What is Corrosion?

Study Notes

Corrosion is the gradual destruction of metals by chemical, electrochemical, or biochemical interaction with the environment.

Metals occur in nature as their oxides, sulfides, carbonates, etc. and are thermodynamically more stable in these forms.
When extracted from ores, metals are in a higher energy state, which is thermodynamically unstable.
Metals try to return to their stable state by chemical or electrochemical interaction with the environment.

Metals occur in a native, free, uncombined state or pure metal and are highly stable and non-reactive with the environment.
Examples include Au, Pt, Ag, Pd, Cu, etc.

Except for noble metals, all other metals are reactive and form stable compounds as their oxides, sulfides, chlorides, and carbonates.
Examples include FeO, ZnO, PbS, CaCO₃, etc.

Corrosion results in the wastage of metal in the form of its compounds.
Valuable metallic properties like conductivity, malleability, ductility, etc. are lost due to corrosion.
Life span and efficiency of metallic parts of machinery and fabrications are reduced.
Efficiency of the machine decreases.
Plant shutdowns occur due to failure.
Product contamination can occur.
Toxic products of corrosion can cause health hazards.
Overdesign is necessary to allow for corrosion.

Direct chemical action of atmospheric gases like oxygen, halogens, H₂S, etc. on metals in a dry environment.
Oxidation corrosion: direct attack of oxygen on metal surface in the absence of moisture.
Alkali and alkaline earth metals are corroded at low temperatures.
Most metals except Au, Pt, and Ag are oxidized at high temperatures.
Corrosion due to other gases: SO₂, CO₂, Cl₂, H₂S, F₂, etc.

Wet corrosion occurs due to the electrochemical action of moisture and oxygen on metals.
Occurs when two dissimilar metals or alloys are in contact with each other in the presence of an aqueous solution or moisture.
Occurs when the metal is exposed to an electrolyte with varying amounts of oxygen.