Abstract
The electrochemical behavior of sputter-deposited mixed Cr2O3/Fe2O3 oxide films with Cr2O3-contents of 10, 20, 50 and 90% was studied with in situ X-ray absorption near edge spectroscopy (XANES). At low Cr oxide concentrations the films dissolve when cathodically polarized - due to reductive dissolution of the Fe2O3 compound - and are dissolution resistant in the anodic direction. At high Cr2O3 concentrations, dissolution occurs when the oxides are anodically polarized - due to oxidative dissolution of the Cr2O3 compound, but the films are stable against cathodic dissolution. In the intermediate Cr oxide concentration range the oxides neither dissolve under anodic nor cathodic polarization. In all cases the species remain electrochemically active, i.e. the redox reactions Fe3+ <-> Fe2+ and Cr3+ <-> Cr6+ also take place reversibly when no dissolution occurs. A solid state conversion of Fe2O3 takes place during reduction, whereas Cr2O3 oxidation proceeds via dissolution of the outermost layer. An acidic environment accelerates both anodic and cathodic dissolution, which must be associated with chemical dissolution of iron oxide. Critical composition threshold values for a high stability of the oxide were found. These values are different for anodic and cathodic reactions and depend on the solution chemistry.