|9- Stress corrosion cracking (SCC)|
Stress corrosion cracking (SCC) is a process
involving the initiation of cracks and their propagation, possibly
up to complete failure of a component, due to the combined action
of tensile mechanical loading and a corrosive medium. Indeed, it
is the presence of tensile stresses that is dangerous, compressive
stresses exerting a protective influence.
SCC frequently occurs in media that are little or non-aggressive
towards the metal or alloy concerned in the absence of tensile loading
(e.g. austenitic stainless steels in high temperature water and steam).
The associated weight losses are generally very small and even insignificant
compared to the extent of the overall damage incurred. This form
of corrosion is of great practical importance and represents a permanent
risk in numerous industrial installations, in terms of both the economic
consequences and the safety considerations involved (personnel, equipment
reliability, respect of the environment). There is no known category
of commercial metals and alloys that is fully immune to SCC. Even
materials such as glasses, plastics and rubbers can also be prone
to this type of attack in certain conditions.
The time necessary for a part to fail by SCC can vary from a few minutes to several years.
Means of reducing or preventing stress corrosion cracking are : elimination of residual stresses by stress relieving heat treatments, purification of the medium, choice of the most appropriate material, improvement of the surface condition, avoid surface machining stresses, perform peening treatments on welds to induce surface compressive stresses, apply external protection methods (cathodic protection, inhibitors and organic or inorganic protective coatings).