Intergranular Corrosion in High Purity Ferritic Stainless Steels: Effect of Cooling Rate and Alloy Composition^⋆

The susceptibility of high purity Fe-Cr alloys containing 17 to 26% Cr and 0 to 1% Mo has been determined by the Huey test after subjecting the alloys to various heat treatments. The alloys are highly resistant to intergranular corrosion when 0.15 cm thick specimens are water quenched from temperatures between 954 C (1750 F) and 1120 C (2050 F). Air cooling from temperatures between 1010 C (1850 F) and 1120 C (2050 F) causes an increase in intergranular corrosion compared to base annealed material, the effect being severe in the 17 and 20% Cr alloys, but small in the 26% Cr alloys. The presence of 0 to 1% Mo appears to be slightly beneficial for the resistance of the alloys to sensitization following air cooling. Furnace cooling restores most of the intergranular corrosion resistance in the Mo-free alloys. Alloys containing 1/2 to 1% Mo, however, remain sensitized when furnace cooled. This effect in alloys containing 26% Cr is small. These phenomena are discussed in terms of the chromium depletion theory, the rates of diffusion of carbon and nitrogen to the grain boundaries, their precipitation as carbides and nitrides, and the rate of chromium rediffusion into the depleted zones.