Metal dissolution, electrode kinetics

The thermodynamic and electrode-kinetic principles of cathodic protection have been discussed at some length in Section 10.1. It has been shown that, if electrons are supplied to the metal/electrolyte solution interface, the rate of the cathodic reaction is increased whilst the rate of the anodic reaction is decreased. Thus, corrosion is reduced. Concomitantly, the electrode potential of the metal becomes more negative. Corrosion may be prevented entirely if the rate of electron supply is such that the potential of the metal is lowered to the value where it is found that anodic dissolution does not occur. This may not necessarily be the potential at which dissolution is thermodynamically impossible. 

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. 

The discussion of concentration polarization so far has centred on the depletion of electroactive material on the electrolyte side of the interface. If the metal deposition and dissolution processes involve metastable active surface atoms, then the rate of formation or disappearance of these may be the critical factor in the overall electrode kinetics. Equation (2.69) can be rewritten for crystallization overvoltage as... 

The formation or dissolution of a new phase during an electrode reaction such as metal deposition, anodic oxide formation, precipitation of an insoluble salt, etc. involves surface processes other than charge transfer. For example, the incorporation of a deposited metal atom (adatom [146]) into a stable surface lattice site introduces extra hindrance to the flow of electric charge at the electrode—solution interface and therefore the kinetics of these electrocrystallization processes are important in the overall electrode kinetics. For a detailed discussion of this subject, refs. 147—150 are recommended. 

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-... 

Activation control of an overall dissolution rate can, of course, reside in the reduction process, in the oxidation process, in a mixture of both, or in a mixture including some transport control. The reduction process is usually more influential in determining the overall rate. Thus, in the absence of transport control, the kinetics of the electrode process for reduction of hydrated protons, or water molecules, or dissolved molecular oxygen plays the major role in metal dissolution kinetics. Indeed the literature confirms the conclusion that many of the systems seen in experiment or in practice are diffusion controlled that most of the rest are under mixed diffusion and activation control and that those with some activation control... 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

Although the author believes that the generalized concept was originally responsible for the electrochemical treatment of corrosion processes by the early workers, it appears that Hammett and Lorch (23) and Frumkin (24) were among the first to specifically describe metallic dissolution according to this concept. Wagner and Traud (16) showed that the electrode kinetics for hydrogen evolution are not affected by the simultaneous dissolution of the metallic ions. 

V wilh respect to metal dissolution and - 0.8 V with respect to oxygen reduction. In most studies of electrode kinetics, however, the experiment is set up in such a manner that only one reaction can take place in the range of potential studied, and the overpotential is unambiguously defined. 

The rate of a corrosion reaction is affected by pH (via H reduction and hydroxide formation), the partial pressnre of O, (the solubility/concentration of oxygen in solution), fluid agitation, and electrolyte condnctivity. Corrosion processes are analyzed using the thermodynamics of electrode reactions, mass transfer of the cathode reactants O2 and/or H, and the kinetics of metal dissolution reactions [157, 158]. 

Whether the total corrosion process is determined by the kinetics of anodic metal dissolution or the cathodic process depends on the size of the cathode and the kinetics of the partial electrode processes. The slowest reaction is the rate-determining step, as is usual in kinetics. In the case of a well-passivated valve metal, this is most probably the cathodic reaction, whereas for metals with semiconducting oxides, the rate-determining step win he anodic metal dissolution. In order to study the partial reactions of pitting corrosion separately, potentiostatic experiments are preferred. The cathodic process is replaced hy the electronic circuit of the potentiostat to investigate the anodic metal... 

It is obvious that this reaction can only lead to metal dissolution, if the metal electrode potential is negative from the hydrogen electrode potential. This is the reason for the classification of metals into noble metals (the equilibrium potential is more positive than the standard hydrogen potential) and non-noble metals (the equilibrium potential is more negative than the standard hydrogen potential). The kinetic of the total process can be described by the Butler-Volmer equation for the two partial reactions. 

Anions of common strong acids, such as C104, S04, CF, NOa , etc. exhibit as a rule only weak complexing interactions, if any. Nevertheless, even weak complexation may be of importance in electrode kinetics if the complex ion undergoes electrode reaction more easily than the free metal ion, as is often the case, especially with chlorides. In such cases, the complex takes the role of an electroactive species, as already discussed for the hydroxo complexes. Thus, e.g., nickel can hardly be anodically dissolved at all if chloride ions are not present in the solution. In sulfate electrolytes, the oxidation product (some oxygen-containing species) forms a passive film and further dissolution is blocked soon after an anodic overpotential is imposed upon the electrode. The phenomenon of passivity is discussed elsewhere (cf. Volume 4). At this point, one should note that passivity is absent in the presence of chlorides. 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

Usually the equilibrium potential of metal dissolution is much more negative than that of the cathodic reaction of the redox system. Iron dissolution in sulfuric acid with hydrogen evolution corresponds to this situation. The potential of the electrode shifts by -0.059 V pH and its kinetics slow down due to a smaller exchange cur-... 

The corrosion rate depends on the electrode kinetics of both partial reactions. If all of the electrochemical parameters of the anodic and cathodic partial reactions are known, in principle the rate may be predicted. According to Faraday s law, a linear relationship exists between the metal dissolution rate at any potential Vm the partial anodic current density for metal dissolution... 

Metal dissolution is a general phenomenon in molten salts, and dissolved metals are responsible for the major loss in current efficiency due to their reaction with the anode product. So-called metal fog is a visual phenomenon associated with metal deposition from molten salts. Results from experimental studies have shown that metal fog consists of small metal droplets formed by homogeneous nucleation from a supersaturated solution of dissolved metal [1]. The electrode kinetics for metal deposition reactions are known to be very fast. Therefore, limitations due to nucleation and diffusion are more important for the metal deposition process. Nucleation may be of significance both for solid and liquid metal products. 

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