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 TA 6V (Ti-6% Al-4% V) alloy were investigated. Both voltametric (potentio-dyna mic) and chrono-amperometric (constant voltage, either applied directly or progressively increased during the first steps of the treatment) experiment s are conducted. The electrolyte is a chromic acid (CA) (0.5 mol l(-1) Cr2O 3) 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 dup lex 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 aro und 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 m easurements reveal a complex growth process of the porous film, in which th e residual current participates in the thickening of both compact and porou s layers of the film. The overall electrochemical efficiency is small and d ecreases with the treatment time. A growth mechanism involving a poisoning (by Cr-VI-containing ions) and antidote (by F-containing species) competiti on is proposed. (C) 1999 Elsevier Science Ltd. All rights reserved.