Oxide growth on evaporated thin film and bulk iron exposed to humid air and HCl or HNO3 vapours

High purity iron was used in the form of thin films evaporated on glass and in the form of bulk material. Evaporated films exhibited a geometrical pattern with rectangular or square facets of about 0.2 μm in size. Samples were exposed at ambient temperature to air and to vapours of 1.0 M HNO3 and 1.0 M HCl aqueous solutions for up to 18 h and then examined by atomic force microscopy (AFM); bulk iron was examined additionally by XPS and ac impedance in a borate solution. XPS indicated the presence of iron oxides in corrosion products, possibly of Fe2O3, FeOOH and/or Fe3O4; their content increased for the exposures in the sequence: air≤HNO3<HCl. On evaporated iron, oxides grew in the form of separated features (small columns, spikes, needles) and some of these features appeared to grow on individual facets of the substrate, whereas on bulk iron, oxides grew at random in the form of irregular hills. It was suggested that the unidirectional growth of oxides in HCl vapour was associated with the hindrance of a lateral growth by Cl-containing species. The impedance element Q1 of the equivalent circuit Ro(Q1(R1(Q2R2))) differed for various exposures in agreement with the AFM and XPS data, suggesting that it can be used for characterisation of the morphology of surface products.