Corrosion Types

Corrosion is a broad term that encompasses various types of deterioration and degradation processes that materials undergo due to chemical or electrochemical reactions with their environment.  Corrosion is nature's wasteful way of returning metals to their ores.  The chemistry of corrosion emphasizes the basic corrosion reaction \(M^o\) to \(M+\)  + electron, where \(M^o\) is the metal and \(M+\) a positive ion of the metal.  As long as the metal \((M^o)\) retains posession of the electrons, it retains its identity as a metal.  When it loses possession of them by any means whatever, it has experienced corrosion.

• Tags: Structural Steel Corrosion  

Corrosion types Index

• Common Corrosion Types
  • Uniform Corrosion
  • Galvanic Corrosion
  • Pitting Corrosion
  • Crevice Corrosion
  • Stress Corrosion Cracking
  • Intergranular Corrosion
  • Selective Leaching
  • Erosion Corrosion
  • Microbiologically Influenced Corrosion
    • Oil and Gas Production
  • High-Temperature Corrosion

Common Corrosion Types

• Uniform Corrosion (UC)  -  This type of corrosion is characterized by a relatively even or uniform loss of material thickness over the entire exposed surface.  It occurs when the entire surface of a material is exposed to a corrosive environment, leading to a consistent corrosion rate across the surface.  It is often caused by exposure to atmospheric conditions or chemical environments where the corrosion rate is relatively constant.
  • Even Degradation  -  The corrosion proceeds uniformly across the material surface, resulting in a consistent loss of material thickness.
  • Predictable Corrosion Rate  -  Under stable environmental conditions, the corrosion rate remains relatively constant over time, allowing for relatively straightforward prediction and estimation of material loss.
  • Visible Signs  -  Uniform corrosion typically results in a dull or matte appearance on the surface of the material due to the formation of corrosion products, such as oxides or salts.
  • Limited Localized Damage  -  Unlike some other forms of corrosion, such as pitting corrosion or crevice corrosion, uniform corrosion does not typically lead to localized damage or severe structural degradation.  Instead, it results in gradual thinning of the material.

• Galvanic Corrosion (GC)  -  Also called bimetallic corrosion or dissimilar metal corrosion, is an electrochemical process in which one metal corrodes preferentially when in electrical contact with a different type of metal in the presence of an electrolyte.
  • Electrochemical Cell Formation  -  When two different metals are in contact with each other and exposed to an electrolyte (such as saltwater or moisture), an electrochemical cell is formed.  This cell consists of an anode (the metal that corrodes) and a cathode (the metal that remains protected).
  • Galvanic Series  -  Metals have different electrochemical potentials, and they can be arranged in a galvanic series based on their tendency to corrode.  When two metals with significantly different positions in the galvanic series are in contact, the one with the lower (more negative) position will act as the anode and corrode, while the one with the higher (more positive) position will act as the cathode and remain protected.
  • Corrosion  -  The anodic metal undergoes oxidation, releasing electrons into the surrounding electrolyte.  These electrons flow through the conductive path provided by the metals, causing the cathodic metal to undergo reduction.  The anodic metal corrodes, leading to the degradation of the material.

• Pitting Corrosion (PC)  -  This corrosion is a localized form of corrosion characterized by the formation of small pits or cavities on the surface of a material.  Unlike uniform corrosion, which causes a relatively even loss of material thickness over the entire surface, pitting corrosion leads to the formation of small, isolated areas of damage.  These pits can penetrate deeply into the material, even if they are relatively small in size.
  • Localized Damage  -  Pitting corrosion is characterized by the formation of small pits or craters on the material's surface.  These pits are often irregular in shape and can vary in size from microscopic to visible to the naked eye.
  • Rapid Penetration  -  Despite their small size, pits created by pitting corrosion can penetrate deeply into the material, compromising its structural integrity.  This can lead to catastrophic failure, especially in critical components or structures.
  • Initiation and Propagation  -  Pitting corrosion typically starts with the localized breakdown of the material's protective passive film or oxide layer, followed by the accelerated corrosion of the exposed surface within the pit.  Once initiated, pitting corrosion can propagate rapidly, leading to the formation of multiple pits across the material surface.
  • Challenging Detection  -  Pits created by pitting corrosion can be difficult to detect visually, especially if they are small or located in inaccessible areas.  However, advanced inspection techniques such as non-destructive testing (NDT) methods may be employed to identify and assess pitting corrosion damage.

