Common Types of Corrosion and Their Causes

Corrosion is the gradual deterioration of metal through electrochemical or chemical reactions with its environment, causing material loss and weakening the structure. Different types arise from different mechanisms and environmental interactions. 

Corrosion repair costs are difficult to predict and ongoing; its harmful effects can arise at any time. Below is an overview of common corrosion types and how they form:

Electrochemical corrosion

Galvanic corrosion

• MechanismOccurs when dissimilar metals contact each other in an electrolyte. The more active metal (anode) corrodes while the more noble metal (cathode) is protected. Influencing factors include:
  • Potential difference (per Galvanic series.
  • Electrolyte conductivity.
  • Anode-to-cathode area ratio (a small anode corrodes faster).
• Example: Galvanized steel acts sacrificially to protect the base steel.

Consequences of electrochemical corrosion are severe and include:

• Reduced metal service life: Corroded metal deteriorates quickly, reducing its service performance.
• Asset loss:Electrochemical corrosion causes major economic losses in industries such as construction and transportation.
• Safety hazards:Electrochemical corrosion can threaten people and the environment.

Crevice corrosion

• MechanismOccurs in confined spaces (such as beneath gaskets, bolts, or washers), where trapped liquid creates an oxygen concentration difference. The oxygen-depleted area inside the crevice becomes the anode, while the oxygen-rich area outside acts as the cathode, forming an oxygen concentration cell This accelerates metal dissolution inside the crevice.
• Example: Stainless steel and aluminum alloys are vulnerable in chloride-rich environments.

Contact-point corrosion (TPC)

Contact-point corrosion (TPC) occurs when wood and metal objects interact, creating corrosion sites in the metal.

This corrosion is common in piping at contact points between a pipe and its support, or wherever pipes touch other objects. Water and corrosive agents can become trapped in the gap; movement breaks scale that would otherwise slow corrosion. The damage is difficult to monitor because it is usually hidden until the pipe is lifted for inspection.

Pitting corrosion

• MechanismCharacterized by small, deep pits caused by localized breakdown of the protective oxide layer. Chloride or sulfide often initiates pits by creating an acidic microenvironment. Anion migration (such as Cl⁻) into the pit maintains acidity and accelerates metal dissolution.
• Example: Stainless steel exposed to seawater.

Chemical corrosion

Uniform corrosion

• MechanismUniform material loss across a surface, usually from moisture or acid exposure. It is predictable but gradually reduces material thickness.
• SolutionProtective coatings and cathodic protection.

Sulfidation

Sulfidation results from naturally occurring chemical reactions in crude oil that attack carbon steel and other alloys used in refinery equipment. It can occur in both liquid and vapor phases, causing corrosion failures in piping, heater tubes, pressure vessels, and other refinery equipment.

According to API RP 939-C and API RP 571, carbon steel and other alloys can corrode when they react with sulfur compounds at temperatures above 400°F (204°C).

Hydrogen accelerates sulfidation, and hydrogen in H₂S streams increases its severity above 500°F (260°C). Piping with no silicon or low silicon content has shown higher sulfidation rates, while adding chromium improves resistance.

Mechanical corrosion

Erosion corrosion

• MechanismCombines flow or abrasive particles with chemical attack. Turbulence or cavitation removes protective films and accelerates localized wear.
• Example: Pipes and pump impellers.

Fretting corrosion

• MechanismRepeated friction between surfaces in a corrosive environment breaks down the passive film.
• Example: Bolted joints or bearings.

Microbiologically influenced corrosion (MIC)

Microbiologically influenced corrosion occurs naturally due to biological growth on materials. Bacteria can come from many sources; materials exposed to untreated natural water, oil, or emulsions are especially at risk.

This corrosion particularly attacks carbon steel and stainless steel assets commonly used in:

• Pipelines
• Vessels
• Storage tanks
• Steel cables
• Pipelines

MIC pits can seriously reduce the strength and hardness of steel assets, making early detection and repair an important investment.

Stress corrosion cracking (SCC)

• MechanismCombines tensile stress and a corrosive environment to propagate cracks. Mechanisms include:
  • Hydrogen embrittlementHydrogen atoms diffuse into stressed metal and cause embrittlement.
  • Anodic dissolutionStress concentrates at the crack tip and accelerates corrosion.

• Susceptible systemsStainless steel in chloride and carbon steel in alkaline environments.

Corrosion under insulation (CUI)

Insulation can hide corrosion from view, allowing serious damage to develop before visible signs appear.

CUI is corrosion beneath insulation caused by moisture collecting on the asset’s outer surface. It mainly affects carbon and low-alloy steels but can affect any insulated asset.

Insulation can trap water between the metal and insulation, making CUI difficult to prevent. Any insulation-system failure can expose metal to CUI, commonly in pipes, vessels, and other insulated assets.

Intergranular corrosion (IGC)

• MechanismAttacks grain boundaries due to impurities or depletion of alloying elements (such as chromium in stainless steel). Anodic dissolution weakens boundaries and causes grains to detach.
• Example: Aluminum alloys and sensitized stainless steel are susceptible to IGC.

Uniform surface corrosion

Uniform surface attack is the most common corrosion type in aerospace components. When paint or protective coatings wear away, exposure to moisture and oxygen can cause corrosion. Harsh operating conditions accelerate damage and threaten safety and material integrity.

Aircraft components can also experience stress, localized, seam, filiform, and galvanic corrosion.

Filiform corrosion

• MechanismOccurs beneath paint or coatings, forming thread-like tracks as ions (Cl⁻, SO₄²⁻) and moisture migrate.
• CauseDamaged coating or moisture penetration.
• Example: Marine steel structures, ship hulls, and containers.

Stray current corrosion

• MechanismCurrent from an external source (railways or electrical systems) flows through metal and causes electrochemical dissolution.
• Cause:
  • Poor grounding.
  • Metal contacts soil or water carrying current.
• Example: Gas pipes near electric railways or ship hulls connected to shore power.

Selective leaching/dealloying

• MechanismOne alloying element dissolves preferentially, leaving a porous structure.
• Typical forms:
  • DezincificationZinc corrodes from brass (Cu-Zn), leaving porous copper.
  • GraphitizationIron corrodes from cast iron, leaving graphite carbon.
• Example: Brass pipes in seawater systems.

High-temperature oxidation

• MechanismMetal reacts with oxygen above >500°C, forming a scale that can spall.
• Cause:
  • Thermal cycling (heating and cooling) cracks the oxide layer.
  • Insufficient oxidation-resistant alloying elements (Cr, Al).
• Example: Furnaces, gas turbines, and engine exhausts.

Mechanisms and prevention

Corrosion mechanisms depend on:

• Material composition.
• Environmental conditions (temperature, pH, contaminants).
• Mechanical stress.

Prevention strategies:

• Material selection (e.g., corrosion-resistant alloys).
• Coatings (e.g., galvanizing).
• Environmental control (e.g., corrosion inhibitors).
• Design to avoid crevices and water traps.

Methods for detecting internal corrosion

Four common methods are used to detect material corrosion in pipes, steel cables, tanks, vessels, and tubing: ultrasonic testing (UT)pulsed eddy current (PEC), magnetic flux leakage (MFL) and remote visual inspection (RVI).