Effect of equal channel angular pressing as the pretreatment on microstructure and corrosion behavior of micro-arc oxidation (MAO) composite coating on biodegradable Mg-Zn-Ca alloy

Abstract

To improve the mechanical properties and control the long-term corrosion properties of biodegradable Mg-4.71Zn-0.6Ca (wt%) alloy, a composite coating was fabricated on the Mg-4.71Zn-0.6Ca alloy by using micro-arc oxidation (MAO) under the pretreatment of equal channel angular pressing (ECAP). Experimental results showed that ECAP treatment refined the microstructure of Mg-4.71Zn-0.6Ca alloy with an average grain size of 1.6 μm, and made the mechanical properties improve from 206 MPa to 290 MPa. The phase composition of MAO coatings includes the α-Mg matrix phase, Mg2SiO4, MgO, and HA, and the ECAP/MAO coating is denser with a smaller pore size than the as-cast/MAO coating. The electrochemical and immersion test results indicated that the corrosion resistance of the ECAPed sample is better than that of the as-cast sample, and the MAO processing can significantly increase the corrosion resistance of both as-cast and ECAPed samples. The corrosion rate of ECAP/MAO treated Mg-4.71Zn-0.6Ca alloy is the lowest, and the ECAP pre-treatment improves the long-term corrosion properties of the MAOed Mg alloy. Besides, the bioactivity of Mg-4.71Zn-0.6Ca alloy was enhanced by the hybrid ECAP/MAO treatment as well.