Photoelectron spectroscopy study of the inhibition of mild steel corrosion by molybdate and nitrite anions

Passive films formed on mild steel in aqueous 8.6 mM NaCl solutions (pH 8), containing either MoO42- or NO2-, have been studied with X-ray photoelectron spectroscopy. For either anion these films are ∼5 nm deep, and the primary chemical state of iron is Fe3+. Following exposure to MoO42-, the film consists of a sub-layer (∼4.1 nm) composed largely of ferric oxide/hydroxide, overlaid by Fe2(MoO4)3 (∼0.6 nm). As regards NO2-, spectra are consistent with the film being closely related to γ-Fe2O3. Furthermore, a reduction product of NO2-, potentially N2, is present, displaying a depth profile comparable to that of molybdate.