Reaction cathodic

If two redox electrodes both use an inert electrode material such as platinum, tlie cell EMF can be written down iimnediately. Thus, for the hydrogen/chlorine fiiel cell, which we represent by the cell Fl2(g) Pt FICl(m) Pt Cl2(g) and for which it is clear that the cathodic reaction is the reduction of CI2 as considered in section... 

Both cathodic reactions can drive the metal oxidation. Of course, the potentials given above are only standard... 

The dotted lines represent the cases when the above cathodic reactions, (a) or (b), drive the reaction. The solid lines indicate the stability ranges for Fe and its corrosion products (Fe, Fe, Fe O, Fc202, tthcOT). 

It should be pointed out that external polarization differs from the unbiased (open circuit) case in that after application of, say, an anodic voltage only the oxidation reaction takes place on the metal, whereas the cathodic reaction (H — H2) occurs at the external counter-electrode. 

The cathodic reaction (the right-hand cell) is the reduction of Fe + to Fe +... 

Chloiine is pioduced at the anode in each of the three types of electrolytic cells. The cathodic reaction in diaphragm and membrane cells is the electrolysis of water to generate as indicated, whereas the cathodic reaction in mercury cells is the discharge of sodium ion, Na, to form dilute sodium amalgam. 

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

Combining alumina dissolution, equation 16, and the anode and cathode reactions, equations 17—19, gives the overall reaction... 

Iron metal preferentially dissolves the copper does not. Therefore, the anodic and cathodic reactions are as follows, to give the soHd copper. 

At neutral or higher pH, the concentration of ions is too low for this reaction to contribute significantly to the overall corrosion rate. However, as pH decreases, this reaction becomes more important until, at a pH of about 4, it becomes the predominant cathodic reaction. 

The mechanism of the cathode reaction for all three types of Mn02 can best be described by two approximately one-electron steps. 

In a battery, the anode and cathode reactions occur ia different compartments, kept apart by a separator that allows only ionic, not electronic conduction. The only way for the cell reactions to occur is to mn the electrons through an external circuit so that electrons travel from the anode to the cathode. But ia the corrosion reaction the anode and cathode reactions, equations 8 and 12 respectively, occur at different locations within the anode. Because the anode is a single, electrically conductive mass, the electrons produced ia the anode reaction travel easily to the site of the cathode reaction and the 2iac acts like a battery where the positive and negative terminals are shorted together. 

Cathode Reaction. There are many different types of manganese dioxide (18), having varying activity in batteries. The only type suitable for alkaline batteries is y-Mn02, the mineral form of which is nsutite. The chemical composition of has been described (19) by the general formula... 

Zinc—Silver Oxide Batteries. Miniature 2inc—silver oxide batteries have a 2inc anode, and a cathode containing silver oxide [20667-12-3] Ag20. The cathode reaction... 

The cathode reaction involves reduction of silver oxide to metallic silver [7440-22-4J. The reaction is a two-phase, heterogeneous reaction producing a substantially constant voltage during discharge. Some manganese dioxide may be added to the cathode, as in the case of mercury oxide cells. 

The CF cathode reaction is beheved to be a heterogeneous process, initiated by the insertion of lithium ions between the CF planes. It is completed by the extmsion of LiF and the coUapse of the stmcture to carbon. 

The cathodic reaction is the reduction of iodine to form lithium iodide at the carbon collector sites as lithium ions diffuse to the reaction site. The anode reaction is lithium ion formation and diffusion through the thin lithium iodide electrolyte layer. If the anode is cormgated and coated with PVP prior to adding the cathode fluid, the impedance of the cell is lower and remains at a low level until late in the discharge. The cell eventually fails because of high resistance, even though the drain rate is low. 

In electro winning, the cathode reaction is the same as for electrorefining (see eq. 31). However, because of the use of insoluble anodes, oxygen is released at the anode. 

The ions, M , formed by this reaction at a rate, may be carried into a bulk solution in contact with the metal, or may form insoluble salts or oxides. In order for this anodic reaction to proceed, a second reaction which uses the electrons produced, ie, a reduction reaction, must take place. This second reaction, the cathodic reaction, occurs at the same rate, ie, = 7, where and are the cathodic and anodic currents, respectively. The cathodic reaction, in most cases, is hydrogen evolution or oxygen reduction. 

The three elements necessary for corrosion are an aggressive environment, an anodic and a cathodic reaction, and an electron conducting path between the anode and the cathode. Other factors such as a mechanical stress also play a role. The thermodynamic and kinetic aspects of corrosion deterrnine, respectively, if corrosion can occur, and the rate at which it does occur. 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Reduction of oxygen is one of the predominant cathodic reactions contributing to corrosion. Awareness of the importance of the role of oxygen was developed in the 1920s (19). In classical drop experiments, the corrosion of iron or steel by drops of electrolytes was shown to depend on electrochemical action between the central relatively unaerated area, which becomes anodic and suffers attack, and the peripheral aerated portion, which becomes cathodic and remains unattacked. In 1945 the linear relationship between rate of iron corrosion and oxygen pressure from 0—2.5 MPa (0—25 atm) was shown (20). 

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