Time-dependent role of Mg in regulating the degradation behaviour of LPBF-fabricated Zn3Mg alloy

Although Zn3Mg has been widely reported as a promising biodegradable Zn-based alloy with favourable mechanical performance, its role in regulating degradation behaviour, particularly under the unique microstructural conditions introduced by laser powder bed fusion (LPBF), remains highly controversial. In this study, the long-term in vitro degradation behaviour of laser powder bed fusion (LPBF) fabricated Zn3Mg alloy was systematically investigated in simulated body fluid over a period of up to 28 days and compared with that of pure Zn. The results reveal a pronounced time-dependent dual role of Mg during degradation. In the early (0–3 days) and intermediate–late (14–21 days) stages, the release of Mg²⁺ contributes to interfacial buffering and stabilises protective corrosion products, whereas in the intermediate stage (3–14 days), densification of the degradation layer inhibits Mg²⁺ transport, thereby diminishing its regulatory effect. In the final stage (21–28 days), local pH elevation and Cl⁻ enrichment weaken the protective effect of Mg, leading to a stabilised degradation rate of approximately 0.09 mm·year⁻¹. This study elucidates the dynamic competition between micro-galvanic corrosion and Mg²⁺-mediated interfacial regulation in LPBF-fabricated Zn3Mg alloy, providing mechanistic insights for the composition and microstructure design of biodegradable Zn-based implants.