Upcoming SlideShare
Loading in...5







Total Views
Views on SlideShare
Embed Views



0 Embeds 0

No embeds



Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
Post Comment
Edit your comment

Corrosion Corrosion Presentation Transcript

  • Corrosion Impact ofCathodic Protection onSurrounding 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 ANODE CATHODE
  • 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 ZINC ANODE 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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 Routine  Rectifier current read normal  Pipe/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 of current 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
  • 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
  • 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 8 7 6 5 4 Volts Amps 3 2 1 0 1 2 3 4 5 6 7 8 9
  • 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