Effect of cold-working in the passive behavior of 304 stainless steel in sulfate media

Extralow sulfur, cold-rolled 304 stainless steel (SS) samples, annealed and cold-worked to different degrees in an industrial plant, together with a ferrite-rich SS, have been studied by electrochemical and microscopic techniques to analyze the effect of cold-working on its passive behavior in sulfate media. The techniques employed were potentiostatic and cyclic polarization, optical microscopy, transmission electron microscopy, X-ray diffraction, ex situ scanning tunneling microscopy, and ex situ and in situ atomic force microscopy. The cyclic polarization measurements showed that the passive currents in neutral sulfate + chloride and in sulfuric acid solutions were similar and increased with cold-work. The pitting susceptibility also increased with cold-work, this effect being more evident as the chloride concentration increased. The dependence of the pitting potential on cold-work was explained using the bilayer model, the sulfate ingress in the passive film during anodic oxidation forming a coulombic barrier against chloride penetration. The microscopic examination of the samples after long time polarization in 1.0 M sulfuric acid showed that the surface profile of the passive film depended on the metallurgical structure of the metallic substrate. The higher passive currents and the increased susceptibility to pitting corrosion of the work-hardened samples were explained by the formation of a much more defective oxide during its anodic oxidation, with easy paths that enhanced sulfate ingress. The growth of such a much more imperfect oxide was related to the formation of defects in the grains and of more defective interfaces in the bulk of the material, both resulting from the accumulation of internal stresses during cold-rolling. Comparison with previous work highlighted the role of such defects in front of the absolute amount of martensite in the material. (C) 2002 The Electrochemical Society.