Oxidation mixed-potential theory

In the active region, the anodic electrochemical reaction is metal oxidation. Mixed potential theory governs the alloy corrosion in this r on. The corrosion potential and... 

Mital et al. [40] studied the electroless deposition of Ni from DMAB and hypophosphite electrolytes, employing a variety of electrochemical techniques. They concluded that an electrochemical mechanism predominated in the case of the DMAB reductant, whereas reduction by hypophosphite was chemically controlled. The conclusion was based on mixed-potential theory the electrochemical oxidation rate of hypophosphite was found, in the absence of Ni2 + ions, to be significantly less than its oxidation rate at an equivalent potential during the electroless process. These authors do not take into account the possible implication of Ni2+ (or Co2+) ions to the mechanism of electrochemical reactions of hypophosphite. 

In the mixed potential theory (MPT) model, both partial reactions occur randomly on the surface, both with respect to time and space. However, given the catalytic nature of the reductant oxidation reaction, it may be contended that such a reaction would tend to favor active sites on the surface, especially at the onset of deposition, and especially on an insulator surface catalyzed with Pd nuclei. Since each reaction strives to reach its own equilibrium potential and impose this on the surface, a situation is achieved in which a compromise potential, known as the mixed potential (.Emp), is assumed by the surface. Spiro [27] has argued the mixed potential should more correctly be termed the mixture potential , since it is the potential adopted by the complete electroless solution which comprises a mixture of reducing agent and metal ions, along with other constituents. However, the term mixed potential is deeply entrenched in the literature relating to several systems, not just electroless deposition. 

The incorporation of a third element, e.g. Cu, in electroless Ni-P coatings has been shown to improve thermal stability and other properties of these coatings [99]. Chassaing et al. [100] carried out an electrochemical study of electroless deposition of Ni-Cu-P alloys (55-65 wt% Ni, 25-35 wt% Cu, 7-10 wt% P). As mentioned earlier, pure Cu surfaces do not catalyze the oxidation of hypophosphite. They observed interactions between the anodic and cathodic processes both reactions exhibited faster kinetics in the full electroless solutions than their respective half cell environments (mixed potential theory model is apparently inapplicable). The mechanism responsible for this enhancement has not been established, however. It is possible that an adsorbed species related to hypophosphite mediates electron transfer between the surface and Ni2+ and Cu2+, rather in the manner that halide ions facilitate electron transfer in other systems, e.g., as has been recently demonstrated in the case of In electrodeposition from solutions containing Cl [101]. 

Marmatite has a narrower band gap than (Zn, Cu)S, thus it should be more easily oxidized than (Zn, Cu)S. But in fact, the sphalerite after Cu activation has the most excellent flotation response using xanthate. These phenomena can be explained by the mixed potential theory. 

According to the mixed potential theory, an anodic reaction can occur only if there is a cathodic reaction proceeding at finite rate at that potential (Rand and Woods, 1984). For the flotation systems, the cathodic reaction is usually given by the reduction of oxygen. The corresponding anodic reaction involves interaction of xanthate on the sulphide minerals in various ways, including the reaction of xanthate with the sulphide mineral (MS) to form metal xanthate and the oxidation of xanthate to dixanthogen (X2) at the mineral surface. 

Online mass spectrometry data presented and discussed in the previous sections suggest that catalytic hypophosphite oxidation on nickel in D2O solutions proceeds via the coupling of anodic (19.11) and cathodic (19.12) half-reactions at the catalyst surface. The classical mixed-potential theory for simultaneously occurring electrochemical partial reactions [14] presupposes the catalyst surface to be equally accessible for both anodic (19.11) and cathodic (19.12) half-reactions. Equilibrium mixtures of H2, HD, and D2 should be formed in this case due to the statistical recombination of Hahalf-reactions (19.11) and (19.12) for example, the catalytic oxidation of hypophosphite on nickel in D20 solution under open-circuit conditions should result in the formation of gas containing equal amounts of hydrogen and deuterium (H/D=l) with the distribution H2 HD D2= 1 2 1 (the probability of HD molecule formation is twice as high as for either H2 or D2 formation [75]). Therefore, to get further mechanistic insight, the distribution of H2, HD, and D2 species in the evolved gas was compared to the equilibrium values at the respective deuterium content [54]. 

According to mixed potential theory, any electrochemical reaction consists of partial reduction and oxidation reactions. In any redox reaction, such as the corrosion of a metal, there is no net accumulation of electric chaise and the rate of the oxidation must equal the rate of reduction. At the intersection of the cathodic and anodic kinetic lines (see Fig. 3.8), the rates of oxidation and reductions are equal. This point represents the corrosion potential, Eco .> and the corrosion current, At the... 

Solution pH, velocity, and oxidizer concentration change the properties of the anodic curve of the active-passive metal. For example, the equilibrium potential of the cathodic reaction shifts according to the Nemst equation in the noble direction by increasing the oxidizer concentration. Mixed potential theory, in this case, may predict the intersection of the cathodic and anodic Tafel fines and corrosion rate or extent of passivation of the metal. 

Under steady state conditions, mixed potential theory can be used to evaluate galvanic corrosion rate and potential. To maintain electroneutrality at the electrode interfaces, this theory requires the rate of oxidation at the anode be equal to the reduction at the cathode. Under these conditions, the potential of both electrodes is equal to the corrosion potential, while the current density represents the corrosion current density. The corrosion current of two electrically connected dissimilar metals is proportional to the galvanic corrosion current. Graphical evaluation using mixed potential theory is illustrated for two coupled corroding metals in Fig. 6.8. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

This example establishes the tenets of the mixed potential theory (1) that the reaction surface is at the same potential, (2) that the electrons released in the anodic reaction are consiuned by the reduction reaction, which means that (3) the total currents for the anodic (oxidation) and cathodic (reduction) reactions have to exactly balance each other. In this regard, one can say that the total cirrrent, J, of a system is equal to 0 ... 

However, closer examination showed that the Cu-Ni system, immediately after the interruption of current, acquires an open circuit potential (or a mixed potential) which hes above the nickel reversible potential and below the copper reversible potential. In addition, the pH of these solutions, i.e. pH = 4, does not allow the formation of nickel oxides on the surface of the nickel. Therefore, in accordance with the mixed potential theory, copper continues to deposit at the expense of nickel dissolution, forming a classic galvanic corrosion cell. This process does not... 

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