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The mechanism of transpassive dissolution of ASS N25 austenitic stainless steel in sulphuric acid solution

The transpassive dissolution of ASS N25 nitrogen alloyed austenitic stainless steel austenitic stainless in a 0.5 M sulphate solution with pH 0.4 was studied using a cyclic voltammetry on static and rotating electrode and electrochemical impedance spectroscopy. The main process in the transpassive potential region was found to be the release of soluble Cr(VI). Secondary passivation readily occurs for ASS N25 alloys whereas chromium dissolves at high current densities in the whole transpassive potential range. The secondary pasivation occurs, probably due to enrichment of Fe in the outermost layer of the surface film. The obtained results are compared with pure chromium as the main alloys constituents. A kinetic model of the processes is proposed, including a two step transpassive dissolution of Cr via a Cr(VI) intermediate species and the dissolution of Fe(III) through the anodic film.

Cyclic voltammetry; Kinetic model; Nitrogen alloyed austenitic stainless steel; Transpassive dissolution

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