Formation process of bright spots on surfaces of die-cast magnesium alloy components after chemical conversion treatment

A component for a mobile phone, made of AZ91D alloy, is manufactured by die cast. After chemical conversion treatment, it was found that there were some bright spots on the component surface. The formation mechanism of the bright spots was studied by means of SEM and metallgraphic microscope. The corrosion resistance of coatings was measured by potentiodynamic polarization test. The reason of bright spots formation is that the coarse primary α is dissolved into a dent during acid activation process because of little netted β and little aluminum. Besides, the coatings on bright spots are thin, the dimensions of primary α grains are large, and the surfaces are smooth relatively, so they are prone to reflecting light and become bright spots.