CORROSION BEHAVIOR OF AZ91D MAGNESIUM ALLOY IN CITY ATMOSPHERE

The surface morphology of corrosion layer, corrosion products and structure of rust layer for AZ91D magnesium alloy in natural city atmosphere were studied by SEM and XRD. The results show that Mg(OH)2 layer forms at the initial corrosion period, then film thickens and cracks appear with continued corrosion under cyclic wet/dry conditions, in the end these cracks develop to reticulate structure. The water vapour is easily condensed and the corrosives gases, salt particles are easily absorbed in the cracks. Meanwhile, the cracks provide the diffusion paths for oxygen and other corrosion pollutants into matrix, which results in severe local corrosion. The major corrosion products are Mg(OH)2, Al(OH)3, Mg2CO3(OH)2·3H2O and Mg2(OH)3Cl·4H2O, which plays a partial role in protecting the matrix, therefore slowing down the average corrosion rate of magnesium alloy in the latter periods.