ANODIC BEHAVIOUR OF TITANIUM IN HCL-METHANOL SOLUTIONS

The anodic behaviour of titanium has been studied in 0·1-11 ·0 M HCl-methanol solutions. At cds up to 1 A/cm2 dissolution occurs with 100 per cent efficiency based on the reaction Ti → Ti3+ + 3e. Anodic disintegration occurs at these cds, the disintegration product being particles of α-titanium of similar size to that of the metal grains in the anode. The amount of disintegration product formed on the passage of constant electrical charge increases proportionately with the reciprocal of the cd and this relationship has been derived theoretically. The presence of hydride films on dissolving titanium (TiH1·9) has been confirmed by X-ray diffraction. Disintegration is accompanied by surface pitting. In the presence of > 1 per cent H2O the amount of disintegration decreases until at > 10 per cent H2O disintegration does not occur and the features of the surface are similar to those in HCl(aq) solutions. When anodically polarized in 0·1-11.0 M HCl-methanol solutions titanium dissolves at cds of up to 50-100 mA/cm2 under activation control and Tafel behaviour is observed. The Tafel slope is 0·115 ± 0·004 V at 25°C and is independent of HCl concentration. The corrosion cds (0·22-0.34 mA/cm2) are also independent of HCl concentration. At cds > 50-100 mA/cm2 a limiting cd is observed, although this is only clearly defined in 11·0 M solutions. The corrosion rate of titanium as determined from immersion tests is in agreement with that deduced from kinetic studies. The mechanism of dissolution of titanium is discussed and it is concluded that the over-all dissolution reaction is of the type. [TiClads]- → [TiClads]2+ + 3e. Solvation of such species with methanol is also possible but there is as yet insufficient evidence to decide upon the individual rate-controlling step. © 1968.