Corrosion, abbreviated as CRSN, is 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. Examples of non metallic corrosion include the dissolution of ceramic materials or the discoloration and weakening of polymers by the sun's ultraviolet light.
The corrosion resistance of a pipe or a metal is the ability of the material to resist the corrosive effects of its environment. Internal corrosion is caused by the reaction of the fluid in a pipe to the pipe proper. External corrosive forces would be the reaction of the pipe to the soil or air. By isolating the flow line from its corrosive environment by lining the pipe or wrapping the pipe, the corrosive effects can be mitigated.
Corrosion is a very common chemical reaction typically in the form of oxidation. The loss of material mass has a natural tendency to revert to its natural state when an exposed surface comes in contact with a gas or liquid. Most materials are susceptible to corrosion.
Nomenclature & Symbols
Oil Field Corrosion
Corrosion in the oil field is caused by carbon dioxide, hydrogen sulfide or organic acids dissolved in the produced fluids.
Variables affecting Corrosion
Like many chemical reactions, the rate of corrosion generally increases when the temperature increases. As a rule of thumb, the reaction rate doubles for every ten degrees Centigrade increase in temperature.
The main concern with pressure and its affect on corrosion is that under lower pressures, dissolved corrosive gases can break out of the solution which may increase the corrosivity.
Usually velocity's contribution to erosion is that under very low flows, localized corrosion (pitting) is likely. With casing gasses in CVR systems, corrosion will likely present on the bottom of the pipe as condensate precipitates out of the casing gas as it cools. It moves slowly relative to the gas in the line.
With very high velocities, the corrosion can be present in corrosion-erosion.
Forms of Corrosion
- General Corrosion
- Galvanic Corrosion - Caused by dissimilar metals
- Pitting - Localized on metal surface
- Crevice - Portion of the surface is isolated from the environment
- Intergranular - Corrosion at the grain boundaries
- Selective Leaching - Only one metal in an alloy is attacked
- Stress Corrosion Cracking
- Corrosion Erosion - Velocity aggravated corrosion
- Corrosion Fatigue - Corrosion on cyclic loads
- Cavitation - Corrosion gas bubble formation & immediate collapse
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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 Polarisation - The electrochemical state changing of an electrode's potential moving in a corroding positive direction.
- 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.
- Brackish water - Water having salinity values ranging from approximately 500 to 5,000 parts per million.
- Buffer - A chemical substance which stabilizes pH values in solutions.
- Buffer capacity - A measure of the capacity of a solution or liquid to neutralize acids or bases.
- Camera pig - A configuration pig with a camera and light source recording the inside of the pipeline.
- 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.
- Cleaning pig - A utility pig with brushes, cups, and scrapers for cleaning foreign matter from the inside of the pipeline.
- 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.
- Deactivation - The process of prior removal of the active corrosive constituents, usually oxygen, from a corrosive liquid by controlled corrosion of expendable metal or by other chemical means.
- Defective weld - A weld having one or more defects.
- Diffusion - The spread of gases, liquids, or solids from areas of high concentration to areas of low concentration.
- Diluent - A liquid used in coatings to reduce the consistency and make coating flow more easily.
- Direct current - An electric current that flows in only one direction.
- Dry corrosion - See gaseous corrosion or hot corrosion.
- Elastic modulus - The ratio of the stress applied to a body or substance to the resulting strain within the elastic limits.
- 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.
- External corrosion - When the outside of a pipe is decayed or eroded by chemical or electrochemical processes or any other environmental conditions.
- Flux - Chemicals used to protect metals from oxidation.
- Free corrosion potential - Corrosion potential in the absence of net electric current flowing to or from the metal surface.
- Fretting corrosion - Takes place where there is friction between two metal surfaces.
- Gaseous corrosion (dry corrosion or hot corrosion) - Corrosion with gas as the only corrosive agent and without any aqueous phase on the surface of the metal.
- Gouging - Mechanical removel of metal from the surface of the pipe.
- Hardness - The property of a material that enables it to resist plastic deformation, usually by penetration.
- Holiday - A discontinuity in painted or coated surfaces.
- Hot corrosion - See dry corrosion or gaseous corrosion.
- Incomplete fusion - A weld break where complete fusion did not occur between the weld material and the faces or adjoining weld material.
