SGK Schweizerische Gesellschaft für Korrosionsschutz Technoparkstrasse 1, CH-8005 Zürich Tel. +41 44 213 15 90 Fax +41 44 213 15 91 www.sgk.ch

Managing AC and DC Interference with Smart Cathodic Protection

Dr. Markus Büchler

Cathodic protection of steel

• Durability of pipelines is assured by:

– Coating of the pipeline

– Cathodic protection of the coating defects

• A properly adjusted cathodic protection results in corrosion rates below 10 µm/year

Electrochemical conditions of steel

Fe Fe2++ 2e-

Corrosion Passivity Immunity

Fe2++ 2e- Fe 3Fe+4H2O Fe3O4+8H++8e-

Cathodic O2 reduction O2 + 2H2O + 4e- 4OH-

Mechanism of cathodic protection

• The current causes the potential to decrease

• The electrochemical reactions cause the pH-value to increase

• The formation of a passive film results in corrosion protection

Corrosion potential of steel in aerated soil with 50 Ωm

Eon -0.80 V CSE of a coating defect with 10 cm2

Eon -0.82 V CSE

Eon -0.83 V CSE

Eon -0.84 V CSE

Eon -0.88 V CSE

Eon -0.90 V CSE

Eon -0.92 V CSE

Eon -0.94 V CSE

Eon -0.98 V CSE

Eon -1.02 V CSE

Eon -1.08 V CSE

Eon -1.14 V CSE

Eon -1.20 V CSE

Eon -1.30 V CSE

Eon -1.40 V CSE

Eon -1.60 V CSE

Eon -2.00 V CSE

Konsequenzen für die Wechselstromkorrosion

• a.c. corrosion is caused by repeated formation and dissolution of a passive film

• Oxidation of one atomic layer per cycle results in a corrosion rate of 70 mm/a

• Corrosion can be stopped if the dissolution of the passive film is prevented Steel Passive film Rust

Curr

ent

ca

tho

dic

anodic

Protection against a.c. corrosion

• No formation of a passive film at high protection current density

• Strong increase of alkalinity

• Strong hydrogen evolution

• Interference

• No dissolution of the passive film at small current density

• Insufficient polarisation

ca

tho

dic

a

no

dic

C

urr

en

t

ca

tho

dic

a

no

dic

C

urr

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Steel Passive film Rust

Threshold values

• Corrosion is stopped at high and low CP current density if an a.c. interference is present

• The threshold values of EN 15280 are based on current densities

• This requires the installation of coupons

• Current densites are linked to potential values by Ohms law

Relation between current and potential

• Calculation of the potential based on kinetic and thermodynamic data

• Determination of the dependence of Jdc on the pH-value

• Calculation of the spread resistance depending on ρ and pH

• The meeting of the protection criterion can be checked based on Eon, Uac, and ρ

PE coated pipeline in heterogeneous terrain

• Separation of the sections and protection with different strategies

Time dependence of a.c. interference

• Determination of the a.c. voltage over a representative period

• An active control allows for automatic adjustment

time

Uac [

V]

The active cathodic protection

• Meaurement of the soil resistivity, a.c. interference and d.c. interference

• Determination of the critical pipeline sections

• Each section with an individual interference source is equipped with remote monitoring

• Measurement all 5 minutes and transfer all hours

• Adjustment of the optimal Eon every hour

The control process

The control algorithm

• The average values of Eon and Uac may not exceed the threshold

• This ensures maximum CP with minimum a.c. risk

Example 1

Example 2

Problems

• Loss of communication or crash of the server can bring the pipeline in a critical status

• Defective reference electrodes will cause critical potentials

• The setup allows for automatic control of the reference electrodes and checking the plausibility of the data

Requirements for the protection

• Insufficient CP causes galvanic corrosion and a.c. corrosion

• Too negative on potentials increase the risk of a.c corrosion

• The effectiveness of CP may never be compromised since most recent results and field experience indicates that a.c. corrosion stops at a certain depth

Conclusions

• An active control of CP can be implemented

• The average of Eon and Uac are suitable as control parameters

• On interfered pipelines the corrosion risk is minimized at an increased level of CP

• The time required for determining the CP effectiveness is decreased

• How should the effectiveness of CP be assessed?

Acknowledgement

This work was possible thanks to the support of DVGW, ENBW Regional AG, Open Grid Europe GmbH, MERO Pipeline GmbH, MVV Energie AG, Westnetz GmbH, ONTRAS - VNG Gastransport GmbH, and GASCADE Gastransport GmbH