Anodic oxidation of titanium and TA6V alloy in chromic media. An electrochemical approach

In order to optimise anodisation surface treatment of titanium alloys, the electrochemical conditions of anodic oxidation of pure titanium and of a TA6V (Ti-6% Al-4% V) alloy were investigated. Both voltametric (potentio-dynamic) and chrono-amperometric (constant voltage, either applied directly or progressively increased during the first steps of the treatment) experiments are conducted. The electrolyte is a chromic acid (CA) (0.5 mol l(-1) Cr2O3) solution with and without hydrofluoric acid (HF) addition (9.5 x 10(-2) mol l(-1)). A thin oxide compact film is formed in CA electrolyte and a duplex film composed of a compact layer surmounted by a columnar porous layer grows from the fluorinated electrolyte. The voltametric results indicate a breakdown of the compact film (in non-fluorinated medium) for potential around 3 V/SCE, and a strong influence of the alloying elements of TA6V on the formation of porous films in fluorinated medium. The chrono-amperometric measurements reveal a complex growth process of the porous film, in which the residual current participates in the thickening of both compact and porous layers of the film. The overall electrochemical efficiency is small and decreases with the treatment time. A growth mechanism involving a poisoning (by Cr-VI-containing ions) and antidote (by F-containing species) competition is proposed. (C) 1999 Elsevier Science Ltd. All rights reserved.