Barrier oxide layer

Fig. 4 Summary of the defect generation and annihilation reactions envisioned at the interfaces of the barrier oxide layer on a metal, according to the PDM [37], = cation vacancy...

Iron hydroxide ion formation, Fe(OFI), depends on solution pFl, that is, the availability of OH ions. As bivalent Fe ions are formed through Eq. (12.6), OH ions are transported from the bulk to the surfice to maintain electroneutrality. Flydroxide ions are also produced via the cathodic oxygen reduction reaction, causing an increase in surface pH. At high pH, the formation of adsorbed [Fe(OH)]ads on the iron surface becomes more favorable than bivalent Fe ions, Eq. (12.6). The electrode potential tends to shift into a more anodic direction to accommodate the formation of Fe(OH)". As time increases, the Fe(OH) concentration at the surface increases. In the next step, Fe(OH) oxidizes to ferric oxide at e° = —0.084Vvs.SCE, resulting in a barrier oxide layer ... 

The fields marked Fe203 and Fe304 are sometimes labeled passivation on the assumption that iron reacts in these regions to form protective oxide films. This is correct only insofar as passivity is accounted for by a diffusion-barrier oxide layer (Definition 2, Section 6.1). Actually, the Flade potential, above which passivity of iron is observed in media such as sulfuric or nitric acid, parallels line a and b, intersecting 0.6 V at pH = 0. For this reason, the passive film (Definition 1, Section 6.1) may not be any of the equilibrium stoichiometric iron oxides, as is further discussed in Chapter 6. 

While these six generalizations are not all encompassing, in that exceptions may exist, they are sufQcient to differentiate between various theories that have been proposed for the growth of barrier oxide layers on metals and alloys. A number of models that have been developed to describe the growth of anodic oxide films on metals are listed in Table 4.4.2, together with some of their important features and predictions. Of the models listed, which were chosen because they make analytical predictions that can be tested and because they introduced new concepts into the theory of passivity, only the point defect model (PDM) in its latest form (D. Macdonald [1999], Pensado-Rodriguez et al. [1999a,b]) accounts for all of the observations summarized above. 

Chapter 4 Applications of Impedance Spectroscopy Metal I Barrier Oxide Layer... 

According to the point defect model (D. Macdonald [1999]), the steady state thickness of the barrier oxide layer is given by Eq. (109), which is reproduced here as... 

Figure 3-22. Blocking effect of adsorbed sulfur preventing the recombination of adjacent hydroxyl groups to form the barrier oxide layer of the passive film.

Posttest exarrrirratiorts included deterrrrirration of weight loss and SEM/EDS exarrrirration of the steel surface. A typical SEM micrograph of the corroded surface is displayed in Fig. 55. The form of attack was observed to be scaling with sections where it was apparent that exfoliation had occurred (Fig. 55b), as observed in other studies. " The exfoliated material is almost certainly the outer layer, which from passivity theory forms via the hydrolysis of cations (Fe ) that are trarrsmitted as cation irrterstitials across the barrier oxide layer from the metal/barrier layer interface to the barrier layer/outer layer (solution in the pores of the outer layer) to... 

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