Comparison of rolled and heat treated SS304 in chloride solution using electrochemical and XPS techniques

Abstract

SS304 in the rolled condition (66% cold work) and in the heat treated condition (solution quenched from 1100 °C) was assessed for its electrochemical behavior in de-aerated 3.5% NaCl by potentiodynamic polarization and AC impedance techniques. Surface analysis of films formed on the two materials in air and in solution was done using XPS. Open circuit potential (OCP) and pitting potential of the rolled SS304 were seen to be −0.25 V vs standard calomel electrode (SCE) and 0.25 V vs SCE respectively, while the heat treated sample showed −0.33 V vs SCE and 0.06 V vs SCE respectively. The rolled material showed a passive current density of 0.9 μA cm −2, which was about one order lower than that shown by the heat treated material. The rolled material, in addition, exhibited repassivation at 0.085 V vs SCE during reverse scanning of potential. Repassivation was not observed in the heat tread sample. XPS showed that films on rolled material were richer in chromium as compared with those on heat treated alloy. Better passivation characteristics shown by the rolled material were attributed to thicker passive film with a higher Cr:Fe ratio, the higher Cr being the result of enhanced diffusion through the oriented grains in the rolled material.