Corrosion Impact ofCathodic Protection onSurrounding Structures         Robert A. Durham, PE         D2 Tech Solutions    ...
Introduction   Corrosion not new topic – since history   Loss of material leaving a metal   Flow through a medium   Re...
History   Sir Humphry Davy, 1824   British ships copper clad corrosion   Proposed attaching zinc   Considered impresse...
Takes Many Forms   Oxidation, rust, chemical, bacteria   All are result of electrical current   Treatments: chemical, c...
Mandatory                   Cathodic Protection   Underground metal pipe    with hazardous gas or liquids   Underground ...
Fundamentals                         Components   Anode sacrifices metal, pos battery   Cathode receives metal, neg batt...
Fundamentals                               CircuitFor corrosion to exist:  1. Metal conductor  2. An electrolyte  3. A pot...
Cause & Mitigation   Same elements that cause corrosion    can be used to control   Al electronegativity = 1.61   Fe el...
Cause & Mitigation   If force Al to more negative (cathodic)   Fe molecules through electrolyte to Al   Al is protected...
Problem   CP is common practice on vessels, wells    and cross-country pipelines   CP is designed to protect pipe or ves...
Case 1   Pipeline systems     2 with rectifiers     1 without, not petro   Rectifier at major lake crossing   Nearby ...
Case 1   Problems @ residences     Corrosion of underground lines     Ground wires corroded     Electric shock from wa...
Case 1   Routine rectifier readings   Complete path     Not intended     Through residence metal   Investigation, bre...
Case 1   For corrosion to occur    need electrical circuit   Without direct path thru anode, will find    alternate path...
Case 1   Corrosion of water & sewer       Costly & inconvenient   More serious     Electrical ground electrode conduct...
Case 2   Pipeline systems      3 with rectifiers      1 without, not petro   Rectifier on hill, ¾ mile from residence...
Case 2   Pipeline systems had –1.45 V pipe to soil   8 month period of problems     All copper tubing in concrete floor...
Case 2   Electrical safety       Shock by water from shower       Shock when touch metal of pre-engineered        build...
Case 2   Problems     Rectifier grounding electrode, 178 Ohm     >5 times NEC allowance     Ground rod driven only 5’ ...
Case 2   Problems pump station     1 pump 277 V 1-phase      w/ no ground whatsoever     Other sites ground electrode r...
Case 2   CP failure source of corrosion       Plumbing and electrical   Pump station was source of shock   Inadequate ...
Case 3   Well casing     6500 feet, 5.5” steel     Penetrate variety of soils     High pressure gas     Known corrosi...
Case 3   Routine     Rectifier current read normal     Pipe/soil readings not routine   3 years, corrosion of pipe   ...
Case 3   Investigation     Tank bottoms like new     Pipeline pristine     Casing eaten up   Hammer union insulating ...
Electrical Bonding   NEC requires grounding electrode   NEC requires bonding metal to ground   Problems       Steel, d...
Electrical Bonding   Bonding to ground will short CP to earth     Do not bond to CP system     Precludes using large me...
Standards   Cases emphasize importance of proper    C/P maintenance   Beyond monthly current reading   Preserve integri...
Standards                                       DOT 12/29/03Protected           a) Tests for corrosion once per yearPipeli...
Standards   Record keeping       Show location of CP piping, CP facilities, anodes       Neighboring structures bonded ...
Standards   49 CFR Part 192   49 CFR Part 195   40 CFR Part 280   UL 1746   NACE RP0169   NACE RP0177   NACE RP0193...
Installation & Maintenance   Initial     Imperative to isolate protected pipe     Visual and testing     Check resista...
Installation & Maintenance   Periodic current     Show drastic changes     Failed rectifier, broken connection   Trend...
Installation & Maintenance   Annual     11 or 13 month cycle     Over time will see all seasons and      climatological...
Conclusions   Corrosion Happens   CP sacrifices one metal to protect other   Requires complete path   Failure may caus...
Corrosion
Upcoming SlideShare
Loading in …5
×

Corrosion

1,508 views

Published on

Published in: Technology, Business
0 Comments
1 Like
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
1,508
On SlideShare
0
From Embeds
0
Number of Embeds
4
Actions
Shares
0
Downloads
150
Comments
0
Likes
1
Embeds 0
No embeds

No notes for slide

Corrosion

  1. 1. Corrosion Impact ofCathodic Protection onSurrounding Structures Robert A. Durham, PE D2 Tech Solutions Marcus O. Durham, PhD, PE THEWAY Corp.
  2. 2. Introduction Corrosion not new topic – since history Loss of material leaving a metal Flow through a medium Returns to metal at different point ANODE CATHODE
  3. 3. History Sir Humphry Davy, 1824 British ships copper clad corrosion Proposed attaching zinc Considered impressed current Batteries not perfected
  4. 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. 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. 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 CATHODE CHEMICAL ANODE -CATHODE
  7. 7. Fundamentals CircuitFor 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
  8. 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. 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. 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. 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. 12. Case 1 Problems @ residences  Corrosion of underground lines  Ground wires corroded  Electric shock from water exiting faucets Indications of compromised ground system
  13. 13. Case 1 Routine rectifier readings Complete path  Not intended  Through residence metal Investigation, break in rectifier lead
  14. 14. Case 1 For corrosion to occur need electrical circuit Without direct path thru anode, will find alternate path thru adjacent metal RECTIFIER STRUCTURE + - BREAK ANODE SOIL ALTERNATE METAL PATH CORROSION POINT
  15. 15. Case 1 Corrosion of water & sewer  Costly & inconvenient More serious  Electrical ground electrode conductor gone  Propane lines damaged Routine maintenance may not catch slow trends
  16. 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. 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. 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. 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. 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. 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. 22. 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
  23. 23. Case 3 Routine  Rectifier current read normal  Pipe/soil readings not routine 3 years, corrosion of pipe $350,000 replacement
  24. 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. 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. 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. 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. 28. Standards DOT 12/29/03Protected a) Tests for corrosion once per yearPipelines b) By Dec. 29, 2003, accomplish objectives of NACE RP0169-96 c) Inspect removed pipe; if corrosion, inspect adjacent and correctUnprotected Pipe Electric corrosion survey every three yearsRectifier Electrically check once every 2 monthsReverse Current Electrically check once per yearSwitchDiodes Electrically check once per yearCritical Electrically check once every 2 monthsInterference BondsInterference Bonds Electrically check once per yearBreakout Tanks Inspect system per API RP 651
  29. 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. 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
  31. 31. 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
  32. 32. Installation & Maintenance Periodic current  Show drastic changes  Failed rectifier, broken connection Trend over time  Decrease I  Increase V  Shows failing anode or connection 8 7 6 5 4 Volts Amps 3 2 1 0 1 2 3 4 5 6 7 8 9
  33. 33. 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
  34. 34. 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

×