Revealing anti-corrosion mechanism of low-alloyed Mg-Gd-Zn-Zr alloy with ultra-low corrosion rate

Poor corrosion resistance is the inherent weakness of common Mg alloys, and consequentially, it would be a long-standing challenge to improve the corrosion resistance of Mg alloys to reach or approach that of other stainless metals while maintaining mechanical properties. Here, we successfully develop a low-alloyed Mg-2Gd-0.6Zn-0.3Zr (wt%) alloy having an ultra-low corrosion rate of 0.09 mm y⁻¹ coupled with good mechanical properties. Its corrosion resistance in 3.5 wt% NaCl solution is superior to that of ultra-high-purity Mg and other comparative Mg alloys. The ultra-high corrosion resistance is mainly attributed to three key factors. The relatively uniform microstructure of alloy substrate in terms of potential is the basic factor. The corrosion film exhibits superior corrosion resistance and passivation effect as well as self-healing ability. The enrichment of alloying elements and the formation of ZrO₂, Gd₂O₃, and ZnO with Pilling Bedworth Ratio between 1 and 2 in the corrosion film are conducive to reduce macroscopic defects and thus improve the stability and compactness of the corrosion film, which is another key factor. Amorphous ZrO₂ plays an important role in rapid formation of compact film. The more important finding of this study is that this corrosion film has amorphous characteristics to a large extent, which is believed to avoid microscopic defects from grain boundaries as rapid channels for Cl^- transport, and this is the third key factor for further reduction to achieve the ultra-low corrosion rate for the developed alloy. Our findings are expected to inspire the development of real stainless Mg alloys.