ADSORPTION OF SULFATE AND CHLORIDE-IONS ON ALUMINUM

Radioactive labeling method, electrochemical techniques and X-ray phot oelectron spectroscopy (XPS) were applied to study sulfate and chlorid e adsorption/incorporation at the passive films on pure aluminum in 0. 1 mol dm(-3) NaClO4 containing sulfate or chloride ions. We found that the anion adsorption was pH dependent and occurred over a broad elect rode potential range. Under open-circuit conditions, the adsorption of sulfate anion is controlled by solution pH, surface charge and the st ability of the aluminum-oxide film. The surface concentration of chlor ide is smaller than that of sulfate and also depends on pH, but slight ly. The degree of adsorption irreversibility between the two anions is different, most likely due to a dissimilar extent of the incorporatio n of sulfate and chloride in the oxide film. We demonstrate that the r elative amount of irreversibly bonded sulfate increases with the solut ion pH and exposure time. Under well-controlled potential conditions, the sulfate coverage increases with the electrode potential until the passive film breakdown occurs. When aluminum undergoes pitting, sulfat e removal from the surface is observed. In contrast to sulfate, the su rface coverage of chloride increases with anodic polarization even abo ve the pitting potential. The cathodic polarization reduces both the s ulfate and chloride coverage due to the high local pH and the destabil ization of the passive him. (C) 1998 Published by Elsevier Science Ltd . All rights reserved.