Corrosion behaviour of titanium anodised in DES solvents: electrochemical study combined with ellipsometric investigations

A sandwich structure of TiO 2 layer was proposed, i.e., a dense inner layer and a porous outer layer, for anodising of technical titanium in a DES solvent composed of choline dihydrogencitrate and oxalic acid in a 1:1 M ratio. The first seconds of logarithm growth of thickness from a single nm to about 10 nm formed a barrier inner layer with resistance of 28 MΩ cm 2 , low electric capacitance (15 μF cm −2 ) and not so high outer layer resistance (∼0.6 MΩ cm 2 ). It was accompanied by a distinct decrease in anodic current density in the passive range in 0.05 mol dm −3 NaCl solution. Extension of the anodising time from 10 s to 20 min, despite the fact that the layer thickness increased from about 10 to over 30 nm, did not deepen this initial trend due to the possible development of amorphous features more similar to titanium(IV) hydroxide than to titanium(IV) oxide.