Microstructure and Corrosion Properties of Fine-Grained Mg-Based Alloys

Abstract

The influence of various processing routes (casting, extrusion, ECAP) on microstructure evolution and electrochemical properties of the magnesium alloy AZ80 were investigated. Both extrusion and ECAP were found to result in significant grain refinement (by a factor of 100 in the ECAP alloy) of the as cast AZ80 alloy. The electrochemical impedance spectroscopy was used to characterize the electrochemical properties of the surface of different microstructure states of the alloy. After the exposure to the corrosion environment containig aggressive chloride ions (0.1M NaCl) a complex polarization layer consisting of two different layers was formed on the specimen surface. The first compact layer directly adjoined to the specimen surface while the second porous layer was formed on the first layer. Microstructure changes due to extrusion and equal channel angular pressing were found to influence the stability of these layers. In the as cast alloy the compact layer was broken already during the first 24 hours of the exposure, while the compact layer in the extruded specimen remained unchanged up to 72 hours of the exposure. In the ECAP specimen both layers remained stable even after 96 hours of the exposure and no degradation occurred. Enhanced electrochemical properties were therefore found in the ultrafine-grained ECAP specimen.