Aluminium-magnesium alloys as corrosion resistant coatings for steel

The corrosion behaviour of binary aluminium-magnesium (Al-Mg) alloy coatings deposited on to steel panels using an unbalanced magnetron sputtering technique has been studied. Neutral salt fog tests have shown that the corrosion resistance of sputtered aluminium alloy coatings increases as the magnesium content is raised until an optimum is reached for alloys containing approximately 20 wt% Mg. Increasing the magnesium content above 20 wt% did not give further improvements in protection but instead caused a decline in corrosion performance. Potentiodynamic cathodic polarization sweeps conducted in 600 mmol l(-1) sodium chloride solution have shown that the corrosion current density of sputtered aluminium coatings is increased as the magnesium content is increased, suggesting that there is a reduction in barrier protection afforded by the Al coatings as the Mg level is raised. Open-circuit corrosion potential measurements have shown that the sputtered aluminium alloys become progressively more active as the magnesium content is increased, suggesting that they afford improved sacrificial protection as the Mg level is raised. It is proposed that the peak in performance observed in the neutral salt fog environment represents an optimum balance between the barrier and sacrificial properties of the Al-Mg coatings.