Effect of Sn and Al contents on the corrosion performance of stir-cast Mg-Al-Si alloy under different saltwater conditions

Abstract

This study investigates the corrosion performance of stir-cast Mg-3.0Al-0.5Si-xSn (x = 1.0 and 2.0 wt%) and Mg2.0Al-0.5Si-2.0Sn alloys in three saltwater conditions: 0.1 M NaCl (Sol A), 0.1 M Na2SO4 (Sol B), and a mixed 0.1 M solution (0.05 M NaCl + 0.05 M Na2SO4) (Sol C). The Sn-modified alloys exhibited lower weight loss, reduced hydrogen evolution, and greater electrochemical stability than the base Mg-3.0Al-0.5Si alloy, with the Mg-3.0Al0.5Si-2.0Sn alloy consistently showing the lowest corrosion rates in all media. Potentiodynamic polarization revealed that Sn additions shifted corrosion potentials to more negative values and decreased corrosion current densities, with the greatest improvement in the Mg-3.0Al-0.5Si-2.0Sn alloy. Electrochemical noise analysis further confirmed its reduced potential/current fluctuations, higher noise resistance, and lower susceptibility to localized corrosion. Among the solutions, Sol B provided the highest protection, Sol A was the most aggressive, and Sol C showed intermediate behavior. Surface analysis identified Mg(OH)2, MgCO3, Al(OH)3, and hydromagnesite as major corrosion products, with Sol B promoting the most stable films. Topography and pit assessments showed that the Mg-3.0Al-0.5Si-2.0Sn alloy developed the shallowest and most uniformly distributed pits. The enhanced corrosion resistance of the Sn-modified alloy is attributed to microstructural refinement, including finer grains, reduced cathodic phase content, and smaller precipitates that lower the cathode-to-anode ratio. Mechanistically, Sn addition refines Mg2Si and Mg17Al12 distributions, decreases micro-galvanic coupling, suppresses Mg dissolution, and stabilizes surface Mg(OH)2/Al(OH)3 films, resulting in more uniform and less localized corrosion behavior.