Corrosion

What is Corrosion?

Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration.

Do All Metals Corrode

All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which combines iron and other alloys, is slower to corrode and is therefore used more frequently.

All small group of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include rhodium, palladium, silver, platinum, and gold.

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.

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

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.

 

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.


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