NACE CP Interference January 2008

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CP Interference

January 2008

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IMPORTANT NOTICE

Neither the NACE International, its officers, directors, nor members thereof accept any responsibility for the use of the methods and materials discussed herein. No authorization is implied concerning the use of patented or copyrighted material. The information is advisory only and the use of the materials and methods is solely at the risk of the user.

It is the responsibility of the each person to be aware of current local, state and federal regulations. This course is not intended to provide comprehensive coverage of regulations.

Printed in the United States. All rights reserved. Reproduction of contents in whole or part or transfer into electronic or photographic storage without permission of copyright owner is expressly forbidden.

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Acknowledgements

The scope, desired learning outcomes and performance criteria of this course were developed by the CP Task Group under the auspices of the NACE Education Administrative Committee.

The time and expertise of several members of NACE International have gone into the development of this course—and its task analysis, course outline, student manual, classroom lab manual, presentation slides, and examinations. Their dedication and efforts are greatly appreciated.

On behalf of NACE, we would like to thank the task group for its work. Their efforts were extraordinary and their goal was in the best interest of public service—to develop and provide a much needed training program that would help improve corrosion control efforts industry-wide. We also wish to thank their employers for being generously supportive of the substantial work and personal time that the members dedicated to this program.

CP Interference Course Development Task Group

Paul Nichols, Task Group Chairman Shell Global Solutions, Houston, Texas Brian Holtsbaum CC Technologies Canada, Ltd., Calgary,

Alberta

Kevin Parker CC Technologies, Mt. Pleasant, Michigan David A. Schramm EN Engineering, Woodridge, Illinois Steven R. Zurbuchen EN Engineering, Topeka, Kansas

Steven Nelson Columbia Gas Transmission, Charleston, West Virginia

Donald R. Mayfield Dominion Transmission, Delmont, Pennsylvania

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CP Interference Course Manual

CP Interference

Daily Course Outline

DAY ONE

Introduction, Welcome, Overview

Chapter 1 Stray Current Interference

DAY TWO Chapter 2 DC Interference (Includes Experiment 2-1) DAY THREE MORNING Chapter 2 DC Interference AFTERNOON Chapter 3 AC Interference

(Includes experiments 3-1, 3-2, and 3-3)

DAY FOUR Chapter 3 AC Interference DAY FIVE MORNING Chapter 3 AC Interference AFTERNOON

Chapter 4 Telluric Current Interference

DAY SIX

MORNING

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© NACE International, 2006 July 2007

Introduction

The Cathodic Protection (CP) Interference course is a six-day course focusing on alternating current (AC) and direct current (DC) interference. The course includes in-depth coverage of both the theoretical concepts and the practical application of identifying interference and interference mitigation techniques. Students will learn to identify the causes and effects of interference as well as conduct tests to determine if an interference condition exists and perform calculations required to predict AC interference. The course is presented in a format of lecture, discussion and hands-on, in-class experiments, case studies and group exercises. There is a written examination at the conclusion of the course.

Who Should Attend

This course is designed for persons who have extensive CP field experience, a strong background in mathematics, and a strong technical background in CP.

Prerequisites

• CP 3–Cathodic Protection Technologist certification recommended

• Minimum of 3 years CP work experience

Length

The course begins at 1 p.m. on Sunday and concludes Friday afternoon.

Daily class hours: 8 a.m. to 6:30 p.m. Monday through Thursday and 8 a.m. to 3 p.m. Friday.

Reference Book

Students will receive the CP Interference Course Manual prior to the start of the course. A course manual on CD-ROM will be provided to students on-site.

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© NACE International, 2006 July 2007

Quizzes and Examinations

There will be four (4) quizzes distributed during the week and reviewed in class by the instructors.

This course has a written final examination. The final examinations will be given on Friday.

The written final examination is open-book and students may bring reference materials and notes into the examination room.

Non-communicating, battery-operated, silent, non-printing calculators, including calculators with alphanumeric keypads, are permitted for use during the examination. Calculating and computing devices having a QWERTY keypad arrangement similar to a typewriter or keyboard are not permitted. Such devices include but are not limited to palmtop, laptop, handheld, and desktop computers, calculators, databanks, data collectors, and organizers. Also excluded for use during the examination are communication devices such as pagers and cell phones along with cameras and recorders.

A score of 70% or greater on the examination is required for successful completion of the course. All questions are from the concepts discussed in this training manual.

You will receive written notification of your exam results as quickly as possible. Your results will not be available on Friday.

Introductions

We would like for each of you to stand, one at a time and introduce yourself to the class. Tell us:

• Your name

• Your company’s name and location

• Your job function

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CP Interference Course Manual © NACE International, 2006 June 2007

Course Manual

General Course Information

Daily Course Outline

Introduction

Chapter 1–Stray Current Interference

1.1 Historical Background ... 1:1 1.2 Typical Stray Current Circuit Arising from a Transit

System Operation ... 1:5 1.3 Stray Current Charge Transfer Reactions on a... 1:6 Metallic Structure

1.4 Effects of Stray Current on Metallic Structures ... 1:9

1.4.1 At the Current Discharge Location... 1:9 1.4.2 At Area of Current Pick-Up ... 1:15 1.4.3 Along the Structure ... 1:19

1.5 Summary ... 1:21 Summary of Equations... 1:22

Figures

Fig. 1-1 Early Electric Trolley... 1:1

Fig. 1-2 Pipe-to-soil Potential Changes due to Transit System

Stray Current Activity were Recorded on Smoked Charts.. 1:3

Fig. 1-3 Co-efficient of Corrosion at Different Frequencies for

Iron Electrode Denoted as Average Electrode Loss... 1:4

Fig. 1-4 Typical Stray Current Paths Around a DC Transit System .... 1:6

Fig. 1-5 Typical Stray Current Interference on a Metallic

Underground Structure ... 1:6

Fig. 1-6 Simplified pH Pourbaix Diagram For Iron in Water at 25ºC

Showing Potential Shift Direction for Current Pick-up

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Fig. 1-8 Current Discharge from a Metal Structure to Earth via

an Oxidation Reaction ... 1:10

Fig. 1-9 Superposition of a Stray Current and a Cathodic Protection

Current at a Metal/Electrolyte Interface ... 1:10 Fig. 1-10 Randle’s Electrical Circuit Model of a Metal/Electrolyte

Interface... 1:14 Fig. 1-11 Theoretical Conditions of Corrosion, Immunity and

Passivation of (a) Aluminum at 25ºC and

(b) Lead at 25ºC ... 1:16 Fig. 1-12 Comparison of Zn and Al Coatings for Corrosion

Resistance as Functions of pH ... 1:17

Fig. 1-13 Typical Section Through a Joint in Two Types of PCCP ... 1:18

Fig. 1-14 Cathodic Blistering/Disbondment of Protective Coating ... 1:19 Fig. 1-15a Stray Current Discharge and Pick-Up Around an

Electrically Discontinuous Joint Though the Earth... 1:19 Fig. 1-15b Stray Current Discharge and Pick-Up Through the

Internal Aqueous Medium Around an Electrically

Discontinuous Bell and Spigot Joint on Cast Iron Piping.... 1:20 Fig. 1-16 Stray Current Circuit in an AC Electrical Distribution

System... 1:20

Tables

Table 1-1 Theoretical Consumption Rates of Various Metals and

Substances... 1:12 Table 1-2 Electrochemical and Current Density Equivalence with

Corrosion Rate... 1:13

Chapter 2–DC Interference

2.1 Introduction ... 2:1 2.2 Detecting Stray Current ... 2:23

2.2.1 Mitigation of Interference Effects from Impressed Current Cathodic Protection Systems ... 2:24

a. Source Removal or Output Reduction ... 2:25

b. Installation of Isolating Fittings... 2:26 c. Burying a Metallic Shield Next to the Interfered-with

Structure ... 2:27 d. Installation of Galvanic Anodes on Interfered-with

Structure at Point of Stray Current Discharge... 2:28 e. Installation of an Impressed Current Distribution System

on the Interfered-with Structure at Point of Stray Current Discharge... 2:33 f. i. Installing a Bond Between the Interfered-with and

