Cathodic Protection

cathodic protection banner 7Cathodic protection, abbreviated as CP, is a method used to control corrosion on the surface of a metal which can be above or below ground pipes, tanks, vessels, etc.  This process is accomplished by placing a sacrificial anode (material) connected to the protected metal.

 

 

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Cathodic Protection Types

  • Galvanic anode CP (GACP)  -  Also called sacrifical CP.  Uses a sacrifical anode that corrodes before the material being protected does by an electrochemical reaction (no power source needed).
    • Galvanic anode material  -  aluminum, magnesium, and zinc
    • Galvanic anode pro  -
      • Easy installation
      • No external power source
      • Require virtually no monitoring or maintenance
    • Galvanic anode con  -
      • Limited current, low driving voltage
      • Limited life
      • Limited control, no power source to adjust the power output
  • Impressed current CP (ICCP) -  Uses a sacrificial anode connected to an external DC power source.  DC flows from source to anode, to protected material, to source.
    • Impressed current anode material  -  graphite, high silicon cast iron, and mixed metal oxide
    • Impressed current anode pro  -
      • Current can be controlled, with no limit of driving voltage
      • Can be remotely controlled
      • Can replace anodes when needed
    • Impressed current anode con  -
      • Require regularly monitoring or maintenance
      • Requires power source
      • More monitoring or maintenance means more likely the breakdowns.

 

Cathodic Protection standards

ASME Standards

  • ASME B16.5 - Pipe Flanges and Flanged Fittings: NPS 1/2 through NPS 24 Metric/Inch Standard
  • ASME B16.9 - Factory-Made Wrought Buttwelding Fittings
  • ASME B16.47 - Large Diameter Steel Flanges, NPS 26 Through NPS 60
  • ASME B16.36 - Orifice Flanges
  • ASME B31.4 - Pipeline Transportation Systems for Liquid Hydrocarbons and Other Liquids
  • ASME B31.8 - Gas Transmission and Distribution Piping Systems
  • ASME G8 - Standard Test Methods for Cathodic Disbonding of Pipeline Coatings
  • ASME G14 - Testing Method for Impact Resistance of Pipeline Coatings (Falling Weight Test)
  • ASME G95 - Testing Method for Cathodic Disbonding of Pipeline Coatings (Attached Cell Method)

AWWA Standards

  • AWWA C209 - Cold-Applied Tape Coating for the Exterior of Special Sections, Connections, and Fittings for Steel Water Pipelinss
  • AWWA C214 - Tape Coating Systems for the Exterior of Steel Water Pipelines
  • AWWA C216 - Heat-Shrinkable Cross-Linked Polyolefin Coatings for the Exterior of Special Sections, Connections, and Fittings for Steel Water Pipelinss
  • AWWA C222 - Polyurethane Coatings for the Interior and Exterior of Steel Water Pipe and Fittings

NACE Standards

  • NACE RP0186 - Application of Cathodic Protection for External Structures of Steel Well Casings
  • NACE SP0102 - In-Line Inspection of Pipelines
  • NACE SP0104 - The Use of Coupons for Cathodic Protection Monitoring Applications
  • NACE SP0106 - Control of Internal Corrosion in Steel Pipelines and Piping Systems
  • NACE SP0169 - Control of External Corrosion on Underground or Submerged Metallic Piping Systems
  • NACE SP0188 - Discontinuity (Holiday) Testing of Protective Coating
  • NACE SP0193 - External Cathodic Protection of On-Grade Carbon Steel Storage Tank Bottoms
  • NACE SP0216 - Steel-Cased Pipeline Practices
  • NACE SP0205 - External Corrosion Control of Underground Storage Tank Systems by Cathodic Protection
  • NACE SP0207 - Close-Internal Potential Surveys on Buried or Submerged Metalic Pipelines
  • NACE SP0216 - Sacrificial Cathodic Protection of Reinforcing Steel in Atmospherically Exposed Concrete Structures
  • NACE SP0290 - Impressed Current Protection of Reinforcing Steel in Atmospherically Exposed Concrete Structures
  • NACE SP0502 - Pipeline External Corrosion Direct Assessment Methodology
  • NACE SP0607 - Petroleum and Natural Gas Industries-Cathodic Protection of Pipeline
  • NACE SP21434 - Cathodic Protection Systems for the Mitigation of External Corrosion of Buried and Submerged Metallic Piping Systems at Nuclear Power Plants
  • NACE Publication 1E100 - Engineering Symbols Related to Cathodic Protection
  • NACE TR01105 - Sacrificial Cathodic Protection of Reinforced Concrete Elements
  • NACE TR21447 - Consequences of Coating Failures as Related to Interaction with Cathodic Protection
  • NACE TR21463 - Criteria for Evaluation of Cathodic Protection Methods for Steel in Existing Concrete Structures

 

Cathodic Protection Glossary

A

  • Abradable coating  -  It gives wear resistance to highly abrasive material when rubbed against, while leaving the underlying material damage free.
  • Anion  -  A negatively charged ion.
  • Anode  - The electrons flow away the anode (negative charge) at which corrosion or oxidation occures at the material.
  • Anode Polarisation  -  The electrochemical state changing of an electrode's potential moving in a corroding positive direction.

