Dear Ingmar Bösing ,

You have asked many great questions regarding the transpassivity (and anodic dissolution, in general) of iron.

Transpassivity is often referred to as the increase in current density as more anodic potential are applied (with respect to the passive region), and describes the breakdown of primary passivity. Transpassivity is a bit challenging to define (as an exact point) without much analysis and testing; however, I have found the work of Macdonald and co-workers helpful in this, as they describe it as part of their Point Defect Model (PDM) [Macdonald, D. D. "Some personal adventures in passivity—A review of the point defect model for film growth" Russ. J. Electrochem. 48 (2012) 235–258. doi:10.1134/S1023193512030068.

Additional information regarding the anodic dissolution of iron, in general, can be found in the research of Keddam and co-workers (as well as Bockris and co-workers) where they performed many EIS studies. This book Chapter provides a good starting point: [It is Chapter 3 Anodic Dissolution in Corrosion Mechanisms in Theory and in Practice (3rd ed.), edited by P. Marcus. doi:10.1201/b11020.]

You may also find this work helpful: Avillera, A, Cid, M, and Petit, M.C. Anodic reaction of iron in transpassive range. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, Vol 105, p 149-160. doi:10.1016/S0022-0728(79)80346-0

With regards to your question on the passive film thickness: The dependence of the passive film thickness on potential has been studied by a number of researchers, typically using surface analysis techniques (e.g. XPS). In general, as more anodic potentials are applied, the oxidation of metals to higher valence species occurs. This is true for iron as it transitions from 2+ to 3+ species during anodic polarization. The work of Strehblow and co-workers provides insight as to the potential dependency of the passive film thickness of Fe, Ni and NiFe alloys (e.g. [Haupt, S., Strehblow, H.H. "Combined electrochemical and surface analytical investigations of the formation of passive layers". Corrosion Science. 29 (1989) 163–182. doi:10.1016/0010-938X(89)90027-9.]

For iron, the passive film thickness is relatively potential independent; however, for Ni, the thickness of the passive film is much more dependent on potential. In my paper, I provide a number of references for the passive film of Ni, Fe, and, in particular, NiFe alloys, as well as provide some insight as to the potential dependency of the passive film thickness for nanocrystalline NiFe alloys: Monaco, L., Sodhi, R.N.S., Palumbo, G. and Erb, U. "XPS study on the passivity of coarse-grained polycrystalline and electrodeposited nanocrystalline nickel-iron (NiFe) alloys," Corrosion Science. 176 (2020) 108902. doi:10.1016/j.corsci.2020.108902.

Lastly, it is important to point out that anodic films can exist on iron (and other metals) other than the passive film. For example, a sulfate-based film has often been reported on Ni and Fe (see references above). During anodic dissolution of Ni in sulfuric acid, for example, this film plays an important role in the dissolution mechanism.

Hopefully these references will provide additional insight for you! Best of luck!

Dear Leo Monaco,

thanks for your detailed answer and the helpful references. These are indeed very good starting points for my literature review.

Best

Ingmar