The Passive Film on Iron at pH 1–14: A Potential‐Modulated Reflectance Study

The passive film on iron has been studied by potential‐modulated reflectance (PMR) spectroscopy over a wide range of p H (1–14) and polarization potentials. PMR is a very sensitive in situ technique for which an established interpretation theory for the results obtained with passive films is still lacking. Here it is proposed that the PMR maxima of passive films correspond to absorption edges of compounds in the passive layer. The PMR spectra of the passive film on iron are the same in acid as in alkaline media, which indicates that, as far as electronic transitions are concerned, the nature of the passive film is independent of the medium p H. A wide spectroscopic data review of iron compounds is given. By comparing the PMR results with absorption edges of bulk iron compounds, a composition of the film is proposed in which an Fe(II) compound is present at all potentials, together with a layer of that is increasingly dehydrated into (both α and γ) upon increasing the anodic potential.