Corrosion electrodes, electron transfer

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

The proposed model for the so-called sodium-potassium pump should be regarded as a first tentative attempt to stimulate the well-informed specialists in that field to investigate the details, i.e., the exact form of the sodium and potassium current-voltage curves at the inner and outer membrane surfaces to demonstrate the excitability (e.g. N, S or Z shaped) connected with changes in the conductance and ion fluxes with this model. To date, the latter is explained by the theory of Hodgkin and Huxley U1) which does not take into account the possibility of solid-state conduction and the fact that a fraction of Na+ in nerves is complexed as indicated by NMR-studies 124). As shown by Iljuschenko and Mirkin 106), the stationary-state approach also considers electron transfer reactions at semiconductors like those of ionselective membranes. It is hoped that this article may facilitate the translation of concepts from the domain of electrodes in corrosion research to membrane research. 

There are various ways in which CMEs can benefit analytical applications. These include acceleration of electron-transfer reactions, preferential accumulation, or selective membrane permeation. Such steps can impart higher selectivity, sensitivity, or stability to electrochemical devices. These analytical applications and improvements have been extensively reviewed (35-37). Many other important applications, including electrochromic display devices, controlled release of drugs, electrosynthesis, and corrosion protection, should also benefit from the rational design of electrode surfaces. 

In recent years, electrochemical charge transfer processes have received considerable theoretical attention at the quantum mechanical level. These quantal treatments are pivotal in understanding underlying processes of technological importance, such as electrode kinetics, electrocatalysis, corrosion, energy transduction, solar energy conversion, and electron transfer in biological systems. 

Fig. 11-1. Mixed electrode model (local cell model) for corrosion of metals i = anodic current for transfer of iron ions i = cathodic current of electron transfer for reduction of hydrogen ions.

The cuprous-cupric electron transfer reaction is believed to be the rate-limiting step in the process of stress corrosion cracking in some engineering environments [60], Experimental studies of the temperature dependence of this rate at a copper electrode were carried out at Argonne. Two remarkable conclusions arise from the study reviewed here [69] (1) Unlike our previous study of the ferrous-ferric reaction [44], we find the cuprous-cupric electron transfer reaction to be adiabatic, and (2) the free energy barrier to the cuprous cupric reaction is dominated in our interpretation by the energy required to approach the electrode and not, as in the ferrous-ferric case, by solvent rearrangement. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

Fig. 4. Schematic diagram of passive metals with electrode reactions (1) Metal corrosion (2) film formation (3) redox reactions with electron transfer to or from metal substrate (4a,b) complex formation and enhanced dissolution in the passive state.

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-... 

In this case, it is likely that electron transfer is sluggish through the A1 oxide film on the pure A1 electrode, and this limits the rate at which the ORR can be sustained. The high ORR rate on the Cu-bearing materials was attributed to corrosion and surface roughening that increased the area capable of supporting the ORR. In the case of the intermetallic compound, dealloying may also have contributed to the increase in active surface area. 

We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer will be sufficiently thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface will not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and shifts in the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar cells by these means. 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

Corrosion of the material used is another factor that limits the selection of the electrocatalyst. The electrochemical corrosion of pure noble metals is not as important as in the case of binary or ternary alloys in strong acid or alkaline solutions, since these catalysts are widely used in electrochemical reactors. In the case of anodic bulk electrolysis, noble metal alloys used in electrocatalysis mainly contain noble metal oxides to make the oxidation mechanism more favorable for complete electron transfer. The corrosion problem that occurs from this type of catalyst is the auto-corrosion of the electrode surface instead of the electrode/electrolyte solution interface degradation. The problem of corrosion is considered in detail in Chapter 22. 

If two or more electrochemical half-cell reactions can occur simultaneously at a metal surface, the metal acts as a mixed electrode and exhibits a potential relative to a reference electrode that is a function of the interaction of the several electrochemical reactions. If the metal can be considered inert, the interaction will be between species in the solution that can be oxidized by other species, which, in turn, will be reduced. For example, ferrous ions can be oxidized to ferric ions by dissolved oxygen and the oxygen reduced to water, the two processes occurring at different positions on the inert metal surface with electron transfer through the metal. If the metal is reactive, oxidation (corrosion) to convert metal to ions or reduction of ions in solution to the neutral metal introduces additional electrochemical reactions that contribute to the mixed electrode. 

When two metals or alloys are joined such that electron transfer can occur between them and they are placed in an electrolyte, the electrochemical system so produced is called a galvanic couple. Coupling causes the corrosion potentials and corrosion current densities to change, frequently significantly, from the values for the two metals in the uncoupled condition. The magnitude of the shift in these values depends on the electrode kinetics parameters, i0 and (3, of the cathodic and anodic reactions and the relative magnitude of the areas of the two metals. The effect also depends on the resistance of the electrochemical cir-... 

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