The micro-alloyed Mg-Sn-Ge-Mn alloy with low corrosion rate enabled by Mn-induced microstructural homogenization and impurity passivation

Magnesium alloys are promising lightweight materials, yet their application is limited by poor corrosion resistance, especially when heat treatment triggers detrimental impurity precipitation. Here, a low-cost, rare-earth-free micro-alloyed Mg-0.1Sn-0.1Ge-0.05Mn alloy that achieves a low corrosion rate of 0.06 mm/y (calculated by hydrogen evolution) after 7 days’ immersion in 3.5 wt.% NaCl solution, with minimal localized attack. Microstructural and electrochemical analyses reveal that trace Mn addition refines and homogenizes the secondary phases, promoting relatively uniform corrosion. More importantly, during homogenization at 400 °C, Mn selectively encapsulates Fe-Si impurity particles, forming Mn-enriched symbiotic structures that suppress cathodic activity and endow the alloy with heat-input resistance to a certain extent. The corrosion product film becomes stable after only 5 days of immersion, providing sustained protection with a low-frequency impedance modulus of ∼8000 Ω·cm 2 after 15 days. This work demonstrates that a trace Mn strategy can concurrently optimize as-cast microstructure and mitigate thermal-degradation of corrosion performance, offering a practical pathway for designing cost-effective, high-performance magnesium alloys amenable to industrial thermal processing.