Study On Microstructure And Corrosive Properties Of Biomaterial Of Mg-Zn-Ca-Mn Alloys

For poor corrosion resistance of medical magnesium alloy and the complexityof human environment, the paper has selected good biocompatibility of non-toxicmetallic elements, Ca, Zn and Mn, as alloying elements. The quaternaryMg_100-x-y-z-Zn_x-Ca_y-Mn_zalloy were prepared by vacuum induction melting andmetal mold casting method. The effects of different relative content of Zn, Ca andMn and heat treatment processes on microstructure and corrosion resistance ofmagnesium alloy is studied by this work. The phase and microstructure ofmagnesium alloy is characterized by X-ray diffraction (XRD) and opticalmicroscopy (OM).The corrosion morphology of the corrosion process of magnesiumalloy specimens is analysised by using scanning electron microscopy (SEM) andenergy dispersive spectroscopy (EDS). The corrosion mechanism ofMg_100-x-y-z-Zn_x-Ca_y-Mn_zalloy immersed in the Hank's simulated body fluid isdiscussed by weight loss and electrochemical tests.The results show a typical localized corrosion of pure magnesium in the PBSsolution and Hank's simulated body fluid, which is due to the non-selective of grainboundary crystal of pure magnesium.The relative content of Mn is less than1.5%, the organization of the castmagnesium alloy can be refined. The solution heat treatment could have eliminatedthe micro-segregation in the cast structure, and the growth of the final grain shape issimilar to hexagonal. After aging treatment, the Mg-Zn solid solution and Mg-Mnsolid solution, which are uniformly distributed and have the high electrode potential,can improve the electrode potential of the alloy matrix. Meanwhile, the cathodephase Mg2Ca that the volume fraction had been reduced sharply could improve thecorrosion resistance of magnesium alloys.The cast alloy has localized corrosion seriously. The formation of corrosive stepin the corrosion region and corrosion morphology of the "river-like" and"skeleton-like" can be observed, while the corrosion performance of the heat treatedalloy is relatively uniform.In hank's simulated body fluid, the corrosion of magnesium alloy taken placewith water molecules mainly, as well as a large number of anionic. As the pH valueincreased, resulting in the hydrogen electrode potential becomes negative, thus, ithindered the process of hydrogen depolarization. Ultimately it delayed the corrosionof metals. We also found the concentration polarization of Mg²⁺of film's internal and external, which can accelerate the alloy corrosion rate in the solution and agingalloy's corrosive process. At the same time, the selective adsorption of Cl^-thathappened in the film surface activated pit corrosion.Through the comparative analysis of the average corrosion rate andelectrochemical corrosion rate, the results show that the solution alloy(Mg-2.0Zn-0.3Ca-1.3Mn) has the finest corrosion resistance, and the averagecorrosion rate is0.125mm/a. Furthermore in the process of soaking, the corrosioncurrent density of alloys has no fluctuations, that is to say, the electrochemicalcorrosion rate does not have a mutation. This shows that the degradation of the alloyhas an obvious predictability. For the reason that this material is extremely suitableas substrate materials of biodegradable magnesium alloy stents.