Effects of hydrogen on the passive film and pitting of iron

The effects of hydrogen on the passive film, such as the effects of hydrogen on stability and nature of passive film, and pitting of iron were systematically investigated by various electrochemical methods and surface analytical techniques. The mechanism of the decreased stability of the passive film and increased pitting of iron caused by hydrogen were discussed.; The results obtained from open circuit potentials, polarization curves and ac impedance measurements show that hydrogen decreases the stability of the passive film on iron, as indicated by decreases in the open circuit potential, the charge transfer resistance, and the increase in the anodic current in the passive region. Hydrogen changes the polarization behavior of iron from simultaneous passivation to active/passivation when the hydrogen charging current density reaches a critical value. Impedance analysis shows that increasing the film formation potential cannot improve the stability of the passive film in the presence of hydrogen. The analysis of impedance data also shows that hydrogen decreases the thickness of the passive film.; The results obtained from pitting induction time, pitting potential and electrochemical noise measurements, and the scanning reference electrode technique show that hydrogen significantly increases the pitting susceptibility of iron as indicated by a decreased pitting induction time and pitting potential. Iron covered by a passive film formed at a high film formation potential has a high pitting resistance. Hydrogen significantly increases the number and magnitude of current fluctuations, which are coupled by simultaneous potential fluctuations. This indicates that hydrogen might increase the initiation and growth of metastable pits. Analyses of Power Spectrum Densities (PSDs) of current fluctuations show that hydrogen decreases the repassivation rate of metastable pits, which may be the reason for the increased pitting susceptibility of iron.; The presence of hydrogen in the passive film significantly affects the electronic properties of the passive film. It was found that hydrogen increases donor density in the passive films formed at various film formation potentials and the extent of donor density depends upon the film formation potential. It was also found that hydrogen decreases the band gap energy of the passive films formed at potentials lower than 0.6V. The effects of hydrogen on the donor density and band gap energy of the passive film are correlated with the effect of hydrogen on the pitting resistance of iron.; Surface analyses by AES, SIMS and NRA show that hydrogen decreases the film thickness and the oxygen content in the passive film. It also increases the contents of iron and hydroxyl ions in the passive film.; The effects of hydrogen on the stability of the passive film and pitting of iron have been discussed in terms of the effects of hydrogen on the composition, thickness and structure of the passive film on iron.