Chromium ion implantation for inhibition of corrosion of aluminium

The effect of chromium implantation into pure aluminium targets on the corrosion behaviour of the superficial alloys produced was studied in aqueous carbonate-bicarbonate solutions containing chloride ions. A beam energy of 150 keV was used for implantation of chromium into aluminium with doses varying from 5 x 10(16) to 2 x 10(17) ions cm(-2). Ennoblement of pitting potentials, obtained when using a scan rate of 0.08 mV s(-1), was observed for all concentrations used. A maximum in pitting resistance was found around 8 at.% Cr. For samples with peak chromium concentrations in excess of about 8 at.%, the values of pitting potential remained almost constant. The maximum was also observed when studying chromium implanted into steel. The changes in chemistry of the passive film on chromium-implanted aluminium alloys, at breakdown potentials, were studied using X-ray photoelectron spectroscopy (XPS). The Cr3+ state corresponded to the chromium oxyhydroxide phase. Cr(IV) and Cr(VI) were also detected. A chromium concentration in excess of approximately 8 at.% made pit repassivation difficult. This might be due to a more effective hydrolysis of chromium ions regarding aluminium ions, which maintains the critical pit environment at lower potential and current density.