August 14, 2021

Metal Corrosion Mechanisms

Corrosion of metals is an electrochemical reaction that involves changes in both the metal and the environment in contact with the metal. While the mechanisms of corrosion are the same on a microscopic level, various microstructure, composition, and mechanical design issues will lead to different manifestations of corrosion.

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There are seven common types of metal corrosion

  • Uniform
  • Galvanic
  • Crevice
  • Pitting
  • Intergranular
  • Stress corrosion cracking (SCC)
  • Dealloying

Uniform corrosion occurs over the entire exposed surface of a metal. Rust on a steel structure or the green patina on a copper roof are examples of uniform corrosion. The driving force for this type of corrosion is the electrochemical activity of the metal in the environment to which the metal is exposed.

Galvanic corrosion occurs near the junction between two dissimilar metals.The driving force for the corrosion reaction is the difference in electrode potentials between the two metals.

Crevice corrosion occurs in crevices between components and also under polymer coatings and adhesives. The driving force for the corrosion is the difference between the oxygen concentration inside the crevice and outside the crevice.

Pitted304Stainless_cropped


Pitting occurs in metals that are normally passive, when the passive layer breaks down. Examples of passive metals are aluminum and stainless steel. Pitting is a problem if it leads to weakening or perforation of the metal. In applications where appearance is important pitting is a problem.

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Interganular corrosion involves corrosion along the grain boundaries of the affected metal. The result is that the metal grains fall away and the metal is weakened. Austenitic stainless steels and precipitation strengthened aluminum alloys such as 2xxx alloys are examples of metals that can suffer from intergranular corrosion if the alloys are not properly processed and if they are exposed to corrosive environments.

Stress corrosion cracking involves the combined action of stress and exposure to a corrosive environment. In most cases, the stress or environment by themselves are insufficient to cause degradation of the metal. That is, the stress is below the metal’s yield strength and the metal would not corrode in the specific environment if the stress was absent.

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Dealloying is the selective leaching of one element from an alloy. This results in the formation of a porous structure that is not strong enough to support the applied mechanical loads. One common example is dezincification of brass alloys used for plumbing, where the zinc is leached out of the alloy.

Factors that influence corrosion mechanism

The specific type of corrosion that occurs depends on the several factors including metal composition, metal microstructure, environment, component geometry, stress on the component, contact between metals, and the manner in which components are joined together.

In some cases, the root cause of metal corrosion failures is selection of materials that are inherently incompatible with the environment. In other cases, corrosion is a result of mechanical design, where incompatible metals are joined together or components meet in a manner than results in narrow spaces between the components. Corrosion can also be the result of faulty manufacturing processes that result in microstructures that render an alloy susceptible to corrosion.

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