Influence of Microstructure and CO2 on the NaCl-Induced Atmospheric Corrosion of Semi-Solid Cast and High Pressure Diecast Mgal Alloys

The NaCl induced atmospheric corrosion of MgAl alloys AM50 and AZ91 was studied in the laboratory at 95.0% relative humidity (RH) and 22^oC. The two alloys were investigated both in semi-solid cast (SSC) and in high pressure diecast (HPDC) states. Exposures were conducted in the presence of 400ppm CO₂ and in the absence of CO₂. Crystalline corrosion products were identified by X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was employed to examine the surface film formed in the initial stages of corrosion. Magnesium carbonate and magnesium oxide (oxide/hydroxide) were detected in the top and bottom of the surface film, respectively. Alloy microstructure and corrosion morphology were characterized by scanning electron microscopy (SEM). Corrosion was more localized in CO₂-free environment compared with air with 400ppm CO₂. The same alloy prepared by SSC and HPDC routes showed different corrosion properties. It is suggested that the influence of casting technology on corrosion is caused by differences in the area fraction and distribution of β-phase.