Pitting Corrosion of Titanium Depending on the Properties of the Passive Film

Pitting corrosion behavior of Ti with passive film under breakdown was investigated from the points of film thickness and oxygen-related species in the film. A Ti specimen was passivated in a sulfuric acid solution at 5.0 V Ag/AgCl at 323 and 353 K. A passivation current decreased, increased and then slightly decreased with increasing a passivation time, the periods were denoted as Region (i), (ii) and (iii), respectively. A pitting potential of the specimen passivated in Region (i) was almost independent of, and that in Regions (ii) and (iii) shifted higher with increasing the time. Change in an amount of water desorbed from the film with the time was similar to that in the pitting potential. It was found from XPS test that change in a film thickness with the passivation time was also similar to that in the pitting potential. Furthermore, the desorbed water was derived from hydroxide or both of hydroxide and bound water in the film. The findings indicate that the film passivated in Regions (ii) and (iii) was under breakdown, and suppression of the pitting corrosion is caused by increasing both of the film thickness and the amount of hydroxide and bound water in the film.