Enhancing protective oxide film formation in Mg-Al-Sn-Gd-Nd-Ca alloys via Al-induced heterogeneous nucleation

Magnesium (Mg) alloys are promising lightweight structural materials, but their widespread application is hindered by poor corrosion resistance, primarily due to inadequate corrosion product films and severe galvanic corrosion. These films, composed mainly of Mg(OH) 2 and hydroxides/oxides of alloying elements, are generally porous and provide insufficient protection. This study systematically investigates the influence of Al content on the corrosion behavior of rolled Mg-xAl-Sn-Gd-Nd-Ca (x = 0, 1, 2, 3, 4 wt%) alloys, with a focus on the formation and protective mechanisms of corrosion product films. The results reveal that Al enhances film protectiveness by promoting the preferential nucleation of Al(OH) 3 , which serves as heterogeneous nucleation sites for other hydroxides. This effect arises from two key factors: the shorter time to reach saturation and the faster nucleation kinetics. However, the improved film stability is counterbalanced by an increase in micro-galvanic corrosion sites. An optimal Al content of 1 wt% achieves the best balance, yielding the lowest corrosion rate (5.19 mm y −1 ). These findings are different from the conventional understanding governed by the Pilling-Bedworth ratio, providing new insights into the design of corrosion-resistant Mg alloys.