mostlikely to be cathodic). Any two materials fromsuch a table will (when connected together inan electrically conducting environment) form acorrosion cell in which the more noble of thetwo will be the cathode.

The practical galvanic series from the BasicCourse as referenced above gives potentialswith respect to a copper-copper sulfatereference electrode. A more complete listing iscontained in Table 1-1. This table, refers to thelisting as a “solution potential series"(synonymous with "electromotive series") withpotentials shown with respect to a hydrogenelectrode which has been arbitrarily set at zero.In both tables, materials listed higher in thetable will be anodic (and will corrode) when

connected in a corrosion cell to a more noblematerial which appears at a lower point in thetable.

Although the principal of Tables 1-1 and 1-2 isthe same, the voltage difference between anytwo materials appearing in one table may notagree with the voltage difference between thesame two materials in the other table. There isa good reason for this. Whereas the practicalseries lists typical potential values measuredwith respect to a copper-copper sulfatereference electrode in neutral soils or waters,laboratory determined values such as those inTable 1-1 are typically determined for eachmaterial with reference to hydrogen electrodewhen the material being tested is immersed ina solution of its own salts.

From the above it would appear that thepractical series (Table 1-2) will be more usefulto the corrosion worker dealing withunderground structures than would alaboratory-determined series. But here again,the practical galvanic series should be used asa guide only because if there are local orgeneral deviations from neutral soil or waterconditions along a buried structure, there canbe distortions in the potential values.

As further assistance in determining theanodic-cathodic relationship between materials, (from most anodic to most cathodic) Table 1-2tabulates a series of metals and metal alloysthat may be encountered. It will be noted thatpotential values with respect to a commonreference are not given as is the case withTable 1-1. Table 1-2 is most useful in makingan initial determination of the probableanodic-cathodic relationship. The general orderof the probable potential between them may beestimated by using Table 1-2 and may beconfirmed by test in the specific environment ofthe actual application.

DEMONSTRATION

Hearing and reading about various corrosioncells and how they operate gives an initialbackground. However, there is no better way to

TABLE 1-1

SOLUTION POTENTIAL SERIES (ELECTROMOTIVE SERIES) OF SOME BASIC METALS AND MATERIALS

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Metal or MaterialPotential(Emf, V) Metal or Material

Potential(Emf, V)

Lithium -2.96 Nickel -0.23

Rubidium -2.93 Tin -0.14

Potassium -2.92 Lead -0.12

Strontium -2.92 Iron (ferric) -0.04

Barium -2.90 HYDROGEN 0.00

Calcium -2.87 Antimony +0.10

Sodium -2.71 Bismuth +0.23

Magnesium -2.40 Arsenic +0.30

Aluminum -1.70 Copper (cupric) +0.34

Beryllium -1.69 Copper (cuprous) +0.47

Manganese -1.10 Tellurium +0.56

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Zinc -0.76 Silver +0.80

Chromium -0.56 Mercury +0.80

Iron (ferrous) -0.44 Palladium +0.82

Cadmium -0.40 Carbon +0.84

Indium -0.34 Platinum +0.86

Thallium -0.33 Gold (auric) +1.36

Cobalt -0.28 Gold (aurous) +1.50

NOTE: Polarities shown are applicable to the external circuit between materials.

TABLE 1-2

A GALVANIC SERIES OF CERTAIN METALSAND ALLOYS ARRANGED IN ORDER OF CORROSIVITY

ANODIC (LEAST NOBLE) ENDMaterial Material (continued)

Magnesium Brasses

Magnesium alloys Copper

Zinc Bronzes

Aluminum 2S Copper-nickel alloys

Cadmium Monel

Aluminum 175S Silver solder

Steel or iron Nickel (passive)

Cast iron Inconel (passive)

Chromium-iron (active) Chromium-iron (passive)

Ni-Resist 18-8 Chromium-nickel-iron (passive)

18-8 Chromium-nickel-iron (active) 18-8-3 Chromium-nickel-molybdenum-iron(passive)

18-8-3 Chromium-nickel-molybdenum-iron(active) Hastelloy C (passive)

Lead-tin solders Silver

Lead Carbon and graphite

Tin Platinum

Nickel (active) Gold

Inconel (active)CATHODIC (MOST NOBLE) END

Hastelloy C (active)


get the full impact of the manner in which thesecells work than to see an actual demonstrationof corrosion cells in action.

Colonel George C. Cox (deceased) developeda demonstration lecture for use at theAppalachian Underground Corrosion ShortCourse which illustrates effectively how variouscells work. The following material describeshow the demonstration is accomplished anddetails several experiments undertaken duringthe demonstration.

The problems involved with suchdemonstrations were, first, how to present themso that they could be seen equally well by allmembers of a large group and, second, how toshow that areas which are supposed to beanodic in a given cell actually are performing inan anodic manner - and, likewise, how to showthe cathodic areas.

The first problem was solved by placing theexperimental corrosion cells in transparentcontainers filled with liquid electrolyte. Then,using optical projection equipment, the image ofthe corrosion cell could be projected on ascreen so that all could see. This illustrates thecorrosion cell framework but without someadditions there would be little that could beseen during a reasonably short lecture timeinterval to show that there was anything goingon in the corrosion cell.

This second problem was solved by the use ofchemical indicators in the electrolyte solutionsused in the corrosion cells. For the anodicindicator, 2 or 3 drops (not more than 3) of asaturated aqueous solution of potassiumferricyanide is added to each 100 milliliters ofstock solution. The resulting precipitate will givea brilliant bluish green color by transmittedprojector light in anodic areas. For the cathodicindicator, 5 milliliters of a 1% alcohol solution ofphenolphthalein is added to (and well shakeninto) each 95 milliliters of electrolyte solutionjust before placing in the projection cell. Themeasurements are critical. The phenolphthaleinindicator will show a bright pink or crimson colorby projected light in cathodic areas.

The combination of the on-screen projectioncapability plus the means of positivelyidentifying anodic and cathodic areas makes itpossible for classroom groups actually to seethe action taking place in corrosion cells ofvarious types.

The following sections will illustrate some of thecorrosion cells which can be demonstratedusing the Cox methods. It should be noted thatdemonstrations of this same nature can beused effectively for corrosion worker trainingpurposes within company organizations.Additionally, such demonstrations can be auseful tool when conducting corrosionfamiliarization sessions for companymanagement personnel.

Dissimilar Metal Corrosion Cell

This demonstration cell is used to show thatwhen two dissimilar metals are connectedtogether in a corrosion cell, one will be ananode (and will corrode) while the other will becathodic and unaffected.

The demonstration cell to be used in this caseis shown diagrammatically by Figure 1-3. Notethat the two materials used are steel andplatinum immersed in a low resistivity sodiumchloride solution. Inspection of Table 1-1indicates that we could expect a potentialdifference between ferrous iron and platinum ofapproximately 1.30 volt and that the iron shouldbe the anodic (corroding) member. Accordingly,in the demonstration cell, the indicators shouldshow a bluish-green coloration around the steelanode and a pink or crimson coloration at theplatinum cathode. The high galvanic voltage between steel andplatinum permits fairly rapid action in the cell sothat the indicator colors should develop in lessthan five minutes after the cell is activated bythe demonstrator.

The demonstration visually indicates thatcorrosion can occur when dissimilar metals arein electrical contact with one another in aconducting electrolyte. There are quite a few

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dissimilar metal combinations that can beencountered when working on undergroundmetallic structures.

Some of the more common are:

C Steel and cast ironC Plain steel and galvanized steelC Galvanized steel and cast ironC Copper and steel or cast ironC Brass and steel or cast ironC Lead and copper

Plus other combinations involving stainlesssteel, aluminum, cinders (carbon), etc.

Corrosion Cell Showing Effect of ElectrolytepH Characteristics on Dissimilar MetalCorrosion

The objective of this demonstration cell is toshow that a dissimilar metal corrosion cell willremain active under different pH conditions.

In the preceding demonstration, the electrolytein the steel-platinum dissimilar metal cell was auniform mixture of sodium chloride in contactwith both elements of the cell. In thisdemonstration, as shown by Figure 1-4, thesame two dissimilar metals are used but thesteel is (for the first part of the demonstrationimmersed in a 10% sodium chloride solution(alkaline) while the platinum is immersed in 1%hydrochloric acid solution. A porous bridgeseparates the two electrolytes preventing freeintermixing of the solutions. However, itbecomes saturated with the solution andpermits corrosion current interchange betweenthe steel and platinum.

When the cell arranged as described above isplaced in operation by the demonstrator, thecolor indicators will show the steel in thealkaline environment to be anodic andcorroding as shown by a blue-green colorationwhile the platinum in the acid environment willshow the pink cathodic coloration.

Once the above demonstration is shown on thescreen, the demonstrator will reverse the

positions of the steel and platinum placing thesteel in the acid solution and the platinum in thealkaline environment. Again the color indicatorswill show the steel to be anodic and theplatinum cathodic with generally similar reactiontimes.

A successful demonstration as describedshows that differing pH conditions along anunderground structure do little to change theanode-cathode relationship of corrosion cells.

On underground structures, particularly longstructures such as pipelines, it is not at allunusual to find alkaline, neutral, and acidconditions at different locations along thestructure. Although some pipelines may be inpredominantly neutral soils, they can passthrough industrial areas where contaminationcan produce either alkaline or acid conditions.Some soils can be naturally acid or alkaline.

Corrosion Cell Demonstrating CorrosionCaused by Di f ferent ia l OxygenConcentration

The intent of this demonstration is to show thata single metal (such as steel) in an environmentof uniform chemical composition, (such asuniform soil) can suffer corrosion if part of themetal surface receives a greater supply ofoxygen than others. Further, it is intended toshow that those areas having the least oxygenavailability will be anodic and corroding.

The schematic arrangement of thedemonstration corrosion cell is shown by Figure1-5. Bright steel electrodes are placed in eachof the two cell compartments; these areconnected together to establish electricalcontinuity between the two. This can beaccomplished with a single strip of steel bentinto a U-shape as shown. Each compartment isfilled with a uniform solution of 1% sodiumchloride with color indicators added. A porousbridge is used between the two compartmentsto prevent free interchange of electrolytes fromthe two compartments but still provide electricalcontinuity between the two.

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To obtain the differential oxygen concentration,oxygen is bubbled through one compartment(which will be the anodic side. In Figure 1-5 themeans of accomplishing this is shownschematically for the demonstration cell whichhas been used in the classroom experiments.This is done by using a DC voltage (supplied bya variable transformer and AC to DC rectifier)between two platinum electrodes in a separatecompartment of the demonstration cell. Thisseparate compartment is filled with a 20%solution of sodium hydroxide (NaOH). Oxygenis evolved at the positive platinum electrodeand is channeled, as shown on the figure,through the cathodic half of the demonstrationcell. Likewise, hydrogen is evolved at thenegative platinum electrode and is channeledthrough the anodic half of the cell. The DCapplied voltage is adjusted by the demonstratorto obtain a good flow of gas from the twoplatinum electrodes. This will requireapproximately 18 volts DC.

The reaction time for the demonstration to showthe anodic and cathodic indicator colorations(pink at the cathode and blue-green at theanode) may range from one to two hours. Thisnecessitates the demonstrator placing the cellin operation prior to the start of the classroomdemonstration so that the reaction can beshown during the classroom time period.

A successful demonstration will show that thesteel strip in the oxygen-poor environment willbe anodic while the steel strip in theoxygen-rich environment will be cathodic.

As discussed in the Chapter 2 of the BasicCourse, differential oxygen concentration isreferred to as "differential aeration". A typicalexample of such a corrosion cell is a pipeline ina relatively porous well aerated soil passingunder a paved highway where the pavementrestricts air (and thereby oxygen) access to thepipe. This causes relatively oxygen-poorenvironment around the pipe under thepavement - and this is where the corrosionoccurs in the absence of adequate protectivemeasures.

Corrosion Cell Demonstrating CorrosionCaused by Dissimilar Surface Conditions onSteel

The purpose of this demonstration is toillustrate that there is an anode-cathoderelationship between new steel and rusted steelin electrical contact with each other in aconducting environment - even though bothsteels are of the same alloy. Further, it is theintent to show that the new steel is anodic andwill corrode.

Figure 1-6 shows the schematic arrangement ofthe demonstration corrosion cell. A bright steelelectrode is placed in one compartment of thecell and a rusted steel electrode is placed in theother. Both compartments are filled with a 10%sodium chloride solution with color indicatorsadded. The two electrodes are electricallyinterconnected to complete the corrosion cell.

Once the cell is placed in operation by thedemonstrator, the time required to obtain colorindications will be in the order of 30 to 60minutes. When the reaction becomes apparent,the blue-green anodic coloration will be seen inthe compartment containing the bright steelelectrode. Similarly, the pink cathodic colorationwill show in the compartment containing therusted steel electrode.

A similar result would be obtained if thecomparison were made between a bright steelelectrode and a steel electrode coated with millscale - with the bright steel electrode beinganodic and corroding.

In pipeline work, the new steel-rusted steelrelationship was learned the hard way beforethe advent of adequate corrosion controlmeasures. Typically, when corrosion failuresoccurred in a "hot-spot" area necessitatingreplacement after, say, ten years, the pipe wasreplaced with a new piece of steel pipe of thesame type as used originally. The expectationwas that it would be another ten years beforereplacement would be required again. Thenwhen it was found that the replacement sectiononly lasted, for example, five years, the unusual

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reaction was, "they just don't make steel likethey use to". Actually, the difference inperformance time was not the fault of the steelitself, it was the anode-cathode relationshipbetween new (bright) steel and old (rusted)steel as shown by the demonstration describedabove.

The steel to mill scale anode-cathoderelationship may be encountered on steel froma hot rolling mill from which the mill scale hasnot been properly removed. The mill scale isstrongly cathodic (more noble) with respect tothe base steel. This gives rise to strongdissimilar-surface-condition corrosion cells atbreaks in the mill scale coating. The base steelis anodic and corrodes at such breaks in theabsence of adequate corrosion controlmeasures.

Demonstration Showing the Corrosion CellResulting from Differences in ElectrolyteConcentration

This demonstration is intended to show that asteel structure (such as a pipeline) passingthrough a similar electrolytic environment, but ofdiffering concentration from point to point, canbe affected by corrosion cells caused by thevariations in concentration. Additionally, it isintended to show that the steel structure in themore highly concentrated electrolyte will beanodic (and corroding) with respect to thoseparts of the structure in less concentratedelectrolyte.

A corrosion demonstration cell to illustrate thedifferential concentration effect is shown byFigure 1-7. A bright steel electrode is placed ineach of the two cell compartments. They areelectrically interconnected (a single strip bentinto a U-Shape. as shown accomplishes this).One of the compartments is filled with a 1%solution (low concentration) of sodium chloride.The other compartment is filled with a 10%(higher concentration) of sodium chloride.Anodic and cathodic color indicators are addedto the solution. A porous barrier between thetwo compartments prevents free intermixing ofthe two solutions but permits passage of

electric current between the two compartments.

Once the cell is placed in operation by thedemonstrator, time required to obtain anodicand cathodic color indications will typicallyrange from 30 to 60 minutes. A successfuldemonstration will show the blue-green anodicreaction coloration at the steel electrode in thehigh concentration (10% sodium chloride)compartment and the pink cathodic reactioncoloration at the steel electrode in the lowconcentration compartment.

Various conditions can give rise to thedifferential-concentration corrosion cells onoperating pipelines. An example is a pipelinecrossing a marine tidal flat with some freshwater influx. At high tide, the entire structuremay be covered with concentrated sea waterbut as the tide recedes, part of the structure willbe in concentrated sea water while part will bein diluted sea water. Corrosion will beconcentrated in that part of the structure still inthe more concentrated sea water. This is amoving situation as the tide flows in and out.

Another example can be chemicalcontamination in industrial areas. Where suchcontamination occurs, the concentration willtypically be greatest at the center of the spillarea. This would be the area of greatest anodiceffect. As the concentration tapers off as theedges of the spill area are approached, thestructure becomes relatively cathodic.

Demonstration to Show the Effect ofAnode-Cathode Ratio on Corrosion Rate

The purpose of this demonstration is to showthat if a small anode is coupled to a largecathode in a corrosion cell, the rate of corrosionwill be high at the small anodic area.

The demonstration cell arrangement toaccomplish this is shown by Figure 1-8. In thiscell, one compartment contains a bright steelelectrode which has been given a paint coatingwith two pinholes (approx 1/16" in diameter) cutthrough the coating near one edge of theelectrode. The other compartment contains a

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rusty steel electrode; a precedingdemonstration has shown that the bright steelwill be anodic and the rusty steel cathodic. Thetwo electrodes are electrically interconnected.Both compartments are filled with a 10%solution of sodium chloride to which the anodeand cathode color indicators leave been added.Typically, with this arrangement, the ratio ofcathodic area to the small anodic area will be inthe order of 150:1.

Typically, a successful demonstration will showa relatively intense blue-green anodic colorationshowing at the small anodic area pinholesbefore pink cathodic coloration appears at thelarger rusty steel cathodic area. This is anindication that the corrosion rate at the anodicpin-holes is relatively intense as compared tothat observed during the prior demonstration inwhich a bare bright steel electrode was coupledto a bare rusty steel electrode.

If the situation were reversed such that theanodic area is large and the cathodic areasmall by the same ratio, there would be anintense pink indicator appearing at the smallcathodic area before the blue-green anodiccoloration becomes apparent oil the largeanodic area - an indication that the corrosionrate in the anodic area is much less intensethan in the preceding case.

This all illustrates the basic rule that if anodicmaterials must be coupled to cathodic materialson an underground structure, a small cathodecoupled to a large anode is much to bepreferred to a small anode coupled to a largecathode. This rule does not, however, apply tocathodic protection installations where relativelysmall anodes (galvanic or impressed current)are connected to large cathodic areas beingprotected - but here the anodes are corroded inorder to cathodically protect the workingunderground structure.

Some examples can be cited to illustrate theeffects of anode-cathode ratio in undergroundpiping systems.

As one example, if a brass valve is used in

galvanized steel piping, there is a dissimilarmetal corrosion problem. However, there is asmall cathode (the brass valve) working againsta relatively large anode (the galvanized piping).Under this condition, the corrosive effect isdistributed over a large area of the galvanizedpiping. Further, the corrosion current from thegalvanized piping can polarize the small brasscathode which acts to reduce the corrosioncurrent interchange. Coating the brass valvecan further reduce the current interchange to anegligible amount by reducing the already-smallcathode to just any brass cathodic materialexposed at defects in the valve coating. Thisemphasizes the wisdom of using the smallcathode, large anode relationship onunderground structures.

As a reverse-situation example, if a galvanizedsteel valve were to be used in undergroundcopper piping, the undesirable small anodelarge cathode would result. Under thiscondition, there would be intense corrosioncurrent discharge from the small anode (thegalvanized steel valve). Further, this currentflowing to the relatively large copper pipecathodic area would have little expectation ofpolarizing the copper - so there would be littleor no reduction in the corrosion currentinterchange from this source. Also, coating thegalvanized valve anode in this situation wouldbe the wrong thing to do since the ratio of largecathode to small anode would be made evengreater. Current discharge from the anodesurface exposed at defects in the coating wouldbe even more intense than if the galvanizedvalve were left bare leading to even earliercorrosion penetration of the valve body. Thisemphasizes the wisdom of avoiding the smallanode, large cathode relationship onunderground structures wherever possible.

A final example relates to the use of coatingson non-cathodically protected undergroundstructures. A good coating can preventcorrosion on more than 99% of the structuresurface area. But the remaining surface area isexposed to earth at minor coating defects orpinholes inevitably exist or will develop onunderground structures in practical


applications. Some of the pinholes will be innaturally cathodic areas and some in naturallyanodic areas. Where pinholes exist in arelatively small anodic area working againstpinholes in large cathodic areas on either sideof the anodic area, corrosion current dischargefrom the pinholes in the anodic area can beintense. This can result in the development ofcorrosion penetrations on the non-cathodicallyprotected structure earlier than if the structurehad been installed bare, even though the totalmetal loss is not so great.

Demonstration to Show the Corrosive Effectof Stress in Metal

It is the intent of this demonstration to show thatresidual stresses in metal are anodic withrespect to non-stressed portions of the samemetal sample.

Figure 1-9 illustrates the arrangement to showthe effect. A single compartment demonstrationcell is used. The compartment is filled with a10% solution of sodium chloride to whichanodic and cathodic color indicators have beenadded. A common iron nail is placed in thecompartment. The cold worked nail head andpoint contain residual stresses from the coldworking process. A successful demonstrationwill result in a concentration of the blue-greenanodic coloration around the head and point ofthe nail with the pink cathodic coloration alongthe shank of the nail. Occasionally, dependingon the surface condition of the nail shank, theremay be secondary lesser anodic areas alongthe shank of the nail.

Because of the low driving potential betweenthe stressed and non-stressed portions of thenail, the time required for the color developmenton on-screen projection may be from one to twohours making it necessary to place the cell inoperation prior to the start of the lecture period.

Stress corrosion can occur on working pipelineswhere stress-induced corrosion can initiatecracking and pipeline failure. This is an involvedstudy in itself but appears to be most apt tooccur on the higher strength pipeline steels in

the absence of adequate protective measures.

The use of rivets and bolts on structures in aconducting environment can be a problem.Stress is present in such fasteners but if rivetand bolt materials are selected such that thestressed material is slightly cathodic to thematerial in the body of the structure, thedesirable large anode small cathoderelationship will be attained. Under thiscondition, the stressed bolts or rivets will becathodically protected to a degree by thestructure itself. Since the anodic area (the bodyof the structure) is large, the corrosive impacton it will be slight as discussed in the precedingsection.

Demonstration to Show Effect of CathodicProtection on a Corrosion Cell

In this demonstration, the intent is to show thebeneficial effect of cathodic protection on aknown corrosion cell. This accomplished with atwo-part demonstration.

In the first part of the demonstration, asingle-compartment cell is used as shownschematically by Figure 1-10. A dissimilar metalcouple (bright steel anode and copper cathodeelectrically interconnected as shown) is placedin the compartment. The compartment is filledwith a 10% solution of sodium chloride to whichthe anodic and cathodic color indicators havebeen added. Within five to ten minutes afterplacing the cell in operation, the bright steelanode should show the blue-green anodiccoloration while the copper cathode shows thepink cathodic coloration. This establishes thecorrosion pattern for a non-cathodicallyprotected steel-copper cell.

In the second part of the demonstration, a two-compartment cell is used as illustrated byFigure 1-11. A bright steel-copper dissimilarmetal couple (identical to that used in the firstpart of the demonstration) is placed in onecompartment. Either a zinc or magnesiumelectrode is placed in the second compartment(either one is less noble than both steel andcopper). The zinc or magnesium electrode is

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electrically connected to the steel-coppercouple. Both compartments are filled with a10% solution of sodium chloride to which thecolor indicators have been added. Within fiveminutes (if magnesium is used), both the steeland the copper in the other compartment willshow the pink cathodic coloration. Thisdemonstrates that the steel portion of thesteel-copper couple is now collecting currentfrom the environment (rather than dischargingcurrent as shown in the first part of thedemonstration) and has changed from anodic tocathodic - and is thus cathodically protected.

This demonstration is a simple visual illustrationof an effective application of cathodic protectionusing galvanic anodes. This procedure is usedwidely on pipelines and other undergroundstructures. Further information on the use ofgalvanic anodes is included in Chapter 2.

Where impressed current cathodic protectionsystems are used, the principal is exactly thesame. The only difference is the source of thecathodic protection current. Whereas withgalvanic anode protection dissimilar metals areused as the source of current, impressedcurrent cathodic protection systems use currentfrom an outside voltage source to force currentto flow from low-consumption-ratio anodes tothe structure to be cathodically protected - butthe result is the same.

Impressed current systems are discussedfurther in Chapter 3.

Demonstration Flexibility

Although the various demonstration cells whichhave been discussed show a number ofexamples of corrosion in action, the lecturerconducting the demonstration may choose toadd to or modify some of them for furtherclarification where lecture time permits.

Demonstration Equipment

For those who may wish to prepare similardemonstrations, the following information isincluded in equipment and techniques that have

been used for this purpose to date asdeveloped by Colonel Cox.

The projector used is a 3¼ inch by 4 inch slideprojector. The transparent plastic demonstrationcells are made to fit the slide carrier slot in theprojector. The cells are made of Plexiglass orLucite. Two types are needed. The cellconstruction illustrated by Figure 1-12 includestwo two-compartment cells and one singlecompartment cell. The unit can be moved, asneeded, to place any one of the three cells inthe path of the projection beam.

The cell construction used for the oxygenconcentration demonstration cell is illustratedby Figure 1-13. This cell includes the hydrogenand oxygen generation compartment which isnot needed for the other demonstration cells.

The various projection cells have to be placedin the projector in an upright position in order toprevent electrolyte spillage. When projected inthe normal manner, the image on the screenwill be upside down. Accordingly, the projectoris pointed away from the screen and the imagereversed (appears upright) by using the twofront-reflecting-surface mirrors, one above theother, to deflect the projector toward the screen.Each mirror should be held in a frame which ismounted in a supporting bracket with pivots onthe center horizontal axis so that the mirrorangle can be adjusted and then held in positionwith thumb- screws or wing nuts.

The lower mirror should be 6 by 9 inches in sizewith the long axis horizontal. It should beplaced directly in line with the projector beamwith the center of the mirror approximately 3inches from the projector lens.

The upper mirror should be 9 by 12 inches andplaced directly above the lower mirror. Thecenter point of the upper mirror should be 10 to12 inches (but not more than 12) above thecenter point of the lower mirror. By adjusting theangle of each mirror to about 45 degrees, theprojector beam is deflected across the top ofthe projector toward the screen - and the imageon the screen will be upright.

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With appropriate modifications in the projectioncells, it should be possible to utilize a standardoverhead projector which would give thedemonstrator added flexibility in changing cellsduring the presentation.

In those demonstration cells requiring a porousbridge between the two compartments of a two-compartment demonstration cell, cotton ballsmay be packed into the space between thecompartments. These will become saturatedwith the electrolytes in the compartments andpermit electric current to flow between the twowithout permitting free intermixing of thediffering electrolytes. Nevertheless, bothcompartments should be filled at the same timeand at the same rate in order to preventseepage from one compartment to the otherthrough the porous bridge.

Fresh solutions should be prepared for eachdemonstration. Aqueous solutions should bemade using distilled water. Observe theinstructions given earlier on the addition ofanodic and cathodic color indicators. It ishelpful to have solutions in small separatepouring bottles for easy handling during ademonstration. It takes approximately 20milliliters to fill a single compartment of ademonstration cell.

When using the oxygen concentrationdemonstration cell, the lower compartmentcontaining the 20% sodium hydroxide solutionshould be filled only as far as the bottom of theglass bead columns and the fill holes carefullystoppered to prevent escape of the gas. Thefunction of the glass bead columns is tocondense any entrained vapors and return thecondensate to the reservoir.

The color indicators reach a peak of brillianceat which the projected effect is mostsatisfactory. The length of time required for thisrespect is given for the various demonstrationsdescribed herein but it is desirable that thefirst-time demonstrator make "dry runs" beforean actual presentation in order to gainadequate experience with the cells.

Metal electrode strips should be about a quarterinch wide of approximately 20-gage material.These may be bent into "L" shape - or, in somecases, a "U" shape where the same material isused in both compartments of atwo-compartment cell. Zinc or magnesiumanodes need not be thin sheet but may be rodsor strip that will fit into the d" thick compartmentof a cell. Platinum anodes, where used, areelectrolysis demonstration electrodes stockedby chemical equipment supply houses.

CONCLUSIONS

In this chapter, we have reviewed theelectrochemical features of the basic corrosioncell showing the consumption of steel (the mostprevalent material used in undergroundstructures). Based on this, fundamentalcorrosion formulas for iron are given.

The common flashlight battery was used as agood practical example of a corrosion cellbased on dissimilar materials in electricalcontact with each other in a corrosive medium.The anodecathodic arrangement of dissimilarmetals in tabular form was reviewed.

The various types of corrosion cells affecting aniron (or steel) structure were illustrated by visualdemonstration to show that conditions which, intheory, should cause corrosion do, just that.The relative speed of reaction among theseveral demonstrations show that someconditions resulting in rapid reaction time tendto be more severe in effect than those havingslower reaction times.

Finally, the visual demonstrations wereconcluded by showing that if cathodicprotection is applied to a corrosion cell, thecathodic protection prevents the formation ofthe anodic areas, which would develop had thecathodic protection not been present. Thisdemonstration showed also, that the materialused as a source of the cathodic protectioncurrent is anodic and corrodes to providecathodic protection for the protected structure.


REFERENCE

Col. George C. Cox "DemonstratingElectrochemical Corrosion Reactions by theUse of Transparent Cells in an OpticalProjector." Proceedings of the 3rd AnnualAppalachian Underground Corrosion ShortCourse (1958).

INSTALLATION OF GALVANIC ANODES2-1

CHAPTER 2

INSTALLATION OF GALVANIC ANODES

INTRODUCTION

This chapter is concerned with the use ofgalvanic anodes for cathodic protection onunderground structures. Although use onpipelines is the major application, galvanicanodes may be used on many other types ofunderground metallic structures.

Within the framework of this chapter, it isintended that the following general categoriesof information will be covered:

1. A brief review of the fundamentals applyingto this specific subject.

2. The normally accepted applications wheregalvanic anodes can be used economicallyand advantageously for cathodic protectionof underground structures.

3. General data on the physical and electricalcharacteristics of galvanic anodes madefrom magnesium, zinc, and aluminum.

4. Guideline material on field installationpractices for various types of cathodicprotection installations using galvanicanodes as the source of electrical energy.

REVIEW OF FUNDAMENTALS

The Galvanic Corrosion Cell

The galvanic corrosion cell was discussed indetail in the Basic Course (Chapter 2 -Corrosion Fundamentals) and has beenreviewed in Intermediate Chapter 1 - CorrosionCell in Action.

The galvanic corrosion cell includes four basicparts:

1. An anode

2. A cathode

3. A metallic path between the anode andcathode

4. A conducting electrolyte in which both theanode and cathode are immersed

The fact that there is an anode and a cathodeimplies that there is a driving voltage (potentialor emf) between the anode and cathode whichwill cause corrosion current to flow between thetwo. There will be no corrosion unless currentflows between the anode and the cathode.

Galvanic Anodes and Cathodic Protection

The cells established between steel andmagnesium, zinc or aluminum are stronger(they have a higher driving voltage) than theusual galvanic corrosion cell encountered on asteel structure. Accordingly, the higher drivingvoltage of the galvanic anode cell overcomesthe lower driving voltage of the corrosion cell.The points which had been anodic (dischargingcurrent and corroding) are forced by thegalvanic anode cell to collect current andbecome cathodic. Corrosion is therefore stifled.This is the basic concept of cathodic protectionwith galvanic anodes. Figure 2-1 illustrates thisconcept.

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Galvanic Anodes Plus Coatings andElectrical Isolation

Although, as will be described later, galvanicanode cathodic protection can be used on bareunderground structures, it is most efficientlyused on well coated structures which areelectrically isolated from all other structures.Here's why.

In the Chapter 3 of the Basic Course text, acoating is described as an isolating barrier thatprevents current discharge from areas that tendto be anodic on underground structures. It isalso pointed out that, from a practicalstandpoint, one can always expect that therewill be "holidays" (scrapes, gouges, pinholes,rock penetrations, etc.) which will expose smallareas of the structure surface to thesurrounding environment. The current flow froma galvanic cathodic protection installation willseek out and flow onto the undergroundstructure at the holidays and provide cathodicprotection. Typically, there will not be asignificant measurable amount of the galvanicanode current flowing through sound coatingonto the structure.

Assuming a reasonably good coating carefullyapplied and handled, the actual exposed metalstructure surface area could be less than 1% ofthe total surface area. This simply means thatthe amount of current from a galvanic anodecathodic protection installation needed toprotect one square foot area of bare metal(such as on a bare steel pipeline) couldcathodically protect several hundred squarefeet of coated structure.

Electrical isolation of a coated structure (orportion of such a structure) to be cathodicallyprotected with galvanic anodes may benecessary for an effective system. The currentoutput of galvanic anode installations onunderground structures is typically much lessthan that which can be obtained fromimpressed current cathodic protection systems(See the following Chapter). For this reason,the current output of galvanic anodeinstallations on coated structures needs to be

confined to that part of the structure which is tobe cathodically protected.

As an example, if a one-mile section of aten-mile-total-length coated pipeline were tohave galvanic anodes installed, and if theinstallations were designed to provide onlyenough current to protect the one-mile section,it would not be cathodically protected unless itwere electrically isolated from the other ninemiles in the pipeline. This is because, withoutelectrical isolation, much of the galvanic anodecurrent would flow to the other nine miles ofpipe rather than to the one-mile section to beprotected. In view of the limited currentavailable from the galvanic anode installations,the result could be that no part of the one-milesection and no part of the remaining nine milesin the pipeline would receive full and adequatecathodic protection.

Although galvanic anodes are most efficientlyused on electrically isolated coated structures,they can be used on bare structures. Such usehas been very extensive on bare pipelines. Thedifference is that, in most applications, enoughcurrent for cathodic protection will reach thestructure surface only in the immediate vicinityof the anode (within its potential gradient field).Galvanic anodes (normally magnesium) used inthis manner are said to provide "hot spot"protection. This type of protection and thegradient field concept will be discussed in moredetail later in this Chapter.

GALVANIC ANODE APPLICATIONS

General Uses

As has been indicated in the precedingmaterial, galvanic anode installations tend to beused mostly on underground structures inapplications where cathodic protection currentrequirements are small and where soilresistivities are acceptably low. The importanceof the soil resistivity is that the current output ofa galvanic anode cathodic protectioninstallation is primarily controlled by two factors:the anode-to-structure potential (drivingvoltage) and the resistance to earth of the


galvanic anode installation. The former isrestricted within narrow limits whereas the latteris a function of the soil resistivity. The lower soilresistivities permit obtaining a low enoughresistance to earth to permit the flow of aneconomically acceptable driving voltage.

Specific Uses

There are numerous ways in which galvanicanodes are used on underground structures.The following descriptions comprise arepresentative listing of uses.

1. For sections of piping in distributionsystems having a small cathodic protectioncurrent requirement, galvanic anodeinstallations can be more practical (lessexpensive, less complicated, and easier tomaintain) than an impressed currentinstallation (rectifier or other impressedcurrent power source).

2. Galvanic anodes (usually magnesium) areused for "hot spot" protection on barepipelines. Typically, these are installed atlocations where leaks are repaired or atlocations where corrosion surveys indicatethat corrosion is active.

3. Galvanic anodes can be used tosupplement impressed current cathodicprotection systems. If the overall protectionafforded by an impressed currentinstallation has some deficient areas (notquite enough current reaching them toattain full protective potentials), galvanicanodes can be used in such areas toprovide the additional current needed. Thiscan be less expensive than reinforcing theimpressed current installation and can leadto fully adequate overall protection.

4. Galvanic anodes frequently are well suitedto providing cathodic protection to shortpipeline replacement sections. This is truewhere soil conditions are favorable andwhere the replacement pipe is well coatedand is electrically isolated from theremainder of the pipeline system and from

any other underground metallic structure

5. Galvanic anodes can be used to preventstray current discharge from undergroundstructures (particularly pipelines) where thedegree of interference is not severe. This isaccomplished in the interference area byso placing and installing the anodes thatthe galvanic voltage difference betweenthe anode metal and structure metal (steelfor example) will force the stray currentinterference current to discharge from theanode material rather than from thestructure.