• Crevice Corrosion (CC)  -  A localized form of corrosion that occurs in narrow gaps or crevices between surfaces.  These crevices can be formed by joints, fasteners, gaskets, or any other configuration that creates a confined space where fluid movement may be restricted.  Crevice corrosion typically occurs in environments where stagnant electrolytes can accumulate, such as the crevice itself or areas adjacent to it.
  • Localized Attack  -  Crevice corrosion initiates and progresses within the confined space of the crevice, leading to localized corrosion damage.  The surrounding areas outside the crevice may remain largely unaffected.
  • Stagnant Electrolyte  -  Stagnant or trapped electrolyte within the crevice provides an environment conducive to corrosion.  This can result from limited fluid flow or the presence of contaminants that inhibit fluid movement.
  • Differential Aeration  -  Crevice corrosion often occurs in environments where there is a difference in oxygen concentration between the crevice and the surrounding area.  This differential aeration can create conditions favorable for the corrosion process to occur.
  • Accelerated Corrosion  -  The presence of a crevice can accelerate the corrosion process by concentrating corrosive agents and facilitating the buildup of aggressive chemical species within the confined space.
  • Local Cell Formation  -  Similar to other forms of corrosion, crevice corrosion involves the formation of local electrochemical cells, with the crevice acting as the anodic region where corrosion occurs and adjacent areas serving as cathodic sites.

• Stress Corrosion Cracking (SCC)  -  A form of corrosion that occurs under tensile stress in the presence of a corrosive environment.  It can lead to brittle failure of materials, especially metals and alloys, even at stress levels below their yield strength.  Unlike traditional mechanical failure, which is typically driven by applied loads exceeding the material's strength, stress corrosion cracking can occur at stress levels below the yield strength of the material.
  • Brittle Fracture  -  SCC results in the brittle fracture of a material, characterized by the sudden propagation of cracks without significant plastic deformation.  This is in contrast to ductile failure, which involves extensive plastic deformation before fracture.
  • Environment Dependence  -  SCC occurs only in the presence of a specific corrosive environment that interacts with the material surface.  This environment typically includes a corrosive agent, such as chloride ions in aqueous solutions, that facilitates the cracking process.
  • Susceptible Materials  -  Not all materials are susceptible to stress corrosion cracking.  Certain alloys, particularly those used in critical applications such as aerospace and nuclear industries, are more prone to SCC under specific environmental conditions.
  • Localized Attack  -  Stress corrosion cracking typically initiates at localized sites of stress concentration, such as surface defects, notches, or areas subjected to residual stress from manufacturing or service conditions.
  • Transgranular or Intergranular Cracking  -  SCC can propagate through the material either along the grain boundaries (intergranular cracking) or through the grains themselves (transgranular cracking), depending on the material and the specific corrosive environment.
  • Crack Propagation  -  Once initiated, stress corrosion cracks can propagate rapidly, often leading to catastrophic failure of the affected component or structure.