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2.2.2 Other Sources of DC Stray Current ... 2:41 a. DC Transit Systems ... 2:42 i. Analysis of Transit System Stray Currents ... 2:44 ii. Mitigation of Transit System Stray Currents ... 2:51 b. High Voltage Direct Current (HVDC) Electrical Transmission

Systems ... 2:55 c. DC Welding Operations ... 2:57

Experiment 2-1: To Demonstrate DC Interference

and Its Mitigation... 2:59

Case Study ………. ….. 2.64

Summary of Equations ... 2:65

Figures

Fig. 2-1 Parallel Current Paths in the Earth... 2:1 Fig. 2-2 Parallel Current Paths in a Pipeline Cathodic Protection

Section... 2:2 Fig. 2-3 Parallel Current Paths in Vertically Stratified Soil Conditions 2:3 Fig. 2-4 Parallel Current Paths in Horizontally Stratified Soil

Conditions... 2:3 Polarization Test Results... 4:5 Fig. 2-5 Stray Current in a Metallic Structure Parallel to a

Cathodically Protected Structure ... 2:5

Fig. 2-6 Voltage vs. Distance from a Vertically Oriented Anode... 2:6

Fig. 2-7a Multiple Vertical Anodes Connected to a Common

Header Cable ... 2:7 Fig. 2-7b Multiple Horizontal Anodes Connected to a Common

Header Cable ... 2:8 Fig. 2-8 Hemispherical Electrode... 2:9 Fig. 2-9 Cathodic Protection Circuit Model with Foreign Structure

Intercepting the Anode Gradient... 2:11 Fig. 2-10 Potential Profile along the Interfered-with Structure ... 2:14 Fig. 2-11 Electrical Model for Interfered-with Pipe ... 2:14 Fig. 2-12 Attenuation Model... 2:15 Fig. 2-13 Voltage Gradient in the Earth Around a Cathodically

Protected Bare Pipeline... 2:18 Fig. 2-14 Cathodic Protection Circuit Model ... 2:18 Fig. 2-15 Cathodic Protection Circuit Model with Foreign Structure

Intercepting the Anode Gradient... 2:19 Fig. 2-16 Stray Current in a Foreign Metallic Structure that Intercepts

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the Cathodic Protection Gradient... 2:21 Fig. 2-19 Cathodic Protection Circuit Model for Foreign Structure

Intercepting the Cathodic Voltage Gradient... 2:22 Fig. 2:20 Typical Potential Profile on an Interfered-with Structure

that Intersects both Anodic and Cathodic Voltage

Gradient with the Current Source Interrupted... 2:23

Fig. 2-21 Current Changes In and Near an Interfered-with Structure... 2:24

Fig. 2-22 Stray Current Arising from Installation of Isolating Fittings .... 2:26 Fig. 2-23 Using a Buried Metallic Cable or Pipe as a Shield to

Reduce Stray Current Interference... 2:27 Fig. 2-24 Cathodic Protection Current Model for a Buried Metallic

Shield Connected to the Negative Terminal of the

Transformer-Rectifier... 2:28 Fig. 2-25 Interference Mitigation using Galvanic Anodes at Stray

Current Discharge Location ... 2:29 Fig. 2-26 Electrical Circuit Model for Mitigating Stray Current

Interference at a Stray Current Discharge Site Using

Galvanic Anodes... 2:30 Fig. 2-27 Potential Profile Changes on a Pipeline where Stray

Current is Discharging in an End-Wise Pattern ... 2:33 Fig. 2-28 Interference Mitigation Using a Resistance Bond... 2:34 Fig. 2-29 Measurements Required to Determine Size of Resistance

Bond Re... 2:36 Fig. 2-30 Use of a Dielectric Coating to Mitigate Interference ... 2:41

Fig. 2-31 Typical Stray Current Paths Around a DC Transit System .... 2:42

Fig. 2-32 Typical Structure-to-Soil Potential Recording with Time

Caused by Interference from a DC Transit System ... 2:43 Fig. 2-33 Current Clamp Used to Measure Pipeline Currents ... 2:44 Fig. 2-34 Line Current Survey to Locate Source of Interference

Using IR-Drop Test Stations ... 2:45 Fig. 2-35 Line Current Plots for Example in Figure 2-34 ... 2:46

Fig. 2-36 Exposure Survey to Locate Point of Maximum Exposure... 2:47

Fig. 2-37 Exposure Survey Plots for Example in Figure 2-36... 2:48 Fig. 2-38 Mutual Survey to Confirm Source of Interference... 2:48 Fig. 2-39 Pipe-to-Soil Potential Versus Pipe-to-Rail Potential for

Example in Figure 2-38... 2:49 Fig. 2-40 Exposure Survey Conducted Without the Measurement

Of Pipeline Currents ... 2:50 Fig. 2-41 Exposure Survey Plots for Example in Figure 2-40... 2:50 Fig. 2-42a Typical Embedded Track Installation... 2:52 Fig. 2-42b Typical Direct-Fixation Isolating Fastener ... 2:52 Fig. 2-43 Typical Utilities Drainage System at a Transit Substation ... 2:52 Fig. 2-44 Schematic Showing Circulating Current between Transit

Substations Through Direct Bonds to Utilities ... 2:53 Fig. 2-45 Forced Drainage Bonds Using a Potential Controlled

Rectifier... 2:54 Fig. 2-46 Electrical Schematic for a HVDC System... 2:55

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Experiment Schematic No. 1... 2:59 Experiment Schematic No. 2... 2:60 Experiment Schematic No. 3... 2:61

Tables

Table 2-1 Specific Leakage Resistances and Conductances in

1000 Ω-cm Soil or Water ... 2:13 Table 2-2 Types of Reverse Current Switches ... 2:54

Chapter 3–AC Interference

3.1 Introduction ... 3:1

3.1.1 Electrostatic (Capacitive) Coupling... 3:2 3.1.2 Electromagnetic (Inductive) Coupling ... 3:11 3.1.3 Conductive Coupling (Resistive Coupling) During Powerline Fault

Conditions... 3:14

Experiment 3-1: To Demonstrate the Effects of Electrostatic

Induction... 3:16

3.2 Basic Theory of Electromagnetically Induced Voltages ... 3:19

3.2.1 AC Circuit Theory ... 3:19 3.2.2 The Nature of Induced AC Pipeline Voltages ... 3:34

Experiment 3-2: To Demonstrate the Effects of Electromagnetic

3.3 Induced AC Voltages ... 3:44

3.3.1 Factors that Affect the Longitudinal Electric Field... 3:44 3.3.2 Factors that Affect the Pipeline Voltages... 3:48

Experiment 3-3: To Further Investigate the Effects of Electromagnetic

3.4 Deleterious Effects of AC Interference... 3:60

3.4.1 Electric Shock Hazards... 3:60 3.4.2 AC Corrosion ... 3:67

.1 Theory... 3:67 .2 AC Corrosion Case Histories... 3:75 .3 AC Corrosion Field Test Procedures ... 3:90 3.4.3 Fault Current Effects... 3:93

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3.5.1 Data Gathering ... 3:95 3.5.2 Field Estimation of LEF... 3:97 3.5.3 Measurement and Interpretation of Soil Resistivity Data.... 3:98

3.6 Prediction of Steady-State Induced AC Voltages... 3:102

3.6.1 Introduction ... 3:102 3.6.2 Calculation of Pipeline Electrical Characteristics... 3:102 3.6.3 Sectionalization of Pipeline-Powerline Route ... 3:106 3.6.4 Determination of Longitudinal Electric Field (LEF) ... 3:107 3.6.5 Calculation of Induced Pipeline Voltages ... 3:110

3.7 Prediction of Fault Voltages ... 3:115

3.7.1 Introduction ... 3:115 3.7.2 Conductive Coupling Due to Fault Currents ... 3:115 3.7.3 Inductive Coupling Due to Fault Currents... 3:122 3.7.4 Other Related Calculations... 3:123 (a) Ground Electrode Resistance... 3:123 (b) Step and Touch Potential ... 3:125

