Initial corrosion of AZ91D magnesium alloy in atmosphere containing SO_2

The initial surface morphology of corrosion layer, corrosion products and structure of rust layer for AZ91D magnesium alloy in SO_2 industrial polluted atmosphere were analyzed by metallographic observation, SEM and XRD. The results show that corrosion rate increases with increasing SO_2 content in the atmosphere. The initial attack has the characteristics of localized corrosion. Corrosion starts from α phase, and MgO and Mg(OH)_2 form at first, which provide a protective layer. SO_2 decreases pH in the electrolyte layer, accelerates dissolution process, and promotes the formation of MgSO_3·6H_2O and MgSO_4·6H_2O. These soluble corrosion products result in the loss of protection function and the acceleration of corrosion. Meanwhile, cracks appear with continuation of corrosion, which provide the paths for oxygen and other corrosion pollutants filtering into matrix, further resulting in severe localized corrosion.