A TRANSMISSION ELECTRON-MICROSCOPY STUDY OF STRESS-CORROSION CRACKINGIN STAINLESS-STEELS

Stress-corrosion cracks were produced in a 25 wt% Cr duplex stainless steel and in a 304L austenitic stainless steel by exposure to boiling chloride solutions. Samples from the steels were examined optically an d by scanning electron microscopy, then prepared for transmission elec tron microscopy by ion-beam thinning. In the duplex steel, cracks prop agated in the ferrite, and tended to be blocked by austenite. Cracking in the ferrite was transgranular and cleavage-like. Cracking in the 3 04L austenitic stainless steel was also transgranular and cleavage-lik e. Transmission electron microscopy and microprobe analysis revealed t wo morphologically distinct layers associated with stress-corrosion cr acking: an essentially featureless outer layer which was considered to be corrosion product, and a dealloyed sponge-like region adjacent to the metal. The presence of the sponge-like layer was interpreted as be ing consistent with a film-induced cleavage mechanism of stress corros ion cracking.