Oxide-solution interface layers

The corrosion current due to diffusion of metal ions through the passivating film, and dissolution of metal ions at the oxide-solution interface. Clearly, the smaller this current, the more protective is the oxide layer. 

The nature of the problem in establishing a mechanistic model of the oxide-electrolyte interface, in which chemical and electrostatic energies are described explicitly, can be appreciated by consideration of the adsorption reaction depicted in Figure 2. The adsorption of a hydrogen ion from the bulk of a monovalent electrolyte is considered. The oxide-solution interface is divided conceptually into four regions the bulk oxide (not shown in the figure), the oxide surface at which the adsorption reaction takes place, the solution part of the double layer containing the counterions, and the bulk of solution. 

Free Energies of Electrical Double Layers at the Oxide-Solution Interface... 

The aim of this paper is not to add to the current debate but to present a simple graphical method of analysing the free energy of formation of the electrical double layer at the oxide/solution interface ( 1). This will provide a simple way of visualizing the complementary roles of chemical reactions or surface properties of... 

Although a family of OgS - Jig8 values are allowed under Equation 7 the actual equilibrium state of the oxide/solution interface will be determined by the dissociation of the surface groups and the properties of the electrolyte or the diffuse double layer near the surface. For surfaces that develop surface charges by different mechanisms such as for semiconductor, there will be an equation of state or charge-potential relationship that is analogous to Equation 7 which characterizes the electrical response of the surface. 

The main, currently used, surface complexation models (SCMs) are the constant capacitance, the diffuse double layer (DDL) or two layer, the triple layer, the four layer and the CD-MUSIC models. These models differ mainly in their descriptions of the electrical double layer at the oxide/solution interface and, in particular, in the locations of the various adsorbing species. As a result, the electrostatic equations which are used to relate surface potential to surface charge, i. e. the way the free energy of adsorption is divided into its chemical and electrostatic components, are different for each model. A further difference is the method by which the weakly bound (non specifically adsorbing see below) ions are treated. The CD-MUSIC model differs from all the others in that it attempts to take into account the nature and arrangement of the surface functional groups of the adsorbent. These models, which are fully described in a number of reviews (Westall and Hohl, 1980 Westall, 1986, 1987 James and Parks, 1982 Sparks, 1986 Schindler and Stumm, 1987 Davis and Kent, 1990 Hiemstra and Van Riemsdijk, 1996 Venema et al., 1996) are summarised here. 

Fig. 13.54. Morphology of a thick oxide layer, showing crystallites, grain boundaries, and pores. The oxide/solution interface area is very large when the layer is impregnated with electrolyte solution. (Reprinted from S. Trasatti and K. Weil, Electrochemical Supercapacitors as Versatile Energy Stores, Platinum Metals Rev., 38 (2) 53, Fig. 8,1994, with permission from Johnson, Matthey Co.)...

The pH value at which the oxide surface carries no fixed charge, i.e. Oj = 0, is defined as the point of zero charge (PZC) . A closely related parameter, the isoelectric point (lEP), obtained from electrophoretic mobility and streaming potential data, refers to the pH value at which the electrokinetic potential equals to zero The PZC and lEP should coincide when there is no specific adsorption in the iimer region of the electric double layer at the oxide-solution interface. In the presence of the specific adsorption, the PZC and lEP values move in opposite directions as the concentration of supporting electrolyte is increased. ... 

S. M. Ahmed, Studies of he double layer at oxide solution interface. /. Phys. Chem., 73 (1969) 3546. 

Figure 10,18 Schematic plot of surface species and charge (a) and potential ) relationships versus distance from the surface (at the zero plane) used in the constant capacitance (CC) and the diffuse-layer (DL) models. The capacitance, C is held constant in the CC model. The potential is the same at the zero and d planes in the diffuse-layer model i/fj). Reprinted from Adv. Colloid Interface Sci. 12, J. C. Westall and H. Hohl, A comparison of electrostatic models for the oxide/solution interface, pp. 265-294, Copyright 1980 with kind permission of Elsevier Science-NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.

Diffuse Layer Model The diffuse layer model of the oxide-solution interface (Huang and Stumm, 1973 Dzombak and Morel, 1990) contains the following assumptions ... 

Ray, K.C. and Khan, S., Electrical double layer at zirconium oxide-solution interface, Indian J. Chem., 13, 577, 1975. 

The effect of Ag ions in the solution on the corrosion rate is within the range of the experimental error. Such a result would he accepted if we assume that the effect of Ag ions is limited to the oxide solution interface only. As the oxide metal interface is isolated from the Ag ions hy the corrosion layer, the effect of these ions on the processes on the metal surface is negligible. When silver is alloyed in the metal, however, the corrosion rate is reduced by one order of magnitude. This experimental finding indicates that Ag in the alloy directly affects the corrosion process that occurs on the metal surface [121],... 

At the oxide/solution interface, one may visualize the reaction as occurring by tantalum ions popping out and capturing O" from species in the solution, or suppose that O" ions enter the oxide and the reaction occurs in some sort of transition layer in the oxide surface. 

Let s now examine the second important mechanistic point. As the surface of the oxidic supports is charged in electrolytic solutions, an electrical double layer is formed between the support surface and the solution. Various models have been developed to describe the oxide/solution interface [43, 56-63]. It has been widely accepted that the triple layer model describes better this interface in the most of cases [33-39, 41]. A simplified picture of this model is illustrated in fig. 9. It should be noted that the SOH2+. SOH and SO groups are considered to be localized on the surface of the support (zero plane). On the other hand the centers of the water molecules surrounding the surface of the support particles constitute the so called Inner Helmholtz Plane (IHP). Moreover, the counter ions (of the indifferent electrolyte) are located on the Outer Helmholtz Plane (OHP). Very near to this plane is the shear plane and then the diffuse part of the double layer and the bulk... 

Ellipsometry at noble metal electrode/sohitiOTi interfaces has been used to test theoretically predicted microscopic parameters of the interface. Investigated systems include numerous oxide layer systems [5-10], metal deposition processes, adsorption processes [11], and polymer films on electrodes [12-14]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [15]. Film thickness as a function of state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [16]. The deposition and electroreduction of Mn02 films has been studied [17] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by a thickness increase it started at the oxide/solution interface. 

Both magnetite dissolution and dissolution of the base metal contribute Fe to the solution. Separating the two contributions is at best a difficult procedure. Most of that effort focused on the dissolution rate of the base metal through a passive film. The rate of that process appears to depend on the ratio of film thickness, I, to the metal/solution potential difference, dissolution process implies transfer of metal ions to the oxide phase, followed by ion diffusion through the oxide, transfer through the oxide solution interface, and finally hydration of the Ions. Thus, the potential difference at the Helmholtz layer is given by... 

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