Transpassive dissolution of 420 stainless steel in concentrated acids under electropolishing conditions

Anodic dissolution of 420 stainless steel (Fe--13Cr alloy) in concentrated phosphoric acid, sulfuric acid, and their mixtures has been studied to determine the conditions that lead to electropolishing of the alloy in these electrolytes. A rotating disk electrode system has been employed to study the influence of electrolyte composition, electrode rotation speed, and electrolyte temperature on anodic polarization behavior and the surface finish. Anodic polarization curves in these electrolytes show active--passive--transpassive transitions. Concentrated sulfuric acid is found to be unsuitable for electropolishing, since metal dissolution is insignificant in this electrolyte, even at very high anode potentials. In phosphoric acid and a mixture of phosphoric and sulfuric acids, electropolishing is observed in the transpassive potential region at or beyond a limiting current plateau. At 90 deg C, the measured limiting current densities as a function of the rotation speed following Levich behavior, while at 25 and 60 deg C, convective mass transport effects are masked by surface kinetic steps. Highly reflecting and microsmooth surfaces are obtained under conditions where the dissolution reaction is mass transport-controlled. Graphs, Photomicrographs. 29 ref.--AA