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