XANES INVESTIGATION OF THE ROLE OF CERIUM COMPOUNDS AS CORROSION-INHIBITORS FOR ALUMINUM

Cerium-containing films on aluminum and Alloy 5052 have been investigated using XANES (X-ray Absorption Near Edge Structure). The Ce L(III) and Ce L(I) absorption edges allow a clear distinction to be made between 3- and 4-valent cerium. Measurements were made of the fluorescent X-ray signal with X-rays incident at glancing angles to give surface sensitivity. Cerium in the 3-valent state was deposited by galvanostatic reduction from solutions of sufficiently high pH for oxygen to oxidize the cerium to a 4-valent state. From XANES measurements, it was determined that these films contained cerium mainly in the 3-valent state. Exposure of these films to a NaCl solution converted the cerium to the 4-valent state. Open circuit exposure of aluminum to low concentrations of cerium ions gave films with either 3- or 4-valent cerium depending on the exposure time.