Titanium anodization in deep eutectic solvents: The effect of anodizing time on the morphology and structure of anodic layers

Highlights: • DES as the environment for Grade 2 titanium anodizing process. • Potentiostatic anodic formation of compact nanometric TiO₂ layer. • Time-controlled growth of TiO₂ layer thickness. • Transition from amorphous to nanocrystalline structure with increasing time. Deep eutectic solvents (DES) are successfully used as green solvents for electrochemical polishing of titanium. The tests performed showed that they can also be used for the production of nanometric TiO₂ layers on Grade 2 titanium with thickness and crystallinity controlled by the anodizing time. Anodic polarization at 60 °C in choline dihydrogencitrate salt : oxalic acid DES allows to obtain amorphous TiO₂ layer with a thickness of 5–11 nm in just a few seconds. Extending this time to 30 s causes, apart from increasing layer thickness, a distinct development of its surface. In the layer formed during the 20 min of polarization, nanosized crystalline domains begin to dominate, and the thickness of the TiO₂ layer increases to approx. 30 nm. The process accompanying titanium anodizing is the incorporation of small amounts of "organic" carbon, most likely DES residues, into the structure of TiO₂ layers.