Corrosion is the action and effect of corrosive substances or of a corrosive environment. Corrosion alters a material by chemical or physical-chemical transformation.
Corrosion occurs as a function of the combination of corrosive substance / material and the environment in which it takes place.
For example, water is very corrosive to non-stainless steels. This corrosion is activated by certain mineral salts and other compounds that may be contained in the water (sea water is more corrosive than fresh water), and also by temperature, the presence of oxygen etc.
Composite materials are not affected by corrosion, but many mechanisms that alter their mechanical properties may nevertheless appear. For example, in the case of osmosis of orthophthalic polyesters or after hygrothermal ageing of epoxy resin.