Where stray current interference effectsare stronger, the voltage drop across thestructure-to-ground resistance caused bythe stray current discharge in theinterference area may be greater than thecounter polarity galvanic voltage betweengalvanic anode metal and the structure. Insuch instances, the interference effect maybe reduced but not eliminated unlessunusually elaborate galvanic anodeinstallations are used.

6. Where pipelines are buried in the oceanbottom or in other sea-water-coveredareas, galvanic anode "bracelets" (usuallyzinc) are attached to the well-coatedpipeline as it is assembled on the lay bargeand fed off into the water and underwatertrench or simply laid on the ocean floor.The very low resistivity marine environmentpermits high current output, when needed,from individuals anodes. The anodebracelets are typically placed at closeintervals along such lines. This permitsexcellent uniformity of protective currentdistribution along the pipeline and permitsmaking available enough weight ofgalvanic anode material to ensure longanode life. The relationship between anodeweight and useful life is discussed later inthis Chapter.

7. Well coated warm pipelines (such as oilpipelines) in a permafrost arcticenvironment can be cathodically protected


effectively with continuous strip galvanicanodes (ribbon anodes). A warm oilpipeline creates a thaw zone in thepermafrost along the pipeline. By placingthe strip galvanic anode material parallel tothe pipeline in the surrounding envelope ofthawed earth, uniformly distributedprotective current is attained. Anode lifecan be adjusted by selecting the size andnumber of paralleling continuous stripgalvanic anodes.

It should be noted that this does not workon a cold pipeline in a permafrostenvironment where there is no thawing ofthe environment surrounding the pipeline.This is because earth which is below thefreezing point has a very high resistivity(even though low resistivity when thawed);accordingly, anode-to-earth resistancesare so high that the limited driving voltageavailable with galvanic anodes will notforce the flow of a significant amount ofcurrent.

8. Continuous strip galvanic anodes can beused for the coated internal surfaces ofpipelines carrying a corrosive material. Inthis case, the anodes are best attached tothe interior of each pipe length as it is laidand welded (or otherwise joined) to thepipeline being built. Again, the amount ofanode material can be adjusted to give thedesired installation life.

9. An adaptation of Item 8 involves the use ofcontinuous strip anodes to serve as asource of backup cathodic protection forthe carrier pipe inside a casing at a road orrailroad crossing. As long as the casingpipe remains electrically isolated from thecarrier pipe, current from the overallpipeline cathodic protection system willpass through the casing wall (and anyelectrolyte enclosed by the casing) toreach the carrier pipe and afford protection.Should, however, the casing pipe becomeelectrically short circuited to (develop adirect metallic contact with) the carrierpipe, any cathodic protection current from

the overall system collected by the casingwill flow directly to the carrier through thepoint of metallic contact between the two.No protection will then be available to thecarrier pipe throughout the length enclosedby the casing.

When contact develops as described, stripgalvanic anodes attached to the carrierinside the casing (if such anodes wereinstalled) will automatically providecathodic protection to the carrier pipewhere covered with any conductingelectrolyte which has accumulated in theannular space between carrier and casing.

10. A highly specialized application is the useof two or more galvanic anodes, closelyspaced, used as a high voltage dissipationgap (or grounding cell) connected acrosspipelines isolating joints. High voltages arepossible across isolating joints duringelectrical storms or as a result of faults onparalleling high voltage electrical systems.If these high voltages are not controlled,the isolating joints may arc across andbecome short circuited necessitatingexpensive repair or replacement.

The galvanic anode dissipation gap ismade up with sets of anodes rigidly heldmechanically, at close spacing (½ to 1inch). The anode assembly is packaged ina low resistivity special backfill. Half theanodes (typically one or 2) are connectedto the pipe on one side of the isolating jointwhile the other half are connected to theopposite side.

In normal operation (assuming a DCvoltage across the isolating joint as a resultof different cathodic protection levels onthe two electrically separated systems),current will tend to flow from the systemhaving less (or no) protection through thelow internal resistance of the anode gap tothe system having a higher level ofprotection. However, the anode or anodeswhich tend to collect current polarize in thenegative direction. This causes a “back


voltage” which opposes the flow of currentthrough the anode gap. Because of thischaracteristic, there is little loss of cathodicprotection current through such anodegaps.

When a high voltage is impressed acrossan isolating joint equipped with an anodegap, the high voltage overcomes the backvoltage (which restricts current loss undernormal conditions) and passes current inan amount primarily determined by theimpressed high voltage and the circuitresistance.

As an example, assume that a high voltageis impressed across an isolating jointequipped with a dissipation gap groundingcell having a gap resistance of 0.3 ohms andthat 1000 amperes passes through theanode gap. The current flow through theanode gap restricts the voltage drop acrossthe isolating joint, by Ohm's Law (E = IR), to1000 amperes x 0.3 ohms = 300 volts. In theabsence of the anode gaps, the voltage dropacross the isolating joint could be, easily,thousands of volts.

Advantages and Disadvantages

Although some of the advantages anddisadvantages of galvanic anodes areundoubtedly apparent from the precedingmaterial, the more significant items in eachcategory are summarized below:

Advantages

1. Since galvanic anodes are “self-powered,”they require no external power source.

2. Installations may be designed with sufficientweight of anode material to produce thedesired current output for many years beforereplacement is necessary.

3. The field installation of galvanic anodes issimple. Relatively little training is required toattain acceptable construction skills.

4. Maintenance requirements are very low. Outages on commercial power supplies do notaffect them. They cannot be short circuited.As long as they are in electrical contact withthe protected structure, they will continue toprovide protective current until the anodematerial has been consumed.

5. Galvanic anodes tend to be moreeconomical than impressed currentinstallations where small amounts ofprotective current are needed and whereearth resistivities are low.

6. Galvanic anodes are less likely to causestray current interference on other structuresthan is the case with impressed currentsystems. Such interference is possible withgalvanic anodes if there is not sufficientclearance between the anodes and foreignstructures. Typically, five feet or more will besufficient to prevent significant interferencein most instances.

Disadvantages

1. Galvanic anodes have a low driving voltage.This driving voltage is limited by the galvanicvoltage difference between the metal of thestructure being protected and the metal ofthe galvanic anode.

2. Because of the low driving voltage, galvanicanodes normally are limited to use in lowresistivity soil. This is so that theanode-to-earth contact resistance will be lowenough to permit the flow of useful amountsof cathodic protection current.

3. The usual galvanic anode installations haveinsufficient capacity for controlling dynamicstray current effects on protected structures.

4. Except in unusual situations, galvanicanodes are not an economical source oflarge amounts of cathodic protection current.


GALVANIC ANODE CHARACTERISTICS

It is to be noted at the outset of this section thatthere are different alloys available of the variousgalvanic anode metals and that manufacturersoffer many sizes and shapes of anode material.Accordingly, the corrosion control workershould keep informed of the latest cataloginformation on the available galvanic anodematerials.

Galvanic Anode Materials

The following three metals are the mostcommon galvanic anode materials:

MagnesiumZincAluminum

The characteristics of these anode metals areincluded in the following section along withcomments on their limitations, energy content,efficiency, and backfill requirements.

Magnesium Anodes

The technical data applicable to magnesiumanodes is included in Table 2-1.

Magnesium anodes are used to a greaterextent than zinc for earth burial installations onpipelines and other underground metallicstructures. This is because the galvanic celldriving potential is higher, permitting practicalamounts of cathodic protection current in higherresistivity soils.

There are many different sizes and shapes ofmagnesium anodes avai lable frommanufacturers. The corrosion worker shouldobtain up-to-date catalogs so that correctinformation will be available to permit matchingthe optimum anode weight and shape tospecific design needs.

The most popular magnesium anode forgeneral use is the 17-pound anode packaged inspecial chemical backfill. Most of themagnesium weights and shapes can be

obtained either bare or packaged in specialbackfill. The advantages of special chemicalbackfill for use with galvanic anodes is coveredin a separate section.

Approximately similar weights of magnesiummay be available in short chunky shapes or inlong slender shapes. There is a reason for this.The resistance-to-earth of a galvanic anodedetermines the current output at the fixedgalvanic anode cell potential. A long slenderanode has a lower resistance-to-earth in agiven soil resistivity than does a short chunkyanode. This means that long slender anodescan be the better choice in the higher soilresistivities.

Although either zinc or magnesium installationsmay be designed for use in any soil resistivity,various considerations tend to dictate that zincanodes have their best usage in low resistivitysoils with lesser advantage as the resistivityincreases up to a rule-of-thumb maximum(which may be exceeded in special cases) ofabout 1500 ohm-cm. Allowing for some rangeoverlap, a guide for magnesium usage could bebetween 1000 and 5000 ohm-cm. Again, bothlimits can be exceeded where design conditionswarrant.

Zinc Anodes

The technical data applicable to zinc anodes isincluded in Table 2-2.

As was the case with magnesium anodes, zincanodes can be obtained in many differentweights and shapes. They may be obtainedpackaged in special backfill. Again, thecorrosion worker should obtain the latestcatalog information on currently available zincanodes.

As indicated earlier, zinc anodes work best invery low resistivity environments such as seawater, salt marshes, and similar low resistivitymaterial. In such environments, short chunkyzinc anodes work best. As the resistivity getshigher, the long anodes would be preferred.This could, for example, be in the general range

TABLE 2-1

COMMON ALLOY SPECIFICATIONS - MAGNESIUM

Element High Potential Grade A Grade B Grade C

Al 0.010% max 5.3 to 6.7% 5.3 to 6.7% 5.0 to 7.0%

Mn 0.50 to 1.30% 0.15 to 0.70% 0.15 to 0.70% 0.15 to 0.70%

Zn 0 2.5 to 3.5% 2.5 to 3.5% 2.0 to 4.0%

Si 0.05 % max 0.10% max 0.30% max 0.30% max

Cu 0.02% max 0.02% max 0.05% max 0.10% max

Ni 0.001% max 0.002% max 0.003% max 0.003% max

Fe 0.03 % max 0.003% max 0.003% max 0.003% max

Other 0.05% each or0.30% max

total

0.30 % max 0.30 % max 0.30 % max

Magnesium Remainder Remainder Remainder Remainder

SolutionPotential

-1.80 V -1.55 V -1.55 V -1.55 V

TABLE 2-2

COMMON ALLOY SPECIFICATIONS - ZINC

Zinc (Mil-A 18001) Zinc (ASTM B418-67 Type II)

Seawater Use Underground Use

Element Percent Element Percent

Aluminum 0.1 to 0.3% Special high-grade Zinc

99.99% pure

Cadmium 0.025 to 0.06%

Iron 0.005% max

Special high-grade zinc

Balance

Solution potential -1.10 V Solution potential -1.10 V


of 750 to 1500 ohm-cm resistivity.

A later section in this Chapter will discuss theadvantage of using a lower potential anodesuch as zinc in low resistivity environments.

Aluminum Anodes

Although investigated extensively in the past,there is not currently a type of chemical backfillthat is in common usage to permit the practicaland economical installation of aluminumanodes in earth burial applications. They do notwork well directly buried in the usual earthenvironments.

Proprietary aluminum alloy anodes areavailable which work very well in a sea waterenvironment. Cathodic protection current fromanodes so located but connected to buriedpipelines or other underground structures canprotect such structures effectively.

Table 2-3 contains technical data pertaining toaluminum anodes.

Use of Special Chemical Backfill

For best performance over the long term,magnesium or zinc anodes are best used witha special chemical backfill surrounding theanode. As indicated earlier, suitable chemicalbackfills are not commonly available for usewith aluminum anodes in earth burialapplications.

The special backfills have the followingadvantages:

1. The chemical backfi l l provideshomogeneous mixture contacting the anodesurface as opposed to, typically,heterogeneous earth backfill contacting theanode if the chemical backfill is not used.This uniform environment reduces theamount of self corrosion on the anodesurface with improved anode currentefficiency.

2. The chemical backfill isolates the anode

from soil chemicals which could have apassivating effect - an effect which canreduce or even completely stifle usefulcathodic protection current output from theanode.

3. Typically, chemical backfills used withmagnesium or zinc anodes have lowresistivity compared to most soils in whichthe anodes are installed. Assuming that ananode is to be installed in earth having aresistivity higher than that of the envelope ofchemical backfill being used around it, thenet effect will be an apparent increase inanode size. This means that the anoderesistance to earth will be lower than wouldbe the case if the anode were installedwithout chemical backfill. This lowerresistance in turn means higher currentoutput at the fixed galvanic anode cellpotential.

4. The chemical backfills commonly used withzinc and magnesium anodes have the abilityto absorb and hold moisture. Thischaracteristic is valuable in that it resistsearly anode drying out which would increaseanode resistance to earth and decreaseuseful current output.

Chemical backfills are made with variousmixtures of gypsum (CaSO4), Bentonite clay,and sodium sulfate (Na2SO4). A simple mix thatworks well with zinc consists of 50% Plaster ofParis (calcined gypsum) and 50% Bentoniteclay. The resistivity of this mixture after waterwetting is in the order of 250 ohm-cm.

Another chemical backfill mixture commonlyused with packaged anodes from suppliersconsists of 75% hydrated gypsum, 20%Bentonite clay, and 5% sodium sulphate. Theresistivity of this mixture after water wetting is inthe order of 50 ohm-cm. This mixture can beused both with zinc and magnesium anodes.The low resistivity is a distinct advantage inreducing anode-to-earth resistance as indicatedearlier.

Purchasing galvanic anodes of zinc or

TABLE 2-3

COMMON ALLOY SPECIFICATIONS - ALUMINUM

Element SeawaterGalvalum I

Saline MudGalvalum II

SeawaterBrackish Saline Mud

Galvalum III

Zinc 0.35 to 0.50% 3.5 to 5.0% 3.0%

Silicon 0.10% max - 0.1%

Mercury 0.035 to 0.048% 0.035 to 0.048% -

Indium - - 0.015%

Aluminum Remainder Remainder Remainder

Solution Potential -1.10 V -1.10 V -1.10 V


magnesium complete with attached connectingwire and chemical backfill in a bag or porouscontainer surrounding and centered on theanode is an advantage from the standpoint ofconvenience and speed of installation.

The disadvantage of using anodes completewith packaged backfill, particularly long (5 ft.)anodes in vertically augered holes, is thepossibility that voids may be left unintentionallybelow or around the package. After the backfillbag or porous container deteriorates with time,the chemical backfill can then settle into thevoids leaving part of the anode surface withoutthe benefit of the chemical backfill. In turn, lessthan optimum anode performance will occur.

The above-described hazard may be offset byconstruction methods and practices that willensure proper backfilling of the packagedanode.

Chemical backfills may be purchasedseparately from galvanic anodes. Someoperators prefer this, particularly when installingmultiple anode beds with long anodes.

GALVANIC ANODE INSTALLATIONPROCEDURES

Single Anode Installation

Where a single anode such as a 17-poundstandard size magnesium anode is to be used,general practice is to use a packaged anode.This would typically be installed as shown onFigure 2-2. Care is taken when backfilling theanode to be sure that there are no voids whichwill cause later trouble after the packagecontainer deteriorates.

If longer packaged anodes are used, the augerhole in which the anodes are installed, asshown also in Figure 2-2, should be largeenough so that when the packaged anode isplaced in position in the hole, the backfill maybe placed and tamped to prevent voids.

If, in specific situations, anodes are installed in

congested areas where they cannot be placedat an adequate lateral distance from thepipeline or other structure, they may be placedin deeper holes below the protected structurewith the required clearance between the bottomof the structure and the top of the anode. Thisis illustrated by Figure 2-3.

The installation of a single bare anode andseparate chemical backfill is illustrated byFigure 2-4. Using separate backfill with longvertical anodes makes it possible to place theanode centrally in the hole and place and tampthe chemical backfill powder (which is installeddry) so that the entire annular spacesurrounding the anode is filled completely withno voids.

Multiple Anode Installation

Where amounts of cathodic protection currentare needed at one location which are largerthan available from a single anode, multipleanode installations may be used. Such aninstallation is illustrated by Figure 2-5.

Anodes in a multiple anode installation may beeither packaged or bare with separatelyinstalled chemical backfill. The latter method isless likely to involve later problems with voids,particularly when using long anodes. Separatechemical backfill is preferably installed dry. Itcan either be wet down from the top afterinstallation to start anode activation or allowedto take up moisture from the earth. As thebackfill mix becomes water saturated it tends toswell which further ensures good contact withthe sides of the anode auger hole.

Anode Lead Attachment

The insulated lead wire from the anode may beconnected to the pipeline or other undergroundstructure by some form of exothermic weldingprocess to ensure long term low resistance inthe connection. Welding kits are availablewhich permit making a copper wire-to-steelconnection quickly and economically. Thecopper metal nub at the points of connectionmust be thoroughly insulated with suitable

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coating material to avoid having acopper-to-steel galvanic corrosion cell at thejunction between the two metals.

Test Points

Test points may be desired or required topermit periodic testing of galvanic anodeperformance. In order to accurately determineIR drop considerations, the anode installationsshould be installed to allow testing of currentoutput and interrupted potential readings. Thistype of testing necessitates surface test pointinstallation.

Ready made terminal box test points of variousdesigns are available from suppliers. A typicaltest point installation is illustrated by Figure 2-6.

CONCLUSIONS

This Chapter has described the different typesof galvanic anodes used for cathodic protectionand their general and specific uses. It also listssome of the advantages and disadvantages asthey relate to impressed current systems.

The Chapter also includes some installationsuggestions.

REFERENCE:

Peabody, A. W., “Control of Pipeline Corrosion”,National Association of Corrosion Engineers.

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INSTALLATION OF IMPRESSED CURRENT CATHODIC PROTECTION SYSTEMS3-1

CHAPTER 3

INSTALLATION OF IMPRESSED CURRENTCATHODIC PROTECTION SYSTEMS

INTRODUCTION

The objective of this chapter is to describe thedifferent types of impressed current anodebeds, the materials and equipment used, andthe installation procedures followed, to insureadequate performance of the installed systems.

This chapter will cover the three basic types ofimpressed current anode beds: (1) conventional(2) distributed and, (3) deep anode beds. It willalso discuss materials and installationpractices. The advantages and disadvantagesof each type of anode bed will also bereviewed.

In Chapter 2, Installation of Galvanic Anodes,the parameters for galvanic anode systemswere discussed. The main difference betweenthe two systems is that galvanic anode systemsmake use of the difference in potential betweenthe anode material and the structure while theimpressed current system uses an outside DCpower source.

DEFINITIONS

The following definitions are pertinent to the textof this Chapter:

Impressed Current Cathodic ProtectionSystem

A cathodic protection system that utilizes anexternal source of DC power in order to forcecurrent flow onto a pipeline or otherunderground/submerged structure, hencemitigating the discharge of corrosion current

from the structure to the electrolyte andreducing the loss of metal from the structure.

Conventional Anode Bed

Sometimes referred to as a “remote” or“point-type” anode bed. A group of anodesinstalled remote (usually 100 feet or more) fromthe structure and spaced on 10 to 30 footcenters. Some are installed with anodes placedhorizontally end to end in a continuous bed ofcoke breeze backfill. This type of anode bed isnormally installed perpendicular to the structurealthough it can also be installed parallel to it.

Distributed Anode Bed

A group of anodes installed close to and alonga structure to be protected. The anodes can beinstalled on one side or on both sides of thestructure at a distance of 5 to 20 feet from thestructure and on 25 to 500 foot centers.Distributed anode beds are normally used toprotect sections of pipelines which have poorcoating or which are bare. They are also usedfor localized protection or in areas whereshielding effect does not allow the current froma conventional type anode bed to reach thepipeline/structure. For the latter reason,distributed anode beds are very often used toprotect pipelines/structures within plantcomplexes.

The distributed anode bed is laid out ordistributed along the entire length of thepipeline/structure thus increasing the number ofanodes and the length of cable required. As aresult, installation costs are usually more


expensive than for a conventional anode bed.Because of the long cable runs normallyassociated with distributed anode bed systems,large gauge cables are often required/ desiredto reduce the voltage drop along the cable runs.A distributed anode system used to protect along pipeline may require the installation of tworectifiers, one at each end of the system, toreduce the voltage drop on the cable. Theinstallation of the distributed anode bedrequires more care than the conventional anodebed because the anodes are located close tothe pipeline/structure being protected and caremust be taken to avoid damage to the pipeline/structure.

An advantage of a distributed anode bedsystem, over a conventional anode bed, is thatit can be installed within a complexunderground network with little or no straycurrent problems being encountered on theforeign metallic structures in the area, due tothe close proximity of the anodes to thepipeline/structure and relatively low currentoutput of the individual anodes.

One disadvantage of the distributed anode bedsystem is that due to its close installation to theprotected pipeline/structure, the possibility ofdamage to the system during excavation of thestructure for maintenance or during constructionactivity is greatly increased.

Deep Anode Bed

A group of anodes, a continuous pipe, or wireanodes, installed in a vertically drilled hole atselected points along the route of a structure, orbetween several structures in a plant area. Theholes normally range from 100 to 600 feetdeep; individual anodes are usually located on10 to 30 foot centers. One variation of this typeof anode bed is the semi-deep anode bedwhere the depth of the hole ranges from 50 to150 feet. When it is desirable to install ananode bed system which has many of theoperating characteristics of a conventionalanode bed but for which there is no spaceavailable at the desired site location, the onlyway to install the anodes at a distance from the

pipeline/structure is to install them deep belowthe pipeline/structure.

Deep anode beds are also used when the soilstrata are so arranged that the upper layers ofsoil are of high resistivity with underlying layersof lower resistivity.

A deep anode bed is usually more expensive tobuild than a conventional anode bed ofcomparable capacity. In addition, the anodes ofa deep anode bed are well below grade leveland are therefore inaccessible for maintenanceshould failure occur in the anode material usedor in the connecting cables. It is often moreeconomical to abandon a malfunctioningsystem in place and install a new anode bedclose to the original one than to attempt systemrepairs.

POWER SUPPLIES FOR IMPRESSEDCURRENT CATHODIC PROTECTIONSYSTEMS

There are several types of equipment that canbe used as power sources for impressedcurrent cathodic protection systems. Amongthese are rect i f iers , so lar ce l ls ,engine-generator sets, wind poweredgenerators and turbine-generators. Althoughrectifiers are by far the most commonly usedsource of DC power for impressed currentsystems, a general basic description of each ofthe sources is given in the followingparagraphs.

Rectifiers

A device which takes alternating current from apower distribution system and converts it todirect current. Rectifiers have as their majorcomponents:

C a transformer to step down AC line voltage,which has taps on the secondary winding toallow selecting a wide range of voltages

C a rectifying element which converts the ACto DC


C a cabinet.

Other components normally found in rectifiersare AC circuit breakers, DC output meters,lightning arresters (on both AC and DC sides)and current measuring shunts.

Rectifiers are manufactured with many differentratings for single-phase and three-phase ACservice. The rectifying element consists ofeither selenium stacks or silicon diodes. Figure3-1 shows a wiring diagram for a single-phaserectifier and Figure 3-2 shows a wiring diagramfor a three-phase rectifier.

Rectifiers are manufactured with air cooled oroil cooled enclosures. For hazardous areas, oilcooled explosion-proof cabinets are available.Rectifiers can be of the conventional type(manual tap changing arrangement) or of theautomatic type: 1) constant potential whichmaintains a preset potential between thestructure and permanent reference electrode,and 2) constant current which maintains apreset current output. Figure 3-3 shows a wiringdiagram for a constant potential rectifier.

Solar Cells

In areas where sunlight is available for a largepercentage of the time, solar cells incombination with storage batteries can be usedto produce a steady flow of direct current.Silicon solar cells are made by coating siliconcrystals with phosphorus (produces negativecharge carriers) and with boron (producespositive charge carriers). When light hits thesilicon slices, a photocurrent flows, voltagedevelops, and electricity is generated.

The cathodic protection unit consists of thesolar panels, storage batteries and a controlcabinet. Panels are connected in series toincrease the voltage output and in parallel toincrease the current output.

Solar cells are a viable power source whereregular AC power is not available. Figure 3-4shows a schematic of a cathodic protectionsolar cell.

Thermoelectric Generators

Thermoelectric generators produce power bythe direct conversion of heat into electricity.Power is produced by maintaining atemperature difference across a thermopile, anassembly of semi-conductor thermoelectricelements. Combustion of Natural Gas orPropane provides the heat while naturalconvection provides the cooling required tocreate this temperature differential.

Engine - Generator Sets

These units consist of a fuel-powered engineand an AC generator used to provide the inputpower for a rectifier unit. The fuel required tooperate the engine can be tapped from thepipeline as shown in Figure 3-5, if the productcan be utilized as a fuel source. An alternativesource of engine fuel could be a storage tankinstalled in the area. A typical engine-generatorset may require 1500 gallons of diesel fuel tooperate for about a month.

Turbine - Generator Sets

Turbine generator sets utilize the product orgas flow to drive a small turbine which in turndrives a DC generator which supplies thecathodic protection current for the anode bed.

Figure 3-6 shows a typical turbine-generator setfor a natural gas pipeline. The gas is diverted tothe turbine through a bypass line, installedacross a delivery station where a drop in gaspressure is present, and returned to the system.Note the application of this system can only beused at locations where there is reasonablyconstant gas pressure drop or decrease inavailable product flow.

Wind Powered Generators

These systems provide a practical and veryeconomical electrical power supply for cathodicprotection installations which require acompletely independent power source.

A typical wind-powered generator consists of a

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turbine assembly, generator assembly,rectifying unit and a battery. The turbineassembly, which usually consists of threeblades, approximately 15 feet in length, isdirectly coupled to a slow speed generatorwhich can produce 200 watts at wind speeds of20 mph and can maintain a 700 watt continuousload at a location with average winds of 12mph. The AC voltage from the generator isconverted to DC voltage via a rectifying unit.This DC voltage is used to charge batterieswhich supply the required cathodic protectioncurrent. Figure 3-7 shows a typical windpowered generator set.

ANODE BED MATERIALS

Anodes

Various types of materials are available for useas impressed current anodes. The followingparagraphs present a list of some of thematerials used and some of theircharacteristics:

a. Scrap Iron

Scrap iron in the form of an old pipeline wasthe original impressed current anode used toprotect an underground structure. Scrap ironanodes are still used today, particularly inareas where regular anode materials areexpensive or hard to obtain.

Impressed current systems have beendesigned utilizing anything from scraprailroad car wheels welded together to forman anode for protection of an undergroundstructure to a half-sunken barge used as aremote anode to protect a dock structure.Some of the disadvantages of using scrapiron are the high consumption rates,nonuniform consumption, and coloredcorrosion products which may causediscoloration of the surroundings.

b. Graphite

Graphite anodes are manufactured frompetroleum coke particles. The process from

coke particles to an almost pure graphitecan take as long as 16 weeks because ofthe many steps involved in it. Graphiteanodes are normally impregnated with hotlinseed oil or microcrystalline wax to inhibitmoisture from penetrating the anode andcausing mechanical and chemicaldecomposition. Disadvantages of thegraphite anode are its softness andbrittleness. Therefore, they must be handledwith care during shipping and installation.Figure 3-8 shows a typical graphite anode.These anodes are manufactured in differentsizes.

c. High Silicon Cast Iron

The typical alloy for cast iron anodes isASTM A518 Grade 3. They containapproximately 15% silicon and 4.5%chromium. Like graphite anodes, they arevery brittle and must be handled with care.Figure 3-9 shows a typical high silicon castiron anode. These anodes are alsomanufactured in different sizes.

d. Platinized Titanium

Platinum works very well as an impressedcurrent anode because of its extremely lowconsumption rate and high current density.Because of the cost of platinum, it is usuallyapplied in a very thin layer onto a lessexpensive material. Titanium is used assuch a material. These anodes aremanufactured by either electrodepositing theplatinum on the metal substrate or by thecladding process. If there are voids in theplatinum surface, current can discharge fromthe titanium substrate. Titanium is generallypassivated at low DC potentials and doesnot discharge current to the electrolyte. Atvoltages in excess of 10 volts, thepassivation film breaks down rapidly andcurrent flows to the electrolyte from thetitanium resulting in rapid corrosion of thetitanium in the form of pitting. This can leadto failure of the anode. Some engineersrecommend that the voltage at the surface ofthe titanium substrate be kept below 10

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volts. It is important to note that this 10 voltlimit is the voltage at the titanium substrate,not necessarily the rectifier output voltage.With a packaged anode, the voltage acrossthe titanium/backfill interface is much lessthan the output voltage measured at therectifier terminals.

e. Platinized Niobium/Tantalum

These anodes are used where there isconcern over the use of platinized titaniumanodes because the system design requiresa driving voltage greater than 10 volts.Niobium or tantalum as base metals canwithstand higher driving voltages. (Thebreakdown voltage of niobium oxide film isapproximately 120 volts). These types ofmetal substrates are the most commonlyused for anode bed anodes installed in thesoil. Figure 3-11 shows one type of anodedetail and specification for a platinizedniobium anode.

f. Mixed Metal Oxide Anodes

These anodes consist of a titanium substratewith a dimensionally stable mixed metaloxide coating. Like platinum, mixed metaloxide anodes have an extremely lowconsumption rate and can be used at veryhigh current densities. Like platinizedtitanium anodes, the voltage limitations ofthe titanium substrate must be considered inthe design process.

g. Magnetite

These anodes have been used in Europe formany years. Magnetite anodes have beenimproved in recent years and are nowreadily available in the western hemispherefor underground and marine use.

Magnetite (Fe3O4) is a corrosion productitself therefore, there is little or no corrosioninvolved with the discharge of current fromthe surface of the anode. Although theseanodes are expensive, their long lifeexpectancy makes them an economical

choice in some applications. See Figure3-12 for typical magnetite details andspecifications.

Packaged Anodes

With the exception of scrap iron anodes, all ofthe anodes describe above are available inprepackaged form. The anodes are centered ina galvanized “stove pipe type” container orelectrical conduit, which is filled with lowresistivity backfill such as coke breeze. Figure3-10 shows a typical prepackaged anode andcanister specification.

Anode-Lead Connection Insulation

It is good practice to specify anodes with sometype of epoxy, polyethylene, or Kynar® cap toprotect the lead end of the anode from currentleakage which could lead to premature anodefailure. Figures 3-8, 3-9, 3-11 and 3-12 outlinetypical details of anode/lead connectioninsulations used to prevent these types ofproblems.

Cables

Impressed current cathodic protection systemsnormally require the installation of considerablelengths of direct buried cables. The primaryinsulation material used is high molecularweight low density polyethylene insulation(HMWPE). This insulation material hasoutstanding dielectric strength and moistureresistance. It also has a high resistance tocorrosive chemicals (mineral acids, fixedalkaline petroleum oils, etc.) usually existing inenvironments requiring cathodic protection.

Where severe conditions exist and polyethyleneinsulated cables are not suitable, specialinsulated cables such as Kynar® and Halar®are used. These insulations are resistant tochlorine and therefore are usuallyrecommended for use in deep anode bedswhere chlorine is usually present. Selecting acable size has both a practicable and economicaspect. The cable should be large enough tocarry the intended current based on one of the

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many ampacity tables that are available. Thecable should also be large enough to limit theIR drop which would require a bigger powersource. In addition, the cable should also belarge enough to withstand mechanical stressesencountered during system installation.

Cable Connections

Impressed current cathodic protection systemnegative return cables are normally connectedto the pipeline/structure using an exothermicweld process. Exothermic weld connections aremade by filling a graphite mold (see Figure3-13) which houses the conductor and the weldmetal (copper oxide and aluminum), and byigniting the powder with a flint gun. Theexothermic reaction which results producesmolten copper which flows into the weld cavityand welds the conductor to the pipe. The moldsare specified based on conductor size and onsurface type/pipe size.

Exothermic weld connections should beadequately coated using a minimum of twolayers of coal tar and glass fiber or equal. Patchcoating should overlap existing pipeline coatinga minimum of 2 inches.

When making header cable to header cable oranode lead to header cable connectionsvarious methods can be used, among them aresolder, exothermic weld, and compression typeand split bolt connectors. The compression typeconnection is probably the most recommendedmethod because the connection is madewithout heat and there is no danger of damageto the cable insulation or wire at the connection.The compression connector is applied with aspecial tool which lets the user know thatadequate compression has been reached,assuming that the proper tool and dies havebeen used. This is an advantage over the splitbolt connector where it is up to the user toestimate if sufficient torque has been applied.

Cable Splices/Repairs

The cables associated with the anode bed,connected to the positive terminal of the DC

power supply, are the most anodic part of theimpressed current system. Taking IR drops inthe cables into consideration, the cablesoperate at a higher voltage than the anodesthemselves. The impressed current anodematerial is formulated for low consumptionrates, but copper wires are not. One can readilysee that any damage to cable insulation orsplices in an impressed current system cancause a relatively large amount of current todischarge to earth from a small area. Rapidcorrosion leading to cable failure would occur.

Splice insulation and cable insulation repairmust be absolutely waterproof; otherwisecurrent will discharge to the electrolyte andcable failure will occur. The insulation materialuse must also have high resistance to corrosivechemicals. For this reason, a poured two-partepoxy system has been used with goodsuccess on impressed current cable splicesand repairs. The two-component epoxy systemis poured into a special mold which is placedaround the splice. The connection and adjacentcable insulation must be cleaned and abradedwith emery cloth. Hardening of the epoxyusually takes one-half hour depending onambient temperature. Backfill or immersionshould not take place until the epoxy is fullyhardened. Figure 3-14 shows a typical epoxysplice process for connection of anode leadwire to anode header cable.

An alternative method used to insulate aconnection is a hand wrapped joint. In order toachieve a good joint insulation, the connectionand surrounding area of insulation must first becleaned and abraded using emery cloth. Thenext step is to fill all voids with a soft rubbersplicing compound and then wrappingconnection with several layers of rubber tapefollowed by several layers of PVC electricaltape. Taping should start in the center of thecollection and end in the center. The first layersof tape should be stretched while being appliedand the tension should be relaxed onsubsequent layers to achieve a smooth surface.The final step is painting the taped jointcompletely with electrical shellac compound.Note: Shellac must be allowed to dry

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completely before allowing joint to contactearth.

SELECTING AN ANODE BED SITE

The corrosion control professional should takethe following factors into account whenconsidering an anode bed site:

C Soil ResistivityC Soil MoistureC Interference with Foreign StructuresC Power Supply AvailabilityC Accessibility for Maintenance and TestingC VandalismC Purpose of Anode Bed and Site Availability

Soil Resistivity

Soil resistivity is one of the most importantfactors to consider when selecting an anodebed site location. The number of anodesrequired, the DC voltage output rating of therectifier and the resulting AC power costs are alldirectly related to the soil resistivity. In theory,the lower the soil resistivity, the fewer thenumber of anodes required and the lower therectifier DC voltage output required. Ideally, thearea with the lowest soil resistivity should beconsidered for the anode bed site.

Soil Moisture

Another factor to consider in the selection of ananode bed site is the soil moisture at theproposed anode depth. In most cases, theresistivity of the soil decreases as the moisturecontent increases until the soil becomessaturated. When possible, anodes should beinstalled below the water table (assuming thewater resistivity is not too high), or in areas ofhigh moisture content such as swamps, ditches,creek beds, etc.

