The specificity of environments that will promote stress corrosion cracking is significant. It is important to realize that not all corrosive environments promote the formation of stress corrosion cracks. Those that do will usually be those that do not promote widespread corrosion in the sense of the attack being spread fairly uniformly over all exposed surfaces, since, if for no other reason, this is not likely to lead to the geometry of a crack, which requires that the crack sides remain relatively inactive whilst the tip remains active to maintain propagation into the metal. (reference)
Consequently those environments, such as sea water, that normally promote general corrosion of mild steel, are not likely to promote stress corrosion, whilst those chemicals sometimes used to control corrosion by addition to an otherwise corrosive environment may result in a borderline condition, between general corrosion and no corrosion, wherein the attack can be localized. Thus, the addition of caustic soda to boiler feed waters to reduce the corrosiveness of the latter towards mild steel can result in the form of stress corrosion frequently referred to as ‘caustic cracking’. The important general point is that those environments that cause stress corrosion are frequently highly specific to the particular alloy involved and a list of some environments that have been shown to promote stress corrosion in various materials is given in the following Table.
Combinations of some alloys and environments that have been shown to promote stress corrosion cracking.
|Al alloys||Chlorides, moist air|
|Mg alloys||Chloride-chromate mixtures, moist air|
|Nitric acid, fluorides.|
|Cu alloys||Ammonia, moist air, moist sulfur dioxide|
|C steels||Nitrates, hydroxides, carbonates|
|Austenitic steels||Chlorides, sulfur acid|
|High strength steels||Moist air, water, chlorides, sulfates, sulfides|
|Ti alloys||Halides, methanol|
Although this list of environments that have been shown to promote stress corrosion cracking may appear extensive it is by no means exhaustive. For a given alloy however there are many more environments that do not cause stress corrosion than those that so act. It is possible, by appropriate electrochemical measurements or by laboratory stress corrosion tests properly conducted, to identify potent environments for a given material, although failures continue to occur in circumstances that may not reasonably have been expected, as the following example indicates.
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