Improvement of Pitting Corrosion Resistance of Type 430 Stainless Steel by Electrochemical Treatments in a Concentrated Nitric Acid

Abstract

Electrochemical surface treatments have been attempted in 5 kmol/m³ HNO₃ solution to improve pitting corrosion resistance of Type 430 stainless steel. Three different surface treatments were carried out for the same time period (2.9 h) under conditions of immersion, potentiostatic polarization at 0.9 V vs. Ag/AgCl/sat.KCl reference electrode (E=+0.222 V vs. SHE) and potential cycling between 0.6 and 0.9 V. The pitting corrosion resistance was evaluated by an ordinary pitting potential measurement and droplet tests which simulates atmospheric corrosion in marine environments. It was found that the potential cycling treatment is most effective among them to improve pitting corrosion resistance. The surface analysis by EPMA and XPS indicated that the potential cycling most successfully removes MnS inclusions and increases Cr content in the passive film.