Study On Corrosion Of Mg-Li-Ca Alloys And Their Coatings

Metals as the most popular implant biomaterials played an important role in repairing and replacing bones in recent decades. However, metallic biomaterials, including stainless steels, titanium alloys and nickel-cobalt alloys used have currently some weakness in their bio-compatibility and bio-mechanical compatibility. Magnesium alloy are potential implant materials due to their excellent biocompatibility, good mechanical property and biodegradable. Unfortunately, the high susceptibility of Mg alloys to corrosion and thus too rapid degradation rate in human body limit their applications in the potentially clinical practice. Therefore, the study has been focused on corrosion and protection of magnesium alloys in simulated body fluids.Newly developed Mg-Ca, Mg-Li-Ca and Mg-Li-Ca-Y alloys had been extrued. The corrosion behavior of the alloys with and without sliane film and Ca-P film in Hank's solution had been investigated by immersion tests, hydrogen evolution, and potentiodynamic electrochemical tests. The microstructure before and after the extrusion deformation and the effect of calcium, lithium and yttrium on the corrosion behavior of the alloys were studied. And the preparation and the corrosion resistance of silane film on Mg-Ca alloys and Ca-P coating on Mg-Li-Ca alloy were also investigated. The microstructure, corrosion morphology and coating surface was observed by means of scanning electron microscope (SEM). Meanwhile X-ray diffraction analysis (XRD) and Energy dispersive spectrum (EDS) were employed to characterize chemical makeup of coatings and corrosion products.The results showed that the grain size of magnesium alloy was decreased substantially with the dynamic recrystallization and the second phases were dispersed during the process of extrusion deformation, and refined with an increase in calcium content. The corrosion resistance of magnesium alloys was improved significantly by the extrusion process. Alloying element calcium can refine grains, and enhance the corrosion resistance of magnesium alloys. However, when the content of calcium exceeded a certain range the micro-galvanic corrosion would be promoted due to the formation of more intermetallic compounds Mg2Ca. The corrosion resistance of Mg-Li-Ca alloy was not more pronounced in the early immersion period, in comparison with Mg-Ca alloys. Note that with increasing immersion time corrosion products led to a rapid alkalization and reduction in the corrosion rate of magnesium alloys in Hank's solution. The addition of yttrium into Mg-Li-Ca alloy hinder hydrogen evolution on cathode, thus improved the corrosion resistance of the alloy.The silane film increased the corrosion potential and corrosion resistance of the substrates, and reduced the difference of corrosion resistance among three Mg-Ca alloys. A Ca-P coating was prepared by means of electric-deposition on the Mg-Li-Ca alloy. The average Ca/P atomic ratio ranges between 0.70-0.79. The coating exhibited a plate-like morphology and boosted the corrosion resistance of the Mg-Li-Ca alloy.