Characterisation of the passive film on iron in phosphate medium by voltammetry and XPS measurements

Passivation of iron in phosphate medium was studied by voltammetry and X-ray photoelectron spectroscopy. The voltammetry at low sweep rate has permitted us to determine the different stages of the passivation. The process of electrochemical passivation obeys a dissolution-precipitation mechanism. The film growth is controlled by the resistance of the solution in the pores. The electrochemical impedance measurements show that the passive film on iron is an n-type semiconductor, highly doped (N-D = 4.69 x 10(20) cm(-3)) and its flat-band potential was equal to -570 mV/(SSE). The composition depth-profile of the passive film was measured by sequential use of XPS and sputter etching. It was found that the film consisted mainly of both metallic oxides and phosphate. (C) 1997 Elsevier Science S.A.