Corrosion behavior of biodegradable Mg–1%Zn–0.2%Ca alloy with ultrafine-grained structure

Abstract

Objectives. The aim of this work is to investigate the corrosion resistance of ultrafine-grained magnesium alloy Mg–1%Zn–0.2%Ca processed by severe plastic deformation (SPD) methods, namely equal channel angular pressing combined with extrusion (ECAP-Ex) and circular simple shear extrusion (CSSE). In order to use such materials in the development of biocompatible implants, it becomes necessary to control their degradation rate. Methods. To form an ultrafine-grained (UFG) structure, equal-channel angular pressing combined with extrusion (ECAP+Ex) and circular shear extrusion (CSSE) methods were used. Structural studies were conducted using X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). In order to confirm the reliability of this material for use in biodegradable medical implants, a gravimetric corrosion resistance study was carried out at 36.6°C over a long period (month). Results. The use of ECAP-Ex processing led to the formation of an ultrafine-grained (UFG) structure in the alloy having a grain size of about 1 μm and the appearance of nanotwins, as well as the formation of highly dispersed Ca 2 Mg 6 Zn 3 nanoparticles 10 nm in a size with a volume fraction of 1%. The corrosion rate of the UFG samples was 3.8 mm/year. Following additional annealing at 175°C, the UFG alloy demonstrated a decrease in the corrosion rate to 0.9 mm/year as compared with coarse grained homogenized sample (4.8 mm/year). By way of comparison, the CSSE processing resulted in grain size reduction to 1 μm and a corrosion rate of 1.3 mm/year. Conclusions. The study demonstrates that combining equal-channel angular pressing ECAP+ Ex and subsequent annealing at 175°C is an effective method for improving corrosion properties of the Mg–1%Zn–0.2%Ca alloy. By reducing the corrosion rate to 0.9 mm/year, the alloy becomes promising for medical applications such as biodegradable implants.