Aluminium anodising in low acidity sulphate baths: growth mechanism and nanostructure of porous anodic films

Overall kinetic and chronopotentiometric studies were performed during Al anodising in H₂SO₄, 0–5% w/v, bath solutions pure and saturated by Al₂(SO₄)₃. Peculiarities in film growth mechanism and nanostructure in these cases appeared, like significant differences of porosity and its dependence on film thickness, different critical current density above which pitting appears, salt deposition on pitted surface regions in saturated bath, etc. The different conditions inside pores are responsible for this behaviour like almost depletion of H⁺ during a long initial transient stage in the first case, supersaturation and formation of Al₂(SO₄)₃ nanoparticle micelles on pore surface in the second case, etc. Differences in film growth mechanism also appeared between these and alike baths at higher acidity. Anodising in low acidity saturated baths shows superiority for growing low porosity films at specific conditions. New technologies may be suggested to produce optimal films of desired structure.