(c) Conductor Size ... 3:126

3.8 Equipment for AC Mitigation ... 3:126

3.8.1 DC Decoupling Devices... 3:126 3.8.2 Test Stations... 3:138 3.8.3 Sacrificial Anodes ... 3:139

Group Activity – AC Mitigation System Design ... 3:142

Summary of Equations ... 3:145

Figures

Fig. 3-1a Single Horizontal 3φ Circuit with Shield Wires... 3:2 Fig. 3-1b Distribution System (1φ 4kV Primary and 2φ 240V Secondary

with Neutral)... 3:2 Fig. 3-2 AC Voltage Waveforms in a 3φ Circuit ... 3:2 Fig. 3-3 Elements of a Capacitor ... 3:3 Fig. 3-4 Electrostatic Coupling during Pipeline Construction ... 3:4 Fig. 3-5 Voltage Divider Circuits – Resistive (left) and Capacitive

(right) ... 3:5

Fig. 3-6 Calculation of Typical Capacitance Values for a Pipe

on Skids... 3:6 Fig. 3-7 Calculation of Typical Electrostatically Induced Voltage

for a Pipe on Skids... 3:7

Fig. 3-8 Calculation of Typical Shock Current Resulting from

Electrostatic Coupling ... 3:8 Fig. 3-9 Calculation of Typical Electrostatically Induced Voltage

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Fig. 3-11 Electromagnetic Field Created by Current Flow in a Wire... 3:11

Fig. 3-12 Electromagnetic Induction in a Multiple-Turn, Iron-Core

Transformer ... 3:12 Fig. 3-13 Electromagnetic Induction in a Single-Turn, Air-Core

Transformer ... 3:13 Fig. 3-14 Electromagnetic Coupling Between a Pipeline and an

Overhead AC Powerline ... 3:13 Fig. 3-15 Conductive Coupling During Line-to-Ground Fault

Conditions... 3:14

Fig. 3-16 Determination of Voltage on a Transformer Secondary ... 3:20

Fig. 3-17 Effect of Interconnecting the Secondary Windings ... 3:20 Fig. 3-18 Effect on Polarity on a Series Combination of DC Voltage

Sources ... 3:21 Fig. 3-19 Effect of “Polarity” on a Series Combination of AC Voltage

Sources ... 3:21 Fig. 3-20 In-Phase 60 Hz AC Waveform ... 3:22 Fig. 3-21 Typical Electrical Distribution Transformer ... 3:23 Fig. 3-22 Typical Residential Electrical Service... 3:23 Fig. 3-23 AC Waveforms on a Residential Electrical Service ... 3:24

Fig. 3-24 Plot of General Equation for Sinusoidal AC Waveforms... 3:25

Fig. 3-25 Typical Phasor Diagram ... 3:26 Fig. 3-26 Series Combination of AC Voltage Sources... 3:27 Fig. 3-27 Phasor Diagram for Problem in Figure 3-26... 3:27 Fig. 3-28 Determination of Current through a Capacitor... 3:30 Fig. 3-29 Voltage and Current Waveforms for a Purely Capacitive

Circuit... 3:31 Fig. 3-30 Determination of Current through an Inductor ... 3:32 Fig. 3-31 Voltage and Current Waveforms for a Purely Inductive

Circuit... 3:33 Fig. 3-32 Phasor Representation of a Three-Phase Circuit... 3:33 Fig. 3-33 Electric Model of Single Pipe Section... 3:35 Fig. 3-34 Simplified Electrical Model of Single Pipe Section... 3:36 Fig. 3-35 Simplified Electrical Model of Single Pipe Section... 3:36 Fig. 3-36 Series Combination of Multiple Pipe Sections... 3:36 Fig. 3-37 Series Combination of Two Pipe Sections ... 3:37 Fig. 3-38 Series Combination of Two Pipe Sections (Simplified)... 3:37 Fig. 3-39 Circuit Analysis Using Kirchhoff’s Law... 3:37 Fig. 3-40 Circuit Analysis Using Kirchhoff’s Law... 3:38 Fig. 3-41 Induced AC Voltage Profile Along Two-Section Pipe

Method of Figure 3-39 ... 3:39 Fig. 3-42 Profile of Induced AC Voltages and their Phase Angles

along any Pipeline having Uniform Electrical

Characteristics ... 3:39 Fig. 3-43 Effect of Electrical Length of Pipeline on AC Voltage Profile. 3:40 Fig. 3-44 Double Vertical Circuit ... 3:44 Fig. 3-45 Quadruple Vertical Circuit... 3:44 Fig. 3-46 Single Delta Circuit ... 3:45

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Variation of d/s Ratios (and for the specific case

Where ρ/s2_{= 1Ω/m, s/h=0.3, and I=1000A) ... 3:47}

Fig. 3-50 Simple Pipeline-Powerline Corridor (Plan View)... 3:49 Fig. 3-51 AC Voltage Profile Along an Electrically Short Pipeline

(Uniform Conditions – No Grounding) ... 3:49 Fig. 3-52 Electrical Service Analogy for Pipeline-Powerline Corridor

In Figure 3-50 ... 3:50 Fig. 3-53 AC Voltage Profile Along an Electrically Short Pipeline

(Non-Uniform Conditions – No Grounding)... 3:50 Fig. 3-54 Effect of Grounding One End of Electrical Service

Secondary... 3:51 Fig. 3-55 Effect of Grounding One End of Pipeline in Figure 3-50 ... 3:52 Fig. 3-56 Effect of Grounding Both Ends of Pipeline or Adding

Distributed Grounds... 3:52 Fig. 3-57 Effect of an Insulator at the Midpoint of the Pipeline ... 3:53

Fig. 3-58 AC Voltage Profile Along an Electrically Long or Lossy

Pipeline (Uniform Conditions – No Grounding)... 3:54 Fig. 3-59 AC Voltage Profile Along an Electrically Long or Lossy

Pipeline (Zero Resistance Ground at Distance = 0) ... 3:55 Fig. 3-60 Effect of an Insulator at the Midpoint of an Electrically

Long Pipeline... 3:56 Fig. 3-61 Fibrillating Current vs. Body Weight (Various animals – 3

second shock duration)... 3:61 Fig. 3-62 Possible Body Current Paths... 3:63 Fig. 3-63 Example of Typical Touch and Step Potentials at an

Energized Structure... 3:64 Fig. 3-64a Coefficient of Corrosion at Different Frequencies for

Iron Electrode Denoted as Average Electrode Loss... 3:68 Fig. 3-64b Maximum Penetration Depth as a Function of Test

Duration at Constant Cathode DC Current Density

(2A/m2_{) and Differing AC Current Density ...} _3:71 Fig. 3-65a Effect of CP Potential on AC Corrosion Rate ... 3:73 Fig. 3-65b Effect of CP Potential on AC Current Density... 3:74 Fig. 3-65c Pit Cluster and Pinhole Perforation (Case History No. 1) ... 3:76 Fig. 3-65d Hemispherical Shell of Hardened Soil Surrounding

Anomaly (Case History No. 3) ... 3:80 Fig. 3-65e Hemisphere of Hardened Soil and Corrosion Pit

(Case History No. 3) ... 3:81 Fig. 3-65f Pinhole Corrosion Failure Following Removal of Repair

Clamp (Case History No. 4)... 3:82 Fig. 3-65g Pipeline-Powerline Route (Case History No. 4)... 3:83 Fig. 3-65h Nodule of Corrosion Products Protruding Through

Coating (Case History No. 4)... 3:85 Fig. 3-65i Corrosion Pit After Removal of Coating and Corrosion

Products (Case History No. 4) ... 3:85 Fig. 3-65j Effects of Installing Ground Electrodes at Sites A and B

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Fig. 3-67 Field Estimation of LEF Magnitude Using Horizontal

Wire Method ... 3:98 Fig. 3-68 Soil Resistivity Measurement Using the Wenner Four-Pin

