Corrosion Behavior Of Calcic Magnesium Alloys And Its TiO₂ Coating In The Simulated Body Fluid

Magnesium alloys are promising for application as biodegradable metal hard tissue replacement materials because of their excellent bioactivity, biocompatibility, and particularly their absorbable degradation. However, magnesium alloys can be corroded easily in water due to their active chemical properties, especially in an environment containing chloride, which limits their application as for biodegradable implant material. In order to improve the corrosion resistance of magnesium, alloying elements such as calcium, zirconium, and yttrium were added into magnesium to prepare different components of Mg-Ca, Mg-Ca-Zr, Mg-Ca-Y alloys, and TiO2 coatings were prepared on magnesium calcium alloy in this thesis. The microstructure and corrosion behavior of the alloys and the coating were investigated through electrochemical corrosion test and immersion test, and by using optical microscope, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction.The results showed that Mg-Ca alloys were composed of Mg phase and Mg2Ca phase. With the increase of calcium content, the grains became small and the hardness of the magnesium alloys increased. Among all Mg-Ca alloys, Mg-1.0Ca alloy exhibited the best corrosion resistance when soaked in simulated body fluid. Mg-Ca-Zr alloys were mainly composed of Mg phase and Mg2Ca phase with a small amount of Zr phase. As for the same content of zirconium, the grain of Mg-Ca-Zr alloys refined with the increase of calcium content. While as for the same calcium content with a relatively low one, the alloying elements zirconium could also refine grain. The hardness of all the Mg-Ca-Zr alloys was around HV40. The corrosion resistance of Mg-1.0Ca-1.0Zr alloy was the best one among all Mg-Ca-Zr alloys in this study. The grain size order of Mg-1.0Ca, Mg-1.0Ca-1.0Y, and Mg-1.0Ca-1.0Zr alloys was Mg-1.0Ca-1.0Zr< Mg-1.0Ca-1.0Y< Mg-1.0Ca. While the corrosion resistance to SBF was Mg-1.0Ca< Mg-1.0Ca-1.0Y< Mg-1.0Ca-1.0Zr, indicating that alloying elements Zr and Y can improve the corrosion resistance of Mg-Ca alloy and adding Zr is better than adding Y in improving the corrosion resistance of Mg-Ca alloys.The TiO2 coatings were mainly anatases and rutiles after heat treatment at 500℃, and some hydroxyapatite was also detected after soaked in the SBF. The corrosion current density of the TiO2 coating was 2 orders of magnitude lower than that of Mg-1.0Ca alloy. Mg-1.0Ca alloy substrate had been corrupted after 48h immersion in SBF, but the Mg-1.0Ca alloy with TiO2 coating immersed in SBF solution was corrupted after 336h. The results indicated that TiO2 coating can improve the corrosion resistance and biological activity of Mg-1.0Ca alloy greatly.