Intergranular Corrosion

When austenitic stainless steels are heated or cooled through the temperature range 425-900C (800-1650F), chromium tends to combine with carbon to form chromium carbides. The carbides precipitate preferen- tially at grain boundaries depleting chromium from the adjacent areas. This reduces the corrosion resistance of the chromium depleted areas, sensitizing the alloy to Intergranular Attack (IGA). The extent of carbide formation is dependent upon time at temperature and the carbon content of the alloy. Thus, exposure in the temperature range stated does not automatically mean that sensitization, or IGA will occur.

Sensitization may also result from slow cooling from solution annealing temperatures, or stress relieving - after welding - in the 425 to 900C (800 to 1650F) temperature range. In welded fabrications, sensitization and IGA may occur in corrosive environments in a rather narrow band on either side of or on the side opposite the weld, known as the heat affected zone (HAZ).

It is important to note that even if sensitization does occur, it is not of significant consequence unless the alloy is exposed to a corrosive environment. Sensitized stainless steel performs in a normal manner and safe manner in non-corrosive applications.

Order of Resistance

Methods of Minimizing Intergranular Attack - IGA

1) Solution anneal above 1040C (1900F) followed by a rapid quench.

2) Use type 347, a Cb stabilized grade, or 321, a Ti stabilized grade.

3) Use a low carbon, 0.03% max. carbon grade such as 304L, 316L, 317L or 904L.

With today's technology, carbon is economically reduced to very low residuals. The low carbon grades are the standard for welded fabrication.

ASTM A262 practice A to E are standard tests to determine susceptibility to IGA. Practice E, the Huey test, is widely used.