Method... 3:99 Fig. 3-69 Determination of Pipeline Coating Resistance ... 3:103 Fig. 3-70 Determination of Pipeline Internal Impedance... 3:104 Fig. 3-71 Sectionalization of Pipeline-Powerline Route ... 3:107 Fig. 3-72 Pipeline-Powerline Geometry for Calculation of LEF... 3:109 Fig. 3-73 Typical Series of Curves for Determining LEF... 3:110 Fig. 3-74 Simple Pipeline-Powerline Corridor (Plan View)... 3:111 Fig. 3-75 Simple Pipeline-Powerline Model ... 3:111 Fig. 3-76 Equivalent Circuit for Line-to-Ground Fault ... 3:115 Fig. 3-77 Distribution of Fault Current Along Powerline... 3:116 Fig. 3-78 Distribution of Fault Current Along Powerline... 3:117 Fig. 3-79 Calculation of Earth Voltage at Pipe due to Faulted Tower ... 3:118 Fig. 3-80 Approximate Length of Pipeline Affected by Faulted Tower.. 3:119 Fig. 3-81 Resistance of Coating Holiday to Earth... 3:120 Fig. 3-82 Modified Resistance of Coating Holiday to Earth due

to Localized Soil Ionization Effects ... 3:121 Fig. 3-83 AC Pipeline Voltages Induced by Overhead Faulted

Powerline (Per 1000 A of Fault Current)... 3:123 Fig. 3-84 Motor Operated Valve – Effects of Grounding on Induced

AC and CP Currents ... 3:127 Fig. 3-85 Electrical Isolation of Motor Operated Valve from Pipeline.... 3:128 Fig. 3-86 Electrical Grounding Schematic of Motor Operated Valve

Showing Two Alternative Locations for a DC Decoupling

Device... 3:129 Fig. 3-87 Decoupling Device Installed by Electrical Utility Between

Primary and Secondary Grounds ... 3:130 Fig. 3-88 Isolation-Surge Protector Installed across Isolating Flange... 3:131 Fig. 3-89 Electrical Schematic of One Model of Solid-State DC

Decoupling Device... 3:131 Fig. 3-90 DC Decoupling Device Installed Across Insulating Flange

for Lightning Protection... 3:132 Fig. 3-91 AC Current Being Measured Through a Polarization Cell ... 3:133 Fig. 3-92 Polarization Cell Construction ... 3:133 Fig. 3-93 Corrosion of Plates Within a Polarization Cell ... 3:134 Fig. 3-94 Grounding Cell... 3:135 Fig. 3-95 Electrolytic Capacitor... 3:135 Fig. 3-96 Failure of Electrolytic Capacitors in Stray Current Area ... 3:136 Fig. 3-97 Metal-Oxide Varistors (MOVs)... 3:137 Fig. 3-98 Explosion-Proof Surge Protection Device Installed

Across Insulator ... 3:138 Fig. 3-99 Test Station Varieties (left to right): a) Terminals Exposed

To Public; b) Terminals Covered by a Plastic Cap (Locking or Non-Locking); c) Dead-Front Terminals; d) Aluminum

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Bicarbonate-Rich Soil ... 3:140

Fig. 3-102 Potential of Magnesium Versus AC Current Density

in a Fe-Mg Cell ... 3:141

Tables

Table 3-1 Effects of 60 Hz AC Body Currents on Humans ... 3:60 Table 3-2 Let-Go Currents from Dalziel’s Experiments ... 3:62 Table 3-3 Let-Go Currents from Dalziel’s Experiments ... 3:63

Table 3-4 Voltage Puncture Levels for Various Holiday-Free Coatings. 3:94

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4.1 Background Theory ... 4:1

4.1.1 Distributed Source Transmission Line Equations ... 4:6 4.1.2 Factors that Affect the Induced Electric Field ... 4:8 (a) Solar Cycle Variations... 4:8 (b) Sun’s Rotational Frequency... 4:9 (c) Earth’s Rotation ... 4:9

(d) Plasma Magnetic Field Direction ... 4:9 (e) Proximity of Pipeline to a Sea Coast... 4:10 (f) Pipeline Latitude ... 4:12

4.1.3 Factors that Affect the Pipeline Lineal Impedance (Z) and

Shunt Admittance (Y)... 4:13 (a) Effect of Coating Quality ... 4:13 (b) Effect of Isolating Fittings... 4:14 (c) Effect of Pipeline Directional Change ... 4:15

4.2 Measuring the Geomagnetic Intensity and Determining

the Electric Field (E)... 4:16 4.3 Interference Effects of Telluric Current on Pipelines... 4:18

4.3.1 General Considerations ... 4:18 4.3.2 Corrosion ... 4:18

(a) Theoretical Considerations ... 4:18 (b) Calculating the Corrosion Rate... 4:22 (c) Telluric Corrosion Case Studies on Cathodically

Protected Piping... 4:27 4.3.3 Impact on Accuracy of Current and Potential Measurements

... 4:29 4.3.4 Impact of Telluric Current on Pipeline Coatings ... 4:31 4.3.5 Impact on Output of a CP Rectifier ... 4:32

4.4 Mitigating the Effects of Telluric Current ... 4:33

4.4.1 Mitigating Corrosion Impact ... 4:33 (a) Making the Pipeline Electrically Continuous and

Grounded ... 4:33 (b) Using CP... 4:34 (i) Sacrificial Anodes ... 4:35

(ii) Impressed Current Systems ... 4:39

4.4.2 Compensating for Measurement Error Caused by ...

Telluric Current ... 4:42

4.5 Summary ... 4:49

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Fig. 4-1 Interaction of Solar Particles on the Earth’s Magnetic Field .. 4:1 Fig. 4-2a Plasma Charge Distribution around the Earth during

Quiescent Period ... 4:2 Fig. 4-2b Plasma Charge Distribution around the Earth during

a Magnetic Storm... 4:2 Fig. 4-3 This Plot Shows the Current Extent and Position of the

Auroral Oval in the Northern Hemisphere, Extrapolated From Measurements Taken During the Most Recent Polar Pass of the NOAA POES Satellite for September

16, 2004 at 14:22 UT ... 4:3

Fig. 4-4 Schematic of Geomagnetic Induction Directly into a Pipeline

and the Resulting Change in Pipeline Potential that is

Produced ... 4:3

Fig. 4-5 Quiet Day Variation in the Geomagnetic Field and the

Associated Change in the Electric Field and the Pipe-to-

Soil Potential... 4:4 Fig. 4-6 P/S Potential and Telluric Current in a Long Pipeline

Exposed to an Induced Electric Field of 1 V/km,

Having an Impedance of 0.1 Ω /km and an

Admittance of 0.15 Ω /km... 4:5 Fig. 4-7 Equivalent Circuit for a Short Section of Pipeline ... 4:6

Fig. 4-8 History of Geomagnetic Effects on Ground Technology... 4:8

Fig. 4-9 Pipe-to-soil Potential Variations with Time ... 4:9 Fig. 4-10 Charge Accumulation at the Coast Resulting from Larger

Induced Currents in the Sea Compared to in the Land. The Charge Accumulation Increases the Electrical

Potential of the Earth’s Surface Near the Coast ... 4:10 Fig. 4-11 Electric Field, E, Generated by Seawater Moving with

Velocity, v, Through the Earth’s Magnetic Field, B ... 4:11 Fig. 4-12 Geomagnetic Hazard Percentage of Probability of

Occurrence ... 4:12 Fig. 4-13 Telluric Induced Voltage Profile vs Distance for a

Pipeline with Different Attenuation Constants... 4:13 Fig. 4-14 Calculated Telluric Induced Voltage at the End of a Long

Pipeline as a Function of Coating Conductance for

an East-West Electric Field of 0.1V/km ... 4:14 Fig. 4-15 Effect of Isolating Fittings on the Telluric Induced Voltage

Profile on an Electrically Short Pipeline ... 4:14 Fig. 4-16 Effect of Pipeline Directional Change on the Telluric

Induced Voltage... 4:15 Fig. 4-17 Average Occurrence of 3-Hour Intervals with the Magnetic

Activity Index Kp Equal to or Greater than a Specified

Value. Kp=9 Corresponds to a Severe Magnetic Storm.... 4:16

Fig. 4-18 Peak Electric Field Magnitudes as a Function of Kp ... 4:17 Fig. 4-19 Oxidation Reaction at Pipe Surface During Telluric