B

  • Back ionisation  -  A condition, which may occur during electrostatic application of powder coating where excessive build up of charged powder particles limits further powder to be deposited onto the substrate.
  • Barrier coating  -  A protective layer of material that prevents the contact of corrosive elements.
  • Base  -  A substance that releases hydroxyl ions when disolved in water.
  • Blast cleaning  -  Removal of surface contamination and corrosion products by use of air and mechanical abrasives.
  • Blistering bubbles  -  Formed under the cured powder film, usually caused by the expansion of trapped air, moisture or corrosion.
  • Blooming  -  A hazy appearance on the surface of a coating.

C

  • Cathode  -  The electrons flow toward the cathode reducing the corrosion or oxidation of the material.
  • Cathodic Polarisation  -  The electrochemical state changing of an electrode's potential moving in a non-corroding negative direction.
  • Corrosion  -  The thinning of a pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.
  • Corrosion allowance  -  The amount of material in a pipe or vessel that is available for corrosion without affecting the pressure containing integrity.
  • Corrosion coupon  -  Used to monitor the corrosion rate of a material in a process.
  • Corrosion inhibitor  -  A substance that slows down the chemical reaction rate of corrosion on metal that is exposed to the environment.
  • Corrosion mapping  -  An ultrasonic method that identifies and maps corroded areas in a pipelineby yhe varying material thickness.
  • Crack  -  Cracks can come from fatigue, grith welds, or seam welds.
  • Current  -  The rate of flow of electricity in a circuit, measured in amperes.

D

  • Depolarization  -  When the CP cirrent stops flowing from athe anode to the structure being protected, the polarized structure will then begin to depolarize.
  • Direct current  -  An electric current that flows in only one direction.

E

  • Electrode  -  Refered to as the anode or cathode, whichever is approperate.
  • Electrode potential  -  The potential of an electrode in an electrolyte as a measure against a reference electrode.
  • Electrolyte  -  A chemical substance or mixture, liquid or solid, normally liquid, which conducts electric current.
  • Etching  -  Removal of a layer of the base metal.
  • External corrosion  -  When the outside of a pipe is decayed or eroded by chemical or electrochemical processes or any other environmental conditions.

F

  • Flux  -  Chemicals used to protect metals from oxidation.
  • Fretting corrosion  -  Takes place where there is friction between two metal surfaces.

G

H

I

  • Impressed current anode  -  They are intended to discharge current when being powered by an external DC power source.
  • Incomplete fusion  -  A weld break where complete fusion did not occur between the weld material and the faces or adjoining weld material.
  • Interupter  -  A sophisticated switch that can be used to interrult the operation of a rectifier.
  • Ion  -  An atom or molecular particle having a net charge.  Positive charged ions are cations and negative charged are anions.
  • Isolation gasket  -  Used to stop the current flow across metallic pipelines by separating two flanges.

J

K

L

M

N

O

  • Ohm  -  A unit of resistance.
  • Ohm's law  -  The relationships between power, voltage, current, and resistance.
  • Oxidation  -  The loss of electrons in a chemical reaction in which an element combines with oxygen.  Oxidation and reduction always occur at the same time in equal amounts.

P

  • pH  -  Affects the corrosion rate by affecting the reaction rate of cathodes and anodes.
  • Polarization  -  When CP current flows from the anode to the structure being protected, that structure's potential will shift more electrically negative.  The shift is called polarization.

Q

R

  • Rectifier  -  A power supply that converts AC power to DC power.
  • Rust  -  A corrosion product consisting primarily of hydrated iron oxide.

S

  • Stray current  -  The flow of electric current into the ground by the leakage on industrial currents.

T

U

V

  • Vent pipe  -  A small diameter pipe that has drilled holes or cut slots that allow gasses generated at the anode during the CP process to vent away from the anode.
  • Volt  -  A unit of electrical pressure.
  • Voltage  -  One volt is the amount of pressure that will cause one ampere of current in one ohm of resistance.
  • Voltage drop  -  When the voltage at the end of the cable is less than the beginning of the cable.
  • Voltage rating  -  The maximum voltage at which a cable or insulated conductor can be safetly maintained during continuous use in a normal manner.

W

  • Well integrity  -  An operation of technical, operational, and organizational solutions to reduce fisk of controlled release of formation fluids throught the life cycle of a well.

X

Y

Z

 

Display #
Title
Back Ionisation
Blast Cleaning
Blistering Bubbles
Blooming
Corrosion Allowance

Tags: Equations for Corrosion