The passivation process of pure iron in 85% H3PO4 was studied by conventional and rotating ring-disk voltammetry, impedance spectroscopy, and dc contact electric resistance (CER) techniques. The evolution of the film resistance as a function of potential in the passivation domain, demonstrated that the passivation of iron is most probably due to an adsorbed layer or a fairly conductive precursor film. Rotating ring-disk studies indicated the release of both soluble Fe(II) and Fe(III) in the passivation domain, a peak in the release of Fe(III) coinciding with the active-to-passive transition. Impedance spectra of Fe passivation, with a peculiar shape circling the origin of the complex plane, were found to be qualitatively analogous to those for a range of metals in concentrated acidic solutions. A kinetic model is proposed which reproduces quantitatively the steady-state and ac impedance results in the passivation region.
Bojinov, M., Betova, I., Fabricius, G., Laitinen, T., & Raicheff, R. (1999). Passivation mechanism of iron in concentrated phosphoric acid. Journal of Electroanalytical Chemistry, 475(1), 58-65. https://doi.org/10.1016/S0022-0728(99)00343-5