Current Discharge in the Absence of CP... 4:18 Fig. 4-20 Reduction Reactions During Negative Cycle Telluric

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Polarization... 4:20 Fig. 4-23 Experimental Anodic Polarization Curve of Steel in

Hydroxide (pH 12.0)... 4:21 Fig. 4-24 Telluric Current Discharge from a Cathodically Protected

Pipe ... 4:22 Fig. 4-25 Coefficient of Corrosion at Different Frequencies for

Iron Electrodes Denoted as Average Electrode Loss ... 4:23 Fig. 4-26 Effect on Corrosion Rate of Reversing Direction of

Current Compared to Steady State DC and Length of Time

Between Reversals... 4:24 Fig. 4-27 Corrosion Current Density at a Coating Defect having an

Applied Voltage of 1.0V in 1000 Ω-cm Soil for

Various Coating Thicknesses ... 4:25 Fig. 4-28 Chart Showing the Influence of Anodic Transient Time with

Respect to Corrosion Experienced by Probe in Sandy and Clay Soil. Line (a) Represents the Corrosion Rate

Expected from Faraday’s Law for the Clay Soil, and

Line (B) for the Sandy Soil, Respectively ... 4:26 Fig. 4-29 Corrosion Pit at 112+307 (60 mils/497mils 07:30)... 4:28 Fig. 4-30 Magnetic Field Intensity and Pipe-to-Soil Potential

Superimposed... 4:29 Fig. 4-31 Schematic of Potentially Controlled CP System Used to Mitigate

Telluric Current Effects ... 4:30 Fig. 4-32 Current Flow and Calculated OFF Potentials during a

GIC Incident... 4:31 Fig. 4-33 Telluric Current Through a Bridge Rectifying Element

During a Discharge Cycle ... 4:32 Fig. 4-34 Schematic of a Telluric Bond Switch ... 4:34 Fig. 4-35 Mitigation of Telluric Current Discharge Effects Using

Galvanic Anodes... 4:35 Fig. 4-36 Effect of Connecting and Disconnecting Groups of

Galvanic Anodes to a Pipeline Subjected to Telluric

Current... 4:36 Fig. 4-37 Maritimes DSTL Results Without Flanges ... 4:38 Fig. 4-38 Electrical Schematic at a Constant Voltage Transformer

Rectifier During a Positive Telluric Voltage Fluctuation ... 4:40 Fig. 4-39 Pipe Potential and Rectifier Current Output vs Time for

An Impressed Current System Operating in Potential

Control ... 4:41 Fig. 4-40 Typical Pipe-to-Soil Potential Measurements at Test

Station Having a Steel Coupon and Soil Tube ... 4:42 Fig. 4-41 Typical Pipe-to-Soil Potential Recording at a Test

Station Using a Coupon/Reference Probe... 4:43 Fig. 4-42 Comparison Between Pipe/Coupon Potential with Time

Recorded with Respect to a Copper-Copper Sulfate Reference on Grade and to a Coupon/Reference Probe

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Fig. 4-44 CIPS Method Using One Moving and Two Stationary

Data Loggers ... 4:46 Fig. 4-45 Pipe-to-Soil Potential Measurement Method to Compensate

for Telluric Current Effects During a Close Interval

CP Survey... 4:47 Fig. 4-46 Pipe Potential/Telluric Current Relationship at a Coupon

Test Station... 4:48 Fig. 4-47 Four Wire Test Lead Arrangement for Measuring

Pipe Current... 4:49

Appendices

Appendix A – Curve Matching Appendix B – Pipe Data Table Appendix C – Anode Tables Appendix D – Wire Size Table

Appendix E – Metric Conversion Table Appendix F – Dabkowski Paper NACE RP0177

NACE SP0169

NACE Glossary of Corrosion-related Terms Course Evaluation

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CP Interference Course Manual © NACE International, 2006 January 2008

1.1 Historical

Background

The term “interference” is understood in the pipeline industry as electrical interference and is defined as “any detectable electrical disturbance on a structure caused by a stray current where a ‘stray current’ is defined as a current in an unintended path”.1 This broad definition suggests that the structure, although often a pipeline, could be any metallic network such as electrical power grids and communication systems. Furthermore, although the interfering current is often a direct current (DC) from a cathodic protection (CP) impressed current source, the current can also originate from any electrical system that uses the earth either intentionally or inadvertently as a current path. Thus alternating current (AC) can also be included in the definition.

Electrical interference concerns preceded the use of CP for corrosion control of pipelines. Telegraph systems were reported2 to interfere with the operation of the early telephone systems. Lighting systems, first introduced in about 1880, comprised arcs and incandescent lamps also interfered with the telephone systems, primarily because both the telephone system and the lighting systems used the earth as a current path. Then, in the late 1800s and early 1900s, street railways throughout North America were electrified.3 They ultimately led to the corrosion of cast iron watermains.

Figure 1-1: Early Electric Trolley

(courtesy of East Bay Municipal Utility District, Oakland, CA)4

1_{CP3 – Cathodic Protection Technologist Course, NACE International, June 1, 2004, p.3-1.}

2_{Anderson, John M., The Fight Over the Highways, IEEE Power Engineering Review, December 1997,} p.45.

3_{Anderson, John M., First Electric Street Car, IEEE Power Engineering Review, Oct. 1999, p.32.}

4

Lewis, Mark, Once Vagrant Current, Now Impressed Current Cathodic Protection, MP, Vol 36, July1997.

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Corrosion on watermains as a result of interference from a DC transit system was first reported by Stone & Forbes in 18945, just 6 years after a New England transit system began operation. In 1901, damage to water and gas mains in Toronto, Ontario, was reported6 as being due “to railway currents.” The currents reportedly affected the watermains for two reasons: deterioration of the rail joint bonds and the practice of bonding the watermains to the rails at certain locations. The U.S. Bureau of Standards began studying the stray current traction problem in 1910. The bureau would issue 15 reports by 1921. Many of the investigations involved field studies, during which temporary electrolysis committees were formed consisting of interested utility representatives. The corrosion resulting from stray current was initially referred to as “electrolysis,” a term defined as “the decomposition of a substance by the application of a current”.7 The widespread corrosion of iron watermains by stray transit system currents led to the formation in 1913 of the American Committee on Electrolysis.8

Stray current activity on underground structures arising from transit system operation is not steady-state but dynamic in terms of current and potential amplitude. It often reverses direction. Typical structure potential activity was recorded on smoked charts. These charts collect data as a stylus moving in response to a changing potential input removes the smoke from the chart, which is rotated by a clock drive. The dynamic nature of the stray current effect on pipe potential is shown in Figure 1-2.

5_{Stone, C.A. and Forbes, H.C., Electrolysis of Water Pipes, New England Water Works Association, Vol.} 9, pp.1894-5.

6_{Knudson, A.A., Report on the Joint Investigation and Survey for Electrolysis on the Water and Gas} Mains in the City of Toronto, Ontario, July 1, 1906.

7_{The Oxford Encyclopedic English Dictionary, Oxford University Press, 1991.}

8_{Meany, J.J., A History of Stray Traction Current Corrosion in the United States, NACE, Corrosion’74,} Paper 152, p.3.

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Figure 1-2: Pipe-to-Soil Potential Changes due to Transit System Stray Current Activity were Recorded on Smoked Charts

Because of the variable nature of the stray current activity, it is difficult to predict how much corrosion would occur. The Bureau of Standards conducted a study9 in which iron samples where subjected to AC discharge and current pick-up for different periods of time. The resulting corrosion was compared to corrosion produced by a steady-state DC of the same current density and discharge period. The results of this study, reported in 1916, are summarized in Figure 1-3.

9_{McCollum, B. and Ahlborn, G.H., Influence of Frequency of Alternating and Infrequently Reversed} Current on Electrolytic Corrosion, Technologic Papers of the Bureau of Standards, U.S. Dept. of Commerce, No. 72, 1916.

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1/60S 1/15S 1S 5S 1M 5M 10M 1Hr. 2Days 2Weeks D.C.

