Introduction
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
|
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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
warning.
The best methods of eliminating or minimising this
problem are listed below
- By selecting a material that is not
susceptible to the environment and by ensuring
that the environment is not adversely affected
by cleaning etc are not detrimental.
- By controlling the operating stresses
through design and minimising stress
concentrations to keep them below the critical
value. Residual stresses can be
reduced by heat treatments and careful design
for manufacturing.
- By coating the material and effectively
isolating the material from the environment.
- Minimising the operating temperature -
stress corrosion is very low if the temperature
is maintained below a certain specific value
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