Two-Stage HNO3 Passivation Treatment for Improving the Pitting Corrosion Resistance of Type 304 Stainless Steel

A two-stage HNO3 passivation treatment was introduced in order to improve the pitting corrosion resistance of type 304 stainless steel. In the 1st stage, the steel was immersed in 1.5kmol·m-3 HNO3 solution containing 0 to 5×10-2kmol·m-3 NaF at 313K for 3.6ks. The following 2nd stage was immersion in 1.5 to 10.5kmol·m-3 HNO3 solutions at 323K for 3.6ks. Pitting potentials for the steel subjected to the above mentioned treatments were measured by potentio-dynamic polarization in 1.0kmol·m-3 NaCl solution at 313K to reveal that the combination of 1.5kmol·m-3 HNO3+5×10.3 NaF solution for the 1st stage and 7.5kmol·m-3 HNO3 solution for the 2nd stage was the most preferable condition for the most noble pitting potential of about 1.0V (SCE). XPS analysis of treated steels revealed that Cr content in the passive film extensively increased and F- ions still remained in the film after the 2nd treatment. It is considered that F- ions penetrated in the passive film accelerated a preferential dissolution of iron in the film in the 1st stage, then Cr enrichment in the films was fairly enhanced by the high corrosion potential provided by HNO3 in the 2nd stage. Furthermore, F- ions located in the film and at the film/electrolyte interface seemed to act as an inhibitor to prevent the harmful attack by Cl- ions, resulting in the highly improved pitting corrosion resistance of the steel in chloride containing aqueous solution.