Effects of inorganic ions, organic particles, blood cells, and cyclic loading on in vitro corrosion of Mg[sbnd]Al alloys

Recently, magnesium (Mg) alloys have attracted extensive attention as biodegradable implant materials. However, cyclic loading and the corrosive environment of the body are significant challenges for the practical use of alloys, and there are few studies on this topic. In this study, we conducted a four-point bending fatigue test for 86,400 cycles (12 h) in simulated body fluid (SBF), plasma, and whole blood with an AZ series alloy Mg-9Al-0.5Zn-0.27Mn-0.12Ag, to examine the effects of inorganic ions, organic particles, blood cells, and cyclic loading on Mg alloy corrosion. The Mg²⁺ concentration and solution pH were measured before and after experimentation, and the sample surfaces were characterized by 3D digital microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Our results showed that in the non-loading condition, a porous and weak inorganic product layer (mainly Mg/Ca phosphate and carbonate) formed on the surface of the Mg alloy sample immersed in SBF, which hardly had a protective effect on Mg alloy corrosion. For the samples immersed in plasma, the organic particles promoted the formation of an organic and more compact product layer, which protected the Mg alloy from severe corrosion. For the sample immersed in whole blood, the blood cells affected organic particle deposition on the product layer and thus interfered with the formation of an organic compact product layer, which slightly accelerated the corrosion process. Furthermore, cyclic loading damaged the layer integrity and significantly increased the corrosion rates of all the studied materials compared to the samples not subjected to cyclic loading. Nonetheless, under cyclic loading, blood cells adsorbed on the Mg alloy surfaces, and formed films, which protected the Mg alloy substrate and delayed Mg alloy corrosion.