Barrier film formation, kinetics

Figure 18 shows the dependence of the activation barrier for film nucleation on the electrode potential. The activation barrier, which at the equilibrium film-formation potential E, depends only on the surface tension and electric field, is seen to decrease with increasing anodic potential, and an overpotential of a few tenths of a volt is required for the activation energy to decrease to the order of kBT. However, for some metals such as iron,30,31 in the passivation process metal dissolution takes place simultaneously with film formation, and kinetic factors such as the rate of metal dissolution and the accumulation of ions in the diffusion layer of the electrolyte on the metal surface have to be taken into account, requiring a more refined treatment. 

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-... 

The characteristic feature of solid—solid reactions which controls, to some extent, the methods which can be applied to the investigation of their kinetics, is that the continuation of product formation requires the transportation of one or both reactants to a zone of interaction, perhaps through a coherent barrier layer of the product phase or as a monomolec-ular layer across surfaces. Since diffusion at phase boundaries may occur at temperatures appreciably below those required for bulk diffusion, the initial step in product formation may be rapidly completed on the attainment of reaction temperature. In such systems, there is no initial delay during nucleation and the initial processes, perhaps involving monomolec-ular films, are not readily identified. The subsequent growth of the product phase, the main reaction, is thereafter controlled by the diffusion of one or more species through the barrier layer. Microscopic observation is of little value where the phases present cannot be unambiguously identified and X-ray diffraction techniques are more fruitful. More recently, the considerable potential of electron microprobe analyses has been developed and exploited. 

When the results for oxide growth and anion incorporation172,160 are compared with the kinetics of space charge accumulation in barrier and porous alumina films [see Section IV(1)], it can be concluded that anion incorporation modifies the electrostatics of the external oxide interface, thus influencing oxide dissolution and pore formation.172... 

D.B. Bergstrom, I. Petrov, L.H. Allen, J.E. Greene. Aluminide formation in polycrystalline A1AY metal/barrier thin-film bilayers Reaction paths and kinetics // J.Appl.Phys.- 1997.- V.82, No.l.- P.201-209. 

While thick film passivity has been documented and understood for many years, the difficulties in studying thin film passivity were daunting. It took many years to determine that indeed a film was responsible for the effect, as these films are so thin that they are invisible to the eye (i.e., transparent to radiation in the visible region). Two main types of theories were developed in order to explain the phenomena observed theories based upon the idea of adsorption reducing the corrosion rate, and theories based upon the formation of a new phase, an oxide of the base metal, on the surface. In all cases, an increased barrier to dissolution results upon the increase in potential. This increased kinetic barrier upon anodic polarization contrasts with the exponentially decreased barrier which develops during anodic polarization of an active material. 

The experiments show, however, that many oils with positive B and E coefficients might have low antifoam efficiency. In these cases, the stability of the asymmetric oil-water-air films is very high, and the formation of unstable oil bridges becomes impossible for kinetic reasons (factor 2 is decisive). The repulsive interaction that should be overcome for effectuation of the antifoam globules entry on the solution surface is usually termed the entry barrier. A recently... 

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