Intergranular corrosion is unusual in modern stainless steels because most of the types of austenitic stainless steels which were formerly susceptible to this form of corrosion are now produced with low carbon contents. For this form of corrosion to occur, the steel needs to be exposed to temperatures in the range of 1100-1200°F (600 to 650°C) for sufficiently long time for carbon to combine with the chromium to form chromium carbides on grain boundaries within the stainless steel structure. If that happens, regions adjacent to the carbides in the grain boundaries are depleted in chromium content and becomes susceptible to corrosion if there is subsequent exposure to a corrosive medium. The steel is said to be “sensitized.”
The extent of the sensitization depends on the carbon content and the exposure temperature and time. If the carbon is taken out of the stainless steel, it cannot form chromium carbides. Today’s modern, low carbon or L-grade austenitic stainless steels typically contain less than 0.030% carbon. With these low carbon levels, they would need to be exposed to the sensitizing temperature range for many hours before chromium carbides would begin to form. Practically this only happens during prolonged service at elevated temperatures.
Even the regular grades such as Type 304 typically do not contain more than 0.04% carbon today. This still leaves time for most practical welding situations before sensitization occurs. For this reason, we do not see intergranular corrosion very often.
While Type 304L is more resistant to sensitization to intergranular corrosion compared with the straight grade, Type 304, the low carbon content plays virtually no role with regard to other forms of corrosion. For example, Type 304L and Type 304 are equally resistant (or susceptible) to pitting, crevice corrosion, and chloride stress corrosion cracking.
Alternatively, the low carbon effect can be achieved by tying the carbon up in highly stable compounds so that it is no longer available to cause this reaction. This “stabilization” is achieved by the addition of elements such as titanium or niobium (columbium) in grades such as Type 321 and Type 347, respectively. These grades can operate indefinitely in the sensitizing temperature range without the problem of sensitization to intergranular corrosion.