Abstract
Titanium and titanium passive films have often been taken as standard cases in studying corrosion, passivation, and long term stability of valve metals. Recent work has shown that the corrosion and passivation behaviours and the stability of passive films on titanium depend greatly on the ambient solution. Earlier investigations have shown that the behaviour of the passive films formed in sulphuric acid is not identical to that of those formed in nitric acid. In the present work electrochemical impedance spectroscopy was used to obtain detailed information about the effect of ambient solution on the passive film and the differences in behaviour of different valve metals. The behaviour of anodic passive films formed on tantalum was compared with the behaviour of anodically passivated titanium. A model for the metal–metal oxide–electrolyte system is proposed and discussed. The results reveal that the anodic oxide film formed on tantalum behaves as a perfect dielectric and can be represented by a simple equivalent circuit model; however, passive films on titanium do not show the expected ideal behaviour.
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