Competitive Effect of Grain Size and Second Phase on Corrosion Behavior of Biodegradable Mg-3Zn-1Mn-xSr Alloys

The corrosion behavior and microstructure of Mg-3Zn-1Mn-xSr (x = 0, 0.5, 1.0, 1.5, 2.0 wt.%) alloys are investigated by optical microscope (OM), scanning electron microscope (SEM), electrochemical tests and immersion experiments equipped with a dynamic corrosion device. The introduction of Sr element changes the distribution of Mg₇Zn₃ phase in the Mg-3Zn-1Mn alloy, promoting its precipitation at the grain boundary. Decrease in grain size and second phase spacing can be observed with the increase of Sr content, and the size of semi-continuous second phases (Mg₇Zn₃+ Mg₁₇Sr₂) distributed along grain boundary increases. The corrosion resistance of Mg-3Zn-1Mn-xSr alloys increases with the suitable addition of Sr, but decreases with the addition of excessive Sr, indicating that the critical value exists for the addition of Sr to obtain the optimal corrosion resistance. Optimal corrosion performance (CR = 1.213 ± 0.02mm/y, I_corr= 2.80μA/cm²) of Mg-3Zn-1Mn-1Sr alloy is concluded by immersion tests and electrochemical measurements, and its corrosion morphology manifests uniform corrosion pits and dense structure due to mass accumulation of corrosion products on the damaged surface of Mg matrix. The synergistic effect of the grain size and second phase is used to explain the influence of the variation of Sr content on the corrosion performance of Mg-3Zn-1Mn-xSr alloys.