dear Ali Mohammadnejad ;

The colors of the anodic film on Ti when anodized in sulfuric acid is changed according to the coating layer when the thickness increased by increasing the anodizing voltage 10 V to 15 V.for more details can read;

Article Pure Commercial Titanium Color Anodizing and Corrosion Resistance

Ali,

i think the the reason is that the Ti on the surface gets oxidized quicker in higher voltage. And the color change, I assume different shades of purple, is because of the oxide layer thickness formed on the surface. This happened to me when I was working on NiTi, as well.

Dear Dr. Ali Mohammadnejad,

the nature of electrolyte and the oxide layer can be used to confer to the Ti based alloys surface a colored appearance. In fact when titanium is suspended in an electrolytic bath as an anode and a current is passed through the bath, oxygen is produced at the anode surface. This oxygen combines with the titanium to form titanium oxide. The interference taking place between the oxide and light results in the appearance of colors on the substrate surface and oxides with different thickness (or different chemical composition or morphology specifically generated) can produce different colors.

For more details, please see the following interesting papers, both available on ResearchGate:

- Colouring Titanium Alloys by Anodic Oxidation

G. Napoli, M. Paura, T. Vela, A. Di Schino

Metalurgija -Sisak then Zagreb- 57(1):111 (2018)

- Corrosion Resistance and Color Properties of Anodized Ti-6Al-4V

Ali Karambakhsh, Abdollah Afshar, Pejman Malekinejad

Journal of Materials Engineering and Performance 21(1):121-127 (2011)

Best regards, Pierluigi Traverso