The role of minor yttrium in tailoring the failure resistance of surface oxide film formed on Mg alloys

• Addition of yttrium raised the component ratio of MgO to Mg(OH)2 in the surface oxide film.
• Yttrium helped formation of a stronger film with improved adherence to the substrate.
• Excess amount of yttrium (> 3%) resulted in a weakened film.

The high reactivity of magnesium limits its extensive applications in the transportation industry for the development of lightweight vehicles and aircrafts. In addition to modification of the corrosion potential of bulk magnesium through alloying elements, tailoring its surface oxide film is an alternative approach for enhanced resistance corrosion. In this study, effects of small amounts of a rare-earth element, yttrium, on the strength of surface oxide film formed on Mg-3Al (wt.%) alloy and its adherence to the substrate were investigated experimentally and computationally. It is demonstrated that the yttrium addition decreased the component ratio of hydroxides to oxides, mainly Mg(OH)2 to MgO, in the surface oxide film, leading to markedly enhanced mechanical strength and resistance to scratch with stronger adherence to the substrate. However, as the nominal yttrium concentration exceeded a certain level, the beneficial effect of Y on the film was weakened. Efforts are made to clarify underlying mechanisms.