Influence of solution chemistry on dissolution of artificial passive films

摘要

The dissolution behavior of artificial Fe, Cr, and Fe/Cr passive films (sputter-deposited Fe/Cr oxide films on inert substrates) was studied by in situ X-ray absorption near edge spectroscopy (XANES). Low pH and presence of halides (Cl{sup {minus}} and F{sup {minus}}) accelerate the dissolution due to the solubility of Fe oxides in these solutions. The presence of Fe(2+) in the oxide film strongly accelerates dissolution in acidic solution. For mixed Fe/Cr oxides, F{sup {minus}} leads to a selective dissolution of Fe oxide. Cr{sub 2}O{sub 3} is not attacked by fluoride; hence if mixed oxide films contain a sufficient amount of Cr oxide, dissolution stop is obtained, probably by surface enrichment of Cr oxide. The composition of the oxide film is also crucial for its electrochemical stability. An increased Cr{sub 2}O{sub 3} content increases the resistance against reductive dissolution, increases the resistance in aggressive solutions (low pH, halides), but decreases the resistance against oxidative dissolution.