CORROSION AND PASSIVATION OF TITANIUM IN NITRIC-ACID SOLUTIONS AS REVEALED FROM IMPEDANCE AND PHOTOELECTROCHEMICAL MEASUREMENTS

The electrochemical and photoelectrochemical behaviour of Ti and passivated Ti electrodes in nitric acid solutions was studied and compared with the behaviour of the electrodes passivated in sulphuric acid under the same conditions. The donor density and flat band potential were calculated from the Mott-Sottky plots. The use of high intensive pulse lasers allowed photoelectrochemical investigations with light wavelengths smaller than the band gap of the passive layer formed on titanium. The measurements showed that the behaviour of passive films on titanium in nitric acid was not comparable with its behaviour in sulphuric acid or other solutions. A model for the band structure of the passive film is suggested and the anomalous behaviour of titanium passivated in nitric acid is discussed.