• Intergranular Corrosion (IGC)  -  A type of corrosion that occurs preferentially along the grain boundaries of a metal or alloy.  Grain boundaries are the interfaces between adjacent grains or crystalline regions in a polycrystalline material.  Intergranular corrosion is often associated with certain alloys, particularly those containing susceptible elements such as chromium, carbon, or sulfur.
  • Selective Attack  -  Intergranular corrosion selectively attacks the grain boundaries of a material while leaving the grains themselves relatively unaffected.  This can lead to the weakening and eventual failure of the material along the grain boundaries.
  • Grain Boundary Depletion  -  In intergranular corrosion, the material adjacent to the grain boundaries can become depleted in certain alloying elements or protective passivating agents, rendering these regions more susceptible to corrosion.
  • Sensitization  -  Some alloys are susceptible to sensitization, a condition where the grain boundaries become depleted in chromium due to prolonged exposure to elevated temperatures.  Sensitization can increase the susceptibility of the material to intergranular corrosion.
  • Heat Affected Zones  -  Welded or heat-affected zones in metals are particularly susceptible to intergranular corrosion due to changes in microstructure and composition that occur during welding or heat treatment processes.
  • Susceptible Alloys  -  Alloys containing certain elements, such as carbon in stainless steels, are more prone to intergranular corrosion.  This corrosion can occur in environments where these susceptible elements are present, such as in acidic or chloride rich environments.
  • Stress Corrosion Cracking  -  Intergranular corrosion can also lead to stress corrosion cracking, where cracks propagate preferentially along the grain boundaries under tensile stress in a corrosive environment.

• Selective Leaching (SL)  -  Also called dealloying or parting corrosion, is a type of corrosion process in which one or more elements are preferentially removed from an alloy, leaving behind a porous and weakened structure enriched in the remaining elements.  This phenomenon typically occurs when the alloy is exposed to a corrosive environment that selectively attacks certain constituents of the material.
  • Preferential dissolution  -  Selective leaching involves the preferential dissolution of one or more alloying elements from the material's microstructure.  This dissolution can occur due to differences in the susceptibility of the alloying elements to corrosion in the given environment.
  • Formation of porous structure  -  As the more susceptible elements are removed from the alloy, voids or pores are formed within the material's structure.  This can lead to a porous and weakened matrix, compromising the mechanical properties and structural integrity of the material.
  • Enrichment of remaining elements  -  The elements that are less susceptible to corrosion become concentrated in the remaining matrix of the material.  This can alter the material's composition and properties, potentially affecting its performance in service.
  • Localized attack  -  Selective leaching typically occurs in localized regions of the material, often along grain boundaries or other microstructural features where corrosion preferentially initiates.
  • Stress corrosion cracking  -  The formation of a porous and weakened structure as a result of selective leaching can increase the susceptibility of the material to stress corrosion cracking and other forms of mechanical failure.

• Erosion Corrosion (EC) -  A combined process involving both mechanical wear and chemical corrosion, leading to accelerated material degradation.  It occurs when a material is exposed to a corrosive environment with high velocity fluid flow, causing the protective corrosion products to be removed or disturbed by the fluid's action.  This exposes fresh material to the corrosive environment, accelerating the corrosion process.
  • Mechanical Wear  -  High velocity fluid flow causes the material to experience mechanical wear, including erosion, abrasion, or cavitation, which physically removes material from the surface.
  • Chemical Corrosion  -  Concurrently, the corrosive environment reacts chemically with the exposed surface of the material, leading to corrosion.
  • Synergistic Effect  -  The combination of mechanical wear and chemical corrosion results in a synergistic effect, where the rate of material degradation is significantly higher than the sum of the individual effects acting alone.
  • Localized Damage  -  Erosion corrosion typically leads to localized damage on the material's surface, characterized by pits, grooves, or surface roughness.
  • Corrosion Product Removal  -  The high velocity fluid flow can remove or dislodge the protective corrosion products formed on the material's surface, exposing fresh material to the corrosive environment and accelerating corrosion.

