The phase stability of cerium species in aqueous systems - II. The Ce(III/IV)-H2O-H2O2/O-2 systems. Equilibrium considerations and pourbaix diagram calculations

The use of rare-earth metal compounds and oxides to improve the corrosion resistance of aluminum alloys in aircraft applications continues to show promise as an attractive alternative to chromate conversion coatings. Cerium conversion coatings used for metal protection can be deposited by either spontaneous or nonspontaneous processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is electrochemically initiated. As a result, the process is very dependent on the potential and pH of the solution. In an initial study, a revised E-pH diagram was developed for the Ce-H2O system [J. Electrochem. Soc., 149, C623 (2002)]. Precipitation studies were also conducted to verify selected regions of the diagram. In the actual cerium conversion coating processes, an oxidizer, such as hydrogen peroxide (H2O2), is normally used to convert Ce(III) to Ce(IV). Therefore, an E-pH diagram for the Ce-H2O-H2O2 system is essential to understand and predict the factors in the cerium precipitation in the presence of H2O2. In this paper, the trends and pH regions of oxidation/reduction reactions of Ce(III) and Ce(IV) with H2O2 were calculated and evaluated. (c) 2005 The Electrochemical Society.