Metal dissolution compensation reactions

As shown in Fig. 33, the decreasing mechanism of this fluctuation is summarized as follows At a place on the electrode surface where metal dissolution happens to occur, the surface concentration of the metal ions simultaneously increases. Then the dissolved part continues to grow. Consequently, as the concentration gradient of the diffusion layer takes a negative value, the electrochemical potential component contributed by the concentration gradient increases. Here it should be noted that the electrochemical potential is composed of two components one comes from the concentration gradient and the other from the surface concentration. Then from the reaction equilibrium at the electrode surface, the electrochemical potential must be kept constant, so that the surface concentration component acts to compensate for the increment of the concen-... 

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-... 

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

Corrosion is an electrochemical process that consists of at least two reactions that compensate each other electronically in open circuit conditions. The anodic metal dissolution is counterbalanced by the cathodic reduction of a redox system within the electrolyte. Various processes may serve as cathodic counterreactions. The most common are oxygen reduction and hydrogen evolution in acidic electrolytes. 

Others are the reduction of Fe + and [Fe(CN)6] in solution. These systems are often used for chemical corrosion tests. Pitted metals expose a small area of a few intensively dissolving corrosion pits that are not protected by a passive layer and a large cathode of the passive metal surface. Because of the large size of the cathode, a much smaller cathodic current density is required for the compensating reduction of the redox system in comparison to the active metal dissolution within the pits. However, electronic conduction is still required across the passive layer. Figure 3 depicts the existing sections of a pitted metal surface with the related electrode reactions, the very small metal dissolution /pass, and the redox reaction ha,pass via the protecting oxide film and... 

Eq. (10.5) must turnover twice to compensate the metal dissolution. It was assumed in Eq. (10.4) that the electrochemical reaction order of the metal ion ligands is equal to the stoichiometric number. For both equations = 1. If the difference between the metal... 

Principally any reduction reaction, with an equilibrium potential more positive than the Nemst potential of the corroding metal, can compensate the metal dissolution. In practice, two reactions are of special importance, the reduction of hydrogen ions and the reduction of oxygen. 

A very important part is the metal/elec-trolyte interface. This is the site where anodic metal dissolution and the compensating cathodic reduction of redox systems take place. At this site, adsorption and layer formation occur which might drastically reduce the corrosion rate by inhibition of the corresponding electrode reactions. The adjacent electrolyte is the medium where preceding or consecutive electrode reactions occur, and diffusion or migration of corro-... 

In practice, metal dissolution occurs in combination with the reduction of an oxidizing species. Hydrogen evolution and oxygen reduction are frequent counter reactions compensating partially or totally the metal dissolution of such a mixed electrode. Figure 1.40 presents schematically the reactions at an Me/Me + electrode and the H2/H+ electrode. Both electrodes show their Nemst equilibrium potentials and the anodic and cathodic branches of... 

If the cathodic reaction is fast enough to compensate active metal dissolution, the oxidizing agent creates passivity. Thus, the redox system allows passing the peak of active metal dissolution, and a potential well within the passive range is established. After passivation, the cathodic process has to compensate only the very small passive corrosion density (Figure 1.48). 

As shown in Fig. 24, the mechanism of the instability is elucidated as follows At the portion where dissolution is accidentally accelerated and is accompanied by an increase in the concentration of dissolved metal ions, pit formation proceeds. If the specific adsorption is strong, the electric potential at the OHP of the recessed part decreases. Because of the local equilibrium of reaction, the fluctuation of the electrochemical potential must be kept at zero. As a result, the concentration component of the fluctuation must increase to compensate for the decrease in the potential component. This means that local dissolution is promoted more at the recessed portion. Thus these processes form a kind of positive feedback cycle. After several cycles, pits develop on the surface macroscopically through initial fluctuations. 

The electrochemical reactions induce concentration gradients in the electrolyte present in the crack. When active dissolution takes place at the crack tip and oxygen is reduced further away or at the outside surface, the acidity of the electrolyte near the crack tip increases as a result of metal-ion hydrolysis. Electroneutrality requires that anions, such as chloride, migrate towards the crack tip to compensate the charge of the metal cations produced there. Both effects contribute to stabihze the active state of the crack tip. 

It is worth noting that, in whatever way a pit nucleus was bom, its further development to form a pit embryo (growing metastable pit) depends on the electrolyte concentration which locally sets up in the electrolyte when the metal dissolves. The development of the pit embryo implies the local stabilization of an acidic corrosive medium, differing from the surrounding one. This local acidification induces a local dissolution of the metal in the active state, which is compensated only by the diffusion of the corrosion products. This dissolution in turn provokes an acidification due to the hydrolysis reactions and the process is self-sustained, provided that fire diffusion or the electromigration is slow enough in the electrolyte. Formally [3h,5d], if Jis the anodic current andXrepresents the concentration in corrosion products, one has dX/dt = KJ V, X) - DX, where the metal-electrolyte potential difference F is a control parameter (J increases with V) and parameters K and D represent respectively the production in corrosion products by the anodic dissolution and their dilution into the electrolyte by a diffusion process. The steady state conditions write dX/dt = 0. The system is locally stable when... 

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