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introduction to cathodic protection
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Training Program Skill Group 15.10
Level 1
Cathodic Protection
Corrosion Fundamentals and Principles of Cathodic ProtectionCathodic Protection HardwareCathodic Protection Operation and MonitoringCoatings and Cathodic Protection
Table of Contents
Corrosion Fundamentals and Principles of Cathodic Protection
ContentsIntroductionThe Electrochemical CellOther Common Corrosion ReactionsElectrode PotentialsDetermining the Rates of CorrosionCorrosion Control MethodsPractical Applications of Cathodic Protection
Reference
Petronas Technical Standards
PTS ( Petronas Technical Standard )
API ( American Petrolium Institution )
AS ( Australian Standard )
BS ( British Standard )
DNV
NACE
Introduction
Metallic corrosion is a general term used to describe all the processes by which metals and its alloys are degraded by reactions with their environment. In Oil and Gas production operations, the reactions are primarily electrochemical.
For iron/steel in sea water :
Anodic Reaction : Fe ----> Fe2+ + 2e-
Cathodic Reaction : O2 + 4e- + 2H2O ---> 4OH-
Electrochemistry
Corrosion Cell
ELECTROLYTE
METAL SURFACEANODE
CATHODE
ELECTRONFLOW
Fe Fe
2 2e e --
Fe Fe++++2H H
2+ 1/2 O22OH-
H O2+
3Fe
SFe
CO
OFe2 3-H
2OX
Other Common Corrosion Reactions
Anodic Oxidation Reaction :Fe ----> Fe2+ + 2e-
Cathodic Reduction Reactions :In Near-neutral SolutionO2 + 4H+ + 4e- ---> 2H2O
In Acidic WatersO2 + 4H+ +4e- ----> 4OH-
In Very Acidic Waters2H+ + 2e- ----> H2
Rust Formation4Fe(OH)2 + O2 +2H2O ----> 4Fe(OH)3
Electrode Potentials
DefinitionMeasured potential of an electrode in an electrolyte relative to the potential of a reference electrode
Measurement of Standard Electrode Potentials
Electrode Potentials
Selected Standard Electrode Potentials
Corrosion Rate Determination
Faraday's LawMass of substances dissolved in an electrochemical reaction is proportional to current flowing in the electrochemical cell
W = MiT nF
whereW = mass of substances dissolvedM = atomic or molecular weight of substancesi = corrosion current, ampsT = time, sn = number of electrons per atom involved in the reactionF = the Faraday = 96500 coulombs / gm equivalent
Based on Electrochemical Technique
Corrosion Rate = Kai nD
whereK = constant (for corrosion rate in mpy, micrommeter/yr and mm/yr, the values of K are 0.129,
3.27, and 0.00327, respectively)
a = atomic weight of metali = current density, micro amps/cm2
n = number of electrons lostD = metal density, g/cm2
Corrosion Rate Determination
Based on Metal Loss
Corrosion Rate (mpy) = 534W DAT
whereW = mass loss, mgD = density of specimen, g/cm3
A = area of specimen, sq. inT = time of exposure, in hr
Corrosion Rate Determination
Corrosion Control Methods
Cathodic Protection Design Materials Selection Protective Coatings Chemical Inhibition
Cathodic Protection - Definition
" Cathodic protection is an electrochemical technique for controlling corrosion on metallic structures
immersed in an electrolyte, by applying an external DC current to lower the potential of the structure, to an
extent that the structure becomes cathodic with respect to an external anode connected to the structure "
Practical Application of Cathodic Protection
Types of Cathodic Protection Systems
Galvanic (sacrificial) anode systemImpressed current system
Galvanic (Sacrificial) Anode Cathodic Protection System
By appropriate galvanic coupling; Sacrificial Anode Cathodic Protection (SACP) system
TankMg Anode
Coated Copper Wire
CurrentSoil
Practical Application of Cathodic Protection
Impressed Current Cathodic Protection System
By an external power supply; Impressed Current Cathodic Protection (ICCP) system
TankAnode
CurrentRectifier
CurrentSoil
+-
Practical Application of Cathodic Protection
Cathodic Protection Hardware
ContentsBasic Electrical PrinciplesEquivalent CircuitsPower Sources for Impressed Current SystemsAnodesGalvanic AnodesImpressed Current AnodesTest PointsTransformer RectifierJunction Box
Basic Electrical Principles
Ohm's Law
V = IR where V = Voltage (Volts), I = Current (Amps), R = Resistance (ohms)
Electrical PowerP = VI where P = Power (watts), V = Voltage (volts), I = Current (Amps),
R = Resistance (ohms)
Resistivity p= RA/L where R = resistance (ohms), A = cross-sectional area, L = length
Kirchoff's Lawsum of voltages around a close circuit is zerosum of currents entering one point in a circuit is equal to the sum of current leaving it
Equivalent Circuits
Equivalent Electrical Circuit of Basic Corrosion Cell
