Effect of Micro-alloying and Microstructure on the Corrosion Behavior of As-Cast Mg-6.2 wt.% Zn Alloy

The corrosion behavior of the Mg-6.2 (wt.%) Zn alloys with the micro-addition of Zr (0.6 wt. %) and a combination of Ag (0.4 wt.%), Ca (0.2 wt. %), and Zr (0.6 wt. %) was investigated in freely aerated 3.5% NaCl solution using immersion testing for 20 h. The microstructural observation before and after the immersion test helped to comprehend the effect of grain size and second-phase precipitate (cathodic phase) area fraction along with the chemical composition on the corrosion behavior of the alloys. Fine grain size with a homogeneous distribution of the precipitates in the Mg-6.2Zn-0.2Ca-0.4Ag-0.6Zr (wt.%) was found to have greater corrosion resistance by forming micro-galvanic couples in a more uniform way. The corroded surface after immersion test was characterized using X-ray diffraction and Fourier transformed infrared spectroscopy. Insoluble complex carbonates of Mg and Ca in the corrosion products were found to protect the metal surface of the Ca containing alloy from further corrosive attacks.