Effects of calcium addition on phase characteristics and corrosion behaviors of Mg-2Zn-0.2Mn-xCa in simulated body fluid

The effects of Ca addition on corrosion resistance of as-cast Mg-2Zn-0.2Mn-xCa alloys were evaluated by electrochemical measurements and immersion test at 37 °C in simulated body fluid (SBF). The phase characteristics and surface potentials were proven using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe micro-analyzer (EPMA) and atomic force microscope (AFM), whereas the corrosion products and protective film features were characterized using X-Ray diffraction (XRD) and confocal laser scanning microscopy (CLSM). The results reveal that the volume fraction of the second phases increased when the Ca concentration increased. Furthermore, these secondary phases with sizes of 50-100 nm were discretely distributed along the grain boundaries and matrix. However, the bio-corrosion resistance of investigated alloys initially increased and then decreased as Ca concentration increased. The electrochemical measurements also show that Mg-2Zn-0.2Mn-0.38Ca has the highest self-corrosion potential, the lowest current density and a dense corrosion film, and the Rct values were 105.7 Ω cm2, 108.1 Ω cm2, 65.5 Ω cm2 and 50.51 Ω cm2. As a result, the corrosion rate is strongly associated with the volume fraction of phase, where the difference in potential at the α-Mg/phase interface forms a micro-galvanic effect and accelerates the dissolution of Mg substrate.