Corrosion Inhibition by Sulfate after Surface Preparation

Abstract

Electropolishing as a surface preparation technique is increasing in popularity in industrial applications and for corrosion studies. Electropolished surfaces have shown a better resistance to pitting corrosion over mechanical polishing, however, the fundamental reason governing the change in corrosion behaviour remains unclear. This study examined the corrosion behaviour of 13Cr4Ni stainless steel (UNS S41500) after five surface preparation techniques and shows that sulfate is incorporated in the oxide film when it is present in the electropolishing solution. Even after removal from the sulfate-containing solution, the sulfate incorporation increases the material’s pitting resistance by lowering the number of sites available for chloride to induce pitting. This work also demonstrates that, when used as a counter electrode, Pt can dissolve and reprecipitate on the working electrode surface during electropolishing. The deposits result in a more noble open circuit potential, indicating an artificial increase in passivity. These artificial changes to corrosion behaviour due to surface preparation method may result in erroneous conclusions. To establish fair comparisons between surface preparation methods, the counter electrode and the sulfate effect should be strictly considered.