Effect of Al/Mn ratio on corrosion behavior of lean Mg-Zn-Ca-Al-Mn alloy processed by twin-roll casting

Adopting twin-roll casting (TRC) technique, lean Mg-1.2Zn-0.3Ca-0.3Al-xMn (x = 0.25, 0.38, 0.5 wt%) alloys with different Al/Mn ratio were prepared. The stoichiometric ratio of Al8Mn5 is taken as dividing line. Under Al/ Mn>1.6, rhombohedral Al8Mn5 is formed, while under Al/Mn <= 1.6, cubic beta-Mn is generated. Because of lower Volta potential of Al8Mn5 than beta-Mn, the alloy with Al/Mn>1.6 shows the lowest average corrosion rate (4.55 mm y-1). The formation of fine rather than coarse Al8Mn5 weakens micro-galvanic corrosion and relieves stress concentration, thus simultaneously endowing the alloy (Al/Mn>1.6) with admirable corrosion resistance and mechanical properties (UTS:-266 MPa, YS:-193 MPa, EF:-20 %).