Pore-size dependence of AAO films on surface roughness of Al-1050 sheets controlled by electropolishing coupled with fractional factorial design

The influences of electropolishing variables, such as agitation rate, solution temperature, applied potential, electropolishing time, and the volume percentage of perchloric acid in ethanol–perchloric acid solutions, on the surface roughness of the industrially pure aluminum sheets (Al-1050, 99.5%) are systematically investigated using fractional factorial design (FFD). The solution temperature, applied potential, and electropolishing time are found to be the key factors affecting the surface roughness of Al-1050 sheets meanwhile the volume percentage of perchloric acid (or ethanol) exhibits complicated interaction effects with these key factors. The average surface roughness of aluminum sheets ranging from 3 to 30 nm (over a 5 × 5 μm scan area) can be controlled and predicted by varying the applied potential and polishing time when electropolishing is performed at 4 °C (i.e., the low level of solution temperature). Highly uniform, self-ordered anodic aluminum oxide (AAO) can be effectively formed from these polished sheets via an anodizing program in an aqueous solution mainly consisting of sulfuric and oxalic acids. The pore size of AAO is gradually decreased from ca. 90 to 60 nm when the surface roughness of Al sheets is increased from 3 to 30 nm.