Electronic band structure of passive film on X70 pipeline steel

Mott-Schottky analyses and photoelectrochemical measurements were used to explore the effects of film formation potentials, time, and chloride ions on the electronic properties of the passive film on X70 pipeline steel in 0.5 M NaHCO₃. Mott-Schottky analyses showed that with increasing film formation potentials, the capacitance and donor density of the passive film decrease, and the flat band potential and thickness of the space charge layer increases. The addition of chloride ions increases the capacitance and donor density of the film and results in a more negative flat band potential and a thinner space charge layer. Photoelectrochemical measurements imply that photo-generated carriers with a low mobility exist in the passive film and the photocurrents of the film increase when the film formation potential becomes more positive. The bandgap energy, E_g, of the passive film decreases with increasing film formation potentials. The extension of film formation time increases the photocurrents and leads to a decrease in the bandgap energy of the passive film. The effect of film formation time on the photocurrents of the film formed at 0.6 V_SCE was less remarkable than that formed at 0.2 V_SCE. The existence of chloride ions in 0.5 M NaHCO₃ decreases the photocurrents and increases the bandgap energy of the passive film.