Electronic structure and pitting susceptibility of passive film on carbon steel

Abstract

The semiconductive properties, electronic structure and pitting susceptibility of passive films formed on A516-70 carbon steel in chromate solutions with and without chloride ions are studied by capacitance measurements, Mott–Schottky analysis and the electrochemical noise technique. The passive film shows features of an amorphous or highly disordered n-type semiconductor. The donor concentration is in the range of 10 26–10 27 m −3 and decreases with the film formation potential (prepassivation potential). The thickness of the space charge layer ranges from 1 to 6 Å. The flat band potential increases with an increase in the film formation potential. The nonlinearity of Mott–Schottky plots, depending upon the film formation potential, indicates the existence of two donor levels in the space charge layer. When the film formation potential is below −50 mV, the two levels are revealed by two slopes in the Mott–Schottky plots. For a film formed at a prepassivation potential of −50 mV, only the shallow donor level can be ionized due to increase in the stability of the passive film. The onset potential and the transitional potential of metastable pitting, determined by electrochemical noise measurements, coincide with the flat band potential of the film formed at the corrosion potential and the critical potential at which both deep and shallow donor levels are ionized. It is suggested that the divalent iron cation vacancies generated by the ionization of the deep donor level are mainly responsible for pitting corrosion.