Effect of Sn addition on the microstructure, mechanical properties, and corrosion behaviour of AZ31 magnesium alloy

This study systematically investigates the effect of adding tin (Sn) to AZ31 magnesium alloy on its microstructure, mechanical strength, and corrosion behaviour. Alloys with 0, 2, and 4 wt.% Sn were processed and characterized using microscopy, X-ray diffraction, hardness testing, hot compression (at 300 °C), and electrochemical impedance spectroscopy (EIS). Quantitative analysis revealed that Sn addition significantly refined the grain size from 137.8 µm to 23.7 µm and promoted the formation of Mg 2 Sn precipitates. Calculations determined that the total strength gain in the AZ31–2%Sn alloy was overwhelmingly dominated by grain refinement and solid solution effects. In the AZ31–4%Sn alloy, while grain refinement remained the largest component, the sharp increase in overall strength was substantially augmented by the presence of Mg 2 Sn Orowan precipitates. Constitutive analysis of hot deformation yielded strain rate sensitivity exponents (m) between 0.17 and 0.20, identifying dislocation climb as the governing deformation mechanism, with a minor contribution from grain boundary sliding in the fine-grained 4% Sn alloy. Equivalent circuit modeling of EIS data demonstrated a trade-off in corrosion performance: the 2% Sn alloy achieved the highest charge-transfer resistance (R ct ≈ 1.7 ×10^5 Ω). This improvement is specifically attributed to Sn4+-induced modification of the MgO/Mg(OH) 2 passive film, resulting in a denser and more stable barrier. Conversely, the 4% Sn alloy suffered from accelerated degradation (R ct ≈ 2.05 ×10^4 Ω) because excessive Sn forms coarse Mg 2 Sn intermetallics, which act as cathodic sites and drive micro-galvanic corrosion. These findings highlight that moderate Sn alloying (2 wt.%) optimizes the property balance by enhancing strength without compromising corrosion resistance. • Microstructural Refinement: Tin (Sn) addition significantly refined the grain size of the AZ31 alloy from 137.8 µm to 23.7 µm and promoted the formation of Mg₂Sn precipitates. • Strengthening Mechanisms: The total strength gain in the AZ31–2%Sn alloy was primarily dominated by grain refinement and solid solution effects. In the AZ31–4%Sn alloy, the substantial strength increase was augmented by Mg₂Sn Orowan precipitates, in addition to grain refinement. • Hot Deformation: Constitutive analysis of hot compression at 300 °C showed that dislocation climb is the governing deformation mechanism, characterized by strain rate sensitivity exponents (m) between 0.17 and 0.20. • Optimal Corrosion Performance: The AZ31–2%Sn alloy demonstrated the highest corrosion resistance, with a charge-transfer resistance (R ct ) of approximately 1.7 ×10 5 Ω. This improvement is attributed to Sn 4+ -induced modification, forming a denser and more stable passive film. • Corrosion Trade-off: Increasing Sn to 4 wt.% led to accelerated corrosion degradation (R ct = 1.7 ×10 5 Ω), as excessive Sn formed coarse Mg₂Sn intermetallics that acted as micro-galvanic cathodic sites. • Property Optimization: The findings conclude that moderate Sn alloying (2 wt.%) is optimal for balancing properties, as it enhances mechanical strength without compromising the corrosion resistance.