Intergranular corrosion (IGC) is a selective attack in the vicinity of the grain boundaries of a stainless steel.
Chromium carbides can be precipitated if the stainless steel is sensitized in the temperature range 550–850°C (1020–1560°F), for example during heat treatment or welding.
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If the temperature lies in the critical range for too long, chromium carbides will start to form in the grain boundaries, which then become susceptible to intergranular corrosion.
The area adjacent to the grain boundaries becomes depleted in chromium (the chromium reacts with carbon and forms carbides) and this zone, therefore, becomes less resistant to intergranular corrosion.
ELC (extra low carbon content) steels, i.e. steels with maximum 0.030% carbon, have very good resistance to intergranular corrosion.
Steels stabilized with titanium (Ti) or niobium (Nb), since these types of steel form niobium or titanium carbides instead of chromium carbides, thus avoiding the critical decrease in the chromium content.
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ASTM A262 is a common intergranular corrosion testing method that can quickly screen batches of material to determine corrosion susceptibility.