Passive films thickness

Figure 5 shows the relationship between the passive film thickness of an iron electrode and the electrode potential in an anodic phosphate solution and a neutral borate solution.6,9 A passive film on an iron electrode in acidic solution is made up of an oxide barrier layer that increases its thickness approximately linearly with increasing electrode potential, whereas in a neutral solution, there is a precipitated hydroxide layer with a constant thickness outside the oxide barrier layer. 

One of the first mechanistic models of the kinetics of passive film formation was described by Cabrera and Mott [31]. The passive film thickness in their model is controlled by the transport of the metal cations from the underlying metal to the... 

Depending on the potential, chloride concentration in the electrolyte, structure of the alloy, passive film thickness, and the chemical composition of the corrosion environment, pits may appear in various morphologies as illustrated in Fig. 7.4. 

Figure 6.29 Effect of passive film thickness on kinetics of electron tansfer between FelCN) " and Fe(CN)6 on passive iron at pH 8.4 [31].

The passive film thickness at time t can be predicted using eq. (6.1), provided that the passive current density (ip) remains constant in the passive potential range, E [Pg.185]

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Secondly, crystal defects might be expected to affect the corrosion behaviour of metals which owe their corrosion resistance to the presence of thin passive or thick protective films on their surface. The crystal defects and structural features discussed in Section 20.4 might, in principle, be expected to affect the thickness, strength, adhesion, porosity, composition, solubility, etc. of these surface films, and hence, in turn, the corrosion behaviour of the filmed metal surfaces. Clearly, this is the more common situation in practice. 

Calcium hydroxide leached from incompletely cured concrete causes serious corrosion of lead (see Section 9.3). This is because carbon dioxide reacts with the lime solution to form calcium carbonate, which is practically insoluble. Carbonate ions are therefore not available to form a passive film on the surface of the lead . Typically, thick layers of PbO are formed, which may show seasonal rings of litharge (tetragonal PbO) and massicot (orthorhombic PbO) . 

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

For steel, passivation is achieved by the surface formation of a tough, adherent mixture of oxides. The passive film is primarily gamma-magnetite (y-Fe203) but also contains gamma-hydrated ferric oxide (y-FeOOH). The film thickness is perhaps 15 A to 30 A (angstrom units). 

Generally, such a remarkable restriction of metal dissolution results not only from the formation of a thin surface oxide film but also from the formation of a comparatively thick film such as silver chloride or zinc chloride. In this chapter, however, we use the term passive film only for compact and thin oxide films. 

Figure 5. Thickness of the anodic passivating film formed on iron at various potentials.6 9 Lbl and Lr, are the thicknesses of the barrier layer and the precipitated layer, respectively. Temperature is 25°C. , in a 150 mol m 3 phosphate buffer solution at pH 1.85 O, in a 300 mol m 3 borate buffer solution at pH 8.2. (From N. Sato, K. Kudo, and T. Noda, Z Phys. Chem. N. F. 98,271,1975, Fig. 5, reproduced with permission and N. Sato, K. Kudo, and R. Nishimura, / Elec-trochem. Soc, 123,1420,1976, Fig. 1. Reproduced with permission of the Electrochemical Society, Inc.)...

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