Morphology evolution of porous anodic alumina in mixed H3PO4/NH4F electrolytes

Most anodizations of Al to obtain porous anodic alumina (PAA) are conducted in acid solution including sulfuric acid, oxalic acid and phosphoric acid solutions. It is well known that fluoride-containing electrolyte is generally used to fabricate anodic TiO2 nanotubes (ATNTs). Nevertheless, there are few articles reporting anodization processes of Al in NH4F electrolyte and mixed H3PO4/NH4F/ethylene glycol electrolyte. Herein, in order to explore different morphology of anodic alumina film during a series of constant current anodization processes, NH4F solutions, H3PO4 solutions of different concentrations (2 wt%, 4 wt% and 6 wt%), and their mixed electrolytes containing different concentrations of NH4F and H3PO4 were employed. It is the first time for Al to be performed anodization in NH4F solution under constant current. PAA with thick pore wall, thick barrier oxide layer and narrow pores were obtained. The results show that three stages of voltage-time curve are not the requirement of obtaining PAA. Which anodic oxide (porous anodic oxide or compact anodic oxide) occurs only depends on the size of the electronic current. As long as the electronic current is high enough, porous anodic oxide is easily obtained. While the concentration of NH4F increases to 0.6 wt%, the barrier oxide is much thicker than that in H3PO4 electrolyte due to the high ionic current and low electronic current.