Narrow Results

The effect of microconstituents on the corrosion and electrochemical behaviour of AZ91D alloy prepared by die-casting and ingot casting route has been investigated. The studies were carried out in 3.5% NaCl solution at pH 7.25 using constant immersion technique, potentiodynamic polarisation experiments and surface topographic analysis. Microstructural areas with aluminium concentration less than about 8% (primary α phase and surroundings) were found to be more prone to corrosion attack compared with either those with higher amount of aluminium (eutectic α phase and its surroundings) or β phase. Die-cast material with smaller grain size and fine β phase offered marginally higher corrosion resistance and better passivation compared with the ingot.

The influence of chloride ion concentration and pH on the corrosion and electrochemical behaviour of die-cast and ingot cast AZ91D alloy has been investigated. The experimental techniques used include constant immersion test, potentiodynamic polarisation and scanning electron microscopy. Die-cast and ingot showed a linear increase in corrosion rate and I_corr with increase in chloride ion concentration with a change in slope for ingot above 5% NaCl. Open circuit corrosion potential shifted to more negative (more active) values as chloride ion concentration increases. The corrosion rate and I_corr are very high in highly acidic conditions (1-2.0 pH) and tend to decrease as pH increases. A synergism has been observed between the effect of chloride ion concentration, pH and microstructure of the material. Observation of the corrosion morphologies revealed preferential attack on β (Mg₁₇Al₁₂) and adjacent α regions (α regions with higher Al content) in highly acidic and higher chloride ion concentrations.