Properties of colored oxide films formed electrochemically on titanium in green electrolytes under ultrasonic stirring

Titanium anodization was successfully carried out in green electrolytes such as acetic acid and sodium bicarbonate solutions, yielding uniformly colored oxide films as passive coatings. Oxide formation was carried out galvanostatically at 9.7 mA cm −2 up to various cell potential values (in the 20–140 V range), in ultrasonic-stirred electrolyte at room temperature. The anodization rate values varied in the range of 1.8–2.0 nm V −1 for sodium bicarbonate solutions and 2.3–2.7 nm V −1 for acetic acid solutions. Micro-Raman spectroscopy allowed the identification of the anatase crystalline phase in the oxides grown up to the highest potentials; under these conditions, the micrographs obtained by scanning electron microscopy revealed oxides with rough and porous surfaces. A variety of colors were obtained for the titanium oxide films (yellow, blue, brown, purple, pink, and green, and different tones of each of them), depending on the final formation potential and the electrolyte nature and concentration; chromatic coordinates measurements exactly defined the color of the films.