Corrosion Impact of Cathodic Protection on Surrounding Structures Robert A. Durham, PE D 2 Tech...
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Transcript of Corrosion Impact of Cathodic Protection on Surrounding Structures Robert A. Durham, PE D 2 Tech...
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