BONDING CABLE
BRACELET ZINC ANODE
STEEL PIPE
PROTECTIVE COATING
PROTECTIVECOATING REMOVED
STEEL CORE
THERMIT WELD
THERMIT WELD
Pipeline Bracelet Zinc Anodes
Galvanic Anodes
Power Sources for Impressed Current Systems
Types of Power Sources :
Rectifierstakes AC current and rectifies it to Dc current
Solar cellsconvert solar radiation directly to electrical power
Thermoelectric generatorsconvert heat from a heater system to electricity
Batteriesconvert chemical energy to electricity
Anodes
Types of Anodes
galvanic anodesimpressed current anodes
Typical Environment and Operating Characteristics
Galvanic Anodes
Anode Description
Typical Environment for
Cathodic Protection Application
Typical Environment Resistivity (ohm.m)
Environment Resistivity (ohm.m)
Approximate Consumption (kg/A.year)
Zinc All water 0.2 to 15 Controlled by circuit resistance
12
Aluminum Seawater 0.2 to 4 Controlled by circuit resistance
3.5
Magnesium Potable water 5 to 75 Controlled by circuit resistance
7
Higher consumption rates may occur at temperatures above 40o C. Zinc anodes may be unsuitable for use in waters warmer than 45o C.
Platform Aluminum Anodes
Galvanic Anodes
Galvanic Anodes
Tank Zinc Anodes
Vessel Zinc Anodes
Test Points
Test Points
a nominated point on a structure for electrical contactconstruction ranges from simple electrical termination boxes to galvanized cast steel or iron boxes on standposts; or proprietary test leads on standposts
Test Points
Installation Location
at regular intervals along buried pipelinesat likely sources or discharge points for stray currentat rail or road crossings, or waterwaysat cased crossingadjacent to insulating joints or at structure terminationclose to foreign structuresat connection points for galvanic anodes, or structure cables in impressed current systems
Test Points
TestConnection
Test Post
Terminal board
Removable Link
Link box
Cable connections to pipelinePipelin
eAnode
Anode Cable
Ground level
Test Points
Transformer Rectifier
Junction Box
Cathodic Protection Operation and Monitoring
ContentsMonitoring Structure PotentialCathodic Protection Assessment CriteriaSurveys and AuditSafetyDocumentation
Cathodic Protection Operation and Monitoring
Cathodic Protection Monitoring DataTimeAmbient Conditions
Monitoring Structure Potential
Monitoring Structure Potential
Equipment for Measurement of Structure / Electrolyte Potentialreference cell (copper/copper sulfate, silver/silver chloride, zinc)voltmeter or recorder or data loggercables
Monitoring CP Current
Equipment for CP Current Measurement ammeter or magnetic clamp ammeters voltmeter or chart recorder or data logger
Monitoring Current
Clamp On Meters
Monitoring Current
Clamp On Meters
Surveys and Audits
Regular Monitoring and Test Data
Data Collected transformer rectifier (voltage and current) outputpotentials at drain points, structure extremities and critical areas (e.g. areas subject to stray current)
loop resistance to monitor groundbed or anode performance
Surveys and Audits
Potential Testing - When ?
Factors Consideredaccessibility of test pointssystem typetype and location of structurelegislationinterference
Surveys and Audits
Audits
testing on all cathodic protection test point, foreign structure bond and transformer rectifier or galvanic anode outputs
reevaluation for the structure requirement for CP
Safety
Electrical Hazardsexcessive DC voltages at impressed current power supplyexcessive voltage gradients in aqueous electrolytesinduced voltages from parallel electric power transmission lineshigh voltage transients resulting from lighting strikes
Evolution of Dangerous Gaseshydrogen evolutionchlorine evolution
Operating and Maintenance Manual
a description of system and system componentscommissioning reportas built drawingsmanufacturer documentationa schedule of all monitoring facilitiesprotection potential criteria for the systemmonitoring schedules and requirements for monitoring equipmentmonitoring procedures for each type of monitoring facilities installed guidelines for safe operation of the CP system
Documentation
Monitoring and Maintenance Program
Frequency
Sacrificial Anode Cathodic Protection System six monthly
Impressed Current Cathodic Protection System weekly 3 monthly
Coatings and Cathodic Protection
Contents
Coating Systems Used
Coating Systems Used
Commonly Used Coating Types :
coal tar and asphalt enamelwrapping tapesextruded polyethylenefusion bonded epoxyfusion bonded polyethyleneliquid polymeric systems i.e. epoxies and phenolics, coal tar epoxy, polyurethane and polyesters