Cathodic 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.
Nomenclature & Symbols
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).
- 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 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 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 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
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- Abradable Coating - It gives wear resistance to highly abrasive material when rubbed against, while leaving the underlying material damage free.
- Alternating Current - An electric current that reverses its direction over and over.
- Anion - A negatively charged ion.
- Anode - The electrons flow away the anode (negative charge) at which corrosion or oxidation occures at the material.
- Anode Corrosion - The dissolution of a metal acting as an anode.
- Anode Corrosion Efficiency - The ratio of actual to theoretical corrosion based on the total current flow calculated by Faraday's law from the quantity of electricity that has passed.
- Anodic Coating - A film on a metal surface resulting from an electrolytic treatment at the anode.
- Anodic Inhibitor - A chemical substance or combination of substances that prevent or reruce the rate of the anodic or oxidation reaction by a physical, physico-chemical or chemical action.
- Anodic Polarisation - The electrochemical state changing of an electrode's potential moving in a corroding positive direction.
- Anodic Potential - An appreciable reduction in corrosion by making a metal an anode and maintaining this highly ploarized condition with very little current flow.
- Atmospheric Corrosion - A gradual degradation of alteration of a material by contact with substances present in the atmosphere, such as oxygen, carbon dioxide, water vapor, sulfur and chlorine compounds.
- Auxiliary Anode - A supplementary anode positioned so as to raise the current density on a certain area of the catnode and thus obtain distribution of plating.
- Auxiliary Electrode - An electrode commonly used in polorization studies to pass current to or from a test electrode, usually made of noncorroding material.
- Backfill - Material placed in a drilled hole to fill space around anodes amd buried components.
- 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.
- Cathode - The electrons flow toward the cathode reducing the corrosion or oxidation of the material.
- Cathode Cleaning - Electrolytic cleaning in which the work is the cathode.
- Cathode Corrosion - Corrosion resulting from a cathodic condition of a structure usually caused by the reaction of an amphoteric metal with the alkaline products of electrolysis.
- Cathode Disbondment - The destruction of adhesion between a coating and its substrate by products of a cathodic reaction.
- Cathode Efficiency - Current efficiency at the cathode.
- Cathode Film - The portion of solution in immediate contact with the cathode during electrolysis.
- Cathodic Pickling - Electrolytic pickling in which the work is the cathode.
- Cathodic Polarisation - The electrochemical state changing of an electrode's potential moving in a non-corroding negative direction.
- Cathodic Reaction - Electrode reaction equipment to a transfer of negative charge from the electrode to the ionic conductor.
- Catholyte - The electrolyte adjustment to the cathode of an electrolytic cell.
- Catholyte - A positively charged ion that migrates through the electrolyte toward the cathode under the influence of a potential gradient.
- 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 pipeline by the 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.
- Current Density - The measure of current per unit of cross section.
- Current Efficiency - The ratio of the electrochemical equivalent current density for a specific reaction to the total applied current density.
- Depolarization - When the CP cirrent stops flowing from athe anode to the structure being protected, the polarized structure will then begin to depolarize.
- Descaling - Removing the thick layer of oxides formed on some metals at elevated temperatures.
- Dezincification - Corrosion at which zinc is selectively leached from zinc-containing alloys.
- Diffusion - The spread of gases, liquids, or solids from areas of high concentration to areas of low concentration.
- Diffusion Coefficient - Expresses the transfer rate of a substance by random molecular motion.
- Direct Current - An electric current that flows in only one direction.
- Dummy Cathode - A substitute cathode that is used during adjustment of operating conditions.
- Electric Conductivity - The amount of current that a material can conduct.
- Electric Current Density - The measure of current per unit of cross section.
- Electrochemical Cell - A system consisting of an anode and a cathode in metallic contacts and immersed in an electrolyte.
- Electrochemical Corrosion - Corrosion that is accompanied by a flow of electrons between cathodic and anodic areas on metallic surfaces.
- 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.
Faraday's Law of Induction - States that whenever a conductor is placed in a varying magnetic field, an electromotive force is introduced.
- Flux - Chemicals used to protect metals from oxidation.
- Fretting Corrosion - Takes place where there is friction between two metal surfaces.
- Hot Corrosion - An accelerated corrosion of metal surfaces that results from the combined effect of oxidation and reactions with sulfur compounds and other contaminants.
- Hot Cracking - Caused by the segregation at grain boundaries of low-melting constituents in the weld metal.
- Hot Dip Coating - A metallic coating obtained by dipping the base metal into a molten metal.
- Hot Working - Deforming metal physically at such a temperature and strain rate that recrystallization takes place simultaneously with the deformation, thus avoiding any strain hardening.
- Hydrogen Disintegration - Deep internal cracks caused by hydrogen.
- Hydrogen Embrittlement - A process resulting in a decrease of the toughness or ductility of a metal due to the presence of atomic hydrogen.
- Hydrogen Overvoltage - Overvoltage associated with the liberation of hydrogen gas.
- 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.
- Localized Corrosion - Corrosion at discrete sites, stress-corrosion cracking.
- Ohm - A unit of 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.
- 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.
- Rectifier - A power supply that converts AC power to DC power.
- Rust - A corrosion product consisting primarily of hydrated iron oxide.
- Stray Current - The flow of electric current into the ground by the leakage on industrial currents.
- 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 Coefficient of Resistance - The change in resistance with applied voltage.
- 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.
- 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.
- Wetting Agent - A substance that reduces the surface tension of a liquid causing it to spread more readily on a solid.
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