Corrosion behavior of AZ31 and AZ31-0.5Ca in different concentrations of NaCl and Na₂SO₄ at various temperatures

Magnesium (Mg) alloys are used in many structural and automotive applications, though the high susceptibility towards corrosion poses a bottleneck towards its diverse applications. Microalloying Mg is often reported to improve the corrosion resistance of Mg. In this work, the effect of Ca on the microstructure and corrosion characteristics of AZ31 magnesium alloy is investigated. The electrochemical testing of AZ31 and AZ31-0.5Ca was studied in NaCl and Na₂SO₄ electrolytes with different concentrations (0.5-0.25 M) and various temperatures (30-50 oC). The electrochemical corrosion behavior was evaluated by the potentiodynamic polarization. The microstructural investigations revealed the formation of laves phases ((Mg, Al)2 Ca) in the AZ31-0.5Ca. The electrochemical tests showed that the effect of chlorides ions was more vigorous than sulphates ions. The measured average corrosion rates for AZ31 and AZ31-0.5Ca alloys were 55.68 mpy and 78 mpy in 0.05 M NaCl, and 22 mpy and 60 mpy in Na₂SO₄ at 30°C, respectively. Notably, the corrosion rate increased with increasing electrolyte concentration and temperature, consistent with the principles of the Arrhenius law. AZ31-0.5Ca showed superior corrosion resistance as compared to AZ31 in both electrolytes at all the testing conditions. The improved corrosion resistance was attributed to the formation of (Mg, Al)₂ Ca, which reduced the overall fraction of β-Mg₁₇-Al₁₂.