Nitric Acid Passivation Treatment of Type 304 Stainless Steels with Different Surface Polishing Conditions and Changes in Pitting Inhibition Effect of The Treatment with Exposure to Corrosion Environments

Abstract

To develop effects of surface polishing conditions on pitting resistance after passivation treatments, pitting potentials of Type 304 stainless steels finished using different polishing techniques and passivated with HNO₃ solutions of different types were measured in deaerated 3.5%NaCl. Steel finished with electropolishing and then passivated in 30%HNO₃ showed high pitting potential of 1010 mV (vs. Ag/AgCl (sat. KCl)), which is in a trans-passive potential region. Increased pitting potential of 1053 mV was obtained when 0.1 kmol･m^－3Na₂MoO₄-containing 30%HNO₃ was used for passivation. Steel finished with mechanical polishing never showed such high pitting potential. No decrease in pitting potential was observed on steel finished with electropolishing and 30%HNO₃ or 0.1 kmol･m^－3Na₂MoO₄-containing 30%HNO₃ passivation after accelerated corrosion tests such as long term immersion in aerated 3.5%NaCl, salt spraying with 5%NaCl or steaming at 121 °C. To achieve high pitting resistance, combined surface treatment of electropolishing and 30%HNO₃ or 0.1 kmol･m^－3Na₂MoO₄-containing 30%HNO₃ passivation is recommended for Type 304 stainless steels.