Influence of substrate microstructure on the growth of anodic oxide layers

The effects of permanent mold cast microstructure on the growth of anodic oxide layers on three different aluminum substrates (i.e. Al99.8, AlSi10, and AlSi10Cu3, wt.%) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), and laser scanning confocal microscopy (LSCM). The anodic oxidation was performed galvanostatically in 2.25M H2SO4, at 0°C. The oxide layers developed a microscale topography mainly determined by the morphology of aluminum grains and cells. A low amount of insoluble impurities, uniformly distributed, would contribute to the growth of oxide layers with minimum defects and uniform thickness on the pure aluminum substrate whereas for the binary and ternary systems, a fine cell structure and a modified morphology of Si particles would be favorable. The Al–Fe and Al–Fe–Si particles were occluded in the oxide layers next to Si particles, blocking locally the oxide growth whereas Al2Cu particles were preferentially oxidized. In addition, the presence of Si particles in the layer influenced pore morphology by development of deflected pores around the particles.