Corrosion Impact of Cathodic Protection on

Surrounding Structures

Robert A. Durham, PE

D2 Tech Solutions

Marcus O. Durham, PhD, PE

THEWAY Corp.

Introduction

Corrosion not new topic – since history Loss of material leaving a metal Flow through a medium Returns to metal at different point

CATHODEANODE

History

Sir Humphry Davy, 1824 British ships copper clad corrosion Proposed attaching zinc Considered impressed current Batteries not perfected

Takes Many Forms

Oxidation, rust, chemical, bacteria All are result of electrical current Treatments: chemical, coatings, electrical Proper impressed current can stop May not be practical

CATHODE ZINCANODE

ELECTROLYTE

Mandatory Cathodic Protection

Underground metal pipe with hazardous gas or liquids

Underground metal pipe within 10’ of steel reinforced concrete

Water storage tanks >250,000 gallons

FundamentalsComponents

Anode sacrifices metal, pos battery Cathode receives metal, neg battery Electrolyte, non-metallic medium,

with some moisture to support current flow

+ANODE

ANODE

-CATHODE

CATHODE

CHEMICAL

FundamentalsCircuit

For corrosion to exist:

1. Metal conductor

2. An electrolyte

3. A potential difference

#1 & #2 when pipe in soil or water #3 caused by environment

or differences in electrochemical properties

Cause & Mitigation

Same elements that cause corrosion

can be used to control Al electronegativity = 1.61 Fe electronegativity = 1.83

Result = electrochemical attraction Molecules from Al, thru electrolyte, to Fe Protect Fe

Cause & Mitigation

If force Al to more negative (cathodic) Fe molecules through electrolyte to Al Al is protected Can create problems if CP system fails Current flow takes unexpected path Protects and destroys wrong metal

Problem

CP is common practice on vessels, wells and cross-country pipelines

CP is designed to protect pipe or vessel Current can take unintended path Can create negative results on other

metals Three cases examined

Case 1

Pipeline systems 2 with rectifiers 1 without, not petro

Rectifier at major lake crossing Nearby soil some limestone rocks High soil resistivity Near residences

Case 1

Problems @ residences Corrosion of underground lines Ground wires corroded Electric shock from water exiting faucets

Indications of compromised ground system

Case 1

Routine rectifier readings Complete path

Not intended Through residence metal

Investigation, break in rectifier lead

For corrosion to occurneed electrical circuit

Without direct path thru anode, will find alternate path thru adjacent metal

Case 1

STRUCTURE - +

ANODE SOIL

ALTERNATE METAL PATH

CORROSION POINT

BREAK

RECTIFIER

Corrosion of water & sewer Costly & inconvenient

More serious Electrical ground electrode conductor

gone Propane lines damaged

Routine maintenance may notcatch slow trends

Case 1

Case 2

Pipeline systems 3 with rectifiers 1 without, not petro

Rectifier on hill, ¾ mile from residence Nearby soil sandy w/ substantial sandstone High soil resistivity Very remote

Near 1 residence with barns Near petroleum production

Case 2

Pipeline systems had –1.45 V pipe to soil 8 month period of problems

All copper tubing in concrete floor replaced

3/4” copper supply replaced twice Computer monitor & TV failed due to

voltage Multiple motors burned out Fluorescent lights not ignite

Case 2

Electrical safety Shock by water from shower Shock when touch metal of pre-engineered

building Hole burned in bldg from energized ground wire

Ground conductors Electrician measure 40 volts on ground wire at

service entrance Utility measured 90 volts

on ground wire at pump station

Problems Rectifier grounding electrode, 178 Ohm >5 times NEC allowance Ground rod driven only 5’

remainder sticking up Utility

Meter ground corroded in two Ground resistance, 48 Ohms

Case 2

Case 2

Problems pump station 1 pump 277 V 1-phase

w/ no ground whatsoever Other sites ground electrode resistance

of 750 – 1000 Ω Without ground stray currents

travel along metal

Case 2

CP failure source of corrosion Plumbing and electrical

Pump station was source of shock Inadequate grounding Need proper systems maintenance Other systems can complicate matters

Case 3

Well casing 6500 feet, 5.5” steel Penetrate variety of soils High pressure gas Known corrosion problems

CP system Rectifier, 5 anodes

8 Amps impressed

Case 3

RoutineRectifier current read normalPipe/soil readings not routine

3 years, corrosion of pipe $350,000 replacement

Case 3

Investigation Tank bottoms like new Pipeline pristine Casing eaten up

Hammer union insulating flange shorted Current took preferential path thru line &

tank

Electrical Bonding

NEC requires grounding electrode NEC requires bonding metal to ground Problems

Steel, ductile or cast iron sacrifice to copper

Bond Pipe, well casings, tanks etc. Not the grounding electrode w/o bonding, risk of shock

Electrical Bonding

Bonding to ground will short CP to earth Do not bond to CP system Precludes using large metal surface as

grounding electrode CP has inherent personnel protection

Drive potential ~ 1 volt negative Very low circuit resistance < 2Ω

Adequate path for dissipation ofcurrent in a fault

Use resistance bond for close metal

Standards

Cases emphasize importance of proper C/P maintenance

Beyond monthly current reading Preserve integrity of system DOT regulated periodic maintenance Become more stringent

December 29, 2003

StandardsDOT 12/29/03

Protected Pipelines

a) Tests for corrosion once per year

b) By Dec. 29, 2003, accomplish objectives of NACE RP0169-96

c) Inspect removed pipe; if corrosion, inspect adjacent and correct

Unprotected Pipe Electric corrosion survey every three years

Rectifier Electrically check once every 2 months

Reverse Current Switch

Electrically check once per year

Diodes Electrically check once per year

Critical Interference Bonds

Electrically check once every 2 months

Interference Bonds Electrically check once per year

Breakout Tanks Inspect system per API RP 651

Standards

Record keeping Show location of CP piping, CP facilities, anodes Neighboring structures bonded Maintain for life of pipeline

Tests Tests, survey, or inspection per table Demonstrate adequacy Maintain 5 years

Inspection of protected & critical interference bonds Life of pipeline

Standards

49 CFR Part 192 49 CFR Part 195 40 CFR Part 280 UL 1746 NACE RP0169 NACE RP0177 NACE RP0193 NACE RP0285 NACE RP0286 NACE RP0388 API RP 632 API RP 651 STI R892 STI R972

Installation & Maintenance

Initial Imperative to isolate protected pipe Visual and testing Check resistance between protected,

ground, other If not open circuit -> problem

Electrical w/in 5 feet Bond per NEC

Installation & Maintenance

Periodic current Show drastic changes Failed rectifier, broken connection

Trend over time Decrease I Increase V Shows failing anode or connection

0

1

2

3

4

5

6

7

8

1 2 3 4 5 6 7 8 9

Volts

Amps

Installation & Maintenance

Annual 11 or 13 month cycle Over time will see all seasons and

climatological conditions Complete periodic

Same as initial Energized, so measure voltage difference

not resistance Half-cell P-S, and ground bed to soil Rectifier

Conclusions

Corrosion Happens CP sacrifices one metal to protect other Requires complete path Failure may cause unintended path Resultant corrosion can be costly and

compromise safety New regulations in effect Dec 29, 2003

With proper installation, maintenance and inspections CP can be safe and effective