Electrolytic polishing of stainless steels

A study has been made of the electropolishing characteristics of sulphuric acid-glycerine-water baths of different compositions for the polishing of (I) Nickel-free Cr : Mn : N, (ii) 18:8, Cr : Ni and (iii)Ferritic (~ 17% Cr) stainless steels. It has been found that for getting a polish on these steels, at room or higher temperature, the minimum concentration of sulphuric acid in the bath should be 30% by volume. At room temperature, acid concentration was inversely related to current density, whereas at higher temperatures, the reverse was the case. Nickel-free Cr: N stainless steels could be polished at lower current densities than those required for the 18 : 8, Cr : Ni steels and the ferritic stainless steels did not give a comparable bright polish even at very high current densities (of the order of 200 A/dm2). The surface finish could be improved by treatment first at low current densities, followed by that at the optimum limit. The rate of dissolution of metal in terms of thickness decreased with time. (Dr. Y.N. Trehan, Senior Scientific Officer; Mr. S.P.Bhadra, Senior Scientific Assistant; Mr. P.K. Gupte, Assistant Director, National Metallurgical Laboratory