One factor that should be considered whenanalyzing the moisture content of the soil, is theseasonal variations. Moisture content and watertable elevation vary at different times of theyear and these variations would have an impacton the performance of the anode bed. When

anode beds are to be installed near the oceanor a river, tidal variations could have an effecton the water table elevation.

One phenomenon that affects the moisturecontent of the soil around the anodes is calledelectro-osmosis. This phenomenon will be dealtwith in the Advanced Course and is justmentioned in this Chapter to make thecorrosion control worker aware of it. When DCcurrent is discharged to the soil from an anode,this current pushes the water away from thesurface of the anode and could result in anincrease of the anode bed resistance. In thecase of an anode bed being installed in highclay content soil, the removal of moisture fromthe clay may seriously reduce the conductivityof the soil, possibly rendering the anode bedineffective.

Interference with Foreign Structures

Before selecting an anode bed site, localutilities should be contacted and area systemmaps should be obtained to determine if anyunderground metallic structures exist in thearea. Structures such as pipelines,underground metallic sheathed electricalcables, or well casings may be subjected tostray current interference problems therefore,whenever possible, anode beds should belocated away from foreign structures.

When the anode bed must be located in thevicinity of foreign structures, tests should beconducted before the installation of the anodebed to determine the possibility and magnitudeof interference problems that could be expectedand whether corrective action, if required, isfeasible. Subsequent to the installation of theanode bed, cooperative interference testingshould always be conducted to ascertainremedial measures required.

Impressed current interference testing will bediscussed in detail in Intermediate Chapter 5.


Power Supply Availability

The selection of an anode bed site may beinfluenced by the availability of an economicalsource of power to be used to provide therequired cathodic protection DC current.

Accessibility for Maintenance and Testing

The anode bed site selected must beaccessible to construction vehicles for anodebed installation, testing and repairs. Whenpossible, the anode bed should be located nearpublic roadways or where right-of-way canaccommodate vehicular traffic to facilitateperiodic testing and maintenance. If the anodebed is located in an area which is not easilyaccessible due to the environmental conditions,such as in swampy areas, the rectifier and teststations should be located away from the anodebed in some more accessible location.

Anode bed inaccessibility may promote a lackof maintenance/testing which could result inundetected anode bed malfunctions and lack ofprotection for the pipeline/structure.

Vandalism

Anode bed sites near schools and playgroundsshould be avoided due to the possibility ofvandalism.

If system design required the installation of ananode bed in an area which may be considereda problem area as far as vandalism isconcerned, precautions should be taken toprotect the above ground system components.Some steps that could be taken includeinstalling rectifiers in locked sheds, behindfenced areas or where possible at pumpingstations or at terminals. Test stations can beinstalled in underground vaults or in lockedflush mounted test enclosures.

In establishing the need for vandalismprevention measures, the corrosion controlworker should ascertain if the area is one wheretest equipment would have to be installed lateron for extended periods of time (such as

overnight). One such area would be wherevariable stray currents are prevalent and24-hour recordings may be required duringperiodic surveys. Steps could be taken duringthe construction stages of the anode bed toprovide protective enclosure for the testequipment.

Purpose of Anode Bed and Site Availability

Two additional factors that have to beconsidered in the process of selecting an anodebed site are the intended purpose of the anodebed and the possibility of acquiring the requiredright-of-way for the installation. Is the intent ofthe anode bed to protect a long section of apipeline (assuming the pipeline is well coated)or is it the intent to afford localized protection toa short section of a pipeline (poor coating) or asmall structure? In the first case, the approachcould be the installation of one remote typeanode bed near the midpoint of the pipelinesection or perhaps the installation of a deepanode bed near the midpoint area.

In the second case, the approach could be theinstallation of a distributed anode bed.

Once a determination has been made as to theintent and purpose of the installation and themost suitable type of installation, the landavailability should be investigated.

If the installation consists of a distributed anodebed or even a deep anode bed, it is verypossible that the installation can be made withinthe existing right-of-way agreement in whichcase, the only requirement may be to notify theland owner (through the company'sRight-of-Way Department). If on the other hand,a remote type of anode bed is contemplated,the company's Right-of-Way Department shouldbe requested to investigate availability of landat the desired and at possible alternatelocations. It is desirable to select a locationwhere the entire installation can be within oneproperty.


INSTALLATION PRACTICES

Anodes - General

Before installing anodes, the decision to installanodes either vertically or horizontally must bemade. The decision usually depends uponvarious factors such as soil resistivity andequipment availability.

Regardless of the type of installation, graphiteand cast iron anodes are brittle and have to behandled with care. Do not handle anodes by thelead as this could put too much stress on theanode-lead connection, causing the loss of theanode.

Vertical Installation

After determining that the area is suitable forthis type of installation (no shallow underlyingrock), a typical vertical anode can be installedas shown in Figure 3-15. Generally, fewervertical anodes are required than horizontalanodes to protect the same structure. The mainreason for this is that vertical anodes canusually be placed in permanently moist soilwhich generally results in lower anoderesistance.

When installing a vertical anode system, thefirst step is to install the anode header or feedercable. The next step is to auger a hole for theanode. The depth of the hole is based ondesign calculations and type of anode beingused. An extra foot or more of depth is requiredwhen installing an anode which is notprepackaged to allow coke breeze to beinstalled and compacted below and above theanode. After augering the anode hole andinstalling and compacting a layer of cokebreeze, if required, the anode should belowered into hole using support rope tiedaround the anode. Center the anode in the holeand backfill with coke breeze or native soil asrequired. Native soil backfill should be tampedabove anode and coke breeze backfill takingcare not to damage anode lead wire.

After completing the anode hole backfill, the

anode lead wire should be spliced to theheader cable. Sufficient slack should be left onthe lead wire to prevent damage due to soilsettlement.

See Figure 3-16 for a typical prepackagedvertical anode.

For a distributed anode bed system, the exactlocation of the installed anodes and splicesshould be noted on the design drawings forfuture reference.

Horizontal Installation

Where it is necessary to horizontally installanodes because of soil resistivity or equipmentlimitations, the first step, as with the verticalanode installation, is to install the anode headercable. The next step depends on the spacingrequired between the anodes. If the anodespacing is larger than 10 feet, a separateexcavation may be required for each anode tobe installed. If however, the spacing is less than10 feet, it may be more economical to dig onecontinuous trench for the length of the anodebed installation.

Prepackaged anodes can be simply loweredinto the trench by hand or by means of supportropes and backfilled with native backfill tampingperiodically as required.

Anodes requiring the placement of coke breezebackfill during installation require a trench deepenough to allow the placement of at least 6inches of compacted coke breeze on all sidesof the anode. Figure 3-17 shows a typicalhorizontally installed prepackaged anode.

Deep Anode Installation

Installation of a deep anode bed such as theone shown in Figure 3-18 begins with thedrilling of a hole to the required anode depth. Inthe case shown in Figure 3-18 this would be a10-inch x 100 foot hole. The next step in thisinstallation is to install the 40-foot section ofPVC casing followed by the installation of the10-inch diameter, 60 foot long steel pipe

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through the PVC casing. The anodes aresecurely fastened to the 2 inch perforated PVCsupport/vent pipe using straps. Similarly, theanode lead wires are fastened to the pipe andrun to the top of the pipe. The PVC support/ventwith the anodes is then installed and centeredin the hole. The next step is to pump fluidizedcoke breeze backfill into the first 60 feet of thehole, using the PVC support/vent pipe or aseparate hose and pipe inserted into the hole.A cement plug is normally used to seal off thedeep anode. The top 40 feet of the hole isbackfilled with native backfill tampingperiodically as required.

Figure 3-19 shows a similar installation usingan uncased hole. A deep anode using aplatinized anode wire is shown in Figure 3-20.Figure 3-21 shows a capsule type of installationthat can be used to contain one of a number oftypes of anodes.

The last step in the deep anode bed installationwould be the installation of an anodedistribution box and the termination of theanode lead wires therein.

Cable Installation

Anode header cables and system negativereturn cables are normally installed byexcavating a trench and installing the cables ata minimum depth of 2 feet below grade. Beforelaying cables in the trench, remove any largestones or foreign material which may damagethe insulation of tile cable. The first three ormore inches of backfill around the cablesshould be free of stones and foreign materialand thoroughly tamped. The rest of the trenchis backfilled with native soil tamping periodicallyas required.

When installing anode header cables for adistributed anode bed system, it isrecommended that wood planks or plasticwarning tape be installed over the cable asshown in Figure 3-17 to afford some protectionto the ground during later construction in thearea or excavation of the protected structure.

Distributed anode bed header cables, wherethe installation of the wood plank or warningtape is not deemed necessary, can be plowedin. When this is done, extreme care must betaken so as not to damage the cable insulationand more importantly, care must be taken toavoid possible damage to the pipe due to theclose proximity of the header cable to the pipe.

Any damage to the anode header cable or itsinsulation shall be repaired in accordance withone of the methods described in the previoussection, Cable Splices/Repairs. Repairs todamaged negative return cables, shall also bemade as described in that section.

Rectifier Installation

When selecting a rectifier location, effortsshould be made, when possible, to select alocation which is accessible for periodicinspection and maintenance. Rectifiers aretypically either pole mounted, wall mounted, orpedestal mounted as shown in Figures 3-22,3-23 and 3-24 at the end of this Chapter. Typeof mounting used depends on the size andlocation of the unit. Units weighing in excess of450 lbs. are normally pedestal or pad mounted,as shown in Figure 3-22. If the rectifier is to belocated in an area which is prone to floodingproblems the unit should preferably be eitherwall or pole mounted above the maximum waterlevel. When wall or pole mounting a rectifier itshould be installed, if possible, at a convenientworking height, for maintenance and periodictesting.

Before installing a rectifier unit, local electricalcodes should be checked. By conforming tolocal code requirements during installation, timeand possibly money may be saved in areaswhere local electrical inspector's certification isrequired before the local utilities will supply ACpower.

The rectifier cabinet should be grounded forsafety reasons. Rectifiers located near anexisting grounding system should be tied into itif possible. Where no grounding system existsin the area, a ground rod should be installed

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and the rectifier connected to it using No. 6AWG stranded copper cable.

CONCLUSIONS

The Chapter has described the different typesof impressed current anode beds and has alsodescribed the steps that the corrosion controlworker has to take to obtain the necessary datato install an impressed current anode bed.

In addition, the different anode materials andDC Power sources have been described as wellas other miscellaneous items such as backfill,splice and insulation materials, etc. The chapterconcluded with some installation suggestionsfor the different types of anode beds.

CRITERIA FOR CATHODIC PROTECTION4-1

CHAPTER 4

CRITERIA FOR CATHODIC PROTECTION

INTRODUCTION

In Chapter 3 of the Basic Course, the theoryand principles of how cathodic protection workswere presented and discussed. To assure thatcathodic protection is applied in accordancewith these principles, criteria and methods ofassessment are required. This chapterdescribes the industry accepted criteria. Thediscussion found below is a review of the NACEcriteria presented in NACE StandardRP0169-96 (1996 Revision) "Control ofExternal Corrosion of Underground orSubmerged Metallic Piping Systems." Thisdocument "l ists criteria and otherconsiderations for cathodic protection that willindicate, when used either separately or incombination, whether adequate cathodicprotection of a metallic piping system has beenachieved." The document goes on to state thatthe corrosion control programs are not limited tothe primary criteria listed below. It states that"Criteria that have been successfully applied onexisting piping systems can continue to be usedon those systems. Any other criteria used mustachieve corrosion control comparable to thatattained with the criteria here-in," referring tothe three primary criteria described below.Some examples of other criteria that have beenused in the past for underground piping includethe 300 mV Shift, the E-log I, and the NetCurrent Flow Criterion.

CRITERIA FOR STEEL AND CAST IRONPIPING

Three primary criteria for cathodic protection ofunderground or submerged steel or cast iron

piping are listed in Section 6 of NACE StandardRP0169-96.

(1) -850 mV (Cu-CuSO4) With The CathodicProtection Applied

(2) A Polarized Potential of -850 mV (Cu-CuSO4 )

(3) 100 mV of Polarization Decay

The Net Protective Current Criterion is alsolisted in Section 6 under a special conditionssection, for bare or poorly-coated pipelineswhere long-line corrosion activity is the primaryconcern. The application and limitations of eachare given below.

-850 mV With CP Applied Criterion

The full criterion states that adequate protectionis achieved with "A negative (cathodic) potentialof at least -850 mV Cu-CuSO4 with the cathodicprotection applied. This potential is measuredwith respect to a saturated copper/coppersulfate reference electrode contacting theelectrolyte. Voltage drops other than thoseacross the structure-to-electrolyte boundarymust be considered for valid interpretation ofthis voltage measurement." "Consideration isunderstood to mean application of soundengineering practice in determining thesignificance of voltage drops by methods suchas:

Measuring or calculating the voltage drop(s),

reviewing the historical performance of the


cathodic protection system,

evaluating the physical and electricalcharacteristics of the pipe and itsenvironment, and,

determining whether or not there is physicalevidence of corrosion."

Application

Of the three criteria listed above, the -850 mVCriterion with cathodic protection applied isprobably the most widely used for determiningif a buried or submerged steel or cast ironstructure has attained an acceptable level ofcathodic protection. In the case of a buriedsteel or cast iron structure, an acceptable levelof protection is achieved, based on thiscriterion, if the potential difference between thestructure and a saturated copper-copper sulfatereference electrode contacting the soil directlyabove and as close as possible to the structureis equal to or more negative (larger in absolutevalue) than -850 mV. As described above,voltage drops other than those across thestructure-to-electrolyte boundary must beconsidered for valid interpretation of this voltagemeasurement. These voltage drops are a resultof current flow in the electrolyte (soil) and aregenerally referred to as ohmic or IR voltagedrops. IR voltage drops are more prevalent inthe vicinity of an anode bed or in areas wherestray currents are present and generallyincrease with increasing soil resistivity.

For bare or very poorly-coated structures, IRvoltage drops can be reduced by placing thereference electrode as close as possible to thestructure. For the majority of coated structures,most of the IR voltage drop is across thecoating and the measurement is less affectedby reference electrode placement. The IRvoltage drop can also be minimized oreliminated by interrupting all of the DC currentsources of the cathodic protection system andmeasuring the instantaneous "off” potential.The off-potential will be free of the IR voltagedrop errors if all of the current sources,including sources of stray currents, have been

properly interrupted and if long-line currents arenegligible. Long-line currents occur on astructure as a result of the presence of macro-cells. The difference between the on- and theoff-potential indicates the magnitude of the IRvoltage drop error when the measurement ismade with the protective current applied.

This criterion was originally adopted based onthe observation that the most negative nativepotential observed for coated underground steelstructures was about -800 mV Cu-CuSO4. Theassumption was made that macro-cell corrosionwould be mitigated if sufficient CP current isapplied to raise (in the negative direction) thepotential of the entire structure to a value that ismore negative than the native potential of thelocal anodic sites. A potential of -850 mV wasadopted to provide a 50 mV margin ofprotection. The effectiveness of the criterionhas been demonstrated over many years ofapplication.

Limitations

There are a number of limitations with thiscriterion. The potential reading should be takenwith the reference electrode contacting theelectrolyte directly over the structure, tominimize ohmic voltage drop errors in themeasurement and to minimize the extent ofaveraging over large areas of the structure.Alternative criteria may be required where thereference electrode cannot be properly placed,such as at river crossings or road crossings.The criterion also is most commonly used forwell coated structures, where it can beeconomically met. For poorly-coated or barestructures, the high CP currents required tomeet this criterion can be prohibitive, such thatalternative criteria are typically used.

Potentials can vary significantly from one areaof an underground structure to another as aresult of varying soil conditions, coatingdamage, interference effects, etc. This createsthe possibility that potentials less negative than-850 mV Cu-CuSO4 exist between themeasurement points. This problem can beaddressed for pipelines by means of


close-interval surveys. If the close-intervalsurvey establishes that this problem exists, it ispossible to maintain more negative potentials atthe test stations to insure that adequateprotection is achieved on the entire structure.However, the more negative potentials requiredwill result in increased power consumption.

More negative potentials than -850 mV Cu-CuSO4 also are required in the presence ofbacteria or with a hot pipeline. In the lattercase, the current required for CP can increaseby a factor of two for every 10º C (18º F)increase in temperature of the pipe. Thepotential criterion is adjusted to compensate forthe increased anodic current kinetics. Typically,a potential of -950 MV Cu-CuSO4 is used forhot pipelines. In the case of microbes, thekinetics of the corrosion reaction and theenvironment at the pipe surface are alteredsuch that a more negative potential is typicallyrequired to mitigate corrosion. Where thepresence of microbes are confirmed orsuspected, a minimum potential criterion of-950 mV Cu-CuSO4 is typically used.

Care should be exercised to avoidoverprotection, which can result in coatingdamage and may promote hydrogen damage ofsusceptible steels. The potential above whichcoating damage can occur is a function of manyvariables, including the soil composition andmoisture content, temperature, coating type, thequality of the coating application, and thepresence of microbes. The general consensusin the industry is to avoid polarized (instant off)potentials more negative than 1.05 to 1.1 V Cu-CuSO4.

The older steels generally contain higher levelsof impurities, such as sulfur and phosphorus,and exhibit higher susceptibility to hydrogendamage than the newer cleaner steels. In theseolder steels, the microstructures associatedwith hard spots and welds typically are muchmore susceptible to hydrogen damage than themicrostructure of the wrought base metal.Again, the general consensus in the industry isto avoid polarized (instant off) potentials morenegative than 1.05 to 1.1 V Cu-CuSO4 in order

to minimize hydrogen damage in these steels.

Potentials also can vary on a seasonal basis asa result of variation in the soil moisture content.Thus, some pipeline companies perform annualsurveys at the same time each year, such thattrends in the behavior can be properlyinterpreted. It should be cautioned that thisapproach does not preclude the possibility thatthe criterion is not being met on some parts ofthe structure during portions of the year.

Limitations also exist in the ability to accuratelymeasure the potential of the structure in thepresence of telluric currents and whereshielding by disbonded coatings, rocks, thermalinsulation, etc. has occurred. Similarly, theaccuracy of the potential measurement iscompromised by the presence of multiplepipelines in a right-of-way where the pipelineshave varying coating conditions, and by straycurrents that cannot be interrupted.

Dynamic stray currents, from sources such asDC transit systems and mining activities, posea significant challenge in applying this criterion.Where dynamic stray currents are suspected, itis generally necessary to obtain potential valuesover the duration of the stray current activity,typically twenty-four hours or longer. Forexample, for DC transit systems, it is oftenpossible to obtain fairly stable on-potentials ofthe structure in the early morning hours whenthe transit system is not operated. Thesepotentials can provide base-line data for whichto evaluate other measurements. Of course,significant interpretation of such data isrequired. DC stray currents not only affect theability to obtain accurate off-potentials, theyalso influence the polarized potential of thepipe. Nevertheless, the -850 mV Criterion withcathodic protection applied is the most commoncriterion used in areas of significant dynamicstray current activity. It is generally acceptedthat the structure is protected at a test locationif the potential of the structure remains morenegative than -850 mV at all times, even withsignificant fluctuations associated with thedynamic stray currents. It may be necessary toincrease the number of test points and the


frequency of surveys in areas of dynamic strayDC currents over those used on other parts ofa system to insure that adequate protection isbeing achieved.

Polarized Potential of -850 mV Criterion

This criterion states that adequate protection isachieved with "A negative polarized potential ofat least -850 mV relative to a saturatedcopper/copper sulfate [Cu-CuSO4] referenceelectrode." The polarized potential is defined as"The potential across the structure/electrolyteinterface that is the sum of the corrosionpotential and the cathodic polarization." Thepolarized potential is measured directlyfollowing interruption of all current sources andis often referred to as the off or "instant off"potential. The difference in potential betweenthe native potential and the off or polarizedpotential is the amount of polarization that hasoccurred as a result of the application of thecathodic protection. As previously stated, thedifference in potential between the on-potentialand the off-potential is the error in the on-potential introduced as a result of voltage dropsin the electrolyte (soil) and metallic return pathin the measuring circuit.

Application

This criterion is more direct than the previouscriterion (the -850 mV Criterion with cathodicprotection applied) in that it clearly defines themethod by which voltage drops errors in the on-potential are considered; in this criterion theyare minimized or eliminated. The voltage droperrors, which are often referred to as ohmicpotential drop or IR drop errors, occur as aresult of the flow of cathodic protection or straycurrent in the electrolyte (soil) or in thestructure. They are measurement errors in thatthe cathodic polarization at the structure-to-electrolyte interface is the only part of the on-potential measurement that contributes to areduction in the rate of corrosion of thestructure. As described above, polarization isdefined as the difference in potential betweenthe native potential and the off- or polarizedpotential. It is referred to as cathodic

polarization if the potential shift is in thenegative direction.

This criterion is most commonly applied tocoated structures where the sources of DCcurrent can be readily interrupted. An examplewould be a fusion bonded epoxy (FBE) coatedgas transmission pipeline in a rural area with animpressed current cathodic protection system.

Limitations

A significant limitation of this criterion is therequirement that all sources of DC current beinterrupted. For standard survey techniques,the interruption must be performedsimultaneously on all current sources. On gastransmission pipelines, interrupting all currentsources may require the use of a large numberof synchronous interrupters for all rectifiers,sacrificial anodes, and bonds affecting thesection of pipeline that is being evaluated. Insome cases, the number of rectifiers affectinga test section is not known without experimentalverification. On gas distribution systems,sacrificial anodes are more commonly used forcathodic protection and the electrical leads forthe anodes are usually bonded directly to thepipe with no means available to interrupt thecurrent. For these situations, this criterioncannot be used. Achieving the criterion alsomay require the application of high CP currents,resulting in over-protection of some portions ofthe structure, and related problems such ascathodic disbondment of coatings, andhydrogen embrittlement of susceptible steels.As described above, a more negative potentialthan -850 mV Cu-CuSO4 may be required tomitigate corrosion on hot pipelines or in thepresence of MIC, further increasing thelikelihood of over-protection.

Many of the difficulties of accurately measuringstructure-to-electrolyte potentials, describedunder limitations to the -850 mV Criterion WithCathodic Protection Applied apply to thePolarized Potential of -850 mV Criterion as well.These include access to the structure, seasonalfluctuations in the potential between testingtimes, spatial fluctuations in potential between


test stations, the presence of multiple pipelines,having varying coating conditions, in a right-of-way, telluric current effects, and shielding of thestructure surface by disbanded coatings, rocks,thermal insulation, etc.

100 mV of Polarization

This criterion states that adequate protection isachieved with "A minimum of 100 mV ofcathodic polarization between the structuresurface and a stable reference electrodecontacting the electrolyte. The formation ordecay of polarization can be measured tosatisfy this criterion." Of the three criteria, thiscriterion has the most sound fundamental basis.As described in previous chapters, thecorrosion rate decreases and the rate of thereduction reaction on the metal surfaceincreases as the underground structure ispolarized in the negative direction from thenative potential. The difference between thecorrosion rate (expressed as a current) and therate of the reduction reaction is equal to theapplied cathodic protection current. Theseprocesses can be shown graphically in an Eversus Log I diagram, referred to as an Evansdiagram. The slope of the anodic (corrosion)reaction is referred to as the anodic Tafel slopeand typically has a value of about 100 mV perdecade of current. With this Tafel slope, thecorrosion rate of a structure decreases by afactor of ten (order of magnitude) for every 100mV cathodic shift in the polarized potential. Anorder of magnitude decrease in the corrosionrate of an underground structure typically ismore than adequate to effectively mitigatecorrosion.

The cathodic polarization also promotesbeneficial changes in the environment at thepipe surface, such as reducing oxygen,increasing the pH, and moving halides such aschlorides away from the metal surface, furtherdecreasing the corrosion rate. These beneficialchanges in the environment at the metalsurface are referred to as environmentalpolarization in that the environmental changestypically result in a shift in the free corrosionpotential of the pipe in the negative direction.

Thus, the total potential shift from the nativepotential (excluding voltage drops in the soil),includes components due to environmentalpolarization and cathodic polarization.

As described in the criterion, the magnitude ofthe polarization shift can be determined bymeasuring its formation or decay. In order todetermine the magnitude of the shift as a resultof the formation of polarization, it is firstnecessary to determine the native potential ofthe underground structure at test locationsbefore cathodic protection is applied. Thepotential measurement is then repeated afterthe cathodic protection system is energized andthe structure has had sufficient time to polarize.Typically, the on-potential is continuouslymonitored at one test location directly followingenergization of the cathodic protection systemand an off-potential reading is made when thereis no measurable shift in the on-potentialreading with time over a period of severalminutes. The off-potential is then comparedwith the native potential; if the difference isgreater than 100 mV, then the 100 mV criterionhas been satisfied at that location. Off-potentialreadings are then obtained at the other testlocations to determine whether the criterion ismet at these locations. The time required forsufficient polarization to develop is highlydependent on the nature of the structure(coating condition, underground environment,types and number of bonds, etc) and the designof the cathodic protection system. From apractical standpoint, it is wise to re-examine theoverall structure-CP system if a reasonableamount of polarization does not develop withina few hours of energizing the cathodicprotection system.

An alternative method of assessing theformation of cathodic polarization is to measurethe on-potential immediately followingenergization of the cathodic protection systemand then to remeasure the on-potential after afew hours to days of operation of the CPsystem. If the on-potential shifts in the cathodic(negative) direction by more than 100 mV, thenit can be conservatively assumed that thecriterion has been met. This is because the


applied CP current generally decreases withtime, decreasing the magnitude of the IRvoltage drop. Thus, the total shift in the on-potential must be a result of the sum ofadditional cathodic polarization andenvironmental polarization of the structure; bothof which reduce the corrosion rate of thestructure and are included in the 100 mV ofpolarization in the criterion. If this method isused, it should be confirmed that the appliedcathodic protection current decreased with time.

Measuring the positive potential shiftassociated with polarization decay that occursfollowing de-energizing the cathodic protectionsystem is the most common method ofdetermining the amount of polarization. Whena cathodic protection system is de-energized,an instantaneous positive shift in the structure-to-soil potential occurs as a result of theelimination of the IR voltage drop in the soil.The potential measured at this time is referredto as the off-potential, as previously described,and is used as the starting point for assessingthe polarization shift. There may be a spike inthe potential reading immediately followinginterruption of the CP system as a result ofinductive effects of the pipeline and CP system.This spike may last a few hundred millisecondssuch that the off-potential is typically measured200 to 500 milliseconds following interruption.

The potential will then exhibit an exponentialdecay in the positive direction as the capacitoracross the structure to electrolyte boundarydischarges. This component of the potentialshift is the cathodic polarization of the structureas a result of the applied cathodic current. Agradual linear decay in the potential will thenoccur over minutes to weeks as a result of areturn of the environment at the pipe surface toa native condition. This component of thepotential shift is the environmental polarization.To obtain the total polarization shift, the finalpotential after polarization decay is measuredand subtracted from the off-potential. If thisdifference is greater than 100 millivolts, then thecriterion has been satisfied.

Application

The 100 millivolt Polarization Criterion is mostcommonly used on poorly-coated or barestructures where it is difficult or costly toachieve either of the -850 mV Criterion. In manycases, 100 mV of polarization can be achievedwhere the off-potential is less negative than -850 mV Cu-CuSO4. The application of the 100mV Polarization Criterion has the advantage ofminimizing coating degradation and hydrogenembrittlement, both of which can occur as aresult of over protection. In piping networks, the100 mV Polarization Criterion can be used forthe older, poorly-coated pipes while a -850 mVCu-CuSO4 polarized potential criterion can beused for the newer piping in the network.Because of its fundamental underpinnings, the100 mV Polarization Criterion also can be usedon metals other than steel where a specificpotential required for protection has not beenestablished.

Limitations

There are a number of limitations with thiscriterion. The time required for fulldepolarization of a poorly-coated or barestructure can be several days to several weeks,making the method very time consuming andleaving the structure unprotected for anextended period of time. Fortunately, much ofthe depolarization occurs within a few hoursand it frequently is not necessary to wait for thefull decay, except where the total polarization isvery close to 100 mV. Once the criterion hasbeen met, it is not necessary to continuewaiting for further depolarization. At the otherextreme, if a depolarization of less than 50 mVis measured within a few hours, it isquestionable whether the 100 mV PolarizationCriterion can be achieved. At this point, it maybe prudent to assess whether a longer wait fortotal depolarization is justified.

The 100 mV Polarization Criterion is frequentlyused to minimize the costs for upgradingcathodic protection systems, and theassociated increase in power costs, in areaswith degrading coatings. Because of the


complicated nature of the measurements, thecost of conducting surveys for the assessmentof the 100 mV Polarization Criterion isconsiderably higher than for the -850 mVCriteria. Thus, an economical analysis may berequired to determine whether there is actuallya cost savings associated with application ofthe 100 mV Polarization Criterion.

The 100 mV Polarization Criterion should notbe used in areas subject to stray currentsbecause 100 mV of polarization may not besufficient to mitigate corrosion in these areas. Itis generally not possible to interrupt the sourceof the stray currents in order to accuratelymeasure the depolarization. All DC currentsources affecting the structure, includingrectifiers, sacrificial anodes, and bonds must beinterrupted in order to apply this criterion. Inmany instances, this is not possible, especiallyon older structures where the criterion is mostlikely to be used.

The 100 mV Polarization Criterion should notbe used on structures that contain dissimilarmetal couples because 100 mV of polarizationmay not be adequate to protect the active metalin the couple. This criterion also should not beused in areas where the intergranular form ofexternal stress corrosion cracking, also referredto as high-pH or classical SCC, is suspected.This is because the potential range for crackinglies between the native potential and -850 mVCu-CuSO4 such that application of the 100 mVPolarization Criterion may place the structure inthe potential range for cracking. Net Protective Current Criterion

RP0169-96 states under paragraph 6.2.2.2(Special Conditions) that; "On bare orineffectively coated pipelines where long-linecorrosion activity is of primary concern, themeasurement of a net protective current atpredetermined current discharge points fromthe electrolyte to the pipe surface, as measuredby an earth current technique, may besufficient" for cathodic protection to beachieved.

This statement establishes the fourth criterionfor cathodic protection of underground piping,referred to as the Net Protective CurrentCriterion. This criterion was originally based onthe concept that, if the net current at any pointon a structure is flowing from the electrolyte tothe structure, there cannot be any corrosioncurrent discharging from that point on thestructure. The theory of electrochemical kineticsshows that corrosion can occur at a point on astructure that is collecting net cathodic currentfrom the electrolyte as long as the polarizedpotential is more positive than the equilibriumpotential. Nevertheless, the criterion can beeffective, from a practical standpoint, becausethe collection of net cathodic current at anypoint along the structure produces beneficialcathodic polarization and also promotesbeneficial changes in the environment at thestructure surface, as described above.

Typically, the criterion is applied by firstperforming a close-interval structure-to-soilpotential survey or a cell-to-cell potential surveywith the cathodic protection system de-energized in order to locate the anodicdischarge points along the pipeline. For thesurveys to be effective, the CP systems mustbe de-energized for a sufficiently long time suchthat all polarization has decayed. The CPsystem is then energized and the structure isallowed to polarize. A side drain method is thenused at the anodic discharge points todetermine whether the structure is receivingcathodic current at these locations. With theside drain method, the potential differencebetween an electrode placed directly over thestructure and one placed on either side of thestructure is measured. If the electrode locatedover the pipe is negative with respect to theother two electrodes, then current is collectingon the pipe at the location and the criterion issatisfied.

Application

The Net Protective Current Criterion is normallyused on poorly-coated or uncoated structureswhere the primary concern is long-linecorrosion activity. The technique also is


normally only used in situations where othercriterion cannot be easily or economically met.With the exception of these applications, thiscriterion is not a standard criterion forestablishing the effectiveness of a cathodicprotection system.

Limitations

There are a number of limitations with thiscriterion. First and foremost is the fact that thecriterion essentially states that any magnitudeof net current flow to the structure (andtherefore, any amount of cathodic polarizationof the structure) is adequate to mitigatecorrosion. In general, this is not the case andtherefore, the criterion should only beconsidered for use as a last resort. Applicationof the criterion should be avoided in areas ofstray current activity or in common pipelinecorridors because of the possibility ofmisinterpretation of the potential readings. Thecriterion also may not be effective in areas withhigh resistivity soils, for deeply buried pipelines,or where the separation distance of thecorrosion cells is small. Finally, the side drainmeasurements at a given location are onlyindicative of the direction of current flow at thatlocation and are not necessarily representativeof behavior elsewhere on the pipeline. Thus, forthe application of this criterion, it is generallynecessary to perform side-drain measurementsat close intervals (2 to 20 feet) along thepipeline.

Other Criteria For Steel And Cast Iron

The four listed for steel and cast Iron piping arethe only acceptable criterion listed inRP0169-96 for underground or submergedmetallic piping. However, other criterion can beused on a piping system where they have beenused in the past and it can be demonstratedthat their use has resulted in effective cathodicprotection. Other criteria also can be used onunderground structures such as reinforcedconcrete pipe, piling, etc. The two mostcommon other criteria that have been used inthe past for underground structures are the 300mV Potential Shift Criterion and the E-Log I

Curve Criterion.

300 mV Potential Shift Criterion

The 300 mV Potential Shift Criterion wascontained in the original version of RP0169 andstated that adequate protection is achieved with"A negative (cathodic) voltage shift of at least300 mV as measured between the structuresurface and a saturated copper-copper sulfatehalf cell contacting the electrolyte.Determination of this voltage shift is to be madewith the protective current applied." Thiscriterion is similar to the 100 mV PolarizationCriterion, where the latter is assessed based onthe formation of polarization on a structure.With both criteria, it is first necessary todetermine the native potential of theunderground structure at test locations beforecathodic protection is applied. The potentialmeasurement is then repeated after thecathodic protection system is energized and thestructure has had sufficient time to polarize.The difference between the two criteria is that,in the case of the 300 mV Potential ShiftCriterion, the on-potential is used forassessment of the criterion while, in the case ofthe 100 mV Polarization Criterion, the off-potential is used for assessment. In the 300 mVPotential Shift Criterion, it is stated that "TheCorrosion Engineer shall consider voltage (IR)drops other than those across the structure-electrolyte boundary for valid interpretation ofthe voltage measurements." Thus therelationship between the 300 mV Potential ShiftCriterion and the 100 mV Polarization Criterionis analogous to the relationship between the -850 mV (Cu-CuSO4) With Cathodic ProtectionApplied Criterion and the Polarized Potential of-850 mV (Cu-CuSO4) Criterion.

The 300 mV Potential Shift Criterion has mainlybeen used for mitigation of moderate rates ofuniform corrosion of bare steel structures. It hasbeen applied for protection of entire structuresand also for hot-spot protection. On thesestructures, native potentials of 200 mV to 500mV Cu-CuSO4 are commonly observed and ithas been found that a 300 mV shift is adequateto mitigate corrosion in some instances. Thus,


the development of the criterion was empirical.The 300 mV Potential Shift Criterion is moreapplicable to impressed current CP systemsthan to galvanic anode systems becausegalvanic anodes may not have sufficient drivingvoltage to meet the criterion where negativenative potentials are encountered.

Probably the most successful application of thiscriterion has been on steel reinforced concretestructures. These structures typically havenoble native potentials (200 to 400 mV Cu-CuSO4), and passive steel surfaces, with theexception of hot spots, such that a potentialshift of 300 mV can be readily achieved.Application of this criterion avoids problemsassociated with over-protection.

