Effect of heat treatment on the stress corrosion cracking behavior of cast Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy in a 3.5 wt% NaCl salt spray environment

Abstract

This study was undertaken to investigate the influence of heat treatment on the stress corrosion cracking (SCC) behavior of a low-pressure sand-cast Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy in a 3.5 wt% NaCl spray environment. Results indicated that T4 heat treatment (540 °C × 10 h) could effectively improve the corrosion resistance, compared with as-cast and T6 alloys. The relatively lowest volume fraction of high-potential (HPOT) phases observed in the T4 alloy primarily accounts for its lowest corrosion rate. Slow strain rate tensile (SSRT) tests were conducted to evaluate the influence of heat treatment states on the SCC resistance. The lowest SCC susceptibility index ( I SCC I = 0.403 and I SCC II = 0.165) was obtained in T4 state, while the as-cast alloy possessed the highest SCC susceptibility. The present work revealed that hydrogen-assisted cracking (HAC) played a key role in the SCC process. Cracking was effectively suppressed in the T4 alloy owing to its retarded hydrogen diffusion. The structure and compositions of the multi-layer corrosion product film were characterized in detail. Solution treatment applied in this work was confirmed to promote the formation of protective and dense RE oxides layer, primarily contributed to the relatively highest SCC resistance achieved in the T4 alloy. • SCC susceptibility decreases in the order of as-cast > T6 > T4. • HPOT phase dissolution and hydrogen-diffusion suppression strongly improve the SCC resistance. • The structure of multi-layer corrosion product film is established. • Protective RE-oxides layer is characterized by SIMS.