Enhancing corrosion resistance of Mg-Alloys by regulating precipitates at grain boundaries using rare earth oxides

Abstract

This study delved into the corrosion behavior of ZK60 Mg alloy in saturated NaCl solution, particularly focusing on the effects of the addition of rare earth oxide, namely CeO₂ (forming ZKC alloy) and La₂O₃ (forming ZKL alloy). The results indicate that the introduction of CeO₂ and La₂O₃ promotes the precipitation of T-(Mg_1−x, Zn_x)₁₁RE phases (Mg-Zn-RE phases, where RE represents Ce or La) at grain boundaries. The presence and distribution pattern of the T-phase have a profound impact on the corrosion resistance of the Mg alloy. Specifically, the ZKC alloy exhibits the most outstanding corrosion resistance. This superior performance is attributed to the uniform distribution of (Mg_1−x, Zn_x)₁₁Ce phase at grain boundaries in ZK60-0.5 wt% CeO₂, effectively hindering the penetration of corrosive media into the matrix. Additionally, scanning kelvin probe force microscopy (SKPFM) analysis reveals that the (Mg_1−x, Zn_x)₁₁Ce phase exhibits the smallest potential difference with the matrix, significantly mitigating the tendency for galvanic corrosion. In contrast, the ZKL alloy displays less precipitation and uneven distribution of the (Mg_1−x, Zn_x)₁₁La phase, resulting in inferior corrosion resistance compared to the ZKC alloy. The disparities in the precipitation of the two phases, as derived from first-principles calculations, stem from the spontaneous reduction of CeO₂ under Mg conditions, whereas the reduction reaction between La₂O₃ and Mg cannot proceed spontaneously. Furthermore, SKPFM analysis and CALPHAD method found that as the addition of CeO₂/La₂O₃ increases, the atomic ratio of Zn in the Mg-Zn-RE ternary phase rises, accompanied by an increase in the potential difference between the Mg-Zn-RE phase and the Mg matrix. This suggests that fine-tuning the addition of rare earth oxides can modify the atomic ratio of the Mg-Zn-RE ternary phase, thereby enhancing the corrosion resistance of the Mg alloy. In summary, this study not only unravels the specific mechanisms of how CeO₂ and La₂O₃ affect the corrosion behavior of ZK60 Mg alloy but also provides new strategies and insights for the development of low-cost, high-performance corrosion-resistant Mg alloy materials.