Grain size effect on tribocorrosion kinetics in ultrahigh-purity magnesium

Tribocorrosion readily removes the protective corrosion product, creates new reactive corrosion sites and thus accelerates material loss in metallic materials. This is evidenced by a pronounced or gradual decline in open circuit potential (OCP) during tribocorrosion assessments. Here we report that grain refinement can not only enhance wear resistance in dry conditions, but also induce an anomalously stable OCP variation and fortify tribocorrosion resistance in ultrahigh-purity magnesium during tribocorrosion. The tribocorrosion tests revealed that the fine-grained Mg (FG-Mg) sample exhibited a wear rate (4.56 × 10⁻⁴ mm³/Nm) approximately half that of the coarse-grained Mg (CG-Mg) sample (7.87 × 10⁻⁴ mm³/Nm). CG-Mg showed a gradual OCP decrease, associated with a thin, unprotective tribocorrosion layer, even thinner than that resulting from dry sliding. Conversely, FG-Mg exhibited stable OCP evolution and quasi-linear tribocorrosion kinetics over time, attributed to a thick, protective tribocorrosion layer. Transmission electron microscopy data suggest that high-diffusivity pathways for oxygen along grain boundaries at the early tribocorrosion stages facilitate the formation of a continuous, protective MgO layer and an adjacent oxidized layer with a depth-dependent oxygen content gradient, enhancing tribocorrosion resistance in FG-Mg. Our findings offer valuable insights for strategically tailoring tribocorrosion resistance by modulating the OCP variation of highly active metals and alloys.