Studies on the influence of chloride ion and pH on the corrosion and electrochemical behaviour of AZ91D magnesium alloy

The influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of die-cast and ingot-cast AZ91D alloy have been studied with a focus on the stability of microconstituents in these environments. The experimental techniques used include immersion studies, potentiodynamic polarization, X-ray diffraction and optical and scanning electron microscopy. The corrosion rate for the ingot and die-cast was very high in highly acidic solutions (pH 1-2) as compared to that in neutral and highly alkaline solutions (pH 4.5-12.0), and the rate increased with chloride ion concentration at all pH levels. In general, the die-cast showed a lower corrosion rate at all pH values and chloride ion concentrations. The open circuit corrosion potential shifted to more negative (more active) values with increase in concentration of chloride ions. Corrosion morphologies revealed more attack on primary alpha and eutectic alpha with increasing chloride concentration. In highly acidic conditions, corrosion attack was found on beta (Mg17Al12) and eutectic alpha phase (alpha regions with higher Al content) while at pH 12.0 the ingot exhibited a pitting type of morphology. The corrosion product consisted of magnesium hydroxide, fallen beta particles and magnesium-aluminium oxide; the amount of each component was found to be a function of chloride ion concentration and pH.