Effects of hydrogen on the electronic properties and stability of the passive films on iron

Abstract The effects of hydrogen on the electronic properties and stability of passive films on iron formed at various potentials in 0.3 M H 3 BO 3 +0.075 M Na2B4O7·10H2O were investigated by polarization curves, ac impedance, and induction period measurements. It was found that the polarization behavior of iron changes from self-passivation to active/passivation when the hydrogen charging current density reaches a critical value; the charge transfer resistance decreases with increasing hydrogen charging current density and finally reaches a relatively stable value. Mott–Schottky analyses of capacitance data show that hydrogen results in an increase in donor density of the passive films formed in the range of investigated film formation potentials and the increased amount decreases with increasing film formation potentials. The effect of hydrogen charging current density on the donor density of a passive film formed at 0.2 V is stronger than that of a film formed at 0.6 V, resulting in a much larger decrease in pitting corrosion resistance for a film formed at 0.2 V than for one formed at 0.6 V. These results are explained by the reduction of iron oxide by hydrogen.