Many of the limitations associated with the 100mV Polarization Criterion are applicable to the300 mV Potential Shift Criterion as well. Theseinclude the time required for polarization, thepossibility of moving the potential into thecracking range for SCC, and difficulties in areascontaining stray currents or galvanic couples. Ingeneral, the 300 mV Potential Shift Criterionshould not be used where high-pH SCC isconfirmed or suspected, or where stray currentsor galvanic couples are present. In the originalversion of RP0169 it is stated "This criterion ofvoltage shifts applies to structures not incontact with dissimilar metals."

Probably the single greatest limitation of the300 mV Potential Shift Criterion is thatsituations will exist in the field where thecriterion appears to be applicable yet corrosionmay not be adequately mitigated. In somesituations, the majority of the potential shift willbe the result of IR voltage drops in the soil oracross the coating and very little polarization ofthe structure will occur. This is the reason thatthis criterion was removed from the primary listof criteria in the 1992 version of RP0169.

E-Log I Curve Criterion

The E-Log I Curve Criterion also is found in theoriginal version of RP0169. The criterion statesthat adequate protection is achieved with "A

voltage at least as negative (cathodic) as thatoriginally established at the beginning of theTafel segment of the E-Log-I curve. Thisvoltage shall be measured between thestructure surface and a saturated copper-copper sulfate half cell contacting theelectrolyte." The criterion was originallydeveloped based on an incorrect interpretationof an E-Log I curve. It was thought that a breakin the cathodic curve exists as the structure ispolarized from the native potential and that the"break" has some fundamental significance.This break was thought to occur at thebeginning of the "Tafel Region." A review of thetheory of cathodic protection indicates that thenet cathodic current measured at any appliedcathodic potential is equal to the differencebetween the rate of the reduction reaction andthe rate of the oxidation reaction. An Evansdiagram (potential versus log current plot)shows that there is a smooth transition fromzero current, at the native potential, to the linearTafel region. The Tafel region starts when therate of the oxidation (corrosion) reaction isnegligibly small in comparison to the rate of thereduction reaction. Depending on the Tafelslopes for the oxidation and reductionreactions, the beginning of the Tafel region canvary between 50 mV and 100 mV cathodic fromthe native potential.

Presently, the E-Log I Curve Criterion is rarelyused for evaluating existing cathodic protectionsystems. However, the measurementtechnique, originally developed for applying theE-Log I Curve Criterion, is now most commonlyused to determine the minimum currentrequired for protection. The structure-to-soilpotential, using a remote reference electrode, isplotted as a function of the current output of acathodic protection system. Typically, it isnecessary to use an interruption technique andoff-potentials for making the E-Log I plot inorder to accurately establish the curve. Thepotential required to achieve a desiredminimum current value is identified on thecurve. This value should be at least as negativeas the value at the beginning of the Tafel regionof the E-Log I curve. Once the potential andcurrent values have been established, future


surveys consist of checking the current outputof the cathodic protection system and thepotential of the structure with respect to theremote reference electrode, placed in the samelocation as that used in the original E-Log Itests.

Because of the elaborate nature of thetechnique, its use is generally limited tostructures where conventional means ofassessment are difficult. Examples include rivercrossings for pipelines, well casings, and pipingnetworks in concentrated areas such asindustrial parks. The technique can giveerroneous results in areas of stray currents.The reference electrode must be placed in thesame location each time the potential ismeasured. Furthermore, there is no guaranteethat a repeat E-Log I curve will yield the sameresults as the original curve.

CRITERION FOR ALUMINUM PIPING

In RP0169-96, there is a single criterion foraluminum piping. Under paragraph 6.2.3.1,"The following criterion shall apply; a minimumof 100 mV of cathodic polarization between thestructure and a stable reference electrodecontacting the electrolyte. The formation ordecay of this polarization can be used in thiscriterion." This criterion is identical to the 100mV Polarization Criterion used for cast iron andsteel.

There are two precautionary notes, underSection 6.2.3.2, that are unique to aluminumpiping; one dealing with Excessive Voltages(Paragraph 6.2.3.2.1) and one dealing withalkaline conditions (Paragraph 6.2.3.2.2). UnderParagraph 6.2.3.2. 1, it is stated that:

"Notwithstanding, the minimum criterion inSection 6.2.3.1, if aluminum is cathodicallyprotected at voltages more negative than 1200mV measured between the pipe surface and asaturated copper/copper sulfate referenceelectrode contacting the electrolyte andcompensation is made for the voltage dropsother than those across the pipe-electrolyteboundary, it may suffer corrosion as a result of

the buildup of alkali on the metal surface. Apolarized potential more negative than 1200 mVshould not be used unless previous test resultsindicate that no appreciable corrosion will occurin the particular environment."

Under Paragraph 6.2.3.2.2, it is stated that:"Aluminum may suffer from corrosion underhigh-pH conditions and application of cathodicprotection tends to increase the pH at the metalsurface. Therefore, careful investigation ortesting should be made before applyingcathodic protection to stop pitting attack onaluminum in environments with a natural pH inexcess of 8.0.”

The basis for these cautionary notes is theincompatibility of aluminum in high pHenvironments. The protective passive films onaluminum break down in high pH electrolytes,leading to significant increases in the corrosionrate, even at relatively negative potentials.

In addition to these precautionary notes,several of the limitations for the 100 mVPolarization Criterion for steel and cast ironalso apply to aluminum. These include the timeconsuming nature of the measurementtechnique, difficulties associated withinterrupting all current sources, and limitationsin applying the criterion on structures withdissimilar metals and in the presence of straycurrents. Because no other criterion isapplicable to aluminum, good engineeringpractice must be used to address theselimitations. For example, sources of straycurrent should be identified and eliminated, ifpossible. Aluminum piping should be isolatedfrom other metals before cathodic protection isapplied (Isolation of aluminum is required forthe cathodic protection criterion of dissimilarmetals under Section 6.2.5 of RP0169-96, seebelow).

CRITERION FOR COPPER PIPING

In RP0169-96, there is a single criterion forcopper piping. Under paragraph 6.2.4.1, "Thefollowing criterion shall apply; a minimum of 100mV of cathodic polarization between the


structure and a stable reference electrodecontacting the electrolyte. The formation ordecay of this polarization can be used in thiscriterion."

This criterion is identical to the 100 mVPolarization Criterion used for cast iron, steeland aluminum. There are no precautionarynotes with this criterion but several of thelimitations with the 100 mV PolarizationCriterion for steel and cast iron also apply tocopper. These include the time consumingnature of the measurement technique,difficulties associated with interrupting allcurrent sources, and limitations in applying thecriterion on structures with dissimilar metalsand in the presence of stray currents. Sourcesof stray current should be identified andeliminated, if possible. Since copper is a noblemetal, steel, cast iron, or other metals usuallywill undergo preferential galvanic attack whencoupled to copper. Therefore, it is desirable toeliminate such dissimilar metals couples beforethe application of cathodic protection.

CRITERION FOR DISSIMILAR METALPIPING

In RP0169-96, there is a single criterion fordissimilar metal piping. Under paragraph6.2.5.1, the following criterion is listed: "Anegative voltage between all pipe surfaces anda stable reference electrode contacting theelectrolyte equal to that required for theprotection of the most anodic metal should bemaintained." There is one precautionary note,under Paragraph 6.2.5.2:

Amphoteric materials that could be damaged byhigh alkalinity created by cathodic protectionshould be electrically isolated and separatelyprotected." Amphoteric metals includealuminum, titanium and zirconium.

In practice, this criterion only applies wherecarbon steel or cast iron is coupled to a morenoble metal such as copper. In this situation,either of the 850 mV criterion would apply (850mV (Cu-CuSO4) With The Cathodic ProtectionApplied or A Polarized Potential of 850 mV (Cu-

CuSO4). Other criteria, such as the 100 mV ofPolarization Decay Criterion would not beapplicable.

ACKNOWLEDGMENT

This chapter was written by J. A. Beavers andK. C. Garrity and was edited by the AUCSCCurriculum Committee.

STATIC STRAY CURRENT INTERFERENCE TESTING5-1

CHAPTER 5

STATIC STRAY CURRENT INTERFERENCE TESTING

INTRODUCTION

The objective of this chapter is to provideenough information to allow the CorrosionProfessional, first to recognize an interferenceproblem based on pipe-to-soil potential dataand then determine what steps are required toeliminate or control the problem. The primary cause of interference currents orstray currents was initially related to operationof direct current traction systems, such as railsystems, mining carts, and the like. Due to theincreased use of impressed current cathodicprotection systems for pipelines and otherunderground and submerged metallicstructures, these types of cathodic protectionsystems have also become another majorcause of interference problems. This chapter will discuss how to determine thesource of the interference current, determinethe point of maximum exposure, and describemethods to mitigate the problem. DEFINITIONS

Interference currents, or stray currents, aredefined as currents performing work in onegrounded plant, which leaks to, and flowthrough electrolytic paths, and are picked up bynearby grounded plants. Interference currentsare normally separated into two mainclassifications as follows: 1. Static Interference - These stray currents

are those which maintain constantamplitude and constant geographical

paths. Examples of typical sources arerailroad signal batteries and impressedcurrent cathodic protection systems.

2. Dynamic Interference - These stray

currents are those, which are continuallyvarying in amplitude and/or continually,changing their electrolytic paths. They canbe man-made or caused by naturalphenomena. Typical examples ofman-made sources are DC weldingequipment and DC electrical railwaysystems. Natural sources such as telluriccurrents are caused by disturbances in theearth's magnetic fields caused by sun spotactivity.

Although the Corrosion Professional needs agood working knowledge of both types ofinterference currents, we will only focus onstatic interference for this chapter. DynamicInterference will be discussed in the AdvancedCourse. STATIC INTERFERENCE During this chapter you will find that all forms ofinterference correction require considerablefield work and testing. It can be time consumingand therefore very expensive. Much of theexpense can be saved if careful planning isdone in the design stage before a cathodicprotection system is installed. This means thatall involved know of the potential problem, andare aware of foreign lines, and otherunderground structures, in the vicinity of thenewly proposed system.


One of the easiest ways of keeping abreast ofpossible interference problems in an area isthrough a Corrosion Coordinating Committee.These committees are sometimes known as"electrolysis committees” and are composed ofindividuals representing companies thatoperate, maintain, or engineer undergroundstructures, such as pipelines, cables, tanks andtheir associated cathodic protection systems.By being involved or requesting informationfrom one of these committees, the CorrosionProfessional can possibly avoid costly,time-consuming field testing in the designphase of the system. Most companies conduct periodic corrosions u r v e y s o n t h e i r u n d e r g r o u n dstructures/pipelines. Pipe-to-soil potentialmeasurements are normally taken along thepipeline during the survey and the data plottedon a long strip chart or graph, distance(horizontal axis) vs. potentials (vertical axis).Figure 5-1 shows a typical potential survey plotwith no interference problems. The pipe-to-soilpotential survey is the best way to ensureproper operation of a company's own cathodicprotection system.

This survey is also a means of determining theexistence of interference problems. Figure 5-2and 5-3 show potential plots for coated andbare pipelines respectively, which areexperiencing stray current interference. Basedon the plotted data, interference may besuspected if: C Voltage curve profile shows abnormal

variation from previous survey curves. C High negative values are noted remote from

any cathodic protection system on thesurveyed line.

C Low negative or positive voltages are

present. Note: Plots of coated lines normally havesmoother voltage attenuation curves asopposed to bare lines which often have jaggedcurves making them more difficult to interpret.

Small variations in potential can normally beignored provided that adequate levels ofcathodic protection are maintained. Although Figures 5-2 and 5-3 only show onesurvey or one year’s worth of data, theCorrosion Professional can now utilizecomputer software which can plot several yearsof pipe-to-soil data on one graph. This allowsfor an easy comparison of voltage curves ofprevious surveys. INTERPRETATION OF DATA Interpretation of data can best be illustratedthrough an example. Let us look at a coatedpipeline section being cathodically protected bya conventional impressed current anode bed.There are four foreign pipelines crossing ourprotected line, as shown in Figure 5-4, and atest station exists at each line crossing us, asshown in Figure 5-5. All pipe-to-soil potentials should be taken usinga high impedance (10 megohms or greater)voltmeter, connected as shown in Figure 5-5,while interrupting the rectifier associated with orinfluencing the line being tested. When takingpotentials at line crossings, the referenceelectrode should be placed in contact with thesoil directly above the pipe crossing. A pipelocator should be utilized to locate this point.

When interpreting data on multiple pipelines, itis best to look at each crossing individually, asshown in the examples. Crossing A - Figure 5-6

The example shows that with our rectifier "on"the pipe-to-soil potential for our line is -0.89.Pipeline A has a pipe-to-soil potential of -0.86.When the rectifier is switched "off" our potentialbecomes more positive (-0.85). Where PipelineA has become more negative (-0.88).

The pipeline under test is considered to beprotected, based on the recorded test data. Itcan be expected however, that a coatingholiday exists near the point of crossing. This

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conclusion is based on the fact that thepotential of the foreign line decreased (orbecame more positive) when our rectifier wasswitched "on”. This is an indication that there isan appreciable amount of current flowing to ourline, thus creating an area of more negative soillocally around the foreign line. The foreign line at the crossing is cathodicallyprotected. Its potential at the crossing (a localcondition) is decreased by the energization ofour rectifier. However, the reduction is notsufficient to indicate loss of protection. In thisparticular case no corrective measures wouldbe required.

Crossing B - Figure 5-7

The example shows that with our rectifier "on"the pipe-to-soil potential for our line is -1.85.Pipeline "B" has a pipe-to-soil potential of -0.48.When the rectifier is switched "off" our potentialbecomes more positive (-1.04). Where Pipeline"B" has become more negative (- 0.71). The data indicates that our pipeline iscathodically protected. The foreign pipeline atthis crossing is not fully protected because itspotential is well below 0.85 volts with ourrectifier "off”. When the rectifier was energized,the foreign line potential was drastically shiftedin the positive direction, indicating that severedamage may be occurring on the foreign line. Inthis example corrective measures would bedeemed necessary. Due to the close proximity of the foreignpipeline to the anode bed, as shown, theinterference can be expected to occur as aresult of current picked up by the foreignpipeline where it passes through the gradientfield surrounding the anode bed. Two testsshould be conducted to verify the influence ofthe anode bed. First, if the potential of our lineis the same or almost the same with respect toa remote electrode as it is to the electrodedirectly over the point of crossing, this wouldtend to indicate that no coating damage ispresent on our line. Second, if the potential ofthe foreign pipeline, measured with respect to

a reference electrode placed directly above it,in areas near the anode bed, swings in thenegative direction when the rectifier is switched"on”, current pick-up by the foreign line isindicated. Where interference is found, it is necessary todetermine if the actual crossing is the point ofmaximum exposure (explained later in thischapter). This is done by moving a referenceelectrode, in contact with the electrolyte,several feet at a time, first in one direction andthen in the other, away from the point ofcrossing and directly above the foreign line. Ifan area exists where the positive swing isgreater than at the actual crossing, theelectrode should then be moved in smallerincrements until the maximum point of exposureis located in this area. All information such aslocation, "on" and "off" potentials, and voltagechanges ()V) at this point should be recorded. It would rarely be necessary to move theelectrode more than 100 feet in either directionfrom the point of the actual pipeline crossing.The point of maximum exposure may belocated in an area other than the point of theactual crossing if the soil resistivity variesconsiderably in the crossing area or it thecoating (if any) on the foreign pipeline varies inquality. Crossing C - Figure 5-8

This example shows, that with our rectifier "on"the pipe-to-soil potential for our line is -0.71.Pipeline "C" potential is -0.75. When ourrectifier is switched "off" our potential becomesmore positive (-0.65), where Pipeline "C"potential show no change (-0.75).

Based on the data, our line is not receivingadequate levels of cathodic protection in thisarea. This could be due to interference causedby the cathodic protection system for Pipeline"C". If the potential of our line is measured withrespect to a reference electrode locatedremotely to both the foreign and our line andthe potential is found to be representative ofnormal protective potentials, the less negative

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potential at the crossing is shown to be a localcondition. The foreign pipeline crossing mostlikely causes this condition. Correctivemeasures must be taken in this case.Determine the length of the tested line that isbelow -0.85, can be done by taking closepipe-to-soil potential readings directly above theline in both directions from the point of crossing.Measured data should be recorded and thenplotted, potential versus distance. If theresulting graph is similar to the one shown inFigure 5-9, then interference from the cathodicprotection system on the foreign line isconfirmed.

Another test to confirm this interference is tointerrupt the rectifier on the foreign line andtake pipe-to-soil potentials on our line, with thereference electrode located directly over thecrossing. If the potential of the pipe shifts in thepositive direction when the rectifier isenergized, then the interference is confirmed.The foreign pipeline in this case is notadversely affected. Crossing D This example is shown in Figure 5-10. Thisexample shows, that with our rectifier "on" thepipe-to-soil potential for our line is -0.97.Pipeline "D" potential is -0.65. When ourrectifier is switched "off" our potential becomesmore positive (-0.93), where Pipeline "C"potential shows no change (-0.65). Based on the potentials our line appears to beadequately protected. The potentials measuredon Pipeline "C" indicate that this line is notcathodically protected or the level of protectionachieved is not satisfactory. The cathodicprotection system protecting the line under testhas no effect on the foreign line. Therefore, nomitigation is required. Finding the Source of Static Interference

As discussed in Chapter 2 of the Basic Course,the presence of static stray currents is notreadily apparent due to their steady statecharacteristics. In such cases, the presence of

static stray current effects is picked up bydetailed potential measurements on a structure,which reveal typically concentrated areas ofanodic condition. Structure currents flowing inboth directions toward an area of currentdischarge may also reveal them. As indicated,a detailed potential survey in the vicinity of theprotected structure is normally required tolocate the source of the interference. Potentialmeasurements are normally taken with respectto a copper-copper sulfate reference electrode.If low negative or positive pipe-to-soil potentialsare noted, this is a good indication that aforeign structure may exist nearby, whoseprotective current is affecting the surveyedstructure. That foreign structure should beconsidered as the prime suspect for containinga current source which is creating interferenceproblems and should be investigated. The first step is to locate the structure andidentify it. The next step is to search for acurrent generating device or another structurein the vicinity of the known structure, which maycontain a current source. The following stepsshould be followed in the search: 1. Inquire of its Owners.

2. Follow the foreign pipeline geographically.

3. Examine its route maps. In the case where no foreign structure isevident in the vicinity of the low negative orpositive pipe-to-soil potential measurements, itwill be necessary to conduct an investigationthrough local utility companies, local citizens, oras previously mentioned through informationavailable from Corrosion CoordinatingCommittees in the area. Corrosion Professionals can use other tests,such as earth current readings, to track thepath of the current back to the source. Theeasiest way to trace the current is with the useof two identical reference electrodes and a highimpedance voltmeter. By measuring thepotential between the two electrodes spacedapproximately 25 feet apart, the direction of

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RECTIFIER

CURRENT

INTERRUPTER

STATION 1915 + 00

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LINE D

OUR LINE

FOREIGN LINE CROSSING D

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current flow can be determined and its pathtraced. When the suspected source is found, testing asshown on previous examples must beperformed to confirm that this is indeed thecause of the abnormal pipe-to-soil potentials.Because these tests require interrupting thecurrent source, you must receive the Owner'spermission, prior to conducting any testing. Ifthe suspected source is not the cause of theabnormal potentials the search must go on untilthe actual current source is located. Determining the Point of MaximumExposure The point of maximum exposure is defined asthe geographical location at which the mostadverse electrolytic effect exists. This pointcoincides with the location where the bulk of thestray currents are flowing from the metallicstructure/pipeline into the electrolyte. It isimportant to understand that at this pointelectrolytic corrosion will occur on the affectedstructure unless the interference current ismitigated. The point of maximum exposure can be locatedby measuring the pipe-to-soil potentials in thesuspected drainage area while interrupting theinterfering current source. The location wherethe maximum positive voltage change ismeasured when the current source is energizedindicates the point of maximum exposure asshown in Figure 5-11.

The interference at the point of maximumexposure must be cleared. This may not alwaysbe the most convenient place to establish acurrent drainage bond to the interferingstructure. If the drainage bond is installedelsewhere, the interference at the point ofmaximum exposure must still be cleared. Figure5-12 shows a bond installed between the twopipelines at the fence. This bond, however, hasto clear the interference at the point ofmaximum exposure, to mitigate the problem.

MITIGATION OF STATIC INTERFERENCE Mitigation by Bonds After it has been established that aninterference problem exists and the point ofmaximum exposure has been located, a bondcan be installed between the two structures. Abond is a means of draining the stray currentsoff the affected structure by a non-electrolyticcondition. This is due to the fact that electrolyticcorrosion occurs at the point of maximumexposure or where current enters the soil(electrolyte). To mitigate the electrolyticcorrosion process, a low resistance currentdischarge path must be provided between thetwo structures. A metallic conductor can beused to provide this path if the polarity is correctand the potential difference between the twopipes is large enough to draw sufficient currentthrough the conductor. At the point of connection or bond, the potentialof the interfering structure must be morenegative than the structure effected. If this isnot the case, current will not flow in the properdirection. If the current flows in the wrongdirection, from the interfering structure to theeffected structure, the results would beincreased interference. This condition ofreversed current flow is known as negativeresistance. An ideal point to connect the bond would be thenegative terminal of the DC source ofinterference. In the case where the DC sourceis a cathodic protection rectifier, the effectedpipe/structure could be bonded to the structurethrough the negative side of the rectifier. Theinterfering structure is actually an extension ofthe rectifier's negative terminal. Bond or drain conductors are normally installedbetween structures at the point of crossing.This keeps bond conductor lengths short, thuskeeping the resistance low and maximizingcurrent flow. Bond conductor size is aresistance factor and therefore, must be takeninto account. Cable sizes for bonds typicallyrange from AWG No. 6 to AWG No. 10 but in

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some rare cases these sizes may not be largeenough. It is also important to install the proper"shunt" size or rating. If the current to bedrained is 1 amp, the shunt must be rated for 1amp or slightly larger. If the shunt is rated lowerthan the 1 amp the bond connection could belost.

Note: The Owner or operator of thepipeline/structure causing the interference mustbe in agreement with the remedial measuresprior to installation or connection to theirpipeline/structure. You must obtain theirpermission prior to making any connections. Itis also recommended to receive permissionfrom all effected parties prior to removal of anyestablished bond or remedial measures. The bonding method is used extensively andwhen installed correctly, provides some benefitto the effected pipeline/structure, but at thesame time always results in some loss ofprotection on the interfering structure. Variousfactors such as the distance between the anodebed and the affected or foreign structure, therectifier current output, and the coatingresistance of both structures are some of theitems which determine the degree of benefitand detriment. The following is an example of mutual effectswhich could occur when bonding interferingpipelines/structures. It is only a guideline; eachlocation must be tested properly and evaluatedon an individual basis (see Figure 5-13). 1. A coated cathodically protected line

bonded to a bare unprotected line has adetrimental effect on the coated line and anegligible effect on the bare line.

2. A bare cathodically protected line bondedto a bare unprotected line has adetrimental effect on the bare line and littleeffect on the unprotected line.

3. A bare cathodically protected line bondedto a coated unprotected line has anegligible effect on the bare line but has agreat effect on the coated line.

4. A well-coated cathodically protected linebonded to a well-coated unprotected linehas moderate effect on both lines.

Mitigation by Addition of CathodicProtection A cathodic protection system utilizing galvanicanodes or an impressed current system may beregarded as a device which "draws" current offthe protected structure through a metallic pathand "pumps" some of that current back to thestructure through an electrolytic or solution pathgiving it protection. The galvanic anode(s) orthe drain rectifier, whichever is used, providesthe driving potential for the drained current. Agalvanic anode or impressed current system ofsufficient strength will cause the flow of currentback to the interfered structure in a greatermagnitude than was draining off, causing aprotective current flow. All drain current thenuses the metallic lead, which connects to eitherthe galvanic anode or the negative terminal ofthe drain rectifier. If the pipe-to-soil potential ofthe draining pipeline/structure is maintained ata potential more negative than -0.85 both at thepoint of maximum exposure, there will no longerbe a detrimental flow of current. The mostattractive aspect of using a galvanic or rectifieranode bed drain as opposed to a bond, is thatthere is no detrimental effect placed upon theinterfering protection system Rarely is a rectifierand anode bed installed for the sole purpose ofmitigating stray currents. However in areaswhere pipelines/structures of severalcompanies are adequately protected and testsshow that at crossings no structure is lessnegative than -0.85 volt, then anode beds areeffectively acting as mitigationdrains. Galvanic anode drains are commonly used inlieu of bonds where small amounts of draincurrents flow. In areas of large current drains,the use of galvanic anode drains would beimpractical due to the high consumption rate ofthe anode, which would result in frequent anodereplacement. The amount of drain current canbe determined experimentally by installingtemporary galvanic drainage to clear the

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interference and measuring the current throughthe drain wire to the anode(s) using anammeter. The measured current can becorrelated with consumption rates of the anodematerial to determine the theoretical life of thegalvanic anode. The consumption rate can beobtained from the manufacturer of the anode.One test anode can be used to obtain datarequired to extrapolate the requirements foradequate interference clearance. Natural Potential Criterion The natural potential criterion is often used todetermine if the clearance of interferencecurrents through remedial measures such asbonds, galvanic or impressed current anodebed drains, has been achieved. The basis ofthis criterion is that the effected structure has itspipe-to-soil potential restored to its original ornatural potential which existed at the point ofmaximum exposure before the interferingcurrent shifted it in the positive direction. Theinstallation of a galvanic anode usuallyincreases the pipe-to-soil voltage of thestructure to a more negative level than thenatural potential. If requested or required avariable resistor in the galvanic anode drainlead could be used to limit the drain current toa level where the natural potential is restored.This will also lower the consumption rate andincrease the life of the galvanic anode. The natural potential is the most commonlyused criterion in the setting of interferencemitigation measures, because in mostinstances it is the simplest and mosteconomical approach. It is also considered, bymost, to be an equitable solution to theinterference problem as the foreign line isreturned to its original potential. Figure 5-14shows a simple example of the natural potentialcriterion. The following are step-by-step procedureswhich should be used to set a resistance bondbased on the natural potential criterion. Step 1: Observe and record the naturalpotential of the interfered structure before

energizing the rectifier and before making anyconnections between the two structures. Step 2: Connect a current interrupter in serieswith the positive output terminal of the rectifier.Set interrupter at a convenient cycle, such as10 seconds "on" and 5 seconds "off". Step 3: Place portable copper-copper sulfatereference electrode in contact with theelectrolyte at the point of maximum exposureand observe and record the shift in pipe-to-soilpotentials. Step 4: Install a variable resistor between theeffected and interfering structures. With therectifier "on", vary the resistance value until thepipe-to-soil potential on the effected structurereaches the same value as the natural potentialrecorded during Step 1.

Step 5: Measure the final resistance of the testbond between the two structures and record it. Step 6: Reinstall resistance of a proper type forpermanent installation or fabricate a wireresistor with the same resistance. Make surethe size of the wire used is adequate for themaximum current drain. Step 7: Retest if deemed necessary. Mitigation of the interference current iscomplete when the final bond is installed andthe pipe-to-soil potential of the interferedstructure has been restored to its original ornatural potential.

TYPICAL EXAMPLES OF STATICINTERFERENCE As stated before, every interference problem isa special case because there are too manyvariables involved and therefore, it is doubtfulthat there will be two exactly the same.Interference cases however, can be put intogroups or classifications. If the CorrosionProfessional can learn to recognize into whichclassification(s) an interference problem falls,

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he can use the basic concepts shown in thefollowing examples to mitigate the interference.

Case #1 This is a situation where a protected pipeline iscrossed by several unprotected foreign lines(see Figure 5-15). It is always good practice tobegin testing at the crossing closest to theimpressed current anode bed and then workprogressively away from the anode bed alongthe interfering structure. After all bonds havebeen made to the structures, which areadversely affected, each bond should bereexamined and, if necessary, readjusted asrequired. It is usually found that subsequentbonding will normally reduce all preceding bondcurrents. Adjustment of any given bond maynecessitate the adjustment or readjustment ofother bonds so that all bonds should beexamined and reexamined in a "round robin"fashion until no appreciable readjustment isnecessary. Figure 5-15 shows four foreign lines crossingthe protected line, which is protected by animpressed current system using one rectifier.The four lines cross the protected line in thearea of influence of its anode bed. Foreign Line#1 and #2 appear to be connected to eachother and should be electrically continuous.Since only one current source is involved, the"natural potential" criterion testing procedureshould be used at each crossing. As statedabove, start with Foreign Line #2 because it isclosest to the anode bed. The naturalpipe-to-soil potential of Foreign Line #2 shouldbe measured and recorded. A bond should notbe installed however, until the natural potentialof Foreign Line #1 is measured and recordedbecause it appears to be electrically continuouswith Foreign Line #2. When the naturalpotentials are measured on both lines, a bondshould be installed between the interfering lineand Foreign Line #2. The resistance should beadjusted to clear the interference from ForeignLine #2. Conduct tests at Foreign Line #1crossing to determine if the bond installed onForeign Line #2 has cleared Foreign Line #1 orwhether a tolerable increase of Foreign Line #2

bond current will clear Foreign Line #1. If adjusting the bond at Foreign Line #2 cannotattain Foreign Line #1 clearance, it will then benecessary to treat Foreign Line #1 as aseparate structure and clear the interferencewith a separate bond to the interfering line.

Next, using the same criterion, clear ForeignLine #3 and Foreign Line #4. At this point allnecessary bonds will have been established,but they must be retested to determine if thebond current at any pipeline crossing is nowexcessive. This would be due to the weakenedinfluence of the impressed current anode bed atall crossings. Retesting at crossings andreadjustments of bond resistances (ifnecessary), should be conducted in the sameorder as the original tests. If substantialadjustments are made during the second roundof tests, a third round of testing should beconducted. Case #2 In this case we have a cathodically protectedline, with two or more impressed current anodebed systems, crossing an unprotected foreignline (see Figure 5-16). When more than one interference source suchas many closely spaced rectifiers on a givenpipeline/structure are causing interference on aforeign line, the sum of the interference from allthe current sources must be cleared at the pointof maximum exposure.

The natural potential of the foreign line can bemeasured by de-energizing both/all rectifiers orcurrent sources on the protected line. Rectifiersmay then be re-energized and a current drainbond installed from the foreign line to theprotected line, restoring the foreign line to itsnatural potential.

In a situation where there are many rectifiersprotecting one pipeline and influencing aforeign pipeline, testing may be simplified byinterrupting rectifiers separately. As eachrectifier is de-energized, the effect at the

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crossing should be measured and recorded. Allrecorded changes should then be added andthe sum effect cleared by a bond at thecrossing.

Let's consider the following example: 1. Assume that the measured foreign pipe-to

soil potential at the point of crossing withboth rectifiers energized is -0.30 volts.

2. When Rectifier #1 is de-energized theforeign pipe-to-soil potential shifts from-0.30 volts to -0.60 volts, a voltage changeof +0.30 volts.

3. When Rectifier #2 alone is de-energizedthe foreign pipe-to-soil potential shifts from-0.30 volts to- 0.40 volts, a voltage changeof +0.10 volts.

4. Add both voltage changes, 0.30 + 0.10 =0.40 volts.

5. Install a resistance bond between the twopipelines so that with both rectifiersoperating the 0.40 volt positive shift on theforeign line is cancelled. This condition willbe reached when the foreign pipe-to-soilpotential reads 0.70 volts (which is thenatural potential) with both rectifiersoperating.

The Corrosion Professional can simplify thisprocedure by utilizing synchronized currentinterrupters. This allows us to interrupt severalrectifiers simultaneously. Mitigation ofinterference caused by several sources can bedone at the same time. Case #3 In this situation we have two pipelines whichare both protected by their individual impressedcurrent anode bed systems. A mutualinterference exists between the lines but effectsare tolerable to both structures. If all underground structures are adequatelyprotected there will be no need for mitigation.

Cases such as this frequently exist wherebalanced conditions are present and bothpipeline Owners are satisfied that theinterference from the other is tolerable.

Figure 5-17 shows a somewhat balancedsituation where near-equal mutual influencebetween structures exists. With Rectifier #2energized and Rectifier #1 interrupted aninterference survey is conducted on Pipeline#2. When Rectifier #1 cycles "on", the potentialreadings on Pipeline #2 at points away from thecrossing shift in the negative direction. At thepoint of crossing, potentials are shifted in thepositive direction. Based on this test we cansee that current from Rectifier #1 is beingpicked up by Pipeline #2 from the soil at pointsremote from the crossing, while current is beingdischarged to the soil at the point of thecrossing. Now determine the effects of Rectifier #2 in thisarea, while assuming that it is providingadequate levels of cathodic protection onPipeline #2. Because it is maintaining adequatelevels of protection, a substantial amount ofcurrent is flowing from the soil to the pipethroughout the area influenced by Rectifier #1.As a result, Rectifier #1 current output isadditive with that of Rectifier #2 at pick-upareas remote from the crossing, whilecancelling out a portion of Rectifier #2 currentpick-up at the point of crossing.

Provided that the cancellation of Rectifier #2current is not excessive to the point wherepipe-to-soil potentials become more positivethan -0.85 volts at the point of maximumexposure, there is no cause for alarm. It shouldbe noted that although interference still exists,its detrimental effect is cancelled by theinterfered structures own cathodic protectioncurrent thereby rendering the interferenceharmless. Referring back to Figure 5-17, imagine that animpressed current system was installed on theinterfered pipeline in the vicinity of the crossing.Operated at a typical output, the anode bedsystem could shift the entire curve more

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negative, which would raise the graph to acondition where the lowest point on the curveexceeds -0.85 volts. That being the case, theforeign line's interference would be renderedharmless.

Note: The interference is not eliminated butmerely rendered harmless or consideredtolerable. That is why the term "mitigation" isoften used when dealing with stray currentinterference. It is defined as the act of renderingor making interference milder or harmless. In cases such as this example, CorrosionProfessionals representing each Owner'spipeline/structure should desire adequate levelsof cathodic protection at crossings and shouldendeavor to attain them by: 1. Cooperatively adjusting both rectifier

outputs.

2. The installation of additional cathodicprotection devices installed as required atcrossings. Galvanic anodes can bestrategically located at the crossings.

3. The least preferable approach in this casewould be installing a mitigation bondbetween the two structures.

The installation of a bond is normally requiredonly if one party is exerting an overwhelminginfluence upon the other structure, as is thecase in the next example.

Case #4 Figure 5-18 illustrates an example where twostructures, which are protected by individualimpressed current cathodic protection systems,are experiencing mutual interference but onestructure is exerting an overwhelming influence.As shown, Pipeline #1 crosses Pipeline #2 andeach line has an anode bed which is influencingthe other line, but due to its close proximity, theanode bed of Rectifier #1 has an overwhelmingeffect on Pipeline #2, as can be seen by thefollowing measurements.

1. With Rectifier #2 "on" and Rectifier #1interrupted, Pipeline #2 potential at thecrossing shifts from -0.50 (on) to -0.80 (off)volts; voltage change equals +0.30 volts.

2. With Rectifier #1 "on" and Rectifier #2interrupted, Pipeline #1 potential atcrossing shifts from -1.10 (on) to -1.20 (off)volts, voltage change equals +0.10 volts.

