A two-stage HNO
3 passivation treatment was introduced in order to improve the pitting corrosion resistance of type 304 stainless steel. In the 1
st stage, the steel was immersed in 1.5kmol·m
-3 HNO
3 solution containing 0 to 5×10
-2kmol·m
-3 NaF at 313K for 3.6ks. The following 2
nd stage was immersion in 1.5 to 10.5kmol·m
-3 HNO
3 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 HNO
3+5×10
.3 NaF solution for the 1
st stage and 7.5kmol·m
-3 HNO
3 solution for the 2
nd 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 2
nd treatment. It is considered that F
- ions penetrated in the passive film accelerated a preferential dissolution of iron in the film in the 1
st stage, then Cr enrichment in the films was fairly enhanced by the high corrosion potential provided by HNO
3 in the 2
nd 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.
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