Cathode half-cell reaction

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. 

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-cell reactions which control the rate of corrosion. 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

Knowing that a given combination of anode and cathode half-cell reactions will proceed sponta-neonsly does not ensure that the electrode reaction rates will be sufficiently high for practical applications. Reaction kinetics at the anode and cathode and mass transfer of reactants/products to/from the electrodes may play important roles in an electrochemical cell and may influence the choice of cell design and operating conditions. These important points will be addressed later in this chapter. 

Interpretation of an experimentally determined polarization curve, including an understanding of the information derivable therefrom, is based on the form of the polarization curve, which results from the polarization curves for the individual anodic and cathodic half-cell reactions occurring on the metal surface. These individual polarization curves, assuming Tafel behavior in all cases, are shown in Fig. 6.2 (dashed curves) with Ecorr and the corrosion current, Icorr, identified. It is assumed that over the potential range of concern, the Iox x and Ired M contributions to the sum-anodic and sum-cathodic curves are negligible consequently, Uox = Iox M and Ured = Ired x. At any potential of the... 

If oxidation-reduction conples exist in the reaction, electrons must be transferred Electrochemical reactions, in which electrons flow through the solid from anodic to cathodic electrode surface sites, are best represented by anodic and cathodic half-cell reactions ... 

FIGURE 20.70 Anodic and cathodic half-cell reactions for zinc in an acid solntion show, by their intersection, the corrosion potential and corrosion current density (Fontana 1986). 

Coupled Half Cell Reactions. The oxidation of magnetite under anoxic conditions without the presence of reducible aqueous metal species consists of coupled anodic and cathodic half cell reactions (16)... 

Sharp current minimums in Figure 2 for magnetite (400 mV) and ilmenite (-40 mV) correspond to conditions in which no current is applied and the rates for the anodic and cathodic half cell reactions are equivalent and equal to equation 1. Polarization of the magnetite electrode to potentials less than 400 mV results in the dominance of the reductive dissolution of magnetite as described by equation 6. This reaction consumes electrons by reducing ferric atoms in the magnetite structure and releasing Fe(II) to solution. 

Anodic half-cell reaction Fe Fe ++2e Cathodic half-cell reaction H2O -I- l/202+2e —... 

The electrochemical model for corrosion breaks the process down into anodic and cathodic half-cell reactions, each having their own equations, which may include ... 

In all cases, metal atoms in the solid (s) metallic state (in the electronic conductor) are converted to an oxidized state. These species remain either on the metallic surface or are ejected into the aqueous (aq) solution (the ionic conducting phase). Typical cathodic half-cell reactions in aqueous media involve the reduction of oxygen, protons, ind water at the interface between the metallic and ionic conducting phases. This process involves consumption of electrons liberated during oxidation ... 

Either the anodic or cathodic half-cell reaction can become mass transport limited and restrict the rate of corrosion at co,r- The presence of diffusion controlled corrosion processes does not invalid the EIS method, but does require extra precaution and a modification to the circuit model of Fig. 4. In this case, the finite diffusional impedance is added in series with the usual charge transfer parallel resistance shown in Fig. 4. The transfer function for the frequency dependent finite diffusional impedance, Z fco), has been described [43] ... 

Entropy change in the cathodic half-cell reaction. 108... 

The Ca-Li alloy beads also react with dissolved CaCr04, forming a coating of Ca5(Cr04)3Cl. This Cr(V) compound is the same species that is formed in the cathodic half-cell reaction ... 

Both chemical (open-circuit) and electrochemical etching are practised, the former being more important. These chemical processes involve concurrent anodic and cathodic half-cell reactions. The dissolution of the semiconductor M occurs via release of electrons e or acceptance of holes h ... 

Chloride-induced rebar corrosion. Corrosion damage to reinforcing steel is an electrochemical process with anodic and cathodic half-cell reactions. In the absence of chloride ions, the anodic dissolution reaction of iron. 

Anaerobic sites. Hydrogen evolution is a prime candidate for the cathodic half-cell reaction under anaerobic conditions. Corrosion rates therefore tend to increase with decreasing pH (increasing acidity levels). In the case of ground water saturated with calcium and carbonate, the corrosion product is mainly iron (II) carbonate, a milky white precipitate. On exposure to air this white product will revert rapidly to reddish iron (III) oxides. 

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