- Incomplete weld - A defect in the solder joint that causes cracks or damage to the bond.
- In-line inspection - When the pipeline is inspected by examining the interior of the pipe.
- Inhibitor - Can reduce the corrosion rate by presenting a protective film.
- Instrumented pig - A tool with instruments like recorders and sensors to examine the inside of the pipe.
- Integranulat corrosion - Usally of stainless steals or certain nickle-base alloys, that occures as the result of sensitization in the heat affected zone during the welding process.
- Internal Corrosion - The thinning of the interior pipe wall that is typically caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products.
- 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.
- Lacquer - A fast drying, usually clear coating, that is highly flammable and dries by solvent evaporation only.
- Lamellar corrosion - A form of corrosion in which the expanding corrosion products stack up as layers.
- Magnetic flux - This method can detect wall thickness from corrosion and pitting without affecting the pipe.
- Mapping pig - A configuration pig used to produce an elevation and plan view of the pipeline route with collect data that can be analysed from the inertia sensing or some other technology.
- Material hardness - The property of a material that enables it to resist plastic deformation, usually by penetration.
- Natural gas - Gaseous fuel occuring in nature.
- Neutralizer - A common designation for alkaline materials such as calcite or magnesia used in the neutralization of acid waters.
- 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.
- pH - Affects the corrosion rate by affecting the reaction rate of cathodes and anodes.
- Pitting - A non-uniform corrosion of a metal, not in the form of cracks, whereby a number of cavities, are formed in the surface.
- Porosity - Happens when a contaminent or gas is absorbed into the weld puddle.
- Rupture - There are numerous reasons a rupture can happen, depending on the material: age, brittleness, corrosion, internal pressure, movement, etc.
- Rust - A corrosion product consisting primarily of hydrated iron oxide.
- Sacrificial coating - A coating that provides corrosion protection wherein the coating material corrodes in preference to the substrate.
- Salt fog - A teat procedure that attemps to simulate the corrosion environment caused by road salt and marine spray.
- Saltine water - Water containing an excessive amount of dissolved salts.
- Silicone - A resin used in the binders of coatings.
- Shear stress - Tends to deform the material by breaking rather than stretching without changing the volume by restraining the object.
- Shinning - The formation of a thin, tough film on the surface of a liquid point.
- Shrinkage - A decrease in dimensionsof a coating during process.
- Shrinkage stress - The residual stress in a coating caused by shrinkage during processing.
- Smart pig - Collects information internally about the pipeline with electronic components.
- Solute - A substance which is disolved in and by a solvent.
- Specific gravity - The density or ratio of any substance to another substance.
- Strain - The deformation, stretched or compressed, of a material compared to its original length.
- Strain rate - The time rate of straining for the usual tensial test.
- Stray current - The flow of electric current into the ground by the leakage on industrial currents.
- Stress - The force per unit area of cross-section.
- Stress corrosion cracking - The combined effect of tensile stress and a corrosive environment.
- Surge pressure - See water hammer.
- Sweet corrosion: carbon dioxide - A weak acidic gas found in condensate, crude oil, natural gas, and produced water and becomes corrosive when dissolved in water.
- Tensile strength - The maximum stress a material can resist before it starts to elongate.
- Tension - The force (pulling or stretching) acting on a material.
- Toughness - The ability of a material to absorb considerable energy without fracturing.
- Ultrasonic testing - Used to measure the pipe wall thickness perpendicular to the pipe.
- Utility pig - Used to performing pipeline cleaning of debris and unwanted materials.
- 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.
- Water conductivity - The ability of water to conduct an electric current.
- Waterlogged - Saturated with water.
- Water table - The underground boundary between the surface of the soil and the area where groundwater fills the cracks and openings in the rocks and sand.
- Weld crack - Cracks can appear on the surface, inside the weld or heat effected zone.
- Weld decay - See integranulat corrosion.
- Welding defects - Blow hole, defect of joint shape, incomplete fusion, overlap, slag inclusion, undercut, weld crack.
- 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.
- Yield strength - Yield strength, abbreviated as \(\sigma\) (Greek symbol sigma), also called yield stress, is the minimum stress that leads to permanent deformation of the material.