Corrosion and rusting

Corrosion is the gradual deterioration or destruction of a metal, due to a chemical reaction with the surrounding environment.

It is a natural process that occurs when metals are exposed to the substances that are found in their immediate environment such substances include air, moisture, or other chemicals such as acids and bases.

Types of Corrosion
  1. - Uniform/general corrosion - Evenly distributed over the entire surface
  2. - Localized corrosion - Concentrated in specific areas (e.g., pitting, crevice corrosion)
  3. - Galvanic corrosion - Occurs when two dissimilar metals are in contact
  4. - Stress corrosion - Occurs due to the combined action of tensile stress and a corrosive environment

Special name given to the corrosion of iron and steel is called rusting

Let us emphasize:

Rusting is a specific type of corrosion that occurs on iron and steel when they are exposed to air and moisture. It is chemical process that involves the oxidation of iron.

Click here to read about rusting

Corrosion of some metals

Natural corrosion of some metals are:
  1. Corrosion of iron is called rusting
    Click here to know how this happens
  2. Corrosion of aluminium: when aluminium is exposed to air it appears dull in color due to the formation of a thin oxide layer. This thin layer resists further reactions with more oxygen and other substances. However, aluminium with this protective layer can still corrode under certain conditions such as in the presence of acidic environment.
  3. Corrosion of copper: Copper under the presence of carbon dioxide and water or moisture its surface turns green due to the formation of basic copper carbonate, Cu(OH)2.CuCO3(green in colour). It is a very slow process.
  4. Corrosion of silver: silver tarnishes over time due to the formation of a thin layer of sulphide on its surface, when exposed to the environment containing gases such as hydrogen sulphide (H2S) or sulphur dioxide.
    These react with silver to form silver sulphide (Ag2S) and the colour of the surface of silve becomes brown-black in color.

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