The above data indicates that Rectifier #1 is themost influential of the two units and is shiftingthe potential of Pipeline #2 in the positivedirection by 0.30 volts. Rectifier #2 does notprovide adequate levels of protection at thecrossing, so the Owner of Pipeline #2 isjustifiably concerned by the positive shift. If theOwner of Pipeline #2 requested the installationof a drain bond to raise his pipe potential from-0.50 volt to -0.80 volt, it would be an unfairdemand due to the fact that his rectifier systemcurrent has caused the -0.80 volt potential,while causing a small, but positive potentialshift on Pipeline #1 of 0.10 volts. A fair solution in this case may be to subtractthe smaller voltage change (0.10) from thelarger one (0.30). This way, the bond should beset so that the current flow between thestructures will raise the potential of Pipeline #2by the voltage difference (0.20). The installationof the bond will stabilize Pipeline #2 at-0.70 volts with some resultant decrease inPipeline #1 pipe-to-soil potential. In a case such as this, Pipeline #1 will oftenhave sufficient current available to allow excessbond current to flow so that Pipeline #2 mayobtain adequate current to raise its voltageabove the desired cathodic protection criterion.This help may be afforded to Pipeline #2 in lieuof Pipeline #2 installing an additional cathodicprotection device nearby which wouldcomplicate matters. Or, if the Owner of Pipeline #2 decides to installan additional impressed current system anodebed nearby, the Owner of Pipeline #1 mightoffer temporary help in the form of excessivebond current.

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Case #5

It should be mentioned that the most extremeunbalanced condition encountered would be ifPipeline #2 passes through the anode bed ofPipeline #1. Pipeline #2 would pick up currentnear the anodes. The interference current mustsomehow return to Pipeline #1. If the current onPipe #2 flows away from the crossing, it willdischarge at a remote location. This situation isdescribed as "end-wise interference", a termoften used by Corrosion Engineering personnel.The following measurements are an example ofend-wise interference:

1. As is shown in Figure 5-19, with Rectifier #1interrupted, the current flow on Pipeline #2shifts from +3.52 amperes (on) to +0.44amperes (off), a current change of +3.08amperes. With the polarity shown, the 3.08amperes of current is flowing away from thecrossing and the anode bed for Rectifier #1.In order to mitigate this interference, a bondmust be installed so that the current flow isreversed (or made equal to 0) when Rectifier#1 is interrupted. Mitigation of this conditionis shown in Figure 5-20.

2. After the bond has been installed andadjusted so that the end-wise current flowhas been mitigated, Pipeline #2 should betested at the crossing with Pipeline #1 todetermine if an interference condition existsat that location. If interference still exists atthe crossing with the bond in place, the bondresistance must be lowered to increase thebond current drain so that the pipe-to-soilvoltage on Pipeline #2 is restored to itsnatural potential or -0.85 volts.

Summary of Examples

Corrosion Control personnel usually getalarmed when they see any positive pipe-to-soilpotential shifts as a result of interrupted foreigncurrent sources affecting their plant. Thereason for their alarm is usually due to theirlimited knowledge of the subject and inability toadopt generally accepted test procedures thatwere set up for the "ideal interference problem".

The following important conclusions should bederived from the above examples:

1. A positive potential shift at the point ofmaximum exposure is not always harmfuland may well be tolerable. If the pipeline iscathodically protected at the point ofmaximum exposure, there is no need toinstall mitigation.

2. In a small percentage of situations, the pointof maximum exposure is not at the crossingbut at a remote site, know as currentdraining "end-wise" from the crossing.

Things to Watch Out For The following are some items which theCorrosion Professional needs to take intoaccount when testing for and implementingmitigative measures. 1. Extreme care must be taken when installing

resistance bonds to drain current from lineswhich are not electrically continuous.Pipelines using compression couplings orbell and spigot joints are normally notelectrically continuous. Segmented linessuch as these could be prone to increasedcorrosion. The joint or pipe section which isbonded would not be adversely affected, butthe adjacent sections would be becausethey would tend to discharge to the bondedsection. The installation of galvanic anodeson the draining ends of the joints at theexposure area would normally providesufficient current output to mitigateinterference. An alternative approach wouldbe to install jumper cables across joints tomake the line electrically continuous.

2. The mitigation of interference through theuse of a bond is not possible if thepipe-to-soil potentials of the interfered lineare more negative than the potentials of theprotected line. As previously stated this isknown as negative resistance conditionwhereby the current will drain or flowopposite to the desired direction.

END-WISE INTERFERENCE

FIGURE 5-19

CURRENT FLOW ON PIPELINE #2 WITH

RECTIFIER #1 INTERRUPTED

ON 3.52 A

OFF 0.44 A

I 3.08 A

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PIPELINE #1RECTIFIER #1

(-) (+)

PROTECTED

PIPELINE #2

ANODE BED

VOLTMETER AT

CURRENT MEASURING

TEST STATION

(-)(+)

CURRENT FROM

RECTIFIER #1

I = 58AR

END-WISE INTERFERENCE MITIGATED

FIGURE 5-20

CURRENT FLOW ON PIPELINE #2 WITH

RECTIFIER #1 INTERRUPTED

ON 0.24 A

OFF 0.44 A

I 0.20 A

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PIPELINE #1RECTIFIER #1

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PROTECTED

PIPELINE #2

ANODE BED

VOLTMETER AT

CURRENT MEASURING

TEST STATION

(-)(+)

CURRENT FROM

RECTIFIER #1

I = 58AR

(+)

(-)IB

BOND INSTALLED:

I = 6.0 AB

CORROSION CONTROL FOR PIPELINES6-1

CHAPTER 6

CORROSION CONTROL FOR PIPELINES

INTRODUCTION

Corrosion control measures are taken intoaccount during the initial design of a pipelinesystem and should be taken into account duringthe construction/installation of the pipingsystem. This chapter will discuss design andinstallation practices which should beimplemented to assure that the piping systemmeets its designed service life.

PREFACE

Mr. A.W. Peabody stated in the NACEPublication "Control of Pipeline Corrosion" thefollowing:

First attempts to control pipeline corrosionrelied on the use of coating materials with thereasoning that if the pipeline metal could beisolated from contact with the surroundingearth, no corrosion could occur. This concept isentirely reasonable and logical. Furthermore, acoating will be completely effective as a meansof stopping corrosion if:

1. The coating material is an effectiveelectrical insulator,

2. It is applied with no breaks (holidays)whatsoever and will remain so during thebackfilling process, and

3. It constitutes an initially perfect film, whichwill remain so with time.

This is asking more than can be expected frompresently available coatings which are in a pricerange making them economical for pipeline use.

Although coatings, by themselves, may not bethe one perfect answer to corrosion control,they are an extremely effective weapon whenproperly used. A properly selected and appliedcoating will provide all the protection necessaryon most of the pipeline surface to which it isapplied. On a typical well-coated pipeline thisshould be better than 99 percent.

The engineer who is well informed on pipelinecoating and then advises his company on thebest technical and economical coating to usewill find that this probably is the most importantsingle area where his talents can benefit hiscompany. Other facets of pipeline corrosionengineering are important also but a mistake inthe selection of a coating material can meanmany lost dollars during the useful life of thepipeline. On the other hand, the right coatingmaterial properly used will make all otheraspects of corrosion control relatively easy.

Good practice in modern pipeline corrosioncontrol work comprises the use of goodcoatings in combination with cathodic protectionas the main lines of defense. Supplementarytactics, such as the use of electrical isolation,local environmental control, etc. may be used toreinforce these basic control methods.

In selecting a coating system for a givenpipeline project, the one most importantcharacteristic to design for is stability. By thiswe mean a coating combination, which will:

1. Have a high electrical resistance after thepipeline has been installed and the backfillstabilized and will;


2. Have the least reduction in electricalresistance with time.

The four fundamental elements of a successfulcoating system involve:

1. Material Selection

2. Specification

3. Application

4. Inspection

The desirable characteristics of a coatingsystem, along with the selection, specification,application, inspection procedures, and fieldtest methods will be discussed. Internal andatmospheric coatings, while important, will notbe covered in much detail.

COATINGS

The first line of defense against corrosion is theuse of coatings.

History of the Use of Coating Systems

Pipelines were laid bare (uncoated) in the early20th century and as a result, deterioration of theuncoated system was rapid. This sparked theresearch for practical corrosion controlmethods, which resulted in the development ofcoating systems.

By the late 1920's, forerunners of today's hotenamel coatings were well established, and bythe 1940's, technological advances hadresulted in vast improvements on these enameltype coating materials. The implementation ofcathodic protection systems for the mitigation ofcorrosion on underground piping systems wasalso introduced. Testing and experience soonindicated that corrosion mitigation could bedone more effectively and economically byusing a combination of coatings and cathodicprotection.

The selection, application and performance ofavailable coating materials should beconsidered for each specific corrosion controlsituation.

General

The best method of protecting a structure,whether it is made of steel, concrete, wood orother construction material, is by the use ofprotective paints and coatings.

Paints and protective coatings provideprotection in one of three ways:

1. Providing a barrier between the substrateand the environment.

2. By inhibitive action changing theenvironment to a less aggressive one.

3. Providing cathodic protection by sacrificingthemselves to protect the more noblestructure.

Coatings are used in four areas in pipelinework:

1. Atmospheric Coatings

2. Transition Area Coatings – Air/Soil

3. Linings and Internal Coatings

4. Underground Coatings

Atmospheric Coatings are used to protectsurfaces exposed to the atmosphere. Varioustypes of coatings are specified depending onservice requirements – industrial, marine,urban, high temperature, etc.

Transition Area Coatings are coatings used totransition an underground coating to aboveground service. These coatings normally coverup the underground coating to protect it fromultraviolet light, abrasion, ground movement,etc. and transition from 6 inches above groundto 12 inches below ground.

Linings and Internal Coatings can be brokendown into linings used for internal corrosioncontrol and product quality inside tanks andvessels and internal pipeline coatings used forcorrosion control or improved flow in pipelines.


Underground Coatings are used to provide abarrier coating between the pipeline and theelectrolyte. Without the application of aprotective coating (or cathodic protection) to themetallic surfaces, the surfaces will corrode ifthey are in contact with a conductive electrolytesuch as soil or water. Corrosion will occur dueto the formation of galvanic cells on the surfaceof the metal, which will have anodic andcathodic areas, as shown in Figure 6-1.

If the metallic surfaces are coated with anisolating type coating, it will not be in contactwith the electrolyte and thus no corrosion willtake place as shown in Figure 6-2.

If a conductive type coating is used thatcontains a metallic pigmentation anodic to thesubstrate, it will provide cathodic protection tothe substrate where the coating pigmentation isdamaged all the way to the metallic surface,thus preventing corrosion of the substrate, asshown in Figure 6-3.

To obtain the best of both systems, aconductive inhibitive primer can be used whichis in turn top coated with an isolation typecoating, as is shown in Figure 6-4.

In general, each generic class of coating, suchas epoxy, urethane, chlorinated rubber, vinyl,etc., provides particular performancecharacteristics that should be considered duringthe coating selection process.

Types of Underground Coatings

Since the start of large scale piping installationthere has been, and will continue to be, manydevelopments in coating materials andprotective coating systems. Manufacturers arestriving to find materials or combinations ofmaterials, which have the best possibleelectrical and mechanical strength, ease ofapplication and stability in long termperformance - all at a cost compatible witheconomical pipeline construction. Somematerials now available will be describedbriefly. Detailed information on each type ofcoating may be obtained from manufacturers.

Enamels

This term is usually applied to hot-appliedcoatings of coal tar or asphalt, both of whichhave been used for many years. Only Coal TarEnamel is available today. These coatings areformulated from coal tar pitches or petroleumasphalts by combining the processed basematerials with inert mineral fillers, for improvedmechanical strength and impact anddeformation resistance. The operatingtemperature requirements for this type ofcoating are achieved by modifying thecombined materials with various plasticizers.Application of this coating system requires theuse of heating equipment. Hot enamels areapplied over basic tar cutback or syntheticprimers to an overall thickness of 3/32" (94mils) to 5/32" (156 mils) by mill, yard, or over-the-ditch application methods. Over the ditchapplication commonly known as the "grannyrag" method is only used for tie in welds andrepairs due to the hot coating and the toxicfumes that are generated. Bond strength islargely dependent on the degree of surfacepreparation, primer application and applicationtemperature.

The quality of the materials used in producingenamels has a direct bearing on the long-termstability of the system.

The following are some of the advantages anddisadvantages of enamel coating systems:

Advantages• Over 90 years experience• Minimum holiday susceptibility• Low current requirements for cathodic

protection • Good resistance to cathodic disbondment• Good adhesion to steelDisadvantages• Reduced availability• Air quality problems during application• Subject to hydrocarbon attack• Not recommended for above ground use• Cracking problems at low temperatures

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Fusion Bonded Epoxy - FBE

Commonly referred to as fusion bonded epoxy,FBE or thin film coatings. The first formulationswere introduced in 1959 and becamecommercially available in 1961. Of all the pipecoating systems, fusion bonded epoxy systemsare the most resistant to hydrocarbons, acidsand alkalis. One advantage of the coating isthat it does not cover up pipe surface defectsdue to the thinness of the film, thus permittingexcellent inspection of the pipe after coating.

For proper application, fusion bonded epoxyrequires special care and attention to detail.Items of concern are: surface cleanliness,removal of non visible surface contaminants,proper heating, humidity control, uniformapplication of coating material, adequate cureand effective holiday detection.

The application process consists of uniformlyheating the pipe to the recommendedtemperature (450º - 500º F). Each product hasits own individual application requirements andtolerances that must be strictly adhered to.Powdered resin is applied by electrostaticdeposition to a 12-25 mil thickness.

The following are some of the advantages anddisadvantages of fusion bonded epoxy powdercoating systems:

Advantages• Over 40 years experience• Good resistance to cathodic disbondment• Wide operating temperature: -40º to 180º F• Excellent adhesion to steel• Excellent resistance to hydrocarbons• Permits excellent steel inspection

Disadvantages• High application temperature of pipe

(450º F)• Difficult to apply consistently• Surface preparation is critical• Surface temperature of pipe is critical

Electrical inspection should be performed asrequired by NACE Recommended PracticeRP0274 "High-Voltage Electrical Inspection ofPipeline Coatings Prior to Installation". Piperequiring limited repair, perhaps one holiday perten square feet, due to surface defects orcoating imperfections and other minor defectsare repaired by a heat bondable polymeric hotmelt patch stick. A 100 percent solids, liquidepoxy material may also be used for repair.

For more details on FBE see NACER e c o m m e n d e d P r a c t i c e R P 0 3 9 4“Recommended Practice Application,Performance, and Quality Control ofPlant-Applied, Fusion-Bonded Epoxy ExternalPipe Coating”.

Extruded Plastic Coatings

This system usually consists of high densitypolyethylene or polypropylene extruded overthe surface of the pipe. Normal coatingthickness ranges from 30 to 50 mils and thecoating is applied over approximately 10 mils ofhot applied thermoplastic modified rubberadhesive by a specialized yard coating process.

Coating repairs in the field are normally madeusing plastic tapes or heat shrinkable sleevesmade of the same basic materials.

The following are some of the advantages anddisadvantages of extruded plastic coatingsystems:

Advantages• Minimum holiday susceptibility i.e. the ability

to resist development of holidays with time • Wide operating temperature range• Self healing adhesive• Non-polluting, low energy required for

application • Ease of application• Excellent handling properties - high impact

strength

Disadvantages• Difficult to remove coating


• Higher initial cost• Weld joint coatings can be problematic

Hot Applied Mastics

Mastics are commonly referred to as materialswhich are formulated with selected sands andother inert materials bound with an insulatingcompound, which is usually asphalt. Thesematerials are normally applied hot over basictar cutback primers by a continuous pressureextrusion process at stationary coating yardsand are normally thicker than other coatings incommon use. Typical thickness applied rangesfrom ½" (500 mils) to e" (625 mils).

Hot mastics are used because of good coatingintegrity and performance resulting primarilyfrom their greater than normal thickness.

Protective wrappers or dielectric wrappers (forincreased electrical resistance) are not neededunder normal pipelining conditions because oftheir thickness and hardness. Grades withvarious melting points are available to providecompatibility with pipeline operatingtemperatures.

The following are some of the advantages anddisadvantages of hot applied mastic coatingsystems:

Advantages• 70 years experience• Thickest corrosion coating• Reduces concrete weight requirements• Minimum holiday susceptibility • Excellent resistance to cathodic disbondment• Operating temperature range: 40º to 125º F

Disadvantages• Higher initial cost• Higher freight costs because of its weight• Subject to hydrocarbon attack• Not recommended for above ground use• Requires torch for patching• Reduced flexibility in below freezing

temperature• Poor bonding to steel substrate

Cold Liquid Coatings

Coatings in this category include materialswhich are applied in a cold liquid form andsolidify either by solvent or chemical cure.Evaporative setting coatings include solventcutbacks of tar and asphalt. In the asphalticcategory, in addition to the petroleum-derivedasphalts, are what are known as naturalasphalts. These natural asphalts, which aremined in the form of gilsonite stock, are used tomake coatings which generally have higherelectrical strength than coatings usingpetroleum-based asphalts.

These coatings may be applied with or withoutprimers and fiberglass reinforcement. Normalapplication thickness ranges from 40 to 60 mils.

The following are some of the advantages anddisadvantages of solvent cured cold liquidcoatings:

Advantages• 70 years experience

Disadvantages• Long cure time - 24 hours or more• Subject to hydrocarbon attack• Not recommended for above ground use• Reduced flexibility in below freezing

temperature• Poor bonding to steel substrate

Chemically cured coatings include materialssuch as combinations of epoxy resins and coaltar or other chemical compounds of similarnature. Such materials are normally received astwo components, one of which is a chemicalhardener. Once the two materials arecombined, they will harden chemically within aspecific period of time. The length of timeavailable for application varies with the type ofmaterial and the ambient temperature duringapplication. Normal application thickness may


be up to 20 mils, depending on the materialused.

The following are some of the advantages anddisadvantages of chemically cured cold liquidcoatings:

Advantages• Over 50 years experience• Good resistance to cathodic disbondment• Wide operating temperature: -40º to 180º F• Excellent adhesion to steel• Excellent resistance to hydrocarbons• Permits excellent steel inspection

Disadvantages• Difficult to apply consistently unless sprayed• Surface preparation is critical• Surface temperature of pipe is critical

Backfilling time must be considered when usingall of the above systems because of theirrespective curing time.

Hot Applied Waxes

Hot applied petroleum or microcrystalline wax isa refined and blended long-chain solidhydrocarbon mixture centrifuged from heavy oilstocks. Coating can be used with or withoutprimer. However, when specified, primer mustbe compatible with the coating system. Coatingapplication minimum average thickness shouldbe no less than 20 mils. When wax coatingsystems are applied at a coating mill, an outerwrap is normally applied to provide additionalprotection during storage, shipping andinstallation. This outer wrap usually consists ofsnug fitting a uniform layer of Kraft paper, ragfelt or a plastic film over the coating.

The operating temperature limitations aresomewhat lower for this system than for enamelcoatings.

The following are some of the advantages anddisadvantages of hot applied wax coatings:


protection • Good resistance to cathodic disbondment• Good adhesion to steel

Disadvantages• Subject to hydrocarbon attack• Not recommended for above ground use• Not good at high temperatures

Cold Applied Wax

Cold applied wax coatings are grease typematerials formulated by blending petroleum waxwith plasticizers and inhibitors. These systemsare hand-applied to the pipe surface withoutprimer and over wrapped with a componentwrapper, similar to that used for the hot appliedmicrocrystalline wax system. Coatingapplication thickness is normally a minimum of20 mils.

The following are some of the advantages anddisadvantages of cold applied wax coatings:



Disadvantages• Subject to hydrocarbon attack• Not good at high temperatures

Prefabricated Films and Tapes

Tape materials are being used frequently as afull coating system. Tapes normally used areplastic films of polyvinyl chloride (PVC) orpolyethylene with a self-adhesive backing


applied to primed pipe surfaces, or plastic filmswith butyl rubber backings and plastic films withvarious bituminous backings or combinations ofbituminous material and chemical resins.

The following are some of the advantages anddisadvantages of prefabricated films and tapes:


protection• Good resistance to cathodic disbondment• Good adhesion to steel


Tapes are usually thin film coatings. Bestprotective results are obtained with applicationsranging from 15 to 35 mils, with a ¾" to 1"overlap, maintaining a tension approximately 5pounds per inch of tape during the application.

Heat Shrink Sleeves and Tapes

Heat shrinkable polyethylene sleeves and tapesfor field application became popular in the mid-1980's, particularly the type with an irradiatedcross linked polyethylene backing. They do notgenerally require a primer and have anexceptionally high dielectric strength (greaterthan 500 volts per mil) and adhesioncharacteristics at immersed temperatures up to150º F. The sleeves are used for weld jointsand repairing defect areas. Surface preparationconsists of proper cleaning as outlined inSurface Preparation Standards.

Slick boring of coated pipe will often peel backtapes used on the weld joint areas, resulting inincreased cathodic protection levels or cathodicshielding effect under the disbonded tape.Special sleeves have been developed thatpermit the polyethylene to weld itself together atthe overlap zone, preventing the peel back

under certain circumstances at the joints.

The following are some of the advantages anddisadvantages of heat shrink sleeves andtapes:




Directional Drilled Crossings

Many pipelines today are installed viadirectional drilling, thrust boring or slick boring.Corrosion control considerations for drilledcrossings include the use of a corrosion coatingand the use of an overlay or sacrificial coatingover the corrosion coating.

The overlay coating or sacrificial coating mustbond to the corrosion coating and provideprotection for the corrosion coating during thepipe installation process. The most commontypes of overlay coatings are FBE over FBE, 2part epoxy over FBE or 2 part epoxy over 2 partepoxy. Other materials or combinations areavailable based upon the corrosion coating’sproperties.

Desired Coating System Qualities

As previously indicated, one of the ways inwhich a protective coating system providesprotection to the coated structure is byproviding a barrier between the substrate andthe environment. In order to provide apermanent barrier, the coating system mustpossess the following qualities:


1. Electrical Resistance

An underground coating system should havegood dielectric strength to assure high electricalresistance per square foot of coated area. Forthe coating system to be effective, thisresistance value should not change appreciablywith time.

The resistance of the coating is somewhatrelated to the electrical insulating property ofthe coating. Therefore, since the corrosion ofthe pipe is an electrochemical process resultingfrom current flowing from the pipe, theinsulating properties of the coating will lessenthe probability of corrosion occurring.

Another reason why a coating with highdielectric properties is desirable is based on thefact that the higher its electrical resistance, thelower the amount of current required tocathodically protect the piping system. Thus,the coating system which provides the highestresistance for the operating life of the facility isthe most desirable for the application ofcathodic protection.

2. Moisture Absorption

The moisture absorption of the coating isrelated to its permanent high dielectric strength.The presence of a water solution is required toinitiate and support the electrochemical attackon buried metallic structures in conjunction withthe corrosive elements in the soil. Therefore, ifthe structure could be effectively isolated fromthe surrounding soil moisture, the corrosionprocess can be controlled or eliminated. Lowmoisture absorption properties of the coatingwould therefore limit the influence of theelectrolyte on the buried structure.

3. Water Vapor Transmission

Another desirable coating property is resistanceto water vapor transmission. All coatingmaterials, regardless of generic type, have acharacteristic water vapor transmission rate.When selecting a coating the higher the degreeof impermeability, the better the protective

coating material.

4. Impact and Abrasion Resistance

From the time that the pipe is coated to thepoint when it is installed, the pipe will havebeen subjected to considerable handling. Forthis reason it is desirable to have a coatingsystem that has the ability to withstand physicaldamage. Its ability to withstand physicaldamage depends largely on its impact,abrasion, and ductile properties.

The greater the coating system's resistance toimpact and abuse, the less the chance ofcoating damage resulting during backfilling. Anydamage or holidays on the coated surfacewould tend to concentrate corrosion activity atthose locations and also increase the amount ofcathodic protection current required to protectthe pipeline.

The coating system should have good ductileproperties to insure against damage, which mayotherwise be caused during the installation ofthe pipeline due to associated flexing andbending.

5. Deformation Resistance

Soil surrounding the coated pipeline canimpose stresses on the coating as the earthexpands and contracts, and as it absorbs anddissipates moisture. Some soils may exhibitsufficient gripping action to actually pull thecoating from the pipe surface. A satisfactorycoating must be able to withstand suchstresses without serious damage.

The operating temperature of the pipelineaffects the deformation resistance. If the pipe isoperating at a temperature near, or at, thesoftening point of the coating, it will be moresusceptible to deformation by soil movement.Therefore, when selecting a coating system, theoperating temperature of the piping systemmust be taken into account.


6. Bond Strength

In order for a pipe coating to performsatisfactorily it must possess strong andpermanent adhesion properties. Poor bondingor adhesion to the pipe surface may allowmoisture to accumulate between the pipe andcoating thus possibly creating a corrosiveenvironment.

Voids or gaps created between the coating andpipe surface due to poor bonding mayadversely effect the performance of the coatingand the effectiveness of associated cathodicprotection systems.

Bonding strength must be sufficient to preventthe flexing and handling of the pipe duringinstallation without the loss of adhesion.

It is essential that field joints and coatingrepairs also have excellent adhesion to the pipeand be compatible with the initial coatingsystem used on the structure.

7. Compatibility with Cathodic Protection

Coating systems used in conjunction withcathodic protection systems must possesssome resistance to electrical potentials toprevent disbonding of the coating due tohydrogen evolution between the structure andthe coating.

Coatings must also be compatible with thealkaline environment, which is usually presentwith the use of cathodic protection systems.

8. Environmental Contaminants

When selecting a coating system for use in agiven environment, the presence of chemicalswhich may be aggressive toward the coatingmust be determined and taken into account.Some materials are better suited to, and havea better resistance to, a particular environmentthan others do. Environments which areconsidered acidic, alkaline, or wherehydrocarbons are present, are frequentlyencountered along a piping system.

9. Bacterial Organisms

Organisms such as fungi and bacteria arecommonly found in soils. The severity ofcoating deterioration and resulting corrosiondamage attributed to bacterial activity is acontroversial issue.

Bacterial activity can be arrested by theapplication of cathodic protection, whichincreases the pH level of the soil around thepipe creating an environment in which bacteriacannot exist.

10. Weather Resistance

Coating systems must have some resistance toultraviolet rays and extreme temperaturechanges. These conditions may beencountered by the coated pipe section duringstorage or shipping and therefore must beconsidered.

11. Ease of Application

A good coating system must be easy to apply.A system, which has all the propertiespreviously discussed but is difficult to apply orrequires highly specialized equipment to apply,may not be considered a good coating systemfrom an economical standpoint. The applicationof the coating system must be as holiday freeas possible.

The desired qualities discussed above, when allaccomplished, would establish a so-called"perfect" coating material system. However,such a coating does not exist. Therefore, theindividual responsible for specifying a coatingsystem for a given piping system should, takingthe above qualities into account, determinewhich type of coating will be compatible with theenvironment, the piping material, and theoperating temperature of the system.

Surface Preparation

Buildings, bridges and structures of all typesare no better than their foundations. Thearchitect of a corrosion resistant protection


coating system faces an analogous situation.The "foundation" for a protective coating systemis the surface preparation. To obtain plannedand predictable results, one must start with acontrolled, uniform and known foundation.Surface preparation is a key part of theperformance for all coated surfaces.

There are different types of surface preparationmethods such as:

1. Dry blasting with sand or mineral abrasives

2. Grit blasting

3. Shot blasting

4. Blasting with shot/grit mixtures

5. Acid pickling

6. Wet abrasive blasting

7. Weathering off the millscale

8. High pressure water blasting with watercontaining chemical neutralizers or rustconverter additives

9. Solvent cleaning

10. Power tool cleaning, such as needle guns,buffers, etc.

11. Hand tool cleaning, such as scrapers,wire-brushes, etc.

The degrees of cleanliness that can beachieved are:

1. "White metal" blast, SSPC-SP 5 or NACE 1

2. "Near-white" blast, SSPC-SP 10 or NACE 2

3. "Commercial" blast, SSPC-SP 6 or NACE 3

4. Acid pickling SSPC-SP 3

5. "Brush-off" blast, SSPC-SP7 or NACE 4

6. High pressure water blast to remove allloose material

7. Solvent clean to remove oil and grease, andhand tools to remove all loose particles,SSPC-SP 1, 2 and 3

8. Water wash with additives if needed, toremove or neutralize any chemical

contaminants

Coating Specification

Corrosion control personnel should developappropriate specifications to deal with theaspects of a particular project, whether new orrehabilitative. The purpose of this section is toprovide guidance on the more important pointsto consider when formulating the document.The specifications should state what will or willnot be acceptable and what the vendor orcoating contractor is expected to do whenconditions are found to be unacceptable. Theimportant points for consideration are shownbelow:

1. Condition of the Bare Pipe

a. Pipe Material

b. Size

c. Surface condition

d. Age

2. Handling and Storage - mill, transit, field

a. Pipe handling - bare and coated

b. Handling equipment - acceptable types

c. Protection from weathering and UV

d. Coating material - maximum storagetemperatures and shelf life

e. Stacking

3. Atmospheric Conditions - mill or field duringsurface preparation, application

a. Temperature ranges - ambient, surface

b. Relative humidity

c. Dew point

4. Surface Preparation

a. Methods and standards

b. Pre-cleaning and pre-heating

c. Abrasives, tools, equipment

d. Cleanliness of surface and profile

e. Accessibility and Inspection


f. Remedial work - repair surfaceimperfections

g. Personnel qualification

h. Lighting minimum

i. Compressed air quality (if required)

5. Application

a. Methods and standards, manufacturer'sinstructions

b. Materials, solvents, cleaners

c. Tools and equipment

d. Material temperature -minimum/maximum

e. Surface temperature minimum/maximum

f. Thickness of coating system -minimum/maximum

g. Drying time, cure time, cool time

h. Accessibility and Inspection

i. Remedial work - cutback trim

j. Personnel qualification

k. Lighting minimum

l. Compressed air quality (if required)

6. Inspection

a. Methods and standards

b. Personnel qualification

c. Lighting minimum

d. Work access

e. Instrumentation

f. Forms and reports

g. Acceptance of conforming work

h. Handling non-conforming work

i. Acceptance of remedial work

j. Settling disputes

k. Hold points

7. Post Installation Evaluation

a. Coating efficiency resistance testing

b. Reports

c. Remedial measures

The key to any good coating job is, of course,the design and specification of the system.

There are many causes for coating failures, andthe service life of a coating system isdependent on selection of the coating system,surface preparation, coating application,inspection, and maintenance (where feasible).To obtain a good coating system, severalrequirements must be met: a tight specification;selection of the proper coating system, materialsupplier, and application contractor; andimplementation of adequate inspection. Thespecification stage is a point at which a coatingjob can go wrong before it even begins. A good,tight specification is essential if therequirements for a successful job are to be met.

A good specification spells out what is to becoated, how it is to be done, and what thecharacteristics (thickness, freedom fromdiscontinuities, etc.) of the coating film shouldbe. It should be kept simple and to the point.Selection of a coating system is, of course, veryimportant. If the wrong type of coating is chosenfor the job, the results achieved are certainlynot going to be satisfactory. The coatingindustry is continually making advances indeveloping new products to do a better job andin learning more about the performance of olderproducts.

In selecting a primer, the degree of attainablesurface cleanliness is of great importance. Asurface which has been abrasive blasted iseasily wetted by most primers. Hand cleaned orpower tool cleaned surfaces are not wetted orpenetrated by some of the quick, dry primers.Therefore, slower drying primers are required.

The biggest mistake is to select a coatingbased on cost/mil-ft². Buying coatings on thesole criteria of cost/mil-ft² has led to many poorperformances. Once the corrosion engineer hasselected the generic type of coating suitable forthe intended service, the actual cost differentialbetween one specific coating and another is


usually insignificant when the total cost of thecoating work is considered.

Laboratory panel testing is a method often usedfor determining the quality of coating systems.However, the same system can be givenratings of poor, good, and outstanding whentested by three different laboratories insupposedly the same type of salt fog cabinets.The only true evaluation is field performance.Ask suppliers for their recommendations andcase histories of coating systems, then checkwith the maintenance people where thosesystems have been used.

HANDLING COATED STEEL PIPE

Transportation and Handling of Mill CoatedPipe

Pipe sections should be handled carefullyduring loading for delivery by truck or railwaytrain so that damage will be kept to a minimum.Pipes should be padded and adequatelysecured in order to avoid coating damageduring normal shipping conditions.

When pipe sections are transported by train,the pipes should be transferred directly tostringing trucks if possible to avoid excessivehandling. If not, they should be placed in a fieldstorage location and then loaded onto stringingtrucks. Again, efforts should be made to handlesections as carefully as possible.

When stringing trucks arrive at the job site,facilities must be available for placing thecoated pipe sections on the ground. Sectionsshould not be dumped directly from the truck.Coated pipe sections should not be placeddirectly on the ground unless the area is free ofmaterials, which may damage the coating.Preferably, sections should be placed withskids or supports under the bare pipe endswhere the coating has been cut back.

This is seldom possible however, because ofthe varying lengths of pipes. The best alternateis to place the sections on padded skids. Thecoating manufacturer should be consulted as to

the recommended padding material as well asthe number of pipe layers permissible in thepipe pile.

Coated pipe sections should be moved aboutusing belt slings or end hooks. The use ofchains or cable wrapped around the pipeshould not be permitted, as they can causeserious damage to the coating. Belt slings mustbe wide enough so that they can bear the fullweight of the section without distorting the pipecoating. End hooks, one connected to each endof the section using a cable sling and spreader,should be designed so that their use will notdistort the pipe ends. If a pipe section ishandled with a single belt sling, swing must berestricted using ropes or other suitable meansto avoid accidental swinging into equipment,which could cause coating damage. When thecoated pipe section is taken from the groundafter stringing, for lining up and welding thesection into the pipeline, it should be laid onpadded skids under the coated portions to freethe ends for welding. Suitable padded skidsshould be used to avoid damage to the coating.

Pipe Coating Over the Ditch

Coating pipe sections in the field should bedone in strict accordance with coatingspecifications. Coating materials must behandled carefully in the field. They must be keptclean of dirt and other foreign matter. Wrappingmaterial must be kept dry. Excessive moisturein a wrapping felt or mat generates steam whenit strikes hot enamel and adversely affects thequality of the coating system. Similarly, enamelapplied over a moist or wet pipe section willcreate the same deficiency. The application ofcoating in weather below freezing may result ina substandard coating system if a frozen film ofmoisture forms on the pipe. This frozen film ofmoisture can be so thin that the pipe mayappear dry.

Handling of field coated sections must be donewith care, as described in the previous section.

The manufacturer of the coating materialsshould be consulted if questions or problems


arise concerning the application of its product.The manufacturer will be more than happy to beof assistance because it is very concerned inhaving its material perform well.

Holiday Detection and Repair

Holidays or flaws in the coating system may bedetected by visual inspection or through the useof a holiday detector.

A holiday detector is a device which impressesan electrical voltage across the coating. Anelectrode is passed over the entire coatingsurface. As the electrode passes over a coatingdefect, there is an electrical discharge betweenthe electrode and the pipe. This discharge orspark actuates a signaling device which alertsthe operator that a holiday is detected. Theoperator then marks the defect for the repaircrew and continues. Upon completion ofinspection of the coating system all areas sodesignated should be repaired. Personnelmaking such repairs must be trained to repairdefects properly and apply the repair materialsin such a fashion that the repaired area will beas strong, electrically and mechanically, as theoriginal coating.

