The Effect of Low Temperature Sensitization on Corrosion Resistance of Ultrafine-Grained Type 321 Stainless Steels

The ultrafine grain 321 stainless steel was produced by cold rolling. The microstructure, after 87% cold rolling and annealing at 380 and 450 A degrees C, has been characterized using transmission electron microscopy and x-ray diffraction. The degree of sensitization (DOS) was investigated by means of double loop electrochemical potentiokinetic reactivation test. The results showed that the DOS was higher in samples annealed at 380 A degrees C than that in samples annealed at 450 A degrees C. The strain-induced martensite promoted sensitization at 380 A degrees C and the reversion-transformed austenite-restrained sensitization or promoted healing of chromium-depleted zone at 450 A degrees C. Although the activation and reactivation current densities in samples sensitized at 450 A degrees C were higher than those in samples sensitized at 380 A degrees C, higher corrosion resistance of the former was attributed to lower DOS, which was proven further by electrochemical impedance spectroscopy experiment.