Nonconsumable electrodes

Specific types of consumable electrode are designated in terms of the constituent material (e.g., as a silver electrode ). Nonconsumable electrodes are designated either in terms of the electrode material or in terms of the chief component in the electrode reaction for instance, the terms platinum electrode and hydrogen electrode are used for electrode (1.27). Neither of these names completely describes the special features of this electrode. 

This gives rise to an important conclusion. For nonconsumable electrodes that are not involved in the current-producing reaction, and for which the chemical potential of the electrode material is not contained in the equation for electrode potential, the latter (in contrast to a Galvani potential) depends only on the type of reaction taking place it does not depend on the nature of the electrode itself. 

In the case of redox reactions, polarization also depends on the natme of the nonconsumable electrode at which a given reaction occms (for the equilibrium potential, to the contrary, no such dependence exists). Hence, the term reaction will be understood as reaction occurring at a specified efectrode. ... 

Current flow at electrode surfaces often involves several simultaneous electrochemical reactions, which differ in character. For instance, upon cathodic polarization of an electrode in a mixed solution of lead and tin salt, lead and tin ions are discharged simultaneously, and from an acidic solution of zinc salt, zinc is deposited, and at the same time hydrogen is evolved. Upon anodic polarization of a nonconsumable electrode in chloride solution, oxygen and chlorine are evolved in parallel reactions. 

Among electrolytic processes used to produce materials, we customarily distinguish those in which electrodes are reacting that is, where the metal or other electrode material is involved in the reaction (Chapter 16) from those with nonconsumable electrodes (Chapter 15). A very important industrial process with nonconsumable electrodes is the electrolysis of sodium chloride solution (brine) producing chlorine at the anode and sodium hydroxide NaOH (caustic soda) in the catholyte via the overall reaction... 

Selection of Corrosion-Resistant Materials The concentrated sofutions of acids, alkalies, or salts, salt melts, and the like used as electrolytes in reactors as a rule are highly corrosive, particularly so at elevated temperatures. Hence, the design materials, both metallic and nonmetallic, should have a sufficiently high corrosion and chemical resistance. Low-alloy steels are a universal structural material for reactors with alkaline solutions, whereas for reactors with acidic solutions, high-alloy steels and other expensive materials must be used. Polymers, including highly stable fluoropolymers such as PTFE, become more and more common as structural materials for reactors. Corrosion problems are of particular importance, of course, when materials for nonconsumable electrodes (and especially anodes) are selected, which must be sufficiently stable and at the same time catalytically active. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

Many papers have been published regarding HTSCs used as inert, nonconsumable electrodes for kinetic and mechanistic studies of various electrode reactions occurring at them. Most of these studies were performed at room temperature when the materials were not in their state of superconductivity. Unfortunately, to date a given reaction has rarely been studied at similar temperatures just above and below r , that is, at temperatures where the same material is once in its normal state and once in its superconducting state. The electronic stracture of materials differs sharply between these two states, and quantitative studies under these conditions might provide valuable information as to the mechanism of the elementary act of charge transfer from the electrode to a reacting species, and vice versa. 

Plasma Arc Welding. In the transferred-arc mode of the PAW process, shown in Figure lc, the arc is between a nonconsumable electrode and the base metal, in a manner similar to the GTAW process. The unique feature is the flow of inert gas around the electrode and through a restricted orifice, which constricts the arc to form a plasma jet. A second, outer stream of shielding gas protects the molten metal from atmospheric contamination. In the nontransferred arc mode of the PAW process, the arc is between the electrode and the constricting orifice. This mode is used for plasma spraying... 

Two classes of electrode are the non-consumable or inert type and the consumable or sacrificial type. Nonconsumable electrodes are made from non-reactive materials, whereas consumable electrodes are electro-chemically active and are structurally altered by the passage of current during treatment. 

In accordance with Faraday s Law, the operation of an IDDS requires redox reactions at the electrodes in proportion to the amount of charge passed. For nonconsumable electrodes, contacting an essentially aqueous electrolyte solution, electrolysis of water is the likely redox reaction. Therefore, the reaction at the anode is ... 

Electrodes of 2%-thoriated tungsten are the most frequently used water-cooled nonconsumable electrodes. Water-cooled copper anodes have been widely used in experimental work. Figure 1 shows a typical plasma jet assembly. A reactor chamber may be of any configuration desired to accommodate different feeding and quenching devices. 

The use of consumable electrodes, for example scrap iron, is cheap but consumable electrodes must be replaced at regular intervals [28]. Furthermore, these electrodes cannot sustain high current densities and must therefore be rather bulky. Moreover, the dissolution products from these electrodes tend to contaminate the environment. These disadvantages can be avoided with the use of nonconsumable electrodes such as, silicon-iron [29], lead-antimony-silver [30], platinum-titanium [31, 32] or platinum-tantalum [33, 34]. However, they are expensive. 

Lundin (1966) studied nine compositions in the cerium-holmium alloy system and reported lattice spacings for four of his alloys. He used 99.9 wt% pure cerium (principal impurities were C, O, Fe, Al, Si, Cu, Mo and W) and 99.9 + wt% pure holmium (principal impurities were O, Zn, Ca and other rare earths) in the preparation of his alloys, which were melted under purified argon in a nonconsumable electrode arc furnace, then homogenized at 650°C for 32 hr followed by rapid cooling. Jayaraman et al. (1966) also reported lattice spacings for a 8 phase... 

Lattice spacings in the praseodymium-terbium system have been determined by Speight et al. (1968). The praseodymium and terbium metals from which their specimens were prepared contained approximately 0.02 wt% of common metals and less than 0.10 wt% of other rare earth metals. Weighed amounts of the constituent metals were melted under a purified argon atmosphere in a nonconsumable electrode arc furnace. Alloys were homogenized at 700 to 1000°C for two weeks and rapidly quenched. Filings for X-ray analysis were annealed to relieve stresses. Systematic errors were ehminated by use of the Nelson-Riley extrapolation function. 

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