Cathodic secondary phases enable protective tribocorrosion layers in magnesium alloys

Secondary phases strongly influence the degradation behavior of magnesium alloys, yet their role in tribocorrosion remains unclear. This study demonstrates that cathodic secondary phases can significantly enhance the tribocorrosion resistance of binary magnesium-rare-earth (Mg-RE) alloys through a mechanochemical synergy. Two coarse-grained Mg-RE systems were examined: solid-solution alloys and second-phase reinforced alloys. Although their wear rates were similar under dry sliding, their tribocorrosion responses diverged sharply. In solid-solution alloys, a thin (∼200 nm) nanograined MgO layer repeatedly ruptured and repassivated, leading to a drop in open circuit potential (OCP), severe subsurface deformation and accelerated wear under tribocorrosion. In contrast, second-phase reinforced alloys exhibited exceptional tribocorrosion resistance through formation of a micrometer-thick tribocorrosion layer. The cathodic secondary phases promoted localized oxidation and deep oxygen diffusion along phase boundaries, thereby substantially accelerating the growth of the tribocorrosion layer, while their in-situ oxidation into stable rare earth oxides reinforced the tribocorrosion layer. These mechanisms stabilized the OCP and protected the underlying matrix, which retained coarse grains with minimal deformation. These findings reveal that cathodic secondary phases can be strategically utilized to engineer robust, self-reinforcing tribocorrosion layers, overturning their conventional detrimental role in corrosion.