Intergranular corrosion (IGC) is a selective attack in the vicinity of the grain boundaries of a stainless steel. It is as a result of chromium depletion, mainly due to the precipitation of chromium carbides in the grain boundaries.
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. 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.
Materials with less than 0.05% carbon normally have sufficient resistance against intergranular corrosion after welding. ELC (extra low carbon content) steels, i.e. steels with maximum 0.030% carbon, have very good resistance to intergranular corrosion. The same applies to 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.
Intergranular corrosion (IGC) testing is the only corrosion test performed as a standardized pre-delivery test. Strauss, Huey and Streicher are the common test methods. They are carried out as a rule on samples from finished tubes in the as-delivered condition or on material that has been sensitized, i.e. heated in the critical temperature range in a furnace or by welding.