Characterisation of anodic oxide films formed on titanium and two ternary titanium alloys in hydrochloric acid solutions

The purpose of this paper is to characterise the anodic oxide films formed on titanium and two ternary titanium alloys, Ti-15Mo-5A1 and Ti-10Mo-10A1 i n 1%, 10% and 20% hydrochloric acid solutions at 25, 50, and 75 degrees C. The anodic film on titanium in hydrochloric acid is stable between + 0.6 V (SCE) and + 2.0 V (SCE). For our new ternary titanium alloys, the passive f ilm is formed at about + 0.6 V (SCE) and is stable to + 2.0 V (SCE). The an odic polarization curves for alloys differ from the base metal curve, prese nting two peaks for the critical passivation current density in the active- passive potential range. At the first current peak (the first critical pass ivation potential E-cr1) a porous titanium pentaoxide (Ti3O5) is formed. Wh en the potential reaches the second current peak (the second critical passi vation potential E-cr2) the compact and protective titanium dioxide (TiO2) is formed. The impedance spectra exhibit the typical behaviour for a passiv e film i.e. a near capacitive response illustrated by a phase angle close t o - 90 degrees over a wide frequency range. The oxide film on titanium and its alloys in hydrochloric acid solutions exhibits a high resistance and a low capacitance (with the increase of the potential) attributed to the surf ace roughness decrease as the oxide layer thickens.