STUDIES OF THE FORMATION OF CERIUM-RICH PROTECTIVE FILMS USING X-RAY-ABSORPTION NEAR-EDGE SPECTROSCOPY AND ROTATING-DISK ELECTRODE METHODS

The deposition of cerium-rich films on copper under cathodic polarizat ion was studied as a model system for understanding the mechanism of c orrosion inhibition of copper-containing aluminum alloys. Deposition w as also studied on gold and. iron for comparison with copper. Inhibiti on of corrosion of the aluminum alloys is achieved by deposition of a cerium-rich film on the copper-containing intermetallics that blocks t he cathodic reduction of oxygen at these sites. X-ray absorption near- edge structure measurements show that cerium-rich films precipitated f rom aerated solutions are in the tetravalent state. Thermodynamically, the Pourbaix diagram predicts that under these conditions cerium shou ld be in the trivalent state. This indicates that cerium chemistry is determined by processes in the solution rather than the potential of t he electrode. Cerium-rich film formation is dependent on reduction of oxygen which influences the oxidation of Ce(III) to Ce(IV) in solution and precipitation of the film by changing the local pH at the electro de. The generation of hydrogen peroxide by oxygen reduction is conside red to enhance cerium-rich film formation by oxidizing Ce(III) to Ce(I V) in solution. This was confirmed by addition of hydrogen peroxide to the solution.