Transcript Corrosion Impact of Cathodic Protection on

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
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
ELECTROLYTE
ZINC
ANODE
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
Circuit
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
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 12/29/03
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
Electrically check once every 2 months
Interference Bonds
Interference Bonds Electrically check once per year
Breakout Tanks
Inspect system per API RP 651
Standards

Record keeping




Tests




Show location of CP piping, CP facilities, anodes
Neighboring structures bonded
Maintain for life of pipeline
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