1. Corrosion Impact of Cathodic Protection on Surrounding Structures
Robert A. Durham, PE
D2 Tech Solutions
Marcus O. Durham, PhD, PE
THEWAY Corp.

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

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

4. 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
ZINC
ANODE
ELECTROLYTE

5. 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

6. Fundamentals Components
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

7. Fundamentals Circuit For corrosion to exist: Metal conductor
An electrolyte
A potential difference
#1 & #2 when pipe in soil or water
#3 caused by environment
or differences in electrochemical properties

8. 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

9. 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

10. 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

11. 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

12. Case 1 Problems @ residences Indications of compromised ground system
Corrosion of underground lines
Ground wires corroded
Electric shock from water exiting faucets
Indications of compromised ground system

13. Case 1 Routine rectifier readings Complete path
Not intended
Through residence metal
Investigation, break in rectifier lead

14. Case 1 For corrosion to occur need electrical circuit
Without direct path thru anode, will find alternate path thru adjacent metal
STRUCTURE - +
ANODE
SOIL
ALTERNATE METAL PATH
CORROSION
POINT
BREAK
RECTIFIER

15. Case 1 Corrosion of water & sewer More serious
Costly & inconvenient
More serious
Electrical ground electrode conductor gone
Propane lines damaged
Routine maintenance may not catch slow trends

16. 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

17. 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

18. Case 2 Electrical safety Ground conductors 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

19. Case 2 Problems Utility 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

20. 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

21. Case 2 CP failure source of corrosion Pump station was source of shock
Plumbing and electrical
Pump station was source of shock
Inadequate grounding
Need proper systems maintenance
Other systems can complicate matters

22. Case 3 Well casing CP system 6500 feet, 5.5” steel
Penetrate variety of soils
High pressure gas
Known corrosion problems
CP system
Rectifier, 5 anodes 8 Amps impressed

23. Case 3 Routine 3 years, corrosion of pipe $350,000 replacement
Rectifier current read normal
Pipe/soil readings not routine
3 years, corrosion of pipe
$350,000 replacement

24. Case 3 Investigation Hammer union insulating flange shorted
Tank bottoms like new
Pipeline pristine
Casing eaten up
Hammer union insulating flange shorted
Current took preferential path thru line & tank

25. 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

26. 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 of current in a fault
Use resistance bond for close metal

27. 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

28. Standards DOT 12/29/03 Protected Pipelines
a) Tests for corrosion once per year
b) By Dec. 29, 2003, accomplish objectives of NACE RP
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
Critical Interference Bonds
Interference Bonds
Breakout Tanks
Inspect system per API RP 651

29. Standards Record keeping Tests
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

30. 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

32. 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

33. Installation & Maintenance
Periodic current
Show drastic changes
Failed rectifier, broken connection
Trend over time
Decrease I
Increase V
Shows failing anode or connection

34. 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

35. 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