Preparation of the coated area requiring repairincludes the removal of broken and disbondedmaterial and, for best performance whenworking with enamel or other thick coatings,feathering the edges of the holiday with adrawknife or equivalent tool. This assures abetter bond between the repair materials andthe original coating. If an outer wrap such asKraft paper is present, it must be removed fromaround the holiday area to obtain good bondingof repair materials at the overlap. The repairmaterials themselves must be handled carefullyand in accordance with good coating practice.

Hot enamels used for repairs must be hotenough to give a good bond to properly primedsurfaces. If repriming is required before coatingrepair, a fast drying synthetic primer should beconsidered. When holidays are repaired closeto a big temperature controlled enamel kettle,buckets of heated enamel may be used. When

such kettles are not available, enamel can beheated in buckets with kerosene or propaneflame guns or over fires. Caution must be takenso as not to heat the enamel too rapidly or useexcessive heat which may "coke" the material,driving off much of the volatile material, thusgiving the enamel undesirable coatingcharacteristics.

When holiday repairs consist only of daubing onhot enamel and laying on a patch of felt wrapwithout prior surface preparation of thedamaged area, poor long term performance ofthe pipeline coating can be expected. Poorlyapplied patches are apt to disbond in timeresulting in lower effective coating resistanceand increased cathodic protection currentrequirements.

Inspection

Once the materials and applicator are selected,an important element is inspection. Inspectionbegins from the time pipe is received at thecoating mill and continues right up to the time ofbackfill in the ditch. Inspectors must beknowledgeable in the areas of quality controlmethods, coating systems, plant facilities,handling, shipping, joint coating, fieldconditions, electrical inspection techniques, andrepair methods.

Experience and sound engineering judgment inthe interpretation of specifications and analysisof test results will contribute to obtaining thebest possible coating results.

To assure that we get what we paid for, it isimportant to provide proper inspection duringcoating application and pipeline installation.

The purpose of inspection is to make sure thatthe specifications, which have been preparedso carefully, are being met.

One of the key factors in achieving successfulcoatings performance is the quality of thecoating system application. The "best" coatingsystem will fail prematurely if the applicationprocedures, conditions or workmanship are


unsatisfactory. The majority of coatings failuresresult from poor application techniques.

As we stated above, the specification woulddescribe the material to be used, surfacepreparation, film thickness, etc. It should alsoinclude several limitations that the inspectorshould insure that the coating applicatoradheres to:

1. The coating systems and their dry filmthickness must be as specified.

2. No thinners or additives be added to thecoating except as allowed by the coatingmanufacturer's written instructions.

3. Coatings must be applied in accordance withSteel Structures Painting Council SSPC-PA1Shop, Field and Maintenance Painting andthe specification. The coating manufacturer'sinstructions regarding the materials,equipment and application methods must befollowed unless they are in conflict with thespecification.

4. No coating work shall be performed ifconditions are outside the ranges allowed bythe coating manufacturer for items such asair and/or surface temperature, relativehumidity, etc. Coatings shall be applied onlyif the temperature is above 50ºF (10ºC) andat a relative humidity of 80% or less.

5. Surfaces must receive the specified coatingin a thorough and workmanlike manner inaccordance with standard practices.

6. Coating material that has been readied foruse must be used within the time specifiedby the manufacturer. Coating must be mixedimmediately before use in accordance withstandard practices.

7. Coating materials must be dispensed on a"first-in," "first-out" basis in order to preventthe shelf life from expiring.

8. Well-maintained application equipment mustbe employed.

Procedures for Laying Coated Pipe

Before laying the pipe in the trench, the bottomof the trench must be cleared of any largestones or foreign material which may damagethe pipe coating.

Piping should be lowered into the trench bymeans of belt slings or similar means, whichwould enable the pipe to be lowered slowly, andcarefully into the trench, to avoid coatingdamage. After pipe is placed in the trench, thepipeline should again be inspected for coatingdamage. Damaged coating must be repaired inaccordance with the previous section of thischapter relating to repairs.

HOLIDAY DETECTION AFTER BACKFILLING

There are various techniques that can be usedto detect coating defects after backfilling. Theseinclude the Pearson Survey, the Direct CurrentVoltage Gradient (DCVG) survey and theAlternating Current Voltage Gradient (ACVG)survey.

The Pearson Survey was pioneered by J. M.Pearson in the 1930's. The survey consists ofmeasuring the leakage of an audio signal fromthe pipeline at coating holidays.

A Direct Current Voltage Gradient (DCVG)survey is performed by interrupting the CPsystem or another DC source and measuringDC voltage gradients over the pipe. DC voltagegradients will be present at coating holidays.

An Alternating Current Voltage Gradient(ACVG) survey is performed by inducing AC onthe pipeline and measuring AC voltagegradients over the pipe. AC voltage gradientswill be present at coating holidays.

These methods are discussed more fully in theAdvanced Course text.

SYSTEM MAINTENANCE

During normal operations there frequently areoccasions when the line is excavated for


maintenance work which may result in coatingdamage. Coating repairs should be made withmaterial of equal or better quality than theoriginal coating and should be carried out byqualified personnel.

Record keeping of actual coating performanceencountered during the life of the line will permitcorrosion personnel to evaluate the condition ofthe coating. Vital information can be obtainedby training maintenance crews to identify andreport the condition of a coating whenever asection is excavated and examined.

Measuring the effective coating resistanceperiodically will aid in the assessment ofdeterioration, damage caused by third parties,or other conditions. If testing reveals a rapidrate of deterioration, an investigation should beinitiated to determine the cause. PearsonSurveys, DCVG surveys and ACVG surveysare suitable for this.

CASED CROSSINGS

What is a Cased Crossing?

A cased crossing is a point where the pipelineis routed through a steel or reinforced concretetube/casing as shown in Figure 6-5A. Thecasing is used to provide mechanical protectionto the pipeline. Casings are sometimes installedwhere pipelines cross under roadways andrailways.

Component Parts of a Cased Crossing

Figure 6-5A shows the components of a typicalcased crossing. These components consist ofthe casing, vents (if specified), isolatingspacers, and the end seals.

The casing is normally constructed of steel orprestressed concrete pipe. The size of thecasing pipe used should be a minimum of twosizes larger than the pipeline being protected.

The isolating spacers are installed on the pipebeing protected and are used to electricallyisolate the pipe from the casing. Spacers are

available in various designs ranging from allplastic types to models having isolating blockssecured to steel bands (which may be rubber orplastic lined).

End seals are installed at each end of thecasing. They are designed to provide protectionagainst the entry of water, soil or other backfillmaterial which may short the pipe to the casingor cause corrosion in the annular spacebetween the pipe and casing. End seals areavailable in various designs. One type of designis shown in Figure 6-5A. This is a syntheticrubber sleeve, which is sized to fit both the pipeand casing and are secured to them by meansof metallic straps. Another type of end sealcommonly used is constructed of solid syntheticrubber as shown in Figure 6-5B. The end sealis placed in the annular space between thecasing and the pipe, at the end of the casing.As the bolts are tightened, the elongatedpressure plates compress the rubber, which inturn expands to form a continuous seal.

Proper Methods for Installing CasedCrossings

The first step is to install the casing at therequired location. The casing should then beinspected. It should be determined that thecasing is straight and round to prevent bindingof the pipe during installation. All debris shouldbe removed from within the casing prior to pipeinstallation.

If the casing is to be provided with vent pipes,as shown in Figure 6-5A, it is recommendedthat they be installed prior to the installation ofthe pipe. If vent holes are cut in the casing withthe pipe in place, damage to the pipe coatingmay occur due to the heat of the torch, or theresulting coupon may fall into the casing; thecoupon could cause an electrical short betweenthe pipe and the casing.

Spacers must be installed on the pipe prior toits installation in the casing. They should bedesigned to protect the carrier pipe and itscoating during insertion and to adequatelysupport and electrically isolate the carrier pipe

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from the casing when full of product. Thenumber of spacers used and the spacing atwhich they are installed, depend on the type ofspacer used and the weight of the pipe. Beforeinstalling isolating spacers, the manufacturershould be consulted concerning the numberand size of the isolating spacers required. Thelocation of the spacers on the pipe should beplanned so that when the pipe is pulled into thecasing the end isolating spacers will be close tothe ends of the casing. An inspector shouldmake sure that the spacers are not crushedwhile the tie-in foreman is trying to get that lastfraction of an inch that may be required to setup the line-clamps for the welder.

The final step is the installation of the endseals. As previously mentioned, the installationof the end seals between the pipe and casingreduce the ingress of water and debris.

To insure proper installation, the manufacturerof the seal should be consulted concerning thesize and type required for the particularinstallation and for recommended installationprocedures.

Although the use of cased crossings is widelyaccepted, it should be pointed out that their useis presently under scrutiny due to the corrosionproblems which tend to develop with their use.Alternate means of providing mechanicalprotection to the pipe at crossings are beingexplored.

Testing Cased Crossings for ElectricalIsolation

After the installation of a cased crossing iscompleted, the isolation between the pipe andthe casing should be checked beforebackfilling. The cost of repairs will be less ifthey are made prior to backfill of the crossingarea.

One method of testing the isolation of thepipeline from the casing is by measuring theresistance between the two structures. Thisresistance can be measured by using anammeter, a voltmeter, and an external DC

power supply connected as shown in Figure6-6. Another way to measure the resistance isby using a soil resistivity meter connectedbetween the casing and pipe as shown inFigure 6-7. A resistance measurement, which isvery high or infinite, would indicate that the twostructures are electrically isolated.

An alternate method of testing the isolation isby measuring the pipe-to-soil and casing-to-soilpotentials with respect to a portable referenceelectrode contacting the soil above the pipeline.A current should be impressed on one of thetwo structures using an interruptible DC source.Figure 6-8 shows a typical test set-up indicatingthis type of test.

If available, an existing impressed currentcathodic protection system on the pipeline canbe used for this test, as shown in Figure 6-9,interrupting the rectifier during the test. Thepotential readings on both structures should betaken using a high resistance voltmeter whileinterrupting the current source.

Regardless of which current source is used, ifthe potentials of both structures increase in thesame direction, as the current source turns on,the two structures may be shorted. If thepotential of the structure connected to thepower source increases in the negativedirection as the power is turned on, and there islittle or no increase in the other structure'spotential, then the structures can be consideredas being isolated from one another.

Casings should be periodically checked usingtest stations normally installed at casedcrossings. Casings may be properly isolatedjust after installation, but in time may becomeshorted due to one or more of the followingconditions:

1. Too much strain on pipe when final tie-inwas made.

2. Earth movement or settlement.

3. Movement of the casing for whateverreason.


4. Movement of the pipeline due to expansion,contraction, or internal pressure stresses.

5. Casing isolators being placed too far apartor made of inferior materials.

In the event that a casing is found to be shortedto the pipeline after construction activity in thearea has been completed, there are only twoacceptable alternatives which can beconsidered:

1. Excavate the cased crossing, locate thepoint of contact and clear it.

2. Pump a petrolatum or wax type product intothe entire annular space through the ventpipes in order to inhibit corrosion.

Another approach that has been used in thepast is to allow the short to remain and toincrease the level of cathodic protection at thepipe/casing interface area. This approach doesnot preclude the possibility of corrosion of thepipe within the casing if water has gotten insidethe annular space.

Casing isolation is one of the most importantitems in a well designed pipeline system.Therefore, great effort should be expended todetermine the most effective and economicalmethod of properly installing the spacers andseals to avoid the possibility of future electricalshorts due to improper installation.

ISOLATING JOINTS

What Does an Isolating Joint Do?

Isolating joints such as isolating flanges,unions, and couplings are used to electricallyisolate various components of a pipelinesystem.

They are often used to isolate a pipeline intosections for cathodic protection purposes.

Applications of Isolating Joints

Isolating joints have numerous applications in

transmission and distribution piping systems.The following is a list of some of theapplications of isolating joints:

1. Isolating old pipe from new pipe.

2. Isolating coated pipe from bare pipe.

3. Isolating copper or cast iron pipe fromcarbon steel pipe.

4. Connecting two pipelines of differentownership.

5. Isolating distribution lines fromtransmission lines.

6. Isolating compressor, regulator, anddistribution piping from the main line.

7. Isolating river crossing sections ofpipelines.

8. Isolating sections of piping to reduce themagnitude and effects of stray currents.

9. Facilitating the location of contacts incongested areas.

10. Protecting metering devices by isolatingthe section of pipe on which the meter isinstalled from the meter itself.

11. Isolating protected pipes from unprotectedpipes.

Isolating Flanges

The most commonly used isolating joint is theisolating flange. Isolating flanges are availablein various sizes and configurations and areconstructed from a large variety of materialsdesigned for specific temperature, product andpressure applications. Figure 6-10A shows anillustration of a typical isolating flangeinstallation. An isolating flange has three (3)isolating components. They are the gasket, thesleeves, and the washers.

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A. Gaskets

Isolating gaskets are either conventional flatgaskets manufactured of nonconductivematerials or gaskets consisting ofnonconductive retainer materials such asphenolic or epoxy glass with special grooves toaccommodate O-ring or quad-ring sealelements. They are usually c" thick. Factors toconsider when selecting gaskets are thematerial compatibility with the piping productand external environment, temperaturelimitations and sealing capabilities.

The following is a list of types of gasketscommonly used to isolate flanges, along with abrief description of their material composition,configuration, and operating characteristics:

1. Full Face Gaskets (Type E)

Full face gaskets completely cover theflange face from the flange base to theoutside diameter. Full face gaskets are usedon flat faced flanges, however, they can beused with a raised face flange to eliminatedebris build up in raised areas that couldcause a short.

2. Ring Gasket (Type F)

Ring gaskets are designed to seat inside thebolt circle and are commonly used withraised face flanges.

3. Ring Type Joint (Type D)

The Ring Type Joint (RTJ) is an oval glassepoxy or phenolic gasket designed to workspecifically with RTJ grooved flanges. RTJgaskets are sized by R numbers.

A filler gasket is sometimes used on the IDof the RTJ gasket. This is to eliminate debrisbridging across and causing a short.

4. Type D - BX

BX gaskets are an octagonal shapedphenolic or glass epoxy gasket designed to

work specifically with a BX grooved flange.BX gaskets are machined from phenolictubing or glass epoxy materials.

As previously stated, gasket materials shouldbe selected based on operating requirements ofthe pipe. Some of the gasket base materialsavailable are:

Paper (phenolic) with a seal elementGlass Epoxy with a seal elementCanvas Cotton Fabric (phenolic)Neoprene Faced PhenolicSilicon Glass with a seal element

B. Sleeves

Isolating sleeve materials are designed to fitover standard bolts and within standard boltholes in flange faces. They are normally 1/32"thick and commonly available in Mylar®, glassepoxy, phenolic and polyethylene. Whenfeasible, they should be full length, extendinghalf way into the steel back-up washers on bothsides of the flange. Factors to consider whenselecting sleeve materials include: dielectricstrength, product compatibility, temperaturelimits and physical strength with particularattention to their resistance to cut-through fromthe bolt threads. Mylar® and glass epoxysleeves typically provide good service in mostenvironments.

C. Washers

Isolating washers should fit under the steelback-up washers and be sized to fit over theisolating sleeve material within the flange "spot"face. They should have the same ID and OD asthe back-up steel washers. Factors to considerwhen selecting isolating washer materialsinclude: dielectric strength, compressivestrength, sheer strength, temperature limits andcompatibility to environment. Glass cladphenolic or epoxy typically provide goodservice.

Isolating gasket kits can also utilize a moldedone-piece sleeve and washer. The one-piecesleeve and washer reduces possible error in


washer-sleeve arrangement sequence andallows the inspector to determine if the flangehas been properly isolated in one glance.

Installation of Isolating Flanges

Isolating flange kits can only be installed onout-of-service pipes because the installationrequires the opening of pipeline flanges.Isolating gaskets, bolt sleeves, and isolatingwashers should be installed as per themanufacturer's instructions and the followingrecommendations:

1. Gasket inner diameter should be the sameor slightly smaller than the inner diameter offlange.

2. For buried flanges, isolating washers shouldbe installed only on the unprotected side ofthe flange, allowing cathodic protection to beprovided to studs or bolts.

3. Alignment pins are recommended whenpossible and should be a minimum of 3/32"(2.381 mm) larger than bolt size.

4. It is important that after the isolating flange isinstalled, the void area where the two flangefaces meet be taped with two layers ofplastic tape or equal. Alternatively, aninhibited grease can be used to fill the voidarea. This could avoid the accumulation ofdebris, which may lead to electrical shortingof the flange.

5. Replace any broken or cracked sleeves orwashers, as they will eventually result in anelectrical short and possibly productleakage.

Upon completion of the isolating flangeinstallation, the flange should be tested foreffectiveness. Periodic maintenance/testingshould be conducted on accessible flanges(aboveground and underground with testpoints) and whenever a buried flange isexcavated.

Isolating Unions

Isolating unions are generally installedaboveground to provide electrical isolation forregulator stations, processing plants, gaugelines, fuel supply lines, water lines, and otherpipeline applications.

Isolating unions are available for high and lowpressure applications.

They are comprised of two flanged bodieswhich are screwed onto the end of the pipes tobe joined. One flanged end of each body isexternally threaded enabling the two sections tobe joined using a nut. The nut is isolated fromone of the flanged ends through the use of adielectric material. An O-ring or gasket may belocated between the two mating surfaces.

Various dielectric materials are used in theconstruction of isolating unions, but the mostpreferred material is nylon. These dielectricmaterials are machined and/or molded in sucha fashion so that they will not be damagedwhen the union is tightened. Figure 6-10Bshows a typical isolating union.

Isolating unions should be tested foreffectiveness upon completion of theirinstallation.

Isolating Couplings

The isolation of small low pressure piping (sizesup to 2") is normally accomplished using fittingswith non-conductive interiors. These fittings arenormally constructed of cloth-based phenolic,nylon, or Delrin® interior of a quality whichpermits the machining of the couplings orbushings.

Larger lines can be effectively isolated by theuse of compression - type couplings, with nylonand/or rubber gaskets used to prevent electricalcontact between the two sections.

A short section of plastic pipe may also be usedas an isolator.

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All installations should be tested prior to backfillor upon completion of installation.

Testing Isolating Joints

Testing the effectiveness of the installedisolating joint can be done using one or more ofthe following methods:

Method No .1 - Interrupting Cathodic ProtectionSource

This method consists of setting up a temporaryimpressed current anode bed or utilizing anexisting impressed current cathodic protectionsystem anode bed.

Using the test set-up shown in Figure 6-11,measure the pipe-to-soil potential on each sideof the isolating joint while interrupting thecurrent output of the source. Potential readingsshould be taken on both sides of the joint usinga high resistance voltmeter with respect to aclose copper-copper sulfate referenceelectrode.

If the isolating joint is effective, the potential onthe side of the joint which is connected to theimpressed current system will change as thesource is interrupted, while the potential of theopposite will remain constant or shift in theopposite direction. This method can be used forunderground as well as aboveground isolatingjoints.

Method No. 2 - Testing for Shorted Bolts onIsolating Flanges Using a High ResistanceVoltmeter

Referring to Figure 6-12, a battery or otherexternal DC source is connected across theisolating joint. Moving from bolt to bolt, measurethe potential between the two ends of each boltwhile momentarily closing the switch. If apotential swing is noted when the switch isclosed, this indicates that the bolt is shorted.Satisfactory isolated bolts will show no potentialswing. This method can be used onaboveground joints or fully excavatedunderground joints.

Method No. 3 - Testing for Shorted Bolts onIsolating Flanges using a Magnetic Compass

As opposed to using a voltmeter, as describedin Method No. 2 above, a magnetic compasscan be used to locate shorted bolts. Whileinterrupting the DC source, the compass isplaced across the flange and moved from boltto bolt. The compass needle will be deflectedmarkedly at a shorted bolt because of themagnetic field surrounding the bolt throughwhich current is flowing. If there is no deflectionof the compass needle as it is placed directlyabove a bolt, this is a good indication that thebolt is isolated. See Figure 6-13 for a typicaltest set-up. This method can be used on fullyexposed joints.

In order to determine that the desired testresults can be achieved with the test set-up,and that the current capacity of the DC powersupply is sufficient, the flanges can betemporarily shorted with a screwdriver. If thetest set-up is valid, a deflection of the compassshould be noted as it is placed directly abovethe screwdriver and the test current is applied.

Method No. 4 - Using Isolation CheckerInstrument

This method utilizes a test instrument known asan isolation checker. The isolation checker, asshown in Figure 6-14, has two test probes onthe outside of its casing. These two probes areplaced across the isolating joint and the meterread. A full-scale deflection (high reading) onthe calibrated meter indicates that the joint iseffectively isolating. If, however, the joint isshorted, the meter pointer will be deflected to ornear zero on the scale. This method can beused in exposed joints.

Repair of Shorted Isolating Flange

If an isolating joint is found to be shorted/defective it may be possible to repair it withouttaking the line out of service. This is possible ifthe short is due to an isolating bolt sleeve whichhas broken down or was improperly installed. Ifthis is the case, the shorted bolt can be

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removed and the isolation replaced or properlyreinstalled.

If an isolating flange is found to be shorted, andthe above test methods indicate a faulty gasket,the line must be taken out of service and thegasket replaced.

Before disassembling the joint for thereplacement of the gasket, a thorough visualinspection of the joint should be conducted.This inspection may lead to the discovery offoreign material between the flange faces or theexistence of a metallic conductor across theflanges, which may be shorting the joints.

TEST POINTS

The Purpose of Test Points

Test points are used to electrically contact aburied pipeline to facilitate the monitoring ofcathodic protection levels and conducting testsassociated with corrosion control.

Ideally, test points should be convenientlylocated and well constructed to provide years ofreliable service.

Types of Test Points

Each test point/station has a number of wiresconnected to the pipeline(s) on which tests arebeing performed. Usually, each wire is colorcoded by some scheme developed internally bythe operating company. The color-coding isused to identify the pipe (or pipes) the wires areconnected to and also to indicate the position ofeach wire with respect to other wires on thesame pipe. Each company also developsnames for standard types of test stations. Thus,a "Type 1" test station for company "A" may bea "Type B" test station for company "C.”Whichever way each type of test station isdesignated, knowledge of the type of teststation and the wire identification code isessential.

The following paragraphs contain a briefdescription of the types of tests that can be

conducted at the different types of test stations.

1. Two-Wire Test Station

The two-wire test station is basically astructure contacting test station. (See Figure6-15).

TESTS PERFORMED:

P/S - The Pipe-to-Soil Potentialmeasurement is the primary test made at atest station of this type. The procedure is asfollows:

P/S - Pipe-to-Soil, close - One terminal ofthe voltmeter is connected to one of the testwires. The reference electrode is placeddirectly over the pipe and connected to theother terminal of the voltmeter.

P/SR - Pipe-to-Soil, remote - One terminal ofthe voltmeter is connected to one of the testwires. The reference electrode is placed viaa long wire at remote earth. The distance toremote earth varies with the quality of thepipe coating. It is a location outside of thecathodic soil gradient caused by cathodicprotection or stray current entering the pipe.

2. IR Drop or Current Measuring Test Station

This four-wire test station is used formeasuring line current flow along a pipeline(See Figure 6-16). The wires are colorcoded or otherwise identified so that theirposition on the pipe with respect to somereference such as product flow is known.When the 100 ft. (typical) span between theinner wires is a calibrated span (See Figure6-17) the calibration factor, K, can beapplied to the Emv (actual) reading and themagnitude of the line current flow inamperes determined. The direction of linecurrent flow is also determined by observingthe polarity of the reading. In cathodicprotection testing convention, current flowsfrom (+) to (-). For the polarities indicated onthe sketch, a +Emv reading would indicate aline current flow in the direction opposite to

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To calibrate an IR drop span, carry out thefollowing procedure:

a. Connect the circuit elements as shown ondiagram.

b. Close the switch (make contact toammeter negative terminal).

c. Immediately after the circuit is closed,read current (I) and IR Drop (Emv)simultaneously. Record the readings.

d. Open the switch (break contact).

e. Immediately after the circuit is opened,read the current (I) and IR drop (Emv)again. The current will be "0", but theremay be an IR drop (Emv) because ofexisting line current flow.

f. Record these readings.

Repeat steps a, b, c, d, and e severaltimes in order to avoid possible errors. Ifthere are no apparent errors, and if thereadings vary slightly, (a normaloccurrence because of batteryweakening) calculate average readingsfor I and Emv.

g. Calculate the calibration factor (K) asfollows:

Applying typical numbers that might befound on a 12-inch pipeline:

Note that there was some residual currentflow on the pipe after the circuit was opened(Emv off was not 0). Note also that the Emv offreading was of the opposite polarity,indicating that the residual line current flow

(caused by factors other than the testcurrent) is flowing in the direction of lineproduct flow, as determined by the polaritiesobserved for this test.

The magnitude of the residual line currentflow is easily determined:

A test station having only 1 wire on eachside of the measurement span can be usedin lieu of a four-wire calibrated span teststation. This requires the use of tables thatindicate the amount of current required for aone millivolt drop for a given length, size andwall thickness of pipe. Therefore, if thespacing between the test wire connections isknown, and the pipe size and wall thicknessare known, the calibration factor of themeasurement span can be calculated.

3. Isolating Joint Test Point

As mentioned earlier, isolating joints areused to electrically isolate a pipe for cathodicprotection sectionalizing purposes, or whenownership of the pipe changes, such as at adelivery point at a gas metering station or oilterminal. The inner wires on the diagram(Figure 6-19) are usually heavier (for lowresistance purposes) in case it is necessaryto insert a resistance bond across theisolator.

TESTS PERFORMED:

Pipe-to-soil potential (P/S) on each side ofthe isolator

Internal Resistance (RINT) of the isolator (forchecking isolation resistance) by:

Measurement of Resistance AcrossIsolating Joint or

Pipe-to-Soil Measurement Method (Figure6-20)

In an isolating joint, the effective internal

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resistance measured will be the sum of theresistance to earth (RE) of each pipesection. These resistances are in serieswith each other and in parallel with theactual resistance of the isolating joint. Theresistance to earth of each pipe section isshown in Figure 6-20 for illustrativepurposes.

Procedure:

a. Connect ammeter, battery andvoltmeter No. 1 as shown in Figure 6-20.

b. Close switch (make contact).

c. Read current and pipe-to-soil potential(P/S) values simultaneously. Record thedata.

d. Open switch (break contact).

e. Read current (should be "0") andp i p e - t o - s o i l p o t e n t i a l ( P / S )simultaneously. Record the data.

f. Repeat the last four steps several timesto ensure there are no errors.

g. Remove voltmeter No. 1. Connectvoltmeter No. 2 as shown.

h. Repeat the above procedure.

Calculation:

It is very important to make a sketch of thepolarities used for the test, because thesemust be taken into account when algebraicallyadding ΔP/S1 and ΔP/S2. With the testarrangement polarities as shown, ΔP/S1 willbe of the opposite polarity to ΔP/S2.

In effect, with the polarities shown, pipe No. 2is being made an anode with respect to pipeNo. 1 when the switch is closed (contactmade). Hence, cathodic protection is beingapplied to pipe No. 1. The needle on voltmeterNo. 1 will move to the right, while the needleof voltmeter No. 2 will move to the left.

Potential Across Joint Method

Where it is fairly certain that the isolator iseffective, the following procedure may beused to determine resistance.

a. Connect ammeter, battery and voltmeterNo. 3 as shown in Figure 6-20.

b. Close switch (make contact).

c. Read current and voltage valuessimultaneously.

d. Open switch (break contact).

e. Read current (should be “0") and voltagesimultaneously.

f. Repeat the last four steps to ensure thereare no errors.

Calculation:

Note: In the absence of heavy stray current,a reading other than 0 of V3 OFF, or avoltmeter reading alone (without battery orammeter) is usually indicative of an effectiveisolator.

4. Pipeline Casing Test Station (Figure 6-21)

Casings are used on pipelines where theycross under railroads, highways or otherlocations where heavy loads are expected.Some authorities require casings in front ofschools (to carry product a distance away inthe event of carrier pipe rupture).

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Test stations are normally installed atcasings in various configurations. If thecasing is 50 feet long or longer, a test stationsimilar to the one shown in Figure 6-21 isalso installed at the other end of the casing.

Some casings are installed with vent pipes.Sometimes there are vent pipes at one endonly, and sometimes there are vent pipes atboth ends. The vent pipe can serve as oneof the two wires connected to the casing.

TESTS PERFORMED:

P/S - Pipe-to-Soil Potential, as previouslydescribed.

C/S - Casing-to-Soil Potential, as previouslydescribed for pipelines.

RP-C - Pipe-to-Casing Resistance - The testarrangement and procedures are exactly thesame as described for measuring theinternal resistance (RINT) of an isolating joint.

5. Foreign Pipeline Crossing Test Station(Figure 6-22)

For purposes of this discussion, a "foreign"pipe is any pipe which is not electricallyconnected to the pipe under test or undercathodic protection.

Test stations are necessary at all crossingsof foreign pipelines. If cathodic protection isapplied to either one of the pipes, there isalways a possibility of stray currentinterference and corrosion on the other pipe.

TESTS PERFORMED

P/S - Pipe-to-Soil Potential - Pipe under test

P/S - Pipe-to-Soil Potential - Foreign pipe

RINT - Internal resistance between pipes

EOC - Open circuit potential between pipes

Combinations of these tests and the

calculations of the test results are used fordetermining if stray current interferenceexists, and if the interference exists, thecalculation of the resistance of the bondrequired to clear the interference (RBOND) canbe made (See Chapter No. 5).

Pipe-to-soil potential measurements atforeign crossings should be made with thereference electrode placed directly over thecrossing.

For Open Circuit potential tests, thevoltmeter is connected between one wire ofthe pipe under test and one wire of theforeign pipe, and the reading is taken.

6. Galvanic Anode Test Station (Figure 6-23)

Galvanic cathodic protection anodes aresometimes connected to pipes through teststations so that their performance may bemonitored. Groups of galvanic anodes arealso sometimes connected to a singleheader cable, which is usually brought up toa test station for testing purposes.

TESTS PERFORMED:

P/S - Pipe-to-soil potential, with anodesconnected or disconnected. Pipe-to-soilpotentials are always measured through awire, which is not carrying current.

IANODE - Anode current. This is measured bybreaking the anode to pipe connection andinserting an ammeter between the pipe wireand the anode wire. Current flow through theammeter should be from pipe to anode. SeeFigure 6-24 - Method A.

Depending on the resistance involved, theammeter resistance can significantly alterthe circuit resistance, resulting in a currentreading that is lower than the actual current.

Small current measuring shunts can bepermanently installed in test stations so thatthe anode current can be measured by theIR drop method without disturbing the anode

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circuit. (See Figure 6-24 - Method B). Theresistance of the chosen shunt should bedependent upon the current anticipated atthe test station. Anode currents usually varybetween a few milliamperes up to severalhundred milliamperes, depending upon thenumber of anodes and the soil resistivity.The shunt must be selected to provide agood indication on the millivoltmeter.

Installation of Test Points

Test points/stations should be located so thatthey are readily accessible for testing. Teststations can be either aboveground mounted orflush mounted.

Flush mounted test stations are normally usedin areas where an aboveground mounted teststation may not be feasible due to vehiculartraffic in the area or due to right-of-waylimitations. Flush mounted test stations are lessvulnerable to vandalism and therefore, their usein problem areas may be desirable. Figure 6-25illustrates a typical flush mounted test stationinstallation.

Aboveground post mounted test stations, asshown in Figure 6-26, are normally used oncross country pipeline installations to facilitatethe locating of test points for testing.

Regardless of the type of test station installed,the following basic installation practices shouldbe followed:

1. Test stations should be located as close aspossible to the pipeline(s) being tested,preferably directly above it. This will reducethe chances of test leads being damaged byconstruction in the area of the pipeline.Installation costs would also be reduced andany required troubleshooting simplified.

2. Test lead connections to pipe should bemade using an exothermic weld process.The weld connection should be thoroughlyinspected and tapped lightly with anonferrous metal hammer to make sure it issecure.

3. The weld and surrounding exposed pipe andwire can be coated with one layer of coldapplied coal tar, followed by a 9 in. x 9 in.layer of woven glass fiber. The glass fibershould be worked into the coal tar. Anadditional layer of coal tar followed byadditional glass fiber should be applied. Afinal coat of coal tar should complete thepatch. If the pipe is coated, the patch shouldoverlap the coating all around a minimum of2 inches. If the pipe is bare, the patch shouldextend beyond all exposed copper wire andweld material a minimum of 2 inches.

Other acceptable coating procedures can beutilized as well as prefabricated patchesmade by several manufacturers.

4. Sufficient slack should be left in the testwires just below the test points, to preventany damage or breaks due to soil settlementafter backfill.

5. Any damage to test wire insulation duringinstallation must be repaired. Repairs shouldbe made using two half-lapped layers ofrubber electrical tape followed by two half-lapped layers of plastic electrical tape.

6. Approximately 9 inches of coiled test wireshould be provided inside the test box sothat the wires can be raised out of the boxfor testing.

7. Backfill for test wires should be free ofstones larger than ¼ inch and other foreignmaterial which may damage the wireinsulation.

8. Accurate test station location drawings andelectrical termination details should beprepared showing each test point. Theimportance of color coding and recordingwires and their positions on the pipe cannotbe overemphasized. Inaccurate data may beworst than none at all. Drawings should bekept for permanent record, to be referred tofor future testing.

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Counter corrosion methods should beconsidered when designing a new pipelineinstallation. Corrosion control measures shouldinclude, but not be limited to, the use ofprotective coatings. A knowledgeable coatingsengineer should make the selection of theprotective coating system to be used. Cathodicprotection systems must be used in conjunctionwith a good protective coating to ensure thatthe pipeline meets its designed life expectancy.

Cased crossings are used for mechanicalprotection of a pipeline and isolating joints areused to electrically separate sections of apipeline. Both of these appurtenances can shortout and tests should be conducted to ascertainisolation. Test points are installed along apipeline to allow conducting tests. Others maybe required for cathodic protection purposes.

RECTIFIERS7-1

CHAPTER 7

RECTIFIERS

INTRODUCTION

This chapter will discuss the basic operation ofrectifiers, preventive maintenance proceduresfor rectifiers and important information that willbe very useful when purchasing rectifiers.

As previously discussed in Chapter 3 of thiscourse, the operation of an impressed currentcathodic protection system requires the use ofan external DC power source. The followingsection will briefly review some of the cathodicprotection system power supplies that areavailable for use.

CATHODIC PROTECTION SYSTEM POWERSUPPLIES

As discussed in Chapter 3 of this course,various types of equipment can be used toprovide DC power for impressed currentcathodic protection systems. These sourcesinclude the following:

1. Rectifiers

This is the most commonly used DC sourcefor cathodic protection system applications.A rectifier converts AC power to DC power.

2. Solar Power Supplies

These units are used in areas wheresunlight is available for a large percentage ofthe time. Photovoltaic solar arrays convertsunlight to DC power.

3. Thermoelectric Generators

Thermoelectric Generators produce powerby the direct conversion of heat intoelectricity. Power is produced by maintaininga temperature difference across athermopile, an assembly of semi-conductorthermoelectric elements. Combustion ofNatural Gas or Propane provides the heatwhile natural convection provides the coolingrequired to create this temperaturedifferential.

