The effect of microstructure and temperature on the atmospheric corrosion of Mg-Al alloys
The NaCl-induced atmospheric corrosion behavior of the Mg-Al alloy AM50 produced by rheo-casting (RC) and high pressure die casting (HPDC) techniques was studied in the laboratory. The exposures were performed at 95% RH in the absence and presence of 400 ppm CO2 at 22, 4 and -4ºC. In parallel, 99.97%Mg was investigated as reference. The synergistic effects of microstructure, exposure temperature, CO2, amount of added NaCl and exposure time on the corrosion rate and corrosion products were systematically investigated gravimetrically and by means of OM, SEM/EDX, FIB/SEM, BIB, Tof-SIMS, XRD and FTIR. It is shown that the NaCl-induced atmospheric corrosion resistance of the RC material is consistently better than that of the HPDC material. The better corrosion resistance is attributed to differences in the as-cast microstructures. The RC material contains a higher fraction of intermetallic β phase with more continuous and network-like morphology than the HPDC AM50. The results provide new insights into the effect of microstructure on the atmospheric corrosion behavior of Mg-Al alloys. There is a strong positive correlation between exposure temperature and the rate of atmospheric corrosion. Thus, a decrease in temperature from 22 to -4ºC resulted in considerably slower corrosion both for the RC and the HPDC materials. The influence of temperature is suggested to be mainly related to the Al content. A corrosion product with an unknown phase (by XRD) and a non-crystalline corrosion product (by FTIR) were detected at sub-zero temperature.