Abstract
Low concentrations of thiocyanate substantially increase the active dissolution rate of austenitic and ferritic stainless steels. The mechanism of the effect was investigated by potentiostatic and impedance measurements on pure iron, pure chromium, and Type 430 stainless steel, in 1 M Na2SO4, acidified to pH = 2.5 with H2SO4. Thiocyanate has little effect on the anodic dissolution of pure iron, but increases the dissolution rate of chromium by approximately one order of magnitude; the effect on Type 430 stainless steel is even stronger. In the case of pure chromium, impedance measurements show no change in the dissolution mechanism in the presence of thiocyanate; the reaction rates are simply higher. For Type 430 stainless steel, more prominent crystallographic etching in the presence of thiocyanate and the effect of thiocyanate to increase the dissolution rate strongly indicate that thiocyanate suppresses the passivating effect of Cr-O-Cr networks on crystal ledges.
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