Corrosion behaviour of die-cast AZ91D magnesium alloys in sodium sulphate solutions with different pH values

The corrosion behaviours of die-cast AZ91D magnesium alloys were investigated in 0.1 mol/L sodium sulphate (Na2SO4) solutions with different pH values. The corrosion rates, morphologies, and compositions of the corrosion products were studied by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometry (XRD). The results indicate that the order of corrosion rates in Na2SO4 solutions with various pH values is pH 2>pH 4>pH 7>pH 9>pH 12. The corrosion rates in acidic solutions are higher than those in alkaline solutions, and the corrosion products are mainly magnesium hydroxide (Mg(OH)2) and hydrated sulphate pickeringite (MgAl2(SO4)4·22H2O). The results also indicate that the solution pH can influence the corrosion rate and morphology of corrosion products. Chloride ions and sulphate ions have different pitting initiation time.