Logarithm of Length of Time of One Cycle

Soil + Na2CO3 Soil LEGEND: -10 20 30 50 60 70 80 90 100 0 10 40

Figure 1-3: Coefficient of Corrosion at Different Frequencies for Iron Electrode Denoted as Average Electrode Loss

For short periods of reversals, the corrosion was only a small fraction of the corrosion at steady state. For equal periods of pick-up and discharge, the corrosion coefficient remained below 20% when the cycle remained below one hour. This meant that the corrosion occurring from dynamic stray currents was a function of the frequency. At 60hz the corrosion rate was less than approximately 2% of the steady state value.

R.J. Kuhn, who investigated the effects of transit system stray current activity on iron water mains in New Orleans, Louisiana, is credited with the discovery of CP. It occurred to him in 1928 that “ordinary corrosion could be prevented by reversing these currents”.10 Sir Humphrey Davy11 was the first person on record to use CP by applying zinc castings to protect the copper sheathing on British warships in 1824. Although a technical success, Davy’s application was a

10_{Kuhn, R.J., Cathodic Protection of Underground Pipe lines from Soil Corrosion, API Proceedings, Nov.} 1933, Vol. 14, p.164.

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practical failure because the copper biofouled when the corrosion was stopped— thus reducing the speed of these sailing ships. It appears that neither Kuhn nor any other corrosion practitioner had knowledge of this. Hence, Kuhn is considered by one source12 as the “father” of CP (certainly as it applies to pipelines).

Against this backdrop, stray current interference and its corrosion consequences for underground metallic structures were first evaluated. Today electrolysis committees exist throughout North America, and methods of mitigation that have subsequently been developed are commonly utilized. Sources of stray current interference are not confined to DC transit systems. They now include any electrical source that uses the earth either intentionally or inadvertently as a current path. This course addresses these sources and the mitigation methods that have been developed to mitigate not only the corrosion effects, but other deleterious consequences of stray current activity.

1.2 Typical Stray Current Circuit Arising from a

Transit System Operation

Figure 1-4 depicts stray current paths originating from the operation of an electric transit system. Although it is the intent that the DC operating current returns to the substation via the running rails (IR), some of the load current (IL) will pass

through the earth (Ie) if the rail is in electrolytic contact with the earth. If there is a

metallic structure in the earth, it, too, will carry some of the load current (IS).

Therefore, the load current (IL)—after passing through the locomotive—divides

into parallel paths. The amount of current in each path is inversely proportional to the resistance of each path relative to the total circuit resistance, as Equation 1-1 indicates. R I R I path L T path • = [1-1]

where: Ipath = current in a path

RT = total resistance of parallel paths

Rpath = resistance of current path

IL = load current

12_{von Baeckmann, W., Schwenk, W., and Prinz, W., Handbook of Cathodic Corrosion Protection, 3rd} edition, Gulf Publishing Co., Houston, TX, 1997, p.16.

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Figure 1-4: Typical Stray Current Paths Around a DC Transit System

Hence, as the resistance of the rail path increases or the resistance of the alternative stray current path(s) decreases, a greater percentage of the load current will appear in the stray current path(s).

1.3 Stray Current Charge Transfer Reactions on a

Metallic Structure

Figure 1-5 illustrates the typical stray current situation on an underground metallic structure that is not electrically connected to the source of stray current. The stray current pattern consists of a pick-up of stray current from the earth at one or more locations and the subsequent discharge of stray current to the earth at one or more locations.

I_s I_s

Is stray current_pick-up Is

stray current discharge

Figure 1-5: Typical Stray Current Interference on a Metallic Underground Structure

DC su b-station ground pick-up I_s Is IL I_e IR I_e I_s discharge O/Hpower conductor metallicstructure (e.g.,watermain) running rails

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The principal charge carriers in the earth are ions. They are electrons in the metallic structure. For these reasons, electrochemical reactions must transfer the charge between the structure and earth at both the pick-up and discharge locations.

At the pick-up location(s), it is through reduction reactions that the electrical charges are transferred. Depending on the nature of the electrolytic environment, the reduction reactions can be one or more of the following:

H3O+ + e– Æ HO + H2O [a]

O2 + 2H2O + 4 e– Æ 4OH– [b]

2H2O + 2e– Æ H2↑ + 2OH– [c]

Reaction [b] is favored in well-aerated soils and waters; reduction reaction [a] is favored in acidic soils or waters. Reduction reaction [c], which involves the breakdown of water molecules to hydrogen gas and hydroxyl ions, can occur under all conditions if there is sufficient over-voltage applied.

At the discharge location, one or more of the following oxidation reactions transfers the electrical charge.

M0 Æ Mn+ + ne– [d]

4OH– Æ O2 + 2H2O + 4e– [e]

2H2O Æ O2 + 4H+ + 4e– [f]

Reaction [d] tends to occur on most basic metals such as iron, copper, zinc, and aluminum when the electrolyte has an acid or neutral pH. Reaction [e] is more likely in electrolytes with a high pH. Reaction [f] is more likely to occur when the over-voltage reaches the oxygen line. The oxygen line is line “b” on the Pourbaix diagram for iron (Figure 1-6).

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(assuming passivation by a film of Fe O )2 3

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 immunity corrosion 1 ₂ 3 corrosion pH -1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 current discharge current pick-up -2 0 2 4 6 8 10 12 14 16

Figure 1-6: Simplified pH Pourbaix Diagram for Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at Low pH

The Pourbaix diagram for iron in pure water represents three zones of thermodynamic stability: corrosion, immunity, and passivity based on a potential (SHE) vs pH relationship. Line (a) is the hydrogen line and line (b) is the oxygen line. Water is stable between these two lines. If the potential of iron is shifted to either of these lines, then oxygen is generated at line (b) and hydrogen gas at line (a).

For an iron structure without CP that is exposed to a neutral or low-pH water, a current pick-up will cause the potential to shift in the negative direction toward the immunity zone and afford the structure some CP. Conversely, at the discharge location, the potential is shifted in the electropositive direction into the passive region if not at a low pH—where it would otherwise remain in the corrosion zone.

On a cathodically protected structure as illustrated in Figure 1-7, where the electrolyte at the iron surface normally has a high pH, a current discharge resulting in a positive shift can produce a passive film given by the following reaction:

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(assuming passivation by a film of Fe O )2 3

-1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 immunity corrosion 1 ₂ 3 corrosion pH -1.6 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 1.6 2 current discharge current pick-up -2 0 2 4 6 8 10 12 14 16

Figure 1-7: Simplified pH Pourbaix Diagram for Iron in Water at 25ºC Showing Potential Shift Direction for Current Pick-up and Discharge at High pH

The ferrous hydroxide formed is relatively stable at high pH. Because this reaction also produces hydrogen ions, the pH will decrease with time.

1.4 Effects of Stray Current on Metallic Structures

It is apparent that the effect of a stray current pick-up and a stray current discharge from an iron structure from a thermodynamic perspective can cause corrosion, passivation, or immunity, depending upon the direction of current and the pH of the aqueous electrolyte at the charge transfer location.

1.4.1 At the Current Discharge Location

Identification of the current discharge site receives considerable attention in stray current investigations because it is the location where corrosion damage is most likely to occur on all metallic structures. When a current transfers from a metallic structure to earth (Figure 1-8), it must do so via an oxidation reaction that converts electronic current to ionic current.

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CP Interference Course Manual © NACE International, 2006 January 2008 O X I D A T I O N earth Is Is Is metal structure (ions) (electrons)

Figure 1-8: Current Discharge from a Metal Structure to Earth via an Oxidation Reaction

The generic oxidation reaction is the corrosion of the metal as in Equation 1-2.

Mo Æ Mn+ + ne– [1-2]

For steel, the oxidation reaction is:

Feo Æ Fe++ + 2e– [1-3]

A stray current discharge from a metallic structure may not cause corrosion attack if the structure is receiving CP (Figure 1-9). Whether the superposition of a stray current discharge and a CP current pick-up at a metal/electrolyte interface causes corrosion will depend on time and the relative magnitudes of these two currents.

