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

The effects of hydrogen on the electronic properties and stability of passi ve films on iron formed at various potentials in 0.3 M H3BO3 + 0.075M Na2B4 O7. 10H(2)O were investigated by polarization curves, ac impedance, and ind uction 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 resi stance decreases with increasing hydrogen charging current density and fina lly reaches a relatively stable value. Mott-Schottky analyses of capacitanc e data show that hydrogen results in an increase in donor density of the pa ssive films formed in the range of investigated film formation potentials a nd 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 exp lained by the reduction of iron oxide by hydrogen. (C) 2001 Elsevier Scienc e B.V. All rights reserved.