Voltage transients and morphology of AlSi(Cu) anodic oxide layers formed in H2SO4 at low temperature

Anodic oxidation of three different aluminum substrates (i.e. Al, AlSi10 and AlSi10Cu3) in 2.25 M H2SO4, at 0 °C, for 50 min and using three different current densities (i.e. 3.0, 4.2 and 6.0 A dm−2) was performed in this study. The aim was to examine the growth of thick oxide layers over the macroscopic surface bearing large (1–20 μm) second phases with different oxidation rates relative to aluminum. The results revealed different voltage transients for the three different compositions during the stages of barrier layer growth and steady-state pore growth. In addition, the morphology of the oxide layers, as determined by optical microscopy, scanning electron microscopy (cross-section) and laser scanning confocal microscopy (surface) indicated: (i) thickening of the oxide layers with current density and time regardless of the substrate composition; (ii) entrapment of silicon in the growing layers (for the binary and ternary substrates), associated with a non-uniform thickness, a scalloped substrate/oxide interface and surface cracks; and (iii) appearance of flaws like features from the commencement of anodizing (AlSi10Cu3) and after a certain anodizing time (AlSi10) associated with enhanced oxygen evolution and (partial) oxidation of second phases.