4. Engine Generator Sets

These units consist of a fuel-powered engineand an AC generator used to provide theinput power to a rectifier unit.

5. Turbine Generator Sets

Turbine generator sets utilize the productflow or gas pressure in the pipeline to drivea small turbine that in turn drives a DCpower generator.

6. Wind Powered Generators

These units are used in areas where a fairlyconstant breeze/wind is blowing. The winddrives a turbine assembly that in turn drivesa generator assembly that provides the inputto a rectifier unit.

REVIEW OF ELECTRICAL FUNDAMENTALS

Following is a review of the fundamentalsconcerning the generation of AC power and itsconversion to DC power. The review of theseprincipals will help corrosion personnelunderstand the operation of a cathodic

RECTIFIERS7-2

protection rectifier.

THEORY

A rectifier converts or rectifies alternatingcurrent (AC) electricity to direct current (DC).DC current is defined as a current that is alwaysflowing in the same direction. AC current on theother hand is current which periodicallyreverses its direction of flow. In the UnitedStates, our AC power is supplied at a frequencyof 60 cycles per second, which means that thedirection of the current flow reverses every 8.33milliseconds.

The following discussion will review theprincipals of converting AC power to DC power.During this discussion, we will be assuming“conventional” current flow where current isassumed to flow from positive to negativethrough the load.

A relationship exists between electricity andmagnetism. This relationship provides themeans of converting electrical energy intomechanical energy and vice versa. We can alsoconvert electrical quantities to magnetic andback to electrical quantities again.

Current can be defined as the flow of electronsthrough a conductor, and when current flowsthrough a conductor, a magnetic field is formedaround it. The direction of the invisible magneticlines of force can be determined by using the“right hand thumb rule” as shown in Figure 7-1.The magnitude and direction of the magneticfield is directionally proportional to the amountand direction of electric current flow.

An extension of the “right hand thumb rule” isshown in Figure 7-2. When a current flowsthrough a coil of wire, an additive effect occursbetween adjacent turns of wire thus creating astronger magnetic field.

Just as current flow creates a magnetic fieldaround a conductor, magnetic lines of forceacross a conductor creates a voltage orelectromotive force (EMF) in it. If there is anelectrical path or circuit, electrical current will

begin to flow. The direction of the EMF can bedetermined using Fleming’s “3-Finger Rule” asshown in Figure 7-3.

These relationships are always the same andare used in the operation of generators, motors,transformers, meters, and many other pieces ofelectrical equipment. This is illustrated byFigure 7-4. Figure 7-4A shows that a current isinduced or caused to flow when magnetic linesof force are cut by a conductor. This principal isused in the operation of a current generator thatconverts mechanical energy to electricalenergy.

Figure 7-4B illustrates that when a current flowsin a magnetic field, a force is exerted on theconductor. This is the opposite effect of thatshown in Figure 7-4A. Electrical energy isconverted to mechanical energy. This is theprincipal behind the operation of an electricmotor.

If the magnetic field influencing the conductorwere changed in direction alternately, theinduced current in the conductor would bealternating. This principal is used in theoperation of electric transformers and isillustrated in Figure 7-4C. Figures 7-3 and 7-4show how electrical current can be generatedby moving a conductor within a magnetic field,cutting lines of flux. This principal can beextended to generate alternating current.

AC power is generated as follows (refer toFigure 7-5). Starting clockwise from the positionshown, “A” first moves up through the magneticfield as “B” moves down until they haveexchanged positions. Continued rotation brings“A” down as “B” moves up, reversing thepolarity of the generated voltage. Eachrevolution of this simple generator representsone cycle of alternating current. Alternatingcurrent is plotted with respect to time in theform of a sinusoidal (sine) wave form, as shownin the figure.

The frequency of the AC current refers to thenumber of complete cycles per second (hertz).Half a cycle is the period of time during which

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the AC flows in one direction before reversing.

Figure 7-6 shows a pure sine wave before andafter rectification. If rectification is perfect,meaning there is no forward voltage dropacross the diodes used for rectification, certainrelationships will exist between the peak valueof the voltage waveform and the root meansquare value (RMS - see next paragraph) andthe DC average as shown in the figure. Theaverage value of the unrectified sine wave iszero, and that is what a DC voltmeter wouldread at the output of a sine wave source. Therectified wave does have an average value thatcan be measured with a DC voltmeter. Thisaverage value can also be calculated bymultiplying the peak value of the full waverectified sine wave by 0.636.

A full wave rectified sine wave has been foundto produce more heat (dissipate power) in aspecific resistor than a pure DC voltage (asproduced by a battery) of the same averagevalue as the rectified sine wave. For this reasonanother way of expressing the true value of thewaveform is expressed as the RMS value. RMSstands for “root mean square”, which is amathematical description of the process used.The value of the waveform is first squared.Squaring the value of the waveform determinesits mean or average value. If this value wereused, it would be a voltage squared term. Sincean equivalent voltage is what is desired, thesquare root must be taken, thus the meaning ofRMS.

When using the root mean square process fora sine wave or a full wave rectified sine wave,the RMS value is found to be 0.707 times thepeak value of the waveform. For a single phasefull wave rectifier, the RMS voltage value isgreater than the DC average by a factor of 1.11.

All of the factors discussed above with respectto voltage are also valid for current calculations.The RMS value of the current from a singlephase full wave rectifier is 1.11 times the DCaverage value. Therefore, the total outputpower from a single phase full wave rectifiercan be calculated as follows:

Total Power Output =

1.11 EDC x 1.11 IDC = 1.23 EDC x IDC

Note: There is really 23% more power beingdelivered from a single-phase full wave rectifierthan the DC values of the voltage and currentwould indicate.

Since it is the average DC power whichcontributes to the electrolytic polarizationnecessary for cathodic protection, we stateconversion efficiency rather than rectificationefficiency. The maximum theoretical conversionefficiency of a single phase full wave rectifier iscalculated as:

DC Power Output 100Total Power Output

E I 1001.11E 1.11I

81%DC DC

DC DC

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=

This percentage can be increased by the use ofefficiency filters in the DC output circuit. Filtersreduce the amount of ripple voltage in theoutput and are able to reduce the input powerof the rectifier while essentially maintaining thesame DC power output.

Note: The above formulas apply only to singlephase full wave rectifiers and cannot be usedfor single phase half wave or three phasecircuits.

Rectifying Circuits

The following sections discuss the four mostcommonly used rectifying circuits. Thesecircuits include the single-phase bridge, singlephase center-tap, three phase bridge, and thethree phase wye.

1. Single Phase Bridge

The rectifying stack in single phase bridgeconsists of four legs. A leg is a circuitelement, which conducts or allows the flowof current in only one direction. As AC isapplied to the stack, two of these legs areused on each half cycle. Figure 7-7

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illustrates the wiring diagram and theoperation of the single phase bridgerectifying circuit. The DC produced by asingle-phase bridge is not pure DC, as is abattery output. It is a full wave pulsatingcurrent.

2. Single Phase Center-Tap

The single phase center-tap rectifying circuit,as shown in Figure 7-8, also produces fullwave rectification. This center-tapconfiguration however, requires only twolegs instead of four. Only one leg conductsduring each half cycle.

The single phase center-tap transformer islarger in size than the single phase bridgetransformer and tapping is more difficult. It ishowever, an efficient circuit and offers manydesign opportunities in rectifiers which areelectronically controlled and where tappingis not required.

3. Three Phase Bridge

The most commonly used circuit in cathodicprotection rectifiers when three phase poweris available, is the three phase rectifyingbridge circuit. This circuit consists of sixlegs, but as with the single phase bridge,only two legs conduct current at any specifictime. Figure 7-9 shows a typical three phasebridge wiring diagram and associatedwaveforms.

The three phase rectifying bridge circuit isvery efficient and seldom requires filtering.

4. Three Phase Wye

The rectifying stack in a three phase wyecircuit consists of three legs, as illustrated inFigure 7-10. The DC output of this circuit isnot as smooth as that of the three phasebridge, but it is much better than that of thesingle phase circuits.

The transformer for this type of rectifyingcircuit requires more iron than other

rectifying circuits in order to preventsaturation of the core. This is due to thepulsating DC current present in eachwinding.

Components of a Rectifier

A standard cathodic protection rectifier isconsidered as being a simple electrical device.The heart of the rectifier consists of thetransformer and the rectifying stack(s). Thesetwo components alone could provide the DCpower required for a cathodic protectionsystem. Other components, or accessories, areadded to the unit to enhance its performanceand provide safety functions.

1. Circuit Breakers

The primary function of a circuit breaker is toprovide overload protection for the circuit inwhich it is installed. It also serves as an on-off switch for the unit.

There are three types of circuit breakers thatare commonly used in rectifiers. They are asfollows:

A. Thermal Breakers

These breakers are constructed with abimetallic element through which the circuitcurrent flows, with the breaker in the “On”position. Should the current rating of thebreaker be exceeded, the heat generatedby the excessive current flow will cause thetwo metals to expand. As a result of theirdiffering temperature coefficients, onemetal expands at a different rate than theother. This causes the bonded metallicstrips to bend to one side, thus releasing amovable contact and opening the circuit.See Figure 7-11 for an illustration of atypical thermal breaker.

A disadvantage of this type of breaker isthat it is very sensitive to ambienttemperatures, tending to respond (open)quickly on warm days and slowly on colddays. Often this type of breaker cannot be

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reset immediately after tripping becauseit must be allowed to cool down.

B. Thermal-Magnetic Breakers

The operation of the thermal-magneticbreaker is nearly identical to that of thethermal breaker. Figure 7-12 shows atypical thermal magnetic breaker. Notethe magnetic plate attached to thebimetal element. The purpose of thismagnetic plate is to speed up the openingof the contacts in the event of a shortcircuit or extremely large current surge. Alarge surge of current will set up amagnetic field around the plate, whichcauses it to be attracted to another platelocated near it, immediately opening thecontact. During normal loads or lightoverloads, the magnetic plate performsno function whatsoever.

This type of breaker has a slowerreaction time than the fully magneticcircuit breaker. For this reason they arerecommended for use on rectifiers thatwill be interrupted during system testing,as these breakers will not trip duringtesting.

C. Fully Magnetic Breaker

This type of breaker is considered themost suitable for cathodic protectionrectifiers because it only responds tooverload currents. The current rating ofthis type of breaker is determined by thenumber of turns and wire size of themagnetic coil which is wound around asealed tube in the circuit breaker, asshown in Figure 7-13. This tube containsan iron core retained by a compressionspring immersed in silicone fluid.

As long as the breaker operates within itsrating, the iron core will remain stationary.Should an overload occur however, thecore will be drawn towards the polefaceon the end of the tube by the magneticaction of the coil. The closer the core

comes to the poleface, the stronger themagnetic pull becomes on the arm thatholds the contacts closed. The lever willtrip and the contact will open when themagnetic pull becomes great enough.

Under short circuit conditions, themagnetic action of the coil itself will tripthe breaker instantaneously. Theoperation of this breaker is entirelyindependent of temperature, making itvery suitable for various rectifierapplications.

Circuit breakers are normally placed in every“hot” input line of a rectifier unit. In a 115 voltsingle phase circuit, one breaker would beplaced in the “hot” line and none would berequired in the ground or neutral lead. Forhigher single phase input voltages, such as230, 460, etc., both input lines should beprotected with a breaker. Three phase unitsshould have breakers installed on all three inputlines.

It is recommended that circuit breakers alwaysbe ganged (mechanically tied together) so thatwhen one breaker “trips”, the breakers on allother input lines will be “tripped” or opened.This will help prevent personal injury and/orcircuit damage.

2. Transformers

A transformer consists of a laminated ironcore with one or more coils of wire woundaround it as shown in Figure 7-14A. Atransformer is used to step up a voltage,step down a voltage, or to isolate a voltagefrom its source.

The primary winding of the transformer isconnected to the voltage source. Thesecondary winding of the transformerreceives voltage from the primary windingthrough magnetic coupling. An alternatingmagnetic field is set up in the core when anAC voltage is applied to the primary windingof the transformer. The magnetic fieldinduces a voltage into the secondary

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Primary VoltageSecondary Voltage

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winding at the same volts per turn ratio asthat of the primary. Therefore, the ratio ofsecondary volts to secondary turns is thesame as the ratio of primary volts to primaryturns. This is only true under no-loadconditions. Under load conditions a reducedvoltage is seen on the secondary. Thisreduced voltage is due primarily to losses inthe core and in the lamination.

Placing taps at intervals along the secondarywinding will produce or supply voltagescorresponding to the number of turns atwhich they are connected. By locating thetaps in “coarse” and “fine” arrangements, itis possible to produce a considerablenumber of evenly spaced voltageadjustments with relatively few taps.

The voltage output of the transformer can bedetermined by using the following ratio:

Example:

An ideal transformer (no losses) has 10 turnson the primary and 30 turns on the secondaryor output. A 120 volt, 60 cycles per second(hertz) power supply, is connected to theprimary. The output voltage can be calculatedas follows:

The transformer in this example would bereferred to as a step-up transformer with a 1:3turns ratio.

A desirable feature sometimes added to rectifiertransformers is a grounded shield. Thegrounded shield is installed between theprimary and secondary windings, as shown inFigure 7-14B, to intercept high voltage pulses(switching surges or lightning initiated pulses).These pulses could otherwise damagerectifying elements, particularly silicon diodes.

3. Rectifying Elements

The function of a rectifying element or cell is toallow current to flow in a circuit in only onedirection. This can be accomplished with theuse of various materials, but the mostcommonly used materials in cathodic protectionrectifiers are selenium and silicon.

Selenium Cell

The selenium cell is generally made up of analuminum plate varying in size from 1 to 8inches square, with a deposit of seleniumcrystals covered with a metal film to provide acontact area. Special processing creates abarrier layer on the selenium side of the plate,which prevents current from passing from theselenium to the aluminum. Figure 7-15 showsa typical selenium cell. Advantages of the useof selenium stacks over silicon stacks are asfollows:

a. Selenium stacks will withstand surges due topossible lightning strikes much better thansilicon stacks.

b. Selenium stacks are more economical inlower voltage output circuits where currentrequirements are small.

c. Selenium stacks can withstand severe shortterm circuit overloads.

Some of the disadvantages associated with theuse of selenium stacks are:

a. High voltage and current output units arevery expensive due to the number of cellswhich must be arranged in series andparallel to achieve these ratings.

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b. Selenium stacks cannot be practicallyreplaced.

c. Maintaining an adequate stock ofreplacement stacks can be expensive.

Silicon Diodes

Figure 7-16 illustrates a typical silicon dioderectifying element. Unlike the selenium plate,the silicon diode has a single crystallinerectifying junction. The rectifying area of thediode is many times smaller than that of theselenium plate. As shown the figure, the diodeis manufactured in a physical configuration thatallows it to be installed in a tapped metal plate.The mounting of the diode on the plate willserve as heat sink which draws heat away fromthe junction area thus increasing its powerhandling capacity.

The following list indicates some of theadvantages of silicon diodes over seleniumstacks:

a. The use of silicon diodes in units providinghigh current and voltage outputs is moreefficient and therefore, more economical.

b. Replacement cells are more easily installedand stored.

c. Silicon diodes have a longer operating lifethan selenium stacks.

Some of the disadvantages of silicon diodesare:

a. Additional protection must be added to theseunits to protect them from lightning strikes orother overvoltages.

b. The use of silicon diodes may be moreexpensive initially.

4. Accessory Equipment

Optional equipment such as meters, lightningarresters, filters, and shunts can be suppliedwith the rectifier unit.

Voltmeters and ammeters are normally installedon the rectifier to facilitate the monitoring of itsvoltage and current output. Sometimes, onlyone meter is supplied which, through the use ofa selector switch, will read both volts andamperes.

Lightning arresters are normally installed onboth the AC input and the DC output circuits ofthe rectifier when, due to the location of theunit, the possibility of lightning surges exists.Lightning surges can enter the cathodicprotection rectifier from both the AC and DCsides. Installation of lightning arresters willprevent damage to the unit and its componentsdue to these surges.

Efficiency filters can be added to single phasebridge and center-tap circuits. The filtersimprove the conversion efficiency by reducingthe ripple component in the rectifier output.Ripple in the output can be reduced as much as20 to 25 percent.

Filters can be used to eliminate electronicnoise/interference on electronic circuits and atthe same time provide increased lightningprotection to the DC circuits of the unit. Theinstallation of these efficiency filters mayincrease the cost of the rectifier, but in mostcases they will pay for themselves within thefirst year of operation. Conversion efficiency isusually increased by 10 to 15 percent.

Rectifier units can be provided with shuntsinstalled in series with the positive outputterminals of the unit. These shunts provide ameans of measuring the output current of therectifier, using a portable voltmeter, withoutinterrupting the service of the unit. Thesize/rating of the shunt selected must becompatible with the output rating of the rectifier.

The current output rating of the rectifier mustnot exceed the ampere capacity of the shunt.For example, a typical shunt installed at theoutput of a rectifier rated 40 amps, would berated 50 amps – 100 millivolt. If an 8.7 millivoltdrop where measured across the shunt with avoltmeter, the current output of the rectifier can

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TYPES OF RECTIFIERS

Cathodic protection rectifiers are available withspecialized features and in variousconfigurations/designs. The most commonlyused types of rectifiers are:

1. Air-Cooled Rectifier

This type of unit and its components areenclosed in a steel enclosure with doors thatprovide access to the unit for testing andrepairs. The bottom of the enclosure isusually constructed of steel screening toallow for the circulation of air. This type ofunit can either be wall, pole, or pedestalmounted depending on the size and weightof the unit. See Figure 7-17.

2. Oil-Cooled Rectifier

This type of rectifier is used in areas wheredust, salt air, corrosive fumes, or excessivemoisture may shorten the operating life of anair-cooled unit. The rectifier and itscomponents are installed in a steelenclosure and are completely immersed inoil, thus isolating them from extremeenvironments.

Modified oil-cooled rectifiers are availablewith explosion proof fittings. These explosion

proof rectifiers are for use in refineries,chemical plants, and other areas whereexplosive or flammable vapors, liquids orpowders are present.

Oil-cooled units are normally pedestalmounted due to their weight. See Figure7-18 for a typical oil-cooled rectifier detail.

3. Constant Current Rectifiers

These rectifiers have a special circuit thatenables the rectifier to provide a nearlyconstant current output regardless of loadresistance. This type of unit is used inapplications where the load resistanceschange drastically and the output currentwould be exceeded with a normal rectifier. Atypical constant current rectifier circuit isshown in Figure 7-19.

4. Automatic Potential Controlled Rectifiers

This type rectifier monitors the structure-to-electrolyte potential and maintains it at adesired level. The use of this type of rectifierrequires the use of a permanently installedreference electrode and an additional testwire connection to the structure. Both areconnected to a transistorized control circuitas shown in Figure 7-20. The controlleradjusts the output voltage to keep thereference cell potential at a preset desiredlevel. Automatic potential controlled rectifiersare very useful in the control of corrosion onstructures such as water storage tanks,harbor structures, and where it is necessaryor desirable to maintain a constant potentialon the structure.

PREVENTIVE MAINTENANCE

In order to maintain proper operation, rectifiersshould be inspected on a routine basis. DOTregulated rectifiers must be inspected everysixty days (not to exceed seventy-five days)and this is usually sufficient to ensure that theunits are functioning properly. Units that arelocated in areas that are prone to lightningstrikes or power surges may need to be

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inspected more frequently. This is necessary sothat loss of protection (as from a blown fuse orcomponent failure) will not go undetected forlong periods of time.

Where longer periods between inspections areunavoidable for reasons such as systemremoteness or inaccessibility due to seasonalweather conditions, units can be fitted withremote monitoring units (RMU’s). RMU’s aredevices that can be programmed to downloaddata to computers in order to verify correctoperation.

Routine inspection of rectifiers consistsnormally of reading the DC output voltmeterand ammeter as well as reading the ACkilowatt-hour meter (if one exists). Readingsshould be recorded in a permanent log andcompared to previous readings to observeunusual variations.

If, during maintenance rounds, a rectifier isfound to be off, the inspector should check tosee if the trouble is due to a blown fuse or atripped circuit breaker. These are the two mostcommon causes of rectifier malfunctions. If thisis the case, resetting the breaker or replacingthe fuse may be all that is required in order torestore the rectifier to service.

If the inspector can restore the rectifier toservice he should do so. Regardless of whetheror not they can do so, the responsible corrosionengineer should be notified as soon as possibleso that he can investigate the trouble and takecorrective action as required.

At least once per year all rectifiers should bethoroughly inspected. Following is a typical listof items that should be checked in order toensure that the rectifier continues to functionproperly.

1. Clean and tighten all bolted current-carryingconnections.

2. Clean all ventilating screens in air-cooledunits so that airflow will be completelyunobstructed.

3. Check all meters for accuracy by using aproperly certified portable meter to “zero”meters that are built into the rectifier unit.

4. Replace all wires on which the insulation hasbeen damaged.

5. If the unit is an oil immersed unit, the oillevel and cleanliness of the oil should bechecked. The oil should be clear and nearlycolorless. Failing oil is usually characterizedby a murky or cloudy appearance with lossof transparency and should be replaced withgood grade oil. Facilities are available fortesting the oil and salvaging it by filtrationwhere practical.

6. Check all protective devices (fuses, circuitbreakers, and lightning arresters) to be surethat they are undamaged and in satisfactoryoperating condition. Defective devicesshould be replaced immediately.

7. Turn the rectifier off and feel thecomponents. Watch for excessively hotcomponents or uneven heating of rectifierstacks. Temperatures of electricalconnections are also important. Nocomponent should be too hot to touch.

8. Inspect all components, including lightningarresters, for signs of lightning damage – arctraces across insulators or panels;discolored parts. If their appearance hasbeen altered or if damage is suspected, thecomponent should be replaced.

9. Remove excessive dust accumulations andany type of insect nests. Plug any holesthrough which insects or rodents can enterthe rectifier.

The objective of any good maintenanceprogram should be the prevention of unitfailures before they occur, and prompt repair inthe event that a failure does occur.

RECTIFIERS7-10

EfficiencyE I 100%

3600KNT

=DC DC× ×

Efficiency32 12 100%3600 14 1

89

67.8% =× ×

× × =RECTIFIER EFFICIENCY

As rectifiers get older they tend to become lessefficient as the rectifier stacks age. At somepoint, due to the decrease in efficiency, it maybecome economical to replace the rectifyingstacks. Many manufacturers recommend stackreplacement when an efficiency drop of 20% isexperienced.

An abrupt decrease in rectifier efficiency mayalso indicate the unit is malfunctioning. In orderto detect any changes, the efficiency may becalculated and recorded during periodicinspection/maintenance.

The efficiency of a rectifier can be determinedby using the following formula:

Where:

EDC = DC output voltage measured acrossthe terminals

IDC = DC output current calculated bymeasuring the voltage across the totaloutput shunt, using a voltmeter, andcalculating the current.

K = Kh of watt-hour meter. Shown on face ofwatt-hour meter.

N = Number of revolutions of watt-hourmeter disk. For this method N =1.

T= Time for disk to make one revolution

Example:

The measured DC output of a rectifier is12 amperes @ 32 volts. A watt-hour meter hasa constant (K) of 14, and it takes 89 secondsfor the disk to make one complete revolution.

Note: When checking efficiency, care mustbe taken so that the original load will beduplicated at each efficiency checkbecause efficiency varies with outputvoltage and current. If loading is not thesame, comparison with previously recordedefficiencies will be meaningless.

RECTIFIER SELECTION

Selecting the correct rectifier is a very detailedand important task. It is important to provide themanufacturer with a specification which is asdetailed as possible. This will help assure thatthe unit supplied is exactly that which isrequired. The following list indicates theminimum information that should be included ina rectifier procurement specification.

1. AC Input – Specify voltage, number ofphases (single or three phase), andfrequency. Example: 480 volts, 3 phase, and60 hertz.

2. DC Output – Specify in volts and amperes.Remember to include spare capacity.

3. Rectifier Type – Air-cooled, oil cooled, oilcooled and explosion proof, constantcurrent, or automatic potential controlled.

4. Rectifying Elements and Configuration –Specify element: silicon or selenium. Specifyconfiguration: full wave bridge or full wavecenter tap.

5. Mounting – Indicate desired mountinghardware to be included for either pole, wall,or pedestal mounting.

6. Operating Temperatures – Minimum andmaximum ambient operating temperatures.

7. Instruments – Voltmeter and ammeter.

RECTIFIERS7-11

Specify scales and accuracy.

8. Voltage and Current Control – Specifynumber of transformer taps required formanual adjustments.

9. Protective Devices – Specify ratings andtypes of circuit breakers, fuses, lightningarresters, and filters as required. Shieldedtransformer winding if required.

10. Output Terminals – Specify number ofpositive and negative terminals/circuitsrequired.

11. Shunts – Specify the number of shuntsrequired and their ratings.

When possible, units should be ordered fromthe manufacturer’s standard line. Custom madeunits will be more costly and delivery time maybe lengthy.

CONCLUSIONS

A cathodic protection rectifier converts ACpower into DC power, which is required forcathodic protection of a metallic structure.

Typical rectifiers utilize either full wave bridgeor full wave center tap rectifying circuits withselenium or silicon rectifying elements.

There are various types of rectifiers such as aircooled, oil cooled, constant current, andautomatic potential controlled. Each type isdes igned for use in a spec i f i cenvironment/application. Rectifiers can bepurchased with optional features that willfacilitate testing during periodic maintenance,protect the unit from power surges (lightning),and increase its efficiency.

To make sure that the manufacturer suppliesthe exact type of rectifier desired, a detailedprocurement specification should be written andforwarded to the supplier. The effectiveoperation of the impressed current cathodicprotection system depends on the properoperation of the rectifier. Therefore, a good

maintenance program should be developed tohelp keep the rectifier operating. Malfunctioningrectifiers should be repaired as soon aspossible, as long outages may be detrimental tothe pro tec ted s t ruc ture. Rect i f iertroubleshooting techniques will be discussed inChapter 8 of this course.

CATHODIC PROTECTION SYSTEM MAINTENANCE AND TROUBLESHOOTING PROCEDURES 8-1

CHAPTER 8

CATHODIC PROTECTION SYSTEM MAINTENANCE AND TROUBLESHOOTING PROCEDURES

INTRODUCTION

Corrosion control measures as discussed in theprevious chapters, will be highly effectiveproviding they are properly designed andinstalled as well as adequately maintained.Without a suitable maintenance program, thefunds spent in the design and installation of anycorrosion control system could be wasted.There have been many instances, for example,of system Owners spending the money to havea cathodic protection system designed andinstalled on a section of their system withoutestablishing any procedures for looking after it.Although the Owner may feel that his/hercorrosion related problems are over, this falsesense of security will be short-lived if corrosionfailures continue to occur. Although thetendency is to blame the design, the fault inmost cases is simply the failure of keeping thesystem(s) operating continuously andeffectively. The purpose of this Chapter is topresent suggestions for maintenance programsthat will help keep corrosion control systemsoperating at maximum effectiveness.

MAINTENANCE PROGRAM

A corrosion control system maintenanceprogram should include but not be limited to thefollowing items, as applicable:

1. Periodic Surveys

Periodic Surveys are performed todetermine the status of cathodic protectionand related items. Periodic surveys will be

discussed in detail in one of the followingsections.

2. Coating Maintenance

Steps should be taken to maintain coatingsystems on aboveground structures as wellas on buried or submerged structures.

Coating maintenance on buried orsubmerged structures is more difficult toperform, but steps can be effectively taken.During normal operations of buriedstructures such as pipelines, there arefrequent occasions when it is necessary touncover or make accessible variousappurtenances for other maintenancework. This work may involve damage to orremoval of the coating.

Any apparent damage to the coating, be iton a buried, submerged, or anaboveground structure, should be repairedor the coating replaced. Coating repair orreplacement should be of a quality at leastas good as the original coating system. Inaddition, it is important that any repaircoating be compatible with the originalcoating. Maintenance crews should betrained in good coating applicationprocedures including allowing time forcertain materials to cure before backfilling,care of materials, and compliance with themanufacturer's specifications. Aspreviously discussed, when certain hotapplied enamels are used, particular careis essential when heating small quantities


to be sure that the material is notoverheated and rendered useless. If thishappens, much of the effectiveness of thistype of coating may be lost.

Keeping a factual performance record ofcoating systems will help the corrosionengineer to prepare and presentrecommendations for materials to be usedfor new construction. The general conditionof the coating system in terms of itseffective resistance-to-earth can beobtained from the periodic corrosionsurveys. Other specific information may beobtained by training all systemmaintenance crews to report on coatingconditions whenever possible, such aswhen coated pipelines are excavated.Information to be included in such a reportis the date and specific location of thework, coating type and description, andother identifying information as available orapplicable. The report should also includedata on environmental conditionssurrounding the pipe/structure that couldhave an adverse effect on the coatingsystem, evidence of soil stress effects,bond quality, evidence of cold flow, andevidence of moisture under the coating. Ifthere is evidence of corrosion damage atcoating faults (holidays), i.e., pitting, thereport should include data on the numberof pits, depth range and range of pitdiameters.

This information will enable the corrosionengineer to assess the condition/quality ofthe existing coating system and determinewhat corrective measures are required, ifany.

3. Rectifier and Anode Bed Maintenance

Rectifier maintenance should be conductedas discussed in the previous chapter.Impressed current anode bed maintenanceis normally limited to the visual inspectionof the anode bed site during maintenancerounds.

Any abnormal activity or occurrencesshould be reported to the responsiblecorrosion engineer as soon as possible sothat he or she can determine the effect, ifany, on the anode bed and take correctiveaction as required. For example, if any partof an anode bed is subject to washout bystorm water that will result in exposure ofthe DC output cable(s), arrangementsshould be made to have this areainspected regularly and to have thecable(s) recovered when necessary. Thisshould be done only after making sure thatthere has been no damage to the cableinsulation. Any damage found should berepaired. Keep in mind that if the anodecable conductor is exposed to theelectrolyte, this then will discharge currentand eventually result in a cable break.Corrective action should also be taken toprevent re-exposure of the cable(s) if at allpossible.

If construction activity is to be performed inthe vicinity of an anode bed, the corrosionengineer should have the location of theanode bed staked out so that inadvertentdamage may be avoided.

If the construction activity involves theinstallation of new underground orsubmerged metallic structures, thecorrosion engineer should be notified. Newstructures in the vicinity of the anode bedmay be subject to possible stray currentdamage. Provisions should be made in thedesign of the new structures so that thereare adequate facilities available to test forand to mitigate stray current interference.

4. Galvanic Anode Maintenance

Other than routine electrical testsconducted during periodic surveys,maintenance of galvanic anodeinstallations, as in the impressed currentanode beds previously discussed, is limitedto visual observations by maintenancepersonnel. When damage to anode leadcables, anode header cable(s), pipeline


lead wires, etc. is found, it should bereported to the corrosion engineer so thatprompt repairs can be scheduled.

At test stations that contain galvanicanodes, a clean connection must bemaintained between the anodes and thestructure. Because of the low drivingpotentials associated with galvanic anodes,any resistance in the connections cancause a marked decrease in anode currentoutput. The connections should bechecked on an annual basis and cleanedwhen required.

The use of a test station allows for themeasurement of polarized potentials on thestructure as well as measurement of thecurrent output from the galvanic anodesystem. Measurement of the anode systemcurrent output will aid in determining boththe anode consumption rate and predictionof expected service life.

5. Test Station Maintenance

Test stations are the principal means bywhich the protection afforded to a structurecan be evaluated. If they are to continue tofulfill this function, they must be maintainedin good order.

Test stations may, from time to time,require replacement of box covers or coverretention screens, cover gaskets orterminal nuts or screws within thereceptacle. Occasionally, a receptacle maybe broken or missing as a result ofvandalism. Flush mounted or grade leveltest stations often become filled with silt orcovered by paving material when located inroadways. Efforts should be made to keepthese test points accessible for testing.Corrosion personnel making routinesurveys should carry spare parts andhardware so minor maintenance can beperformed while still on site. In this way,especially on systems utilizing a largenumber of test stations, this type ofmaintenance can be kept up to date.

Screws should be kept greased for ease ofoperation (using graphite lubricant orpetrolatum) and contacting surfaces inelectrical circuits should be kept clean toensure the lowest practicable contactresistances.

If electrically discontinuous test leads arefound during periodic surveys ortroubleshooting testing, these test leadsshould be replaced as soon as possible.Leaving invalid test leads in place can leadto erroneous data that may indicate a lackof cathodic protection, a possible opencircuit, or other types of problems.

PERIODIC SURVEYS

Federal and State laws dictate what surveysmust be made on facilities such as gas piping,oil lines and tanks containing hazardousmaterial. All cathodic protection systems needto be tested periodically, however, and thefollowing data, as required and applicable,should be taken at least on an annual basis.The most common test employed to evaluatethe effectiveness of an existing cathodicprotection system is the structure-to-electrolyteor pipe-to-soil potential measurement. In fact,the majority of the existing criteria fordetermining whether a structure is cathodicallyprotected are all based on the this particularmeasurement.

1. A pipe-to-soil potential survey should beconducted on the protected structure. Thepotential survey is conducted using a highresistance voltmeter and a portablereference electrode. The reference electrodeshould be placed directly above the pipe incontact with the soil. Where the soil is dry,the electrode should be dug in slightly. If thesoil is extremely dry, moisten the soil withwater to assure good reference cell contactwith the earth. It is important to minimize anyinaccuracy in the potential measurement byinsuring that the reference electrode is ingood contact with the electrolyte. Thereshould be no loose stone or debris betweenthe tip of the reference electrode and the


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soil. The presence of petroleum products orchemical contaminants may also affect themeasurement. One method to verify that thepotential measured is an accurate reading isto change the input resistances on ananalog voltmeter with this feature andcompare the potentials measured. If there isboth good reference cell contact andstructure contact, the potentials measuredshould be very similar.

The conventional connection to thevoltmeter has the reference cell connectedto the positive terminal of the meter and thestructure to the negative terminal. This willcause an "up scale" meter movement on ananalog meter. With the introduction of thedigital voltmeter, many corrosion personnelreverse the connection so that a negativevalue is displayed. Whatever polarityconnection is used, make certain to use itconsistently and to note on the data sheetsthe method of connection used. Note: It isgood practice to check the meter reading ontwo different meter scales when using ananalog meter to assure meter accuracy.

Figure 8-1 depicts how a potential survey issometimes conducted over a long length ofpipeline. During this type of survey,potentials are taken along the length of thepipeline and then plotted as a function ofstructure-to-earth potential versus referencecell location. The connection to the structureunder test can be a valve, riser pipe, teststation, etc., any place that is electricallycontinuous with that portion of pipe beingevaluated. It is important to insure that thestructure under evaluation is electricallycontinuous for the length of pipe to besurveyed. Potentials measured on the otherside of an electrically discontinuous pointsuch as an isolating flange, are essentiallypotentials measured to remote earth and arenot indicative of the actual pipe-to-soilpotential.

During the potential survey, the groundvoltage coupling value can be calculated if itis possible to interrupt the current source(s).

Figure 8-2 shows the method for interruptinga rectified system. The ground voltagecoupling value represents the effectivestructure to earth resistance and provideshistorical data on changes in systemparameters such as failures at isolatingjoints. Using the change in the potentialreadings measured while interrupting thecurrent, and the value of the current, theground voltage coupling is calculated. Theground voltage coupling is equal to thepotential change ()V) divided by the outputcurrent (I). The units are volts/amps.