O X I D A T I O N earth Is Is Is metal structure R E D U C T I O N Icp Icp Icp Icp

Figure 1-9: Superposition of a Stray Current and a Cathodic Protection Current at a Metal/Electrolyte Interface

CP current transfers across the metal/earth interface via a reduction reaction, which produces hydroxyl ions in either of the three following reactions:

H3O+ + e– Æ HO + H2O [1-4]

O2 + 2H2O + 4e– Æ 4OH– [1-5]

2H2O + 2e– Æ H2Ç + 2OH– [1-6]

In the presence of a high concentration of hydroxyl ions, a possible oxidation reaction is given in Equation 1-7. The reaction involves the oxidation of hydroxyl ions to oxygen and water.

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This latter reaction does not consume metal atoms; therefore, there is no corrosion damage. Hence, as long as the polarized potential at the structure electrolyte interface is not driven more electropositive than the CP criterion (e.g., –850mVcse for iron or steel), significant corrosion would not be expected.

If the metal has a surface passive film or is a relatively inert material (such as some of the materials used for impressed current anodes), then not all of the stray current need transfer through a corrosion reaction. If the stray current polarizes the metal surface electropositively to the oxygen line on the Pourbaix diagram, then the hydrolysis[13] of water molecules by the following reaction 1-8 is likely.

2H2O Æ 4H+ + O2Ç + 4e– [1-8]

This oxidation reaction does not result in the consumption of the metal surface, but it does produce an acidic pH from the generation of hydrogen ions.

On an iron or steel structure without CP, the oxidation reaction is usually the dissolution of the metal according to Equation 1-9

Feo Æ Fe++ + 2e– [1-9]

The severity of corrosion depends on the magnitude of the stray current and time as related by Faraday’s Law:

corr t I F M W t n = [1-10] where:

Wt = total weight loss at anode or weight of material produced

at the cathode (g)

n = number of charges transferred in the oxidation or reduction reaction

Icorr = the corrosion current (A)

F = Faraday’s constant of approximately 96,500 coulombs per equivalent weight of material (where equivalent weight =

n M

)

M = the atomic weight of the metal that is corroding or the substance being produced at the cathode (g)

t = the total time in which the corrosion cell has operated (s)

13_{Hydrolysis is defined as a double decomposition reaction involving the splitting of water into its ions} and the formation of a weak acid or base or both. CRC Handbook of Chemistry and Physics, CRC Press, 53rd Edition, 1972-1973, PF-83.

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Given the atomic weight of pure iron as 55.85 g and assuming 100% efficiency and pure DC, the consumption rate of iron as illustrated in Table 1-1 is 9.13 kg/A-y.

Table 1-1: Theoretical Consumption Rates of Various Metals and Substances

Reduced Species Oxidized Species Molecular Weight, M (g) Electrons Transferred (n) Equivalent Weight, M/n (g) Theoretical Consumption Rate (Kg/A-y) Al Al+++_26.98 ₃ _8.99 _2.94 Cd Cd++_112.4 ₂ _56.2 _18.4 Be Be++ _9.01 ₂ _4.51 _1.47 Ca Ca++ _40.08 ₂ _20.04 _6.55 Cr Cr+++ _52.00 ₃ _17.3 _5.65 Cu Cu++_{63.54 2 31.77} _10.38 H2 H+ 2.00 2 1.00 0.33 Fe Fe++ _55.85 ₂ _27.93 _9.13 Pb Pb++_207.19 _{2 103.6} _33.9 Mg Mg++ _24.31 ₂ _12.16 _3.97 Ni Ni++ _58.71 ₂ _29.36 _9.59 OH- _O 2 32.00 4 8.00 2.61 Zn Zn++_{65.37 2 32.69} _10.7

On pipelines, the total weight loss is usually less important than the penetration rate. By re-arranging Faraday’s Law, the weight loss per unit time per unit area is shown to be directly proportional to current density (i = I/A) as in Equation 1-11.

i n t F M A W t t ₌ [1-11]

Dividing this equation by the density (d) of the metal or alloy produces the corrosion rate (rcorr), which can be expressed in mm/y (Equation 1-12).

d F M k r s corr n i = [1-12] where: M = atomic weight (g)

n = number of charges transferred in corrosion reaction

i = current density (μA/cm2)

k = unit correction term ≈ 3.156 x 108 mm s/cm yr

d = density (g/cm3)

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Example: Using Equation 1-12 to calculate the penetration rate based on a current

density of 1 A/m2 (10-4 A/cm2):

where: M = 55.85 g i = 10-4 A/cm2 n = 2 d = 7.87 g/cm3 F = 96,500 coulombs then: g/cm 7.87 coulombs 96,500 2 A/cm 10 55.85g yr cm s/ mm 10 3.156 r_corr 8 -4₃ 2 × × × × × = = 1.16 mm/y

Table 1-2 gives the penetration rate, in mpy and 10-3 mm/y, equivalent to a current density of 1μA/cm2_{for a number of common pure metals.}

Table 1-2: Electrochemical and Current Density Equivalence with Corrosion Rate for Some Common Pure Metals

Penetration Rate Equivalent to 1 μA/cm2[1] Metal/Alloy Element/ Oxidation State Density (g/cm3) Equivalent Weight (g) (mpy) 10-3 mm/y[2] Pure Metals Iron Fe/2 7.87 27.93 0.46 11.6 Nickel Ni/2 8.90 29.36 0.43 10.8 Copper Cu/2 8.96 31.77 0.46 11.6 Aluminum Al/3 2.70 8.99 0.43 10.9 Lead Pb/2 11.4 103.6 1.17 29.7 Zinc Zn/2 7.13 32.69 0.59 15.0 Tin Sn/2 7.3 59.34 1.05 26.6 Titanium Ti/2 4.51 23.95 0.69 17.4 Zirconium Zr/4 6.5 22.80 0.45 11.5

Note: [1] A current density of 1 μA/cm2_{is approximately = 1 mA/ft}2

[2] 10-3 _{mm/y = 1 μm/y and 1 mpy = 25.4 μm/y}

The foregoing corrosion rates apply to stray current situations involving a continuous DC discharge. Corrosion rates decrease for periodic reversals of DC and are substantially less for 60Hz AC (Figure 1-3).

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The low corrosion rate for a 60Hz current is attributed to the relatively low impedance of the interfacial capacitance. The structure/electrolyte interface can be modeled electrically by a Randle’s Circuit shown in Figure 1-10.

C_dl R_p

R_e

steel soil (electrolyte)

Eac

where:

double layer capacitance (1-200 F/cm2)μ

C_{dl =}

R_{e =} resistance of steel surface to remote earth

R_{p =} polarization resistance (1-104_Ω-cm )2

E_{oc =} potential difference (volts) I_{ac =} total AC crossing the interface I_{a,rp =}

I_{a,dl =}

total AC through polarization resistance

total AC through double-layer capacitance

I_ac I_ac,r_p

I_ac,dl

Figure 1-10: Randle’s Electrical Circuit Model of a Metal/Electrolyte Interface

This circuit model illustrates that the interface is not simply a resistance but a parallel combination of the polarization resistance (Rp) and a capacitor (Cdl)

called the layer capacitance. Unlike DC, AC can pass through the double-layer capacitance. There is no mass transfer in this current path and hence no corrosion polarization results from current transfer in this path. The proportion of AC (Iac,dl) through the double-layer capacitor is a function of the relative

impedance of this path compared to the polarization resistance.

The reactance (Xcdl) of the double-layer path is given by the following equation:

2 1 Xc_dl dl C f π = [1-13] where: f = frequency (Hz) Cdl = capacitance (farads) Xcdl = reactance (ohms)

Assuming a 1cm2 surface area and mid-range values of both the polarization resistance and the double-layer capacitance as follows, then the proportion of AC through the capacitor can be calculated.

