Electrochemical behavior of Cr 2O 3/Fe 2O 3 artificial passive films studied by in situ XANES

The electrochemical behavior of thin sputter-deposited mixed Cr ₂O ₃/Fe ₂O ₃ oxide films with Cr ₂O ₃-contents of 10, 20, 50 and 90% was studied with in situ X-ray absorption near edge spectroscopy (XANES). These measurements gave information on the chemical states and dissolution rates during anodic and cathodic polarization in different electrolytes. At low Cr oxide concentrations, the films dissolve when cathodically polarized and are resistant to dissolution when polarized in the anodic direction. At high Cr ₂O ₃ concentrations, dissolution occurs when the films are anodically polarized, but the films are stable against cathodic dissolution. In the intermediate Cr oxide concentration range, the oxides neither dissolve under anodic nor cathodic polarization. However, in all the cases, even when no dissolution takes place, the species show electroactivity in that the redox reactions Fe ³⁺ → Fe ²⁺ and Cr ³⁺ → Cr ⁶⁺ can take place under cathodic and anodic polarization, respectively. In the mixed oxides a solid-state conversion takes place in the iron oxide phase during reduction, whereas the oxidation of the chromium oxide phase converts only the outermost layer. An acidic environment accelerates both anodic and cathodic dissolution, associated with chemical dissolution of the iron oxide. The results further show that critical threshold values exist for the dissolution resistance of the oxide. These values are different for anodic and cathodic reactions and further strongly depend on the solution chemistry.