ALUMINUM-MAGNESIUM ALLOYS AS CORROSION-RESISTANT COATINGS FOR STEEL

The corrosion behaviour of binary aluminium-magnesium (Al-Mg) alloy co atings deposited on to steel panels using an unbalanced magnetron sput tering technique has been studied. Neutral salt fog tests have shown t hat the corrosion resistance of sputtered aluminium alloy coatings inc reases as the magnesium content is raised until an optimum is reached for alloys containing approximately 20 wt% Mg. Increasing the magnesiu m content above 20 wt% did not give further improvements in protection but instead caused a decline in corrosion performance. Potentiodynami c cathodic polarization sweeps conducted in 600 mmol l(-1) sodium chlo ride solution have shown that the corrosion current density of sputter ed aluminium coatings is increased as the magnesium content is increas ed, suggesting that there is a reduction in barrier protection afforde d 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 increase d, suggesting that they afford improved sacrificial protection as the Mg level is raised. It is proposed that the peak in performance observ ed in the neutral salt fog environment represents an optimum balance b etween the barrier and sacrificial properties of the Al-Mg coatings.