Passivity of Type 304 Stainless Steel—Effect of Plastic Deformation

The passivity of Type 304 austenitic stainless steel, as affected by plastic deformation, was studied by obtaining the potentiostatic anodic polarization curves of annealed and cold worked materials in hydrogen saturated 1N H₂SO₄ at room temperature. While the effect of 26% cold work is not significant, 50%, and 68% cold work make the attainment of passivity more difficult by shifting the open circuit potential in the active direction and increasing the critical current density. Studies on passive current versus time indicate that cold work does not affect the initial passive current, but the extent of decrease of passive current with time is less for the cold worked material than the annealed material. These results seem to be related to the strain-induced bcc martensite which forms when the metastable austenitic stainless steel is cold worked. This martensitic phase, the presence which has been checked by X-ray and metallography, has a high critical current density for passivity. Also, the active dissolution of a ferritic stainless steel, which does not transform to martensite, is not affected by cold work.