Adsorption of sulfate and chloride ions on aluminum

Radioactive labeling method, electrochemical techniques and X-ray photoelectron spectroscopy (XPS) were applied to study sulfate and chloride 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 electrode potential range. Under open-circuit conditions, the adsorption of sulfate anion is controlled by solution pH, surface charge and the stability of the aluminum-oxide film. The surface concentration of chloride is smaller than that of sulfate and also depends on pH, but slightly. The degree of adsorption irreversibility between the two anions is different, most likely due to a dissimilar extent of the incorporation of sulfate and chloride in the oxide film. We demonstrate that the relative amount of irreversibly bonded sulfate increases with the solution 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, sulfate removal from the surface is observed. In contrast to sulfate, the surface coverage of chloride increases with anodic polarization even above the pitting potential. The cathodic polarization reduces both the sulfate and chloride coverage due to the high local pH and the destabilization of the passive film.