Abstract
The anodic oxidation of titanium, in the majority of electrolytes (sulphuric, phosphoric, boric acid, etc.) results in the formation of a thin homogeneous dielectric oxide film of titanium dioxide (amorphous or crystalline) . At constant current the potential increases with increasing film thickness. If the potential is then maintained constant the ionic current decreases rapidly and the current becomes almost entirely electronic. In these circumstances, providing film breakdown does not occur, film growth practically ceases and oxygen evolution becomes the predominant electrode process. In chloride electrolytes, however, film growth proceeds up to a potential of 12-14V. If the potential is increased above this value film breakdown occurs and titanium ions pass into the electrolyte and precipitate as titanium dioxide. Corrosion of the titanium is confined to a relatively few, widely dispersed, small areas of the metal surface with the consequent formation of gross pits while the rest of the surface, which is coated with the initially formed anodic oxide, remains passive. During a study of the anodic oxidation of titanium in various electrolytes it has been found that formic acid (and formates) give rise to a phenomenon which does not appear to have been reported previously. Within a certain range of temperature and concentration of formic acid, film formation (at constant current) proceeds up to a voltage of 60 V, but as the potential increases above this value very fine pits are formed uniformly over the metal surface (micropitting) and simultaneously titanium oxide is precipitated from the electrolyte. The potential slowly increases to 120 V, indicating film growth, and then remains practically constant and if anodizing is continued the pits are propagated into the metal to a considerable depth. A microscopical examination of the metal surface during the initial stages of micropitting has revealed that different areas on the metal surface have different interference colours showing that the thickness of the surface oxide is not uniform. If, however, the temperature of the electrolyte is greater than 50 C or if the concentration is in the range of 70-96 per cent by vol formic acid, micropitting does not occur and a homogeneous dielectric oxide film is formed. It should be observed that at 25 C, at a concentration of 96 per cent formic acid, film formation can proceed to 400 V without film breakdown, whereas in boric acid the maximum formation voltage is only 250 V. At very high concentrations of formic acid (96.5 per cent to concentrated), a thick dark-yellow non-adherent oxide forms on the metal surface. The phenomenon of micropitting appears to be confined to the titanium-formic acid system as similar results could not be achieved with niobium or zirconium. The mechanism of the phenomenon described is discussed and some tentative theories have been proposed to explain the behaviour of titanium during anodic polarisation in formic acid at various concentrations and temperatures.