Enhanced corrosion behavior and SCC resistance of a cast Mg–0.4Zr alloy after hot extrusion

The impact of hot extrusion on the stress corrosion cracking (SCC) resistance and corrosion behavior of an as-cast Mg–0.4Zr alloy was investigated. Microstructural analysis revealed that extrusion induced dynamic recrystallization (DRX), resulting in a substantial decrease in grain size from 64.5 μm in the as-cast state to 8.3 μm. To assess the SCC resistance, slow strain rate shear (SSRS) testing was employed at ambient temperature with a strain rate of 7.1×10 –5 s –1 , both in air and within phosphate buffered saline (PBS) solution. The respective ultimate shear strength (USS) values ​​recorded for the as-cast and extruded samples were 63.9 MPa and 71.6 MPa in air, which decreased to 61.1 MPa and 67.9 MPa, in the corrosive solution. Results from hydrogen evolution and electrochemical corrosion tests showed that the extruded condition provided higher resistance to corrosive solution penetration, due to a finer and more uniform grain structure, as well as the smaller size and more uniform dispersion of the Zr-rich particles. Consequently, the corrosion rate was reduced from 7.3 mm/year in the as-cast sample to 5.4 mm/year in the extruded sample. The SSRS test is a reliable approach to analyze the SCC of Mg alloys of small dimensions. The results obtained by this test were corroborated by results from immersion and electrochemical corrosion tests.