Interplay between the composition of the passive film and the corrosion resistance of citric acid-passivated AISI 316L stainless steel

Citric acid passivation is an attractive alternative to increase the corrosion resistance of stainless steels. In this work, the correlation of passive film composition and the corrosion properties of citric acid-passivated AISI 316L stainless steel samples was assessed. The concentration of citric acid in the passivating solution was varied from 10 to 30 wt.%. Passive film composition was examined by X-ray photoelectron spectroscopy. The corrosion behavior was investigated by potentiodynamic polarization curves. The citric acid passivation treatment led to Cr2O3 enrichment of the passive film, especially for the 20-wt.% concentration. By increasing the citric acid concentration to 30 wt.%, the corrosion resistance decreased. In addition to chromium compounds, the passivation treatment also affected the relative concentrations of Fe, Mo, and Ni compounds in the passive films. The formation of Mo oxides was triggered at citric acid concentrations of 10 and 20 wt.%, contributing to slower anodic dissolution rate. The corrosion resistance of the passivated samples was favored as the relative concentration of Fe oxides (FeO and Fe2O3) increased with respect to FeOOH.