Effect of fluoride ions on passive performance of 316 stainless steel in acid media

The passivity of 316 stainless steel was investigated in F-containing dilute hydrochloric acid solutions aerated with oxygen gas at 25 degreesC. The polarization curves illustrate that 316 stainless steel can passivate in above solutions at corrosion potential. The anodic polarization curves show that low concentrations of F- ion have no significant influence on the passivity of 316 stainless steel, while the passivity is deteriorated by the higher concentrations (greater than or equal to0.01 mol/L) of F- ion. In 0.1 mol/L NaF solutions, an anodic current peak occurs before pitting potential, which doesn't appear in the solutions with low concentrations of NaF. During anodic polarization, a F--incorporated and Cr-enriched film forms on the stainless steel surface. The F--accelerated reactions mechanism is proposed for the deterioration of the passive film.