Stress Corrosion is evidenced when the metal strength
loss resulting from the combined stress and corrosion
is greater than the effects of stress and corrosion
acting separately. The magnitude of the
combined effect is a measure of the susceptibility of
the material to stress corrosion.
Stress corrosion is generally evidenced as cracks giving an appearance of brittleness in a material which is otherwise has normal ductile properties. The cracks may follow intergranular paths which grow at relatively slow velocities. If the load is constant during the period of crack growth the cracks will eventually reach a critical size to result in material failure.
Some combinations of environment and material known to cause stress corrosion are listed below.
|Material||Environment known to cause stress corrosion|
|Al alloys||Moist air, Sea Water, Chloride, Bromide and iodide solutions|
|Cu alloys||Ammonia solutions and vapours, amines, moist SO2, solutions of acetates, citrates formates, tartrates, nitrites and sodium hydroxide.|
|Ni alloys||Hydroxide solutions and hydrofluoric acid vapour|
|Ti alloys||Solutions of chlorides, bromides, and iodides, liquid N2O4, methanolic solutions and dry salts and elevated temperatures|
|Low strength ferritic steels||Solutions of hydroxides, nitrates, carbonates, phosphates, molybdates, acetates, cyanides, and liquid ammonia|
|High Strength ferritic steels||Moist air, water, aqueous and organic solutions. Susceptibility increases with increasing stregnth of materials|
|Stainless Steels||Solutions of chlorides , fluorides, iodides, bromides, sulphates , Phosphates, nitrates and polythionic acids|
Stress corrosion does not normally occur in
conditions in which the metal suffers from serious
general corrosion. It is therefore often
not noticed resulting in fracture without
The best methods of eliminating or minimising this problem are listed below