Passivity oxide/electrolyte interface

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Fig. 6. Potential diagram for passivated metals for steady and unsteady conditions with an overvoltage r/2 3 at the oxide/electrolyte interface leading to increased dissolution and oxide growth of d to d2 where a new stationary state is reached.

For alloys the corrosion properties, as well as the composition of the passive layers, depend strongly on the chemical properties of the alloy components. For an alloy of chemically very different components, the noble metal tends to stay within the metal matrix, whereas the non-noble partner enters preferentially the oxide matrix or is dissolved more readily. The more-noble component enters the passive layer or is dissolved only if the potential is sufficiently positive. The more-noble component will be oxidized also later on a time scale if the potential is sufficiently positive. Besides thermodynamics also the kinetic properties of the system under study have a decisive influence on the various reactions. This involves the rate of transfer reactions at the metal/oxide and oxide/electrolyte interface, as well as the transfer of the cations and anions across the oxide matrix. 

The point defect model of passivity and its breakdown is a variant of the penetration mechanism [51]. The transport of cations from the metal surface to the oxide-electrolyte interface corresponds to an inward movement of cation vacancies Vm+- This inward transport of Vm+ is supported by their high concentration at... 

Real oxide films are typically nonstoichiometric due to an excess of metal ions or a deficiency of oxygen ions in the film and are often amorphous or nanocrystalline. In the presence of water, hydrated oxides or hydroxides often form, such as Al(OH)3 or AlOOH in the passive layer of Al and Fe203-H20 or y-FeOOH in the passive layer of Fe. Furthermore, the migration or diffusion of defects within the oxide leads to transport of ions within the film and to ion transfer reactions (ITRs) that take place at the oxide-electrolyte interface. Defect concentrations in passive films usually range from 10 to 10 cm [15]. Thus, as CPs are ion exchange polymers, ion transfer across CP-metal oxide interfaces is likely. 

It should be noted that the growth of the passive film may also occur by the inward motion of oxygen anions. In many cases, such as for passive films formed in acid solution on iron, nickel, or chromium, the passive film stops growing because its rate of formation equals its rate of dissolution at the oxide/electrolyte interface. 

On the basis of these ideas, Macdonald and eo-workers [13,14] developed their model of passivity and its breakdown involving the action of vacancies within the passive layer. It is assumed that cation vacancies migrate from the oxide-electrolyte to the metal-oxide interface, whieh is equivalent to the transport of cations in the opposite direetion. If these vacaneies penetrate into the metal phase at a slower rate than their transport through the oxide, they accumulate at the metal-oxide interface and finally lead to a loeal eoneentration. The related voids lead to stresses within the passive film and its final breakdown. The inward diffusion or migration of eation vacaneies is affeeted by the incorporation of Cl ions at the oxide-electrolyte interface aceording to the following mechanism The concentration c of metal ion V, and vacancies Fq2 are determined by the equihbrium of the Schottky pair formation at the oxide-eleetrolyte interface [Eq. (3)], which causes an inverse dependenee of their eoneentrations [Eq. (4)]. 

XPS measurements of passivated Fe and Ni electrodes that have been exposed to aggressive anions (Ni and Fe to F Fe to Cl , Br, and I ) but have not already formed corrosion pits support this mechanism. The quantitative evaluation of the data clearly shows a decrease of the oxide thickness with time of exposure [22,48], Not only F but also the other halides cause thinning of the passive layer (Fig. 8) [48], The catalytically enhanced transfer of cations from the oxide to the electrolyte leads to a new stationary state of the passive layer. Its smaller thickness yields an increased electrical field strength for the same potentiostatically fixed potential drop, which in turn causes faster migration of the cations through the layer to compensate for the faster passive corrosion reaction (1) at the oxide-electrolyte interface (Fig. 2a). Statistical local changes... 

Potential drops Acpi2 and Acp j across the metal-oxide and oxide-electrolyte interfaces and Aq>, across the oxide film for stationary (index s) and instationary (index i) conditions. Instationary conditions after potential increase cause a larger voltage drop Aipi within the layer and an overpotential Tij 3 at the oxide-electrolyte interface where oxide growth and the related O " production occur. (From Strehblow, H.-H., in Passivity of Metals, R.C. Alkire, D.M. Kolb, eds., Advances in Electrochemical Science and Engineering, Vol. 8, Wiley-VCH, Weinheim, Germany, pp. 271-374,2003.)... 

The simple model of a homogenous passive layer of Figure 5.6 becomes more complicated if a second alloy component is present as shown in Figure 5.30. The composition of the passive layer is then determined by the oxidation rates of the components A and B at the metal surface, fheir transfer rates through the film, and their transfer across the passive layer-electrolyte interface, i.e., their individual corrosion rates in the passive state. The reaction rates at both interfaces may be decisive for the layer composition. One example is the preferential dissolution of Fe " ions due to the extremely slow cation transfer of Cr " ions at the surface of the film, which leads to an accumulation of Cr(lll) wifhin the film for FeCr alloys. Another example is the preferential oxidation of A1 of an A1 alloy containing 1% Cu. Cu does not enter the film and is accumulated at the metal surface while an AI2O3 film is formed. These examples are discussed in defail in the following. 

Semiconducting passive layers present band bending relative to the surface of the substrate metal and to the electrolyte. The metal substrate and the electrolyte cannot take over the changes of the applied potential and thus the potential drops are located within the oxide film and at the metal-oxide and oxide-electrolyte interfaces. For a thick film... 

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