Corrosion Impact of Cathodic Protection on Surrounding Structures

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

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

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

4. CATHODE ZINC ANODE ELECTROLYTE 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

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. +ANODE CATHODE CHEMICAL ANODE -CATHODE FundamentalsComponents • Anode sacrifices metal, pos battery • Cathode receives metal, neg battery • Electrolyte, non-metallic medium, with some moisture to support current flow

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

15. Case 1 • Corrosion of water & sewer • Costly & inconvenient • More serious • Electrical ground electrode conductor gone • Propane lines damaged • Routine maintenance may notcatch 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 • 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 • 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 • 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 • 6500 feet, 5.5” steel • Penetrate variety of soils • High pressure gas • Known corrosion problems • CP system • Rectifier, 5 anodes8 Amps impressed

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

24. Case 3 • Investigation • 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 ofcurrent 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 stringentDecember 29, 2003

28. StandardsDOT 12/29/03

29. 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

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