RELATION BETWEEN INTERGRANULAR CORROSION SUSCEPTIBILITY AND PRECIPITATION OF CARBIDES AND NITRIDES IN 17% Cr FERRITIC STAINLESS STEELS.

Carbide and nitride grain boundary precipitation was studied in vacuum melted 17% Cr steels with different carbon and nitrogen contents. Electric resistivity measurements, corrosion tests, and electron microscope observations are described. Intergranular corrosion susceptibility was markedly increased by precipitation of (Fe,Cr)//7C//3 at 550 to 600 C and of an unidentified nitride at 400 to 550 C. A chromium depletion zone along grain boundaries formed by the very rapid precipitation of these special types of carbide and nitride at the intermediate temperatures around 550 degree C is considered the most likely reason for intergranular corrosion.