Effect of ECAP on corrosion behavior of innovative nano MgO/Mg-Zn-Ca composite as a biomedical material in simulated body environment

Abstract

A novel 0.6 wt% nano MgO/Mg-3Zn-0.2Ca composite was designed as medical implanting material. Equal channel angular pressing (ECAP) was adopted to enhance this novel metal matrix composite. The influence of the processing strategy on the corrosion resistance of this novel composite was studied and discussed in this project. The processing strategies used in this study were 300 ℃× 8-pass and 280 ℃× 4-pass+ 220 ℃× 4-pass, respectively. ECAP processing significantly homogenized the microstructure of the composite since the mechanical shear of ECAP dispersed the secondary phase and reduced its size. Besides, the ECAP processing helped to reduce the texture of the material. The corrosion resistance of the samples was tested in simulated body fluid (SBF) at 37 ℃. Compared with the original (as-extruded) sample, both ECAP-ed samples exhibited a lower corrosion current density (Icorr) and higher self-corrosion potential (Ecorr), because of their optimized microstructure. At the beginning stage, the corrosion product Mg(OH)2 thin film could protect the material. Then, more dense calcium phosphorus compound film will reduce the corrosion rate. With the analysis of the pH value, weight loss, and microstructure after immersing in SBF, the sample processed by ECAP under 300 ℃× 8-pass showed the best corrosion resistance. The reason is that reducing-temperature ECAP causes more concentrated stress and dislocation accumulation, which exacerbated corrosion behavior. The ECAP at constant temperature was beneficial to relieve these high-energy defects and led to the improvement of corrosion resistance. This study provides new insight into improving the corrosion resistance of human implants with low-cost strategy.