HYDROGEN EVOLUTION ON ALUMINUM IN CHLORIDE SOLUTIONS

Hydrogen evolution on Al in 0.5 M NaCl aqueous solution was studied in the pH range 2-12. As expected, H-2 evolution occurs either by the discharge of H3O+ present in the bulk of acidic solutions or by direct H2O discharge from solutions with pH 8. However, in the pH range 5-8 H-2 evolves by the discharge of H3O+ ions formed by the dissociation of water, which acts as the rate-determining step. The water splitting appears to be catalysed by the surface film on Al formed spontaneously in the contact of Al with the solution. On heating to above 60-degrees-C, recrystallization of the oxide film occurs and the catalytic properties vanish. Since the corrosion of Al in the absence of oxygen is controlled by the cathodic H-2 evolution, the corrosion of Al in the neutral pH range is primarily controlled by the water-splitting rate, i.e. by the catalytic properties of the oxide film.