• Microbiologically Influenced Corrosion (MIC)  -  Also called biocorrosion, is a type of corrosion caused or accelerated by the activities of microorganisms.  These microorganisms can include bacteria, archaea, fungi, algae, and other microbes that colonize the surface of materials exposed to aqueous environments.  They contribute to the corrosion process by altering the local environment, producing corrosive byproducts, or directly attacking the material surface.
  • Microbial Metabolism  -  Microorganisms can colonize the surface of materials and produce metabolic byproducts that create corrosive conditions. For example, some bacteria can metabolize organic compounds or produce acidic waste products, leading to localized corrosion.
  • Biofilm Formation  -  Microbes often form biofilms on material surfaces, creating a protective layer that can trap moisture, nutrients, and corrosive substances, promoting corrosion beneath the biofilm.
  • Electrochemical Effects  -  Microorganisms can participate in electrochemical reactions that facilitate corrosion processes.  For instance, microbial cells can act as cathodes or anodes in electrochemical cells, accelerating corrosion rates.
  • Local Cell Formation  -  Microbial colonies and biofilms can create localized environments with different chemical compositions and oxygen concentrations, leading to the formation of local electrochemical cells that drive corrosion at specific sites.
  • Pitting Corrosion  -  Microbiologically influenced corrosion often manifests as pitting corrosion, where localized attack occurs beneath biofilms or microbial colonies, leading to the formation of small pits or craters on the material surface.
  • Sulfate Reducing Bacteria (SRB)  -  Certain microorganisms, such as sulfate reducing bacteria, are particularly notorious for their role in MIC.  SRB can produce hydrogen sulfide gas, which is highly corrosive to metals and can lead to sulfide stress cracking and hydrogen embrittlement.
    • Microbiologically influenced corrosion is a significant concern in various industries, including oil and gas production, marine engineering, water treatment, and infrastructure.  It can lead to premature failure of materials, increased maintenance costs, and safety hazards.  Preventive measures to mitigate microbiologically influenced corrosion include:

• • • • Biocide treatment to control microbial growth in aqueous systems.
      • Design modifications to minimize the formation of stagnant water areas where microbial colonies can proliferate.
      • Material selection, choosing alloys resistant to MIC or applying corrosion resistant coatings.
      • Regular monitoring and maintenance to detect and address microbial growth and corrosion damage in its early stages.

• High-Temperature Corrosion (HTC)  -  Also called hot corrosion or high-temperature oxidation, is a type of corrosion that occurs at elevated temperatures typically above 400°C (752°F).  It involves the deterioration of materials exposed to aggressive environments at high temperatures, such as combustion gases, steam, or molten salts.
  • Oxidation  -  One of the primary mechanisms of high-temperature corrosion is oxidation, where the material reacts with oxygen in the surrounding atmosphere or with oxidizing agents present in the environment. This reaction forms oxides on the material's surface, which can degrade its mechanical properties and lead to material loss over time.
  • Sulfidation  -  In addition to oxidation, high-temperature corrosion may involve sulfidation, where sulfur-containing compounds in the environment react with the material to form sulfides. Sulfidation is particularly prevalent in industrial settings such as oil refining, where sulfur compounds are present in combustion gases.
  • Scale Formation  -  As corrosion products accumulate on the material's surface, they can form protective scales or layers that act as barriers against further corrosion. However, these scales can also spall or crack, exposing fresh material to corrosion.
  • Diffusion Processes  -  High-temperature corrosion often involves diffusion processes, where reactive species such as oxygen, sulfur, or other corrosive agents penetrate into the material's bulk, leading to internal oxidation or sulfidation.
  • Accelerated Kinetics  -  The rate of corrosion at high temperatures is typically accelerated due to increased reaction kinetics and diffusion rates. This can lead to rapid material degradation and reduced service life in high-temperature environments.

• • • High-temperature corrosion can affect a wide range of materials, including metals, alloys, ceramics, and refractory materials, used in various industrial applications such as power generation, aerospace, petrochemical processing, and combustion systems.  Preventive measures to mitigate high-temperature corrosion include:
      
      • Material selection, choosing alloys or coatings with high-temperature corrosion resistance.
      • Protective coatings or surface treatments to provide a barrier against corrosion.
      • Control of operating conditions, such as temperature, gas composition, and impurity levels, to minimize corrosive attack.
      • Regular inspection and maintenance to detect and address corrosion damage in its early stages.