A material failure may be accelerated by the combined effect of a corrosion process and a mechanical stress. Two examples of such processes are stress-corrosion cracking and corrosion fatigue. A closer description of these processes is given below.
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The most common type is transgranular stress-corrosion cracking, SCC, that may develop in concentrated chloride-containing environments.
Previously, it was generally considered that an elevated temperature was necessary for SCC to occur.
In recent years, however, SCC has been experienced at ambient temperature on standard grade steels like 304(L) or 316(L) that were exposed to high tensile stresses. In these cases the steel surface was contaminated with solid salt deposits and the humidity of the atmosphere was rather high. These two factors resulted in a thin liquid film saturated with chloride. Other contaminants, such as H2S, may increase the risk of SCC in chloride containing environments. |
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Other environments that may give rise to SCC, particularly on low alloy steels, include very alkaline solutions at high temperatures. A typically SCC attack takes the form of thin, branched cracks as shown in the picture.
Data on SCC resistance for different steel grades are often published in data sheets etc. These data, however, are most often obtained in laboratory tests and can only be used to compare the SCC resistance between different steel grades. Such data cannot be used to predict the risk of SCC in a practical application.