The ground voltage coupling can be used todetermine the amount of current required tochange the potential of a protected structure.

EXAMPLE

The current output of a rectifier is 3 amps.Measured pipe-to-soil potentials are -0.82 V Onand -0.65 V Off with the rectifier outputinterrupted. Determine the additional amount ofcurrent required ()Irqd) to raise the On potentialof the structure to -0.85 V ()Vrqd = 0.03V). Step #1 - Calculate the potential change ()V):

)V = V on - V off

= -0.82 - (-0.65) = 0.17 volt

Step #2 - Calculate the ground voltage coupling(Rvg):

Step #3 - Calculate the additional currentrequired to raise the On potential of thestructure to the 0.85 volt value:

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2. On coated structures, data required for thecalculation of effective coating resistanceshould be taken. Although not essential, thisinformation is valuable for monitoring theperformance of a specific coating system.

This measure of coating effectiveness orquality is based on considering theresistance-to-earth of the line as if it werecomprised of many parallel resistances,each section having a surface area of onesquare foot.

The quality of the coating (averageresistance = Rc) can be calculated using thefollowing formula:

Rc = Rvg x Surface Area of the Structure

Normally, good construction practices willresult in average coating resistance valuesof 300,000 ohm-ft² and greater, uponcompletion of the installation. Theacceptable resistance value for a given setof conditions is a function of various factorsand must be determined by experience andthe judgement of the corrosion engineer.One of the most influential factors is soilresistivity. For example, a line laid in 1000ohm-cm soil will have a lower averagecoating resistance than an identical line laidin 10,000 ohm-cm soil.

EXAMPLE

A 12-inch pipeline has a line resistance(ground voltage coupling, Rvg) of 8 ohms.The length of the pipeline is 32,000 feet.Determine the average coating resistance.

Step #1 - Calculate total surface area (SA)of the pipeline.

SA = ( x diameter (ft) x length(ft)

= (3.14) x (1) x 32,000 = 100,480 ft²

Step #2 - Calculate average coatingresistance (Rc).

Rc = Rvg x Surface Area = 8 x 100,480

= 803,840 ohm-ft²

The sample calculated resistance indicatesa coating of good quality.

3. At each rectifier installation, DC current andvoltage should be measured. See Figures8-3 and 8-4 for the methods to measure therectifier voltage and current. The kilowatthour meter reading should be taken and therectifier efficiency calculated, as described inan earlier chapter. The rectifier should bevisually examined for any signs of damagesuch as a burnt or blistered stack, loosewiring connections, missing equipmentlocks, etc. If it is an air-cooled unit, thebottom screen should be free of anyobstructions. If it is an oil-cooled unit, checkthe oil level and examine the oil for anyevidence of foreign material such as wateror debris.

4. At other DC power sources, the DC currentand voltage as well as pertinentsupplementary information that may apply tothe particular power source should be taken.

5. Data to calculate the resistance of eachimpressed current anode bed should betaken.

6. The current output and resistance of eachgalvanic anode installation should bemeasured. The current output of the anodeis measured by placing an ammeter in serieswith the anode lead, as shown in Figure 8-5,or by connecting a voltmeter across currentshunts installed in an anode distribution boxas shown in Figure 8-6.

7. Potentials of the protected pipeline and offoreign lines at any crossing points shouldalso be measured. Where intersystem bondsexist, measure the bond current anddirection of current flow.

8. At pipeline cased crossings, the resistancebetween carrier pipe and casing should be

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measured plus the potential to a closereference electrode of both the pipeline andcasing.

9. In dynamic stray current areas, verify thatbonds, electrolysis switches or othercorrective measures are operating properlyand are providing the required degree ofprotection. In those areas, recordingvoltmeters should be used for potential andcurrent measurements.

10. Verify that isolating joints are effective andthat any protective lightning arresters,spark gaps or grounding cells areperforming their function effectively.

11. In an impressed current anode systemmeasure the current output of each anodeat the anode distribution box, if available.Again, see Figure 8-6 for the proper meterconnections.

12. Verify continuity between structures wherejumpers were installed as part of thecathodic protection system.

13. Make notes on maintenance records of anyspecial physical features associated withthe corrosion control system. Maintainthese notes with the associated circuits.

The corrosion engineer should establish as partof his/her maintenance program a listcontaining the items listed above that areapplicable to each installation/system, and thefrequency at which each test should beperformed.

In addition to the complete annual survey, arecheck of protective potentials may be made atintermediate intervals. The need forintermediate surveys may be a matter based onoperating experience. In general, intermediatesurveys are usually conducted in congestedareas where stray current interference may bea problem, or where particularly critical orhazardous environmental conditions exist. Atareas that are considered critical, more frequentchecks should be made.

The above compiled data should be recorded ina permanent log so that previous survey datacan be compared and analyzed. As soon as aperiodic survey is completed, data should beanalyzed promptly. The data can becomputerized. With the use of the propersoftware, the computer can compare the datawith that of previous surveys and indicate theproblem areas. Corrective action should betaken immediately.

The importance of clear, accurate, andunderstandable data cannot be over-emphasized. Record all pertinent information onapproved forms. Field sketches that mayaccompany data sheets must be as completeas possible with dimensions given to"permanent" objects. Data sheets should makesense and be clear enough to be analyzed wellafter the fieldwork has been completed.

Information to be included on the data sheets:

C Date and time taken (and by whom)C Structure/circuit designationC Where/at what point on the structure

(sketches are often necessary withdimensions to "permanent" objects)

C Instruments used with model and serialnumbers

C Polarity of all measurements (+/-)C Meter scale used for each readingC Shunt usedC Under what conditions was the data taken:

C Rectifier On/OffC Bonds In/OutC Current sourceC Type of reference cell and its placementC Soil conditions: wet, dry, frozenC Any other unusual conditions that are

pertinent


REPAIRS AND/OR REPLACEMENTS

Based on data compiled during the periodicsurvey or as a result of routine maintenance, itmay be deemed necessary to repair and/orreplace various cathodic protection systemcomponents. Following is a brief discussion ofrepairs and/or replacements that may berequired on a typical cathodic protectionsystem.

1. Coatings

While recoating a buried structure such as apipeline is expensive, it may be necessary insome instances. This is apt to be true wherecoating deterioration has been severe over alength of a pipeline or part of a structure due tounusual environmental conditions. Ifdeterioration has been great enough to divertso much of the cathodic protection current thatsatisfactory protection of other parts of thestructure has been lost, recoating the defectivesection could restore effective levels of cathodicprotection to the entire structure with no changein the cathodic protection system. Theeconomic justification for recoating versussimply adding local cathodic protection in theeffected area would have to be studied by thecorrosion engineer in each such instance. Thematerial used for recoating buried structuresshould be selected specifically for its ability tostand up under the particular environmentalconditions encountered. It is probable that theoriginal coating, assuming that it was properlyapplied, was not a suitable selection forconditions in that environment.

Coating damage/failure on abovegroundstructures detected by maintenance personneland reported to the corrosion engineer shouldbe repaired or replaced promptly to maintainthe integrity of the structure. Coating repair orreplacement shall be made as per coatingmanufacturer's guidelines/recommendations.

2. Rectifiers

As previously discussed, any wires orcomponents found to be damaged during

maintenance checks of a rectifier should berepaired or replaced. If the reason for rectifiermalfunction is not apparent, the following typicalrectifier troubleshooting procedures should befollowed.

a) Rectifier Troubleshooting Precautions

The following precautions should beobserved when troubleshooting rectifiers:

1) Turn rectifier unit OFF when handlingcomponents within the unit. Open the ACdisconnect switch ahead of the rectifieras well as the internal circuit breakers.

2) Consult the rectifier wiring diagramBEFORE starting to troubleshoot.

3) Make certain that meters used introubleshooting are properly connected.The voltmeter should be connectedacross the points where the voltage is tobe measured, while the ammeter shouldbe placed in series with the circuit beingtested. A millivoltmeter, when used tomeasure output current, should beconnected across the terminals on therectifier shunt. Correct polarity must beobserved when using DC instruments.Turn the rectifier OFF before using anohmmeter to avoid harming theinstrument.

b) Troubleshooting Equipment

The following equipment is required for basictroubleshooting:

1) A multimeter for reading AC and DCvoltages and DC current up to 10amperes. This meter should also becapable of resistance measurements.

2) A millivoltmeter that can be used forchecking rectifier DC output current bymeasuring the millivolt drop across theshunt on the rectifier panel.

3) Miscellaneous small tools, shorting


cables and several jumper leadsapproximately three (3) feet long withbooted alligator clips are also needed.

c) Simple Rectif ier TroubleshootingTechniques

Many rectifier malfunctions have symptomsthat are obvious. The obvious should neverbe overlooked. Strange odors, looseconnections, charred components, signs ofarcing, loose connections, etc., indicateproblems which do not require elaborate testprocedures to uncover. Some helpfultroubleshooting techniques are:

1) If there is no current or voltage at theoutput terminals, the trouble and remedymay be:

a) Breaker tripped (or fuse blown). Ifapparently due to steady over-load,reduce the output slightly. If the breakercontinues to trip or the fuse continues toblow even with the output reduced, thecause may be a short circuit in somecomponent. Isolate the component andrepair or replace as required.

If the breaker trips occasionally for noobvious reason, the cause may be:

C line voltage surgesC intermittent short circuits

Check for loose connections or brackets.Check connections using an ohmmeterwhile moving leads, connectors, etc. Checkthat power is turned OFF before usingohmmeter.

2) No AC line voltage. Check with ACvoltmeter at rectifier input terminals. Donot overlook the possibility that the ACpanel-board circuit breaker may havetripped.

3) Open circuited component or connection:

a) Check all connections, fine and course

transformer tap adjustments and stackconnections.

b) Rectifier stacks. Use an AC voltmeter tocheck if voltage is being supplied to therectifying elements. If so, they may beopen-circuited and should be checkedusing an ohmmeter and, if required,replaced.

4) Defective Meters

Faulty meters may give the indication thatthe unit is not operating properly whenindeed it is. The rectifier meters shouldbe checked with portable meters knownto be accurate.

5) Defective Transformer

If AC line voltage, as checked with an ACvoltmeter, is being applied to the primaryof the transformer, but none is present atthe secondary, check to see whetherthere is an audible hum coming from thetransformer. If so, the primary isoperating, but the secondary is probablyopen.

6) If DC voltage is measured at the outputterminals of the rectifier but no currentoutput is measured, there is an open inone of the external DC leads. In order todetermine which external DC lead isopen, proceed with the following steps:

C Turn off power to the rectifier.Disconnect the positive (+) lead fromthe rectifier and install a temporaryjumper cable from the (+) output lug tosome type of grounded structure,other than the pipeline to which therectifier is connected, which can beused as a temporary anode. Thiswould include structures such as acopper ground wire installed at therectifier pole or a metal fence orguardrail. Reduce the AC tap settingsto their lowest setting to preventexceeding the rectifier rating. Restore


power to the rectifier and measure theresultant current output. Increase thetap settings one increment at a timeuntil the current can be measured.

C If installing a temporary anode jumperdoes not result in rectifier current, theproblem may be in the structure lead.Reconnect the (+) lead wire anddisconnect the structure (-) lead andinstall a temporary jumper cable fromthe (-) output lug to the pipeline.Restore power to the rectifier andmeasure the resultant current output.It is often convenient to have astructure connection point readilyavailable near a rectifier in order totroubleshoot a rectifier system.

As previously indicated, most rectifier troublesare simple to detect and do not requireextensive detailed troubleshooting procedures.If, however, the trouble is more difficult to locateit is usually better to systematically isolate therectifier components until the defective part isfound. See Figure 8-7 for a wiring schematic ofa typical three-phase rectifier. The sameschematic applies to a single phase rectifieralthough it is simplified since it will have onlyone transformer.

3. Impressed Current Anode Beds

Impressed current anode bed failure isusually a result of one of the following:

a) Broken or damaged anode leads orheader cable

b) Physically damaged or broken anodes

c) Consumed anodes

Probably the most common reasons forpremature failure are broken or damagedanode lead or header cables. These failurescan often be tied to construction activity inclose vicinity to the anode bed or toimproperly made cable splices. All splicesmust be made completely waterproof using

encapsulating resin splice kits or similarinsulation.

Anodes may be damaged or broken duringinstallation or as a result of constructionactivity near the anode bed. Damaged orbroken anodes are not usually worthrepairing and should be replaced. From atechnical point of view, a consumed anodeshould not be considered a failure eventhough it will no longer provide adequatelevels of cathodic protection current. Ideally,anode life calculations should have beenconducted during system design so thatanodes can be replaced before they arecompletely consumed or consumed to apoint where they can no longer provideadequate protection.

4. Galvanic Anodes

As galvanic anodes approach the end oftheir useful life, current output will diminish.Replacement will be required when they nolonger can furnish enough current tomaintain protective potentials on thestructure. Current output should bemeasured during annual surveys at thoseinstallations having test points installed forthat purpose.

A marked decrease in the output of agalvanic anode which, based on anode lifecalculations, is not reaching the end of its lifemay be an indication of a broken headercable, lead cable, or possibly a physicallydamaged anode. As is the case withimpressed current anode beds, header andlead cables can become damaged orbroken. Repair or replace damaged cablesas required, to restore system operation.Like impressed current anodes, physicallydamaged anodes should be replaced.

5. Test Stations

Broken wires at test stations can be hard torepair if the break location is not accessible.Test wires may snap if sufficient slack wasnot provided during installation to allow for

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soil settlement.

Occasionally, an open circuit indication atthe test station may be the result of adefective or improperly made exothermicweld connection of the test lead to thestructure.

At locations where the test station is locatedclose to a buried structure, the excavationrequired to locate and repair the damage(unless the structure is very deep) is arelatively simple job requiring one relativelysmall excavation. If the break is in a longwire span, however, the question of where todig becomes more difficult and additionaltests are required to determine the locationof the break.

Where the wire has been broken and pulledapart, the break in most cases can belocated by conducting an "over-the-wire"survey using a pipe locator, see Figure 8-8.This is done by connecting the wire beingtested to the pipe locator transmitter terminallabeled "pipe" and connecting the terminallabeled "ground" to a separate ground suchas a ground rod. When the pipe locatortransmitter is turned on, an audio frequencysignal is transmitted along the wire beingtested. Walking over the route of the wirecarrying the pipe locator audio receiver, anaudio signal will be heard. The audio signallevel will gradually decrease as the distancefrom the transmitter increases. An abruptdecrease or loss of the audio signal willindicate the point of the wire break.

If the "over-the-wire" survey does not detecta break in the wire, the route of the wiremust be excavated to locate the break.Excavation should start at the connection ofthe wire to the structure point so that thepossibility of a defective exothermic weldconnection can be ruled out. If the wirebreak is in the long lead wires used formillivolt measurements, it may prove to bemore economical to install a new two wiretest station at the point where the long leadconnections were originally made. This is in

lieu of excavating along the length of thepipeline to find the wire break. Testing donein the future would then require the use of awire reel extended along the groundbetween the two test stations.

Where two test wires are thought to be shortedbelow ground at a test station, the distancefrom the terminal board to the point of the shortcircuit may be determined by measuring theresistance between the two wires, see Figure8-9. Knowing the wire size, the resistance perfoot for that particular size can be obtained froman electrical handbook or a cablemanufacturer's manual. Based on thisresistance, it is possible to calculate thenumber of feet of wire from the terminal boardto the short circuit and back. Half of this lengthwill be the distance from the terminal board tothe short circuit.

EXAMPLE

Using an ohmmeter, the resistance betweenterminals 1 and 2 is measured at the teststation shown in Figure 8-9. The measuredresistance is 0.074 ohm. According to anelectrical handbook, the resistance of No. 12AWG wire is 0.00162 ohm/ft. Locate the shortas follows:

Distance (x) = ½ (measured resistance/wire resistance)

x = ½ (0.074/0.00162) x = 22.84 ft.

The point at which the wires are shortedtogether is approximately 23 ft. from the testpoint terminals.

On occasion, test wires are found to be incontact or shorted to the test station conduit.This usually occurs where the wires emergefrom the conduit edges that have not beenproperly reamed to eliminate sharp edges. Thisis usually a result of incomplete provisions inthe installation specifications or inadequateinspection during construction.

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K (amp / mv)calibration current

calibration voltage dropor

KI

E Ek

k 1

=

=−

Percent LeakageK (E E )

I100

TEST 1

TEST=

× −×

KI

E E34

28 11.26 amps / mv

k

K 1=

−=

−=

Percent LeakageK (E E )

I100

1.26 (4.7 1)

5100

Percent Leakage 93.2%

TEST 1

TEST=

× −×

=× −

×

=

Test station repairs should be made as soon aspossible. Failure to repair test stations may leadto cathodic protection system problems goingundetected for long periods of time.

TESTS USED IN CATHODIC PROTECTIONSYSTEM TROUBLESHOOTING

Percent Leakage Current Tests

Due to lightning or other causes, isolating jointsor fittings may become partially or completelyshorted. Because the integrity of these devicesis generally critical to the proper operation ofthe cathodic protection system, they must betested periodically. In order to measureaccurately the percent of leakage, a test set-upsimilar to that shown in Figure 8-10 is used.

A calibration current (IK) from an external DCsource must be provided. This current isallowed to flow through a short section of thepipeline and return as shown in the figure.

Begin the test by measuring any voltage droppresent in the calibration section (E1). Thenconnect the calibration current (IK). The currentflow will cause a voltage drop (EK). Both thecurrent flow and its associated voltage drop aremeasured so that a calibration factor for the testset-up can be calculated using the followingformula:

Using the external DC source again, makeconnections to the pipe as shown in the figureto enable test current to flow from one side ofthe isolating joint to the other. The voltage dropacross the calibrated section (ETEST), and thetest current (ITEST) are then measured. Thepercent leakage through this isolating joint canthen be calculated using these measured

values and the previously calculated calibrationfactor as follows:

EXAMPLE

A calibration current of 34 amperes (IK) isallowed to flow from the line side of the isolatingjoint through the pipeline causing a voltagedrop of 28 millivolts (EK). A test current of 5amperes (ITEST) is allowed to flow across theisolating joint through the pipe causing avoltage drop of 4.7 millivolts (ETEST). Determinethe percent leakage current. E1 was found to be1.0 mV.

Step #1 - Calculate the calibration factor (k):

Step #2 - Calculate percent leakage current:

System Current Profile

The plotting of the system current profile canhelp to isolate a corrosion problem area withina system. The current profile is developed usingmillivolt drop measurements along the pipelinesystem.

The current profile is a plot of test stationlocation versus percent test current. The currentprofile of a pipeline section with a perfectcoating and no electrical contact with foreignstructures would look like the plot shown in

VOLTMETER:

E AND EK TEST

E ,1

MEASURING PERCENT LEAKAGE

THROUGH ISOLATING JOINT

FIGURE 8-10

PIPELINEISOLATING

FLANGE

ITEST

EXTERNAL DC

SOURCE AMMETER: IK TEST

AND I

A

MV+

+ +

IK

IKAND I

TEST


Figure 8-11.

This plot shows that a uniform amount ofcurrent is entering onto the pipeline per unitlength of pipe.

Figure 8-12 shows a more typical current profileof a protected pipeline section. The profile,when analyzed, provides the followinginformation:

1. The large drop of current between teststations C and D (dashed line ) indicatesthat a large amount of current is flowing ontothe pipeline in this area. This may be a resultof physical contact with a foreign structure.In the case of a "short" to anotherunderground metallic structure, there will bevery little to no current from the protectionsystem on the other side of the contactpoint. Current will pick up on the lowerresistant structure such as a bare waterdistribution pipe and return to the source viathe metallic contact point.

2. The large drop of current and the increasedslope of the plot between test stations C andB (solid line) indicates that a coatingproblem exists in this area. A largerpercentage of the current will be required toprotect the bare steel in that particular area.

After problem areas are located through the useof the current profile, further tests should beconducted in these areas. These tests shouldinclude the use of special electronic equipmentsuch as short locators and/or surface potentialsurvey techniques, or more extensive mill-voltdrop tests.

To determine the amount of current, theresistance of the pipeline span must first becalculated as shown for the Percent LeakageTest. Figure 8-13 depicts the typical testingarrangement for the calculation of resistance fora particular pipeline span. To insure theaccuracy of the calibration factor, it isrecommended that four (4) separateconnections be made to the structure undertest. The calibration test begins with the

connection of a power source such as a 12-voltbattery in series with an ammeter. The sourceof current is then interrupted and the resultingvoltage drop caused by the current flow is thenmeasured between the inside test wireconnections. Calculation of the calibrationfactor or "K" factor is determined by dividing thechange in the test current by the change in thevoltage drop. This calculated K factor thenbecomes part of the permanent record of thepipeline system. Any future testing whichrequires determination of the amount of currentflow on the pipeline at this particular point canuse this K factor to calculate the current flow.This is provided that the voltage drop ismeasured across the same original inside testwires. The original calibration test should berepeated a sufficient number of times to insurethat the data is accurate, particularly in straycurrent areas. The units for the K factor are inamps/mV. See Figure 8-14 for a typical millivoltdrop test.

The direction of current flow can be determinedby noting which polarity connection on thevoltmeter results in a positive meter deflection.The direction of current flow will be frompositive to negative. Therefore, if the upstreamtest lead is connected to the positive terminal ofthe voltmeter and the resulting voltage drop hasa positive polarity, then the direction of currentflow is upstream to downstream.

Knowing the direction of the current flow isalways useful information, particularly whentracing galvanic currents caused by anunderground contact to another metallicstructure. In this case the ultimate goal of thetesting is to determine the point of contact or"short" and eliminate it. Endwise galvaniccurrent flow on a structure can be tracked bymeasuring voltage drops along the structure atvarious points where a physical connection tothe structure can be made. Provided that yourstructure is electrically continuous, the point ofunderground contact will be between those twotest locations where the polarity of themeasured voltage drop will reverse.

When combined with ground voltage coupling

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CALIBRATION OF IR DROP SPAN

FIGURE 8-13

VOLTMETER

SWITCH

12V STORAGE BATTERY

OR DC SOURCE

(+)(-)

AMMETER

PRODUCT FLOW

(+)(-)

OUTER WIREOUTER WIRE

INNER WIRE INNER WIRE

K

I

E

X A mv

MV

� ��

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WHERE: I AMPS

E MILLIVOLTSMV

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INDIRECT MEASUREMENT OF CURRENT

FIGURE 8-14

VOLTMETER

(+)(-)

PRODUCT FLOW

I = E K

%I =

E K 100%

I

CALCULATED MV

TEST

MV

TEST

�

�

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WHERE: K CALIBRATION FACTOR IN AMPS / MV�


measurements (Rvg), the data obtained can beused to calculate the resistance to earth ofeither the entire piping system or of individualsections of pipe within the system. Byincorporating Ohm's Law, Kirchhoff's currentlaw, and current profile measurements, datacan be compiled. When making thesecalculations it is important to remember thatcalculations involving the entire system mustuse total system current and voltagemeasurements. On the other hand, datacalculations for individual sections mustincorporate only the current and voltagemeasurements obtained in that section of thesystem.

Surface Potential Surveys

Surface potential surveys are conducted usingtwo identical reference electrodes and a highresistance voltmeter. The potential measuredbetween the electrodes, due to a test orprotective current applied to the structure undertest, can be used to locate coating holidays oranodes.

Holiday (cathodic) gradients are caused by theflow of current onto a pipeline, leading tovoltage drops in the earth. If cathodic protectioncurrent is applied to the pipeline that hasholidays, about one-half of the total voltagedrop will occur within one foot of the pipesurface, see Figure 8-15.

The voltage gradients caused by an anode(s)dissipating current will follow the same patternas at a holiday except the directions will bereversed. The largest gradient potential will bemeasured closest to the operating anode.

Surface potential surveys used to locateholidays or anodes can be conducted usingeither the one electrode or two electrodemethod. Both surveys are commonly referred toas a "cell to cell" surveys and both surveys usetwo identical reference electrodes. In the oneelectrode survey, one reference electroderemains stationary while the other electrode ismoved during testing. During the two electrodesurvey, both reference cells are moved. When

two reference electrodes are both moved, it isimportant to retain the same separationbetween electrodes as well as the same polarityconfiguration. In this case the polarity sign ofthe potential difference measured will changeonce both electrodes are past either the coatingfault or anode. The use of these two methods isillustrated in the following examples.

Example # 1 - Measuring Holiday Gradients

Referring to Figure 8-16, the two methods ofsurface potentials can be used to measure theholiday voltage gradients caused by a testcurrent applied at the isolating flange whichchanges the pipe-to-soil potential by 2.0 volts.

First, using the one electrode method, thepositive or stationary electrode is place at pointJ. The moving or negative electrode is placed atpoint H. The measured potential difference atthis point is +0.08 volt. Subsequent testingconsists of moving the negative electrode topoints G through A and measuring the potentialat these various points with respect to thereference left in place at point J. Thesereadings will become more positive. In thisexample, the potential difference betweenpoints J and A is +0.50 volt. As the negativeelectrode is moved past point A through pointsB through H and the potential differences aremeasured, the readings become less positive.Plotting both sets of potential measurements ona graph will indicate the location of the holiday.The location of the holiday will coincide withthat of the most positive potentialmeasurement.

The second method, the two electrode method,involves moving both the positive and negativeelectrodes. A test current is applied at theisolating joint as previously illustrated in Figure8-16. A potential difference of +0.06 volt ismeasured when the positive electrode is placedat point H and the negative electrode at point G.The positive electrode is placed at point G andthe negative electrode at point F for the secondreading. The positive electrode is then placedat point F and the negative electrode at point Efor the third reading. All subsequent readings

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must follow this same pattern. The sixth readingwill be the highest positive reading (+0.170volt), which is measured with the positiveelectrode at point A and the negative electrodeat point B. The seventh reading taken betweenpoints A and B, with the positive electrode atpoint A and the negative electrode at point B,will be a negative reading (-0.170 volt).

The change in the potential measurement frompositive to negative indicates a closure ofcurrent around the coating holiday. A potentialreading of zero would be obtained if the positiveand negative electrodes were placed at point Bon either side of point A directly over theholiday. The most positive and most negativereadings will be located on each side of theholiday and the zero reading directly above itscenter.

Figure 8-17 illustrates the two graphs obtainedwhen plotting the data from the examplesdiscussed above. The top graph represents thereadings from the one electrode method. Usingthis method, if one holiday exists on a pipeline,a definite peak will show on the plot. However,if there are two or more holidays located closetogether, the resulting graph will not show twoor more definitive peaks. Rather, the graph willindicate a holiday that is approximately 5 to 10feet long. A base line is defined by the absenceof significant peaks on the graph. Themagnitude of peaks on the graph will beproportional to the size of the holiday.

The lower graph represents the data obtainedusing the two electrode method. As opposed tothe single electrode method, the two electrodemethod will show definite peaks for multipleholidays that are located in close proximity toeach other.

Example #2 - Locating Anodes

Figure 8-18 illustrates the typical voltagegradients created around an operating anodeburied in the earth. These gradients follow thesame pattern as those found at a coating faultexcept that the direction of the gradient field isreversed. Using the one electrode method, the

stationary or negative electrode is placedbetween 50 and 100 feet away from the anodestring. The moving or positive electrode isplaced above the anode string and then movedalong its length. Positive potential peaks will beobserved over each anode. If the anode outputis interrupted, the potential changes will berelated to the pattern of current discharge of theanodes into the earth. A potential change of+1.30 volts is measured between the negativeand positive electrodes located at points J andH respectively. When moving the positiveelectrode over points G through A andmeasuring the potentials with respect to point J,the readings will be come more positive. Themeasured potential in this example betweenpoints J and A is +8.30 volts. Moving thepositive electrode over points B through H andagain measuring the potential difference withrespect to point J, the readings will becomeless positive.

As stated earlier, when using the two electrodemethod, both the positive and negativeelectrodes must be moved with each reading.With the anode output interrupted and thepositive electrode at point G and the negativeelectrode at point H, the potential difference is+0.50 volt. Refer again to Figure 8-18.Subsequent readings must follow the samepattern as that discussed in Example # 1. Apositive potential of +2.50 volts is measured atthe sixth location with the positive electrode atpoint A and the negative electrode at point B.The seventh reading between points A and Bwith the positive electrode on B and thenegative electrode on A will give a negativereading of 2.50 volts. If the positive andnegative electrodes are placed at both points B,on either side of point A directly over the anode,the measured potential difference will be zero.The area between the most positive and themost negative readings will be over the anode.A zero potential difference will be over theanode. Figure 8-19 shows the data plotted forboth types of survey. The spacing betweeneach anode can be determined from the graph.The top graph represents the data from the oneelectrode method. The high positive readings orpeaks occur directly over the anodes. This type

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of survey can be helpful when trying to locate abreak in an anode string since it can identify thelast operating anode. The data obtained usingthe two electrode method is shown in thebottom graph of Figure 8-19.

The one electrode and the two electrodesurveys are useful in locating individualgalvanic anodes directly connected to apipeline. By interrupting the test current acrossan isolating joint, the galvanic anodes willappear as large holidays on the pipeline. There are times when a break in an anodecable is excavated but it is not known whetherthis is the first break in the cable from therectifier and that there is power to this point.One way to determine that there are not otherbreaks in the cable between this point and therectifier is to measure the potential-to-earth ofthe cable at this point to a standard referenceelectrode and compare this value to theoperating voltage output of the rectifier. Thesetwo values should be similar. Proper safetyprecautions should be taken when connectingto the anode cable in the ditch and a highoutput rectifier should be adjusted to a lowersetting for this testing.

Another type of potential survey that uses tworeference electrodes is known as a side drainsurvey that is often used to delineate possibleareas of active corrosion, particularly on bare orpoorly coated pipe. For this survey the firstelectrode is placed directly above the pipe, incontact with the soil. The second electrode isplaced in contact with the soil at a 90-degreeangle to the pipe at a distance approximatelyequal to 2½ times the pipe depth. Duringtesting, the electrode placed directly above thepipe should be connected to the positiveterminal of the voltmeter and the otherelectrode to the negative terminal. Positiveside-drain readings indicate that current isbeing discharged from the pipe at this point,making it an anodic area. Negative side-drainreadings normally indicate that current isflowing towards or onto the pipe at this point,making it a cathodic area. The side-drainmeasurements should be continued at no more

than 5-foot intervals. Tests must be made onboth sides of the pipe, until the extent of thepossible problem section has been determined.However, under certain conditions a relativelystrong localized anodic cell could exist on thebottom of the pipe with the top of the pipeserving as a cathode and negative side-drainreadings could be measured while severecorrosion is actually occurring on the bottom ofthe pipe at this location.

Testing for Pipelines in Contact withCasings

Two types of contacts are typically consideredwhen evaluating whether a pipeline is in contactwith its casing. These two types are known aseither an electrolytic or metallic contact. Anelectrolytic contact refers to a situation wherethe annular space between the pipeline and thecasing is filled with water or some combinationof electrolyte that provides a low resistant pathbetween the carrier pipe and the casing. Ametallic contact refers to the situation where thecarrier pipe is in direct metallic contact with thecasing. There is also the case where both ofthese situations are present at the same time.

A low resistant contact between the carrier pipeand the casing will affect the operation of acathodic protection system. The casing is alarge bare section of steel pipe that will act asa large coating holiday that may divert currentaway from any local coating holidays on thecarrier pipe. The return path for the cathodicprotection current will be through the lowresistant electrolyte in the annular spacebetween the two structures or through the pointof metallic contact. Therefore, piping withcoating faults near a "shorted" casing may notreceive the proper current density required forprotection. A "shorted" casing will also diminishthe amount of current attenuation from aprotection system since the current will bedrawn to this rather large coating fault.

Any areas of bare metal found at coating faultson the carrier pipe inside of a casing filled withelectrolyte will require protection againstcorrosion. In this instance, cathodic protection

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current must pass from the earth to the casing,through the electrolyte, and then on to the pipe.This series of resistances causes a voltagedrop that may result in less than adequateprotection.

Identifying possible "shorted" casings is usuallyaccomplished through a comparison of the"on/off" structure-to-earth potentials of both thecarrier pipe and the casing. Usually there is asignificant potential difference between thecarrier pipe and the isolated casing when thecarrier pipe is well coated and the casing isbare. These potential differences become lesswhen the carrier pipe is poorly coated and thesurrounding soil is of low resistivity. When boththe carrier pipe and the casing exhibit the samepotential values there is a strong indication thatsome type of "short" is present.

Another indication of a "shorted" casing is asignificant change in the amount of current flowmeasured on the pipeline at a point on the farside of the casing from the cathodic protectionsystem. If the carrier pipe has a fairly uniformcoating, the amount of current measured alongthe pipeline should gradually decrease as thedistance from the cathodic protection systembecomes greater. However, since a "shorted"casing has the same effect as a large coatingfault, there should be a significant decrease inthe amount of current measured on the far sideof the casing. This difference in the amount ofline current measured on both sides of thecasing would be approximately equal to theamount of protective current going to the"shorted" casing. This measurement of linecurrent should be made with the test currentinterrupted.

One field test that can be performed todetermine whether a casing is "shorted" to thecarrier pipe involves using the casing as atemporary anode bed. In this instance thecasing is deliberately depolarized as current isdischarged from the casing into the earth andthen on to the carrier pipe. The indication thatthe casing is isolated from the carrier pipe isgiven by casing potential readings becomingless negative with increasing amounts of test

current. The carrier pipe potential readings canvary in either direction, however if the twostructures are not in metallic contact, thepotentials measured on the carrier pipe willprobably become more negative with increasingamounts of test current. Figure 8-20 depicts thisparticular testing arrangement.

CONCLUSIONS

The establishment of a cathodic protectionmaintenance program is vital if the system is tooperate as intended. A system that is notproperly maintained will undoubtedlyexperience premature failure.

Any system malfunctions detected duringperiodic surveys or maintenance operationsshould be repaired as soon as possible. Asystem that is malfunctioning or not operatingfor an extended period of time may bedetrimental to the protected structure.

This Chapter has presented sometroubleshooting techniques that can helpcorrosion personnel to determine where theproblem may be in a cathodic protection systemthat is not functioning properly. The use ofthese techniques and the implementation ofsubsequent repairs will result in reducing thedown time of the cathodic protection system toa minimum.

DC POWER SUPPLY

PIPELINE

CASING

DETERMINATION OF TYPE OF CASING “SHORT”

FIGURE 8-20

LOCAL REFERENCE

ELECTRODE

(+)

(-)

TEMPORARY

REMOTE

STRUCTURE

EOC

Vg

REMOTE REFERENCE

ELECTRODE

TAKE THE FOLLOWING MEASUREMENTS

FOR 3 DIFFERENT CURRENT VALUES WITH

AT LEAST 3 SETS FOR EACH VALUE

1. TEST CURRENT WITH CASING AS ANODE

2. Vg PIPE-TO-SOIL (LOCAL)

3. Vg PIPE-TO-SOIL (REMOTE)

4. Vg CASING-TO-SOIL (LOCAL)

5. Vg CASING-TO-SOIL (REMOTE)

6. Eoc BETWEEN PIPE AND CASING

c:aucscaucsc bia edits 2008intermediate toc 2008 intermediate text 031208.pdf · 2012-11-04 ·...

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