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SOIL RESISTIVITY MEASUREMENTS AND

ECONOMICS OF CORROSION CONTROL

Practical Applications 2017

February 21-23, 2017

Mike Placzek – Senior Engineer, NACE CP Specialist

ARK Engineering 1

Period 8

Introduction

• Soil Resistivity Measurements • Economics of Corrosion Control

February 21-23, 2017 Mike Placzek – Senior Engineer, NACE CP Specialist ARK Engineering 2

2017 Underground Corrosion Short Course Practical Applications – Period 8

SOIL RESISTIVITY MEASUREMENTS

• Definition and Units – Resistivity is the soil property that is used to

calculate the soil’s electrical resistance – Defined as the resistance (Ω) between two parallel

faces of a cube of soil 1 cm on all sides and is measured in units of ohm-centimeter (Ω cm)

– Other units include ohm-meter (100 Ω cm) and Ω ft.


Practical Applications – Period 8 2017 Underground Corrosion Short Course

Uses of Soil Resistivity • Cathodic Protection Design • Galvanic Sytems

– Calculate the resistance of the anode to earth; the soil resistivity is part of this calculation

– Once the anode resistance to earth is known, and knowing the voltage difference (ΔE) between the anode and the pipe, the designer can calculate the anode current output and hence the number of anodes required for the job

• Impressed current protection – Calculate the anodebed resistance to earth – Resistance, combined with other circuit resistances gives the designer data

from which to determine the required rectifier output voltage for the protective current to be applied



Soil Corrosivity

• Many factors combine to create the corrosivity of soil, soil resistivity is a very important component

• Various tables have been created to show levels of corrosivity, from severe to mild, based on soil resistivity

• Soil corrosivity data are used by corrosion control personnel to determine the extent of corrosion control needed for various underground structures



Measurement of Soil Resistivity

• Three common ways to measure soil resistivity: – Wenner four pin method – Single probe method – Soil box method



Wenner four pin method • Four pins, are inserted, in a straight line, about three inches into the earth • The pins are spaced an equal distance apart. • The average soil resistivity to a depth equal to the pin spacing • Pins are wired to an instrument specially designed for use in measuring

soil resistivity • The average resistivity to a depth equal to the pin spacing is calculated

from the following equation: ρ = 191.5 x R x S – Where ρ (Greek letter rho) = soil resistivity in Ω cm 191.5 = constant to convert

metric units to English units, R = resistance between the two center pins (Ω), S pin spacing in feet

– (see next slide)



Wenner 4-Pin Method



Single Probe Method • One instrument that goes with the probe utilizes a

Wheatstone Bridge circuit and has a dial that is calibrated in Ω cm, so it gives you a direct reading of the soil resistivity

• Other instruments utilize different circuitry, but the dial reading still indicates the soil resistivity directly

• The single probe method gives you the resistivity of the soil around the tip of the probe – it is necessary to take a sufficient number of tests at different locations

to insure you have representative data of the area you are testing



Soil Box Method • Miniature version of the Wenner four pin methods

– Resistivity of both soil and liquids can be measured in the soil box • Soil must be well tamped into the box

– The two center pins can be unscrewed and removed to facilitate filling the box. The pins are replaced prior to testing.

• In addition to the four pin soil box, there is also a two pin box, having a terminal at each end. When using the two pin box the C1 and P1 instrument terminals are connected to one of the box terminals and the C2 and P2 terminals to the other box terminal.

• The dimensions of both types of boxes are such that the resistance reading R (galvanometer reading times multiplier) that you get from the instrument equals the resistivity in Ω cm of the sample in the box. The soil resistivity ρ is thus given by Equation 2: ρ = 1.0 x R



ECONOMICS OF CORROSION CONTROL

• Over All Cost of Corrosion • Value of the structure • Value of the corrosion control program • Value of the hardware • Operation and Maintenance • Leakage Progression • Safety



Overall Cost Of Corrosion • In 2007 it was estimated to be about $500 billion in the United States

– includes the cost of failures and replacements as well as the cost of corrosion control – represents about 5% of the US Gross National Product; about $2,000 per person per

year in the US – estimated that about 35% of this figure could be saved if adequate corrosion control was

in use • Management personnel have learned the value, or return on investment, of corrosion control

– The utility sector of the analysis, costs for the gas distribution industry was only $5 Billion.

• Huge sums of money expended because of corrosion • Cost/benefit ratios for corrosion control is the ratio of how much is spent on corrosion

control to the savings generated (Return on Investment) • Experience has shown that the return on corrosion control investment ranges from about 4

to one up to 14 to one. – Ten to one is the generally accepted average – We save about ten dollars in repairs, pipe replacements, unaccounted for gas, and the

costs that mount rapidly in case of a fire or explosion for every dollar spent on corrosion control

February 21-23, 2017 Mike Placzek – Senior Engineer, NACE CP

Specialist ARK Engineering 12


Value of the Underground Piping

• Major investment in your underground piping – must be protected – every repair, replacement, or similar unnecessary

maintenance costs money that could be better spent on new investment or other areas

– These “lost funds” cut into the company’s profit and the dividends it can pay your investors

– These “lost funds” are saved through the investment in corrosion control.



Value of the Corrosion Control Program

• Just as your company has invested large amounts in its piping system; it has also made a big investment in your corrosion control program

• This includes the hardware you have buried and installed to protect the piping from corrosion, and the funds necessary to operate and maintain the corrosion control hardware



Value of the Hardware • Coated pipe costs more than bare pipe • Cost of the coating, along with the required cathodic

protection, in the long term will cost less that maintaining a bare pipe system

• Not only must bare pipeline replacement be anticipated and planned for, but the Federal gas regulations require that bare pipe be evaluated with electrical surveys every three years and protection installed at areas where active corrosion is found.

• The cost of proper protection is soon eclipsed by the costs of these triennial surveys and protection installations



Operation and Maintenance of Cathodic Protection and other Corrosion Control

• Without proper maintenance, cathodic protection, internal corrosion inhibitors, exterior paint, and other corrosion control systems will eventually fail

• There is a cost involved with all maintenance • Maintenance is mandated by the Federal pipeline

regulations • Prudent company management recognizes the

need to maintain the expensive facilities that have been put in place to prevent corrosion

• Maintenance budgets must therefore be established and met



Leakage Progression

• Years of leak history data have shown that once started, leakage on a given portion of pipeline or piping system will increase at an ever increasing, or exponential rate



Safety • Safety is of paramount importance to the pipeline

industry • Safety is the reason that the Federal regulations came

into existence in the first place • It costs money to operate a safe pipeline system, and

the investment in safety procedures is one that management recognizes

• A single fire or explosion can cost millions in lawsuits • Corrosion control is an important part of any safety

program, and investment in corrosion control is an investment in the safe operation of your piping system



Summary

• Safety • Inspection Procedures • Records • Current Flow Measurements

– Shunts – IR Drop Test Station

• Stray Current Control • AC Mitigation



Questions??

• ??



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