Intergranular Corrosion occurs when there is preferential attack of the grain boundaries. This results from depletion of chromium at the grain boundaries which occurs when stainless steel is heated to 500-900ºC. Carbon tends to migrate to the grain boundaries where it bands together with the chromium to precipitate chromium carbides, thus tying up the chromium. Those regions next to the grain boundaries are therefore impoverished in chromium (dechromisation) and, consequently, are liable to corrosive attack. To give the material these stainless qualities, chromium has to be made available.
- This is done by heating the material to above 1000ºC, followed by rapid cooling to prevent the formation of chromium carbide. This technique, applied to austenitic stainless steels, is called quenching.
- In the case of ferritic stainless steels rehomogenisation of the chromium is obtained by holding the material at approximately 800ºC: the diffusion rate of the elements is much faster.
To reduce the susceptibility to intergranular corrosion, either the carbon level can be reduced (L series), or elements combining preferentially with carbon instead of chromium can be added, such as titanium or niobium (DMV 321; DMV 316Ti or DMV).
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