It has been reported that hydrogen decreases the stability of the passive films on iron and stainless steel and promotes their localized corrosion. However, the mechanism of hydrogen-promoted corrosion is still not clear. Potentiostatic, electrical impedance spectroscopy measurements, and Mott-Schottky analysis were used to investigate the effect of hydrogen on the properties of the passive film on X70 microalloyed steel. In the range of passive potentials, hydrogen retards the passive film formation and decreases its stability. Impedance spectra of passive film on X70 steel show that hydrogen decreases the resistance towards charge transfer and ion diffusion and increases the capacitance of the passive film. Mott-Schottky analyses show that hydrogen increases the capacitance and donor density, and decreases the flatband potential and the space-charge layer thickness of the passive film. A new explanation is proposed for the effects of hydrogen on the properties of the passive films. According to this explanation, hydrogen creates an additional electric repulsion on or and oxygen vacancies, and enhances the diffusion rate of cations and anion vacancies. At steady state, hydrogen will not only decrease the concentrations of and but also increase the concentration of in the passive films, resulting in changes in the structure and a decrease in the thickness of the passive films.
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Zeng et al. (2004) studied this question.
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