Uniform coating of a crystalline TiO2 film onto steel plates by electrochemical deposition using staged pulse current

A crystalline TiO₂ (c-TiO₂) film was electrochemically deposited onto a 10 cm × 20 cm hot-dip-galvanized (HDG) steel plate at 60 °C from an alkaline aqueous solution containing 0.1 M titanium potassium oxalate dehydrate and 1 M hydroxylamine. The electrochemical deposition was carried out by a galvanostatic method. First, a current density of 10 mA cm⁻² was applied for 5 min, which led to the formation of a uniform coating of TiO₂ on a 1 cm × 1 cm small HDG plate. A crystalline layer was observed, however, only in the central area, whereas the upper and the edge areas were amorphous. Both calculations and experiments confirmed that this was due to the difference of the local current densities in the vicinities of different areas. Next, three different currents (5 mA cm⁻² (2 min), 10 mA cm⁻² (2 min) and 20 mA cm⁻² (1 min)) were applied continuously so that the local current density for each part of the substrate achieved appropriate deposition conditions. The film thus obtained was crystalline in all areas and of uniform thickness.