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CP Interference Course Manual © NACE International, 2006 January 2008 Assume: Cdl = 100 μf/cm2 Rp = 103 Ω−cm2 Using Equation 1-13: π π 120 10 10 100 60 2 1 Xc 4 6 -dl = _× _× = Ω = = 26.54 8 . 376 10 4

The total impedance Zt to 60Hz AC of the parallel combination of the

polarization resistance (Rp) and the double-layer capacitance is therefore:

54 . 26 1 10 1 Xc 1 R 1 Z 1 3 p + = + = dl t 3 3 3 -10 38.7 10 37.7 10 Z 1 ₌ ₊ _× − ₌ _× − t therefore: = = 25.8Ω 38.7 10 Z 3 t

Then the proportion of AC current through the double-layer capacitance is:

Xc I Z I dl t ac, t dl ac, = 97.4% or I 0.974 I 26.5 25.8

Iac,dl × ac,t = ac,t

Ω =

Accordingly, only approximately 2.6% of the AC would pass through the polarization resistance and only the positive half-cycle of the current would be involved in the corrosion reaction.

1.4.2 At Area of Current Pick-Up

At the area of current pick-up, a negative shift will result in cathodic polarization. If the foreign structure is mild steel, then there is a beneficial effect because the structure is receiving some measure of CP. If the structure is coated and has its

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own CP system, the additional polarization from the stray current pick-up may result in cathodic blistering of the coating.

If the foreign structure is not mild steel but is made of an amphoteric metal such as aluminum, lead, or zinc, then the high pH developed at the structure/earth interface caused by the reduction reaction can effect “cathodic” corrosion. Amphoteric metals such as aluminum are susceptible to corrosion at both high and low pH. Figure 1-11 shows this phenomenon for aluminum.

(a) Aluminum (b) Lead

Figure 1-11: Theoretical Conditions of Corrosion, Immunity, and Passivation of (a) Aluminum at 25ºC and (b) Lead at 25º C

Source: Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, TX, 1974, p.172

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CP Interference Course Manual © NACE International, 2006 January 2008 pH Acid Alkaline 10 20 30 40 50 60 70 Zn Al 1 4 3 2 5 6 7 9 10 11 12 13 14 8

Figure 1-12: Comparison of Zn and Al Coatings for Corrosion Resistance as Functions of pH

One can see that aluminum is particularly sensitive to high pH attack. Aluminum is often used underground for water irrigation systems, gas distribution piping in rural areas, AC secondary distribution conductors, and the sheathing on communication cables. Zinc and lead are also amphoteric metals. The corrosion rate of zinc, as indicated in Figure 1-12, is not as high as aluminum in alkaline conditions but is much greater in acid conditions. Lead sheathing was commonly used on belowground AC power cables. Not only are these amphoteric materials susceptible to corrosion according to Faraday’s Law at rates indicated in Table 1-2 at stray current discharge locations, but also at stray current pick-up locations.

Prestressed concrete cylinder pipe (PCCP) used for both water and sewage transmission is composed of a mild steel inner cylinder, over which a highly stressed steel wire is wound to give the concrete/steel cylinder strength. Typical cross-sections of the two types of PCCP are shown in Figures 1-13a and 1-13b.

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Cement Mortar Placed in Field or Other Protection Concrete Core

Steel Spigot Ring

Rubber Gasket Steel Bell Ring

Grout Joint After Installation

Cement - Mortar Coating

Prestressed Wire Steel Cylinder Prestressing Wire and Wire Fabric

Around Bell or Thicker Bell Ring and Wire Fabric

a. Lined Cylinder Pipe

Cement Mortar Placed in Field or Other Protection Concrete Core

Steel Spigot Ring

Rubber Gasket

Steel Bell Ring Grout Joint

After Installation

Cement - Mortar Coating Prestressed Wire

Steel Cylinder

b. Embedded Cylinder Pipe

Figure 1-13: Typical Section Through a Joint in Two Types of PCCP

Source: Prestressed Concrete Pressure Pipe-Steel Cylinder Type for Water and Other Liquids, AWWA Standard C301, American Water Works Association, Denver, CO

The prestressing wire in these pipes is normally cold drawn steel with a yield strength in the order of 200 ksi. The cold-worked hardened surface of the wire makes it susceptible to hydrogen embrittlement. It is recommended that the polarized potential be limited to –970 mVcse or less negative to minimize the

production of atomic hydrogen. If a stray current causes excessive cathodic polarization, then a catastrophic failure could occur.

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If the foreign structure is coated at the stray current pick-up site, then coating blistering or disbondment can occur. Coating blistering is caused by the pressure build-up beneath the coating due to the movement of water through the coating, due to electroendosmosis. Electroendosmosis is defined as “the inward flow of a fluid through a permeable membrane due to an electric field”. The high pH produced by the reduction reaction at the metal surface can attack the coating adhesion bonds or a surface oxide layer, resulting in coating disbondment.

Figure 1-14: Cathodic Blistering/Disbondment of Protective Coating

1.4.3 Along the Structure

Stray current in a metallic structure does not usually cause damaging effects between the stray current pick-up and discharge locations unless the current is very large or the structure is not electrically continuous. If the structure is electrically discontinuous (as is often the case with cast iron water distribution piping or PCCP transmission piping), the structure resistance (Rs) is greater than

if it were electrically continuous, which reduces the magnitude of Is, but creates a

current discharge/current pick-up pattern at each electrical discontinuity (Figures 1-15a and 1-15b).

Figure 1-15a: Stray Current Discharge and Pick-Up Around an Electrically Discontinuous Joint Through the Earth

In many of these structures not every joint is discontinuous, but localized corrosion will occur on the discharge side of the discontinuous joints. Furthermore, on water and sewer piping, there is not only a soil path for the stray

metal substrate OH_OH _ OH_ OH_ OH_ OH_ OH_ OH_ OH_ H O₂ H O₂ soil DC Is

electrically discontinuous joints

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current but also an internal path through the aqueous medium as illustrated in Figure 1-15b.

rubber seal

aqueous medium

Figure 1-15b: Stray Current Discharge and Pick-Up Through the Internal Aqueous Medium Around an Electrically Discontinuous Bell and Spigot Joint on Cast Iron Piping

Current in an AC distribution system can also affect the transformation characteristics in distribution transformers. At the AC distribution transformer, which supplies the AC service for an impressed current transformer-rectifier, a ground cable is normally run from the AC neutral to a ground rod at the base of the service pole. The ground rod, being relatively close to the groundbed, will pick up stray current. The distribution neutral and the AC phase conductor will carry the stray current to ground at remote transformers because DC does not encounter a high resistance through the primary winding. This circuit is illustrated schematically in Figure 1-16.

CP groundbed Remote Distribution Transformer I_s,₂ L₁ L2 N N I_s I_s,₂ I_s,1 L N T/R CP AC Distribution Transformer I_s

Figure 1-16: Stray Current Circuit in an AC Electrical Distribution System

A DC in the primary or secondary windings of a transformer will produce a magnetic flux in the transformer core that will tend to saturate the core and thus spoil its voltage transformation properties. This is a deleterious effect that is in

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addition to the corrosion damage that results from the stray current discharging off the ground rod at the remote distribution transformer.

1.5 Summary

Stray current is an irrevocable factor to which all metallic underground structures are exposed because so many electrical systems use the earth as a current path. The following list of possible stray current sources is extensive:

• CP systems

• High-voltage AC transmission systems • Low-voltage AC distribution systems • High-voltage DC transmission systems • AC and DC transit systems

• Welding operations

• Geomagnetically induced currents • Low-frequency communication systems • Land-line telephone systems.

Pipeline corrosion control practitioners are often acutely aware of the various sources of stray current, yet impressed current CP systems remain among the most prevalent stray current sources. As public pressure mounts to force more stray current sources into joint-use corridors, stray current control becomes increasingly important and decidedly more complex.

NACE CP Interference January 2008

CP Interference

January 2008

IMPORTANT NOTICE

Acknowledgements

CP Interference

Daily Course Outline

Introduction

Who Should Attend

Prerequisites

Length

Reference Book

Quizzes and Examinations

Introductions

Course Manual

Table of Contents

General Course Information

Daily Course Outline

Introduction

Chapter 1–Stray Current Interference

Chapter 2–DC Interference

Chapter 3–AC Interference

Appendices

1.1 Historical

Background

1.2 Typical Stray Current Circuit Arising from a

Transit System Operation

1.3

Stray Current Charge Transfer Reactions on a

Metallic Structure

1.4

Effects of Stray Current on Metallic Structures

1.5 Summary