Metal dissolution oxygen reduction

It follows from equation 1.45 that the corrosion rate of a metal can be evaluated from the rate of the cathodic process, since the two are faradai-cally equivalent thus either the rate of hydrogen evolution or of oxygen reduction may be used to determine the corrosion rate, providing no other cathodic process occurs. If the anodic and cathodic sites are physically separable the rate of transfer of charge (the current) from one to the other can also be used, as, for example, in evaluating the effects produced by coupling two dissimilar metals. There are a number of examples quoted in the literature where this has been achieved, and reference should be made to the early work of Evans who determined the current and the rate of anodic dissolution in a number of systems in which the anodes and cathodes were physically separable. 

Anodic or cathodic inhibitors This classification is based on whether the inhibitor causes increased polarisation of the anodic reaction (metal dissolution) or of the cathodic reaction, i.e. oxygen reduction (near-neutral solutions) or hydrogen discharge (acid solutions). 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

In addition to the basic corrosion mechanism of attack by acetic acid, it is well established that differential oxygen concentration cells are set up along metals embedded in wood. The gap between a nail and the wood into which it is embedded resembles the ideal crevice or deep, narrow pit. It is expected, therefore, that the cathodic reaction (oxygen reduction) should take place on the exposed head and that metal dissolution should occur on the shank in the wood. 

FIGURE 22.2 Schematic polarization curves for spontaneous dissolution (a) of active metals (h) of passivated metals. (1,2) Anodic curves for active metals (3) cathodic curve for hydrogen evolution (4) cathodic curve for air-oxygen reduction (5) anodic curve of the passivated metal. 

Fig. 13. (a) Schematic representation of the formation of mixed potential, M, at an inert electrode with two simultaneous redox processes (I) and (II) with formal equilibrium potentials E j and E2. Observed current density—potential curve is shown by the broken line, (b) Representation of the formation of corrosion potential, Econ, by simultaneous occurrence of metal dissolution (I), hydrogen evolution, and oxygen reduction. Dissolution of metal M takes place at far too noble potentials and hence does not contribute to EC0Ir and the oxygen evolution reaction. The broken line shows the observed current density—potential curve for the system. 

The most simple Pourbaix diagram is that for water shown in Fig. 7. All Pourbaix diagrams contain the two lines that bound the regions of stability for water, known as the reversible hydrogen and reversible oxygen lines, and labeled as a and b, respectively. For corrosionists the importance of these lines should not be underestimated. As noted in the introduction, for metal dissolution to occur, a reduction reaction must occur. In aqueous solution, the most common reduction reactions are the reduction of dissolved oxygen and the reduction of water... 

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

Corrosion current density — Anodic metal dissolution is compensated electronically by a cathodic process, like cathodic hydrogen evolution or oxygen reduction. These processes follow the exponential current density-potential relationship of the - Butler-Volmer equation in case of their charge transfer control or they may be transport controlled (- diffusion or - migration). At the -> rest potential Er both - current densities have the same value with opposite sign and compensate each other with a zero current density in the outer electronic circuit. In this case the rest potential is a -> mixed potential. This metal dissolution is related to the corro-... 

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... 

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 processes that take place in the pit and in its vicinity are shown in Fig. 20M(a). At the concentration of chloride ions found in seawater, the passive layer on aluminum breaks down, and anodic dissolution of the metal can occur. This happens mostly inside the pit, where the supply of oxygen is slow. On the other hand, oxygen reduction can readily take place on the surface of the metal outside the pits, where its diffusion path is short. Thus, the cathodic area is typically... 

Gu and Balbuena also predict that most solutes dissolved in Pt reduce its stability towards dissolution. They investigated the role of oxygenated intermediates in the oxygen reduction reaction on the dissolution mechanisms of Pt surfaces. They found that the most favorable mechanism for dissolution is an electrochemical mechanism (as opposed to a purely chemical mechanism) that involves oxygenated metal surface atoms. Their investigation focused on single metal atoms or dimers, however, and used small clusters of water to include some solvation effects. They compared the relative stabilities of Ir, Pd, Rh, Ni and Co and found that Pt-Ir was the most stable. 

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