Corrosion Observation and Analysis

Corrosion refers to the phenomenon in which metallic and other solid materials are deteriorated, consumed, or damaged due to chemical or electrochemical reactions with the surrounding environment, leading to loss of original functions. There are various forms of corrosion such as intergranular corrosion, pitting, crevice corrosion, contact corrosion, and stress corrosion cracking (trans granular cracking and grain boundary cracking), all of which differ depending on the material and the environment.
To investigate the causes of corrosion and prevent reoccurrences, corrosion tests are conducted in a wide variety of industries. In particular, corrosion tests are important for products, such as automobiles, that are composed of various materials and whose safety is required in various outdoor environments.
In general, corrosion tests are conducted according to the following procedure.

A. Intergranular corrosion B. Shedding

• Phenomenon:
  Corrosion caused by impurities and inclusions on grain boundaries with low corrosion potential. Shedding, in which intergranular corrosion causes grains to fall off, may also occur.
• Examples:
  This phenomenon occurs at locations where stainless steel or aluminum alloys have not been heat-treated properly or sufficiently. It also occurs at zones affected by welding heat.
• Causes:
  On the material side: Lowered grain boundary chromium concentration, grain boundary segregation of trace constituents, grain boundary deposits, etc.

A. Pitting B. Passivation film

• Phenomenon:
  Corrosion occurring at very limited locations on metal surfaces maintaining a passive state (passivation films) that proceeds deeply in a small hole diameter.
• Examples:
  A passivation film locally damaged by halogen ions (such as Cl-), resulting in corrosion in the shape of a hole. This is triggered by metal non-uniformity such as non-metallic inclusions in aqueous chloride solution of stainless steel and aluminum alloys.
• Causes:
  On the environment side: Halogen ions, dissolved oxygen.
  On the material side: Inclusions, defects, etc.

A. Crevice corrosion B. Passivation film

• Phenomenon:
  Corrosion caused by lack of dissolved oxygen in crevices, which damages passivation films on metal surfaces, causing metals to melt.
• Examples:
  This phenomenon occurs on stainless steel, aluminum, and titanium flange faces.
• Causes:
  On the structure/material side: Crevice structure, oxidized scale, etc.
  On the environment side: Halogen ions, dissolved oxygen (the growing phase is the same as that for pitting).

A. Corroded part B. Base metal C. Noble metal

• Phenomenon:
  Corrosion that occurs on and is promoted on base metals due to the contact between metals having different electrode potentials and the existence of electrolyte solution on the contact point.
• Examples:
  Contact between aluminum and copper corrodes the aluminum, and contact between steel and stainless steel corrodes the steel.
• Causes:
  On the material side: Contact between metals having different electric potentials.

• Phenomenon:
  Corrosion caused by tensile stress (residual stress or external load stress), which locally damages the protective coating on the surface. Concentrated progress of corrosion will lead to cracking.
• Types of cracking forms:
  As the route in which stress corrosion cracking proceeds varies depending on the combination of metal and environment, there are also differences in cracking forms.
  A transgranular crack proceeds through grains while a grain boundary crack proceeds along grain boundaries.
• Causes:
  Transgranular crack: Inclusions, deposits, surface coating, material defects, etc.
  Grain boundary crack: Grain boundaries of trace elements, segregation, grain boundary chromium-depleted-layers, grain boundary deposits, grain boundary irregularity, etc.