Magnetite cathodes

Electrochemical reduction in aqueous acid is useful in the treatment of waste liquors obtained from the formation of chloroacetic acid by chlorination of acetic acid. The liquors contain further chlorination products. These are reduced in an undivided cell at a magnetite cathode and a carbon anode to give excellent conversion to monochloroacetic acid [73]. 

Figure 3.5 Schematic pH eind oxygen concentration profiles in an active tubercle. Below the magnetite shell oxygen concentration decreased sharply. pH rises above the magnetite shell due to cathodic hydroxyl-ion generation emd falls below the shell due to concentration of acidic anion, (Courtesy of National Association of Corrosion Engineers, Corrosion 91 Paper No. 84 by H. M. Herro.)...

Four magnetite rod anodes (All to A14), each weighing 9 kg, in perforated polypropylene protection tubes were used to protect the box headers, the in-flow hood and inlet housing as well as the CrNi steel oil cooler. Protection tubes keep out solid matter carried in the river water. The oil cooler was connected to the protection equipment with its own cathode conductor. 

Conductive films such as magnetite (Fe,0 ) or mill scale on steel, and conductive nonmetals such as carbon can function as cathodes when in contact with anodes... 

The principal cathodic reaction on the upper surface of the membrane is the reduction of Cu " that is formed by the reaction of Cu with dissolved oxygen in the water these Cu ions are provided partly from the diffusion through the pores in the oxide membrane from within the pit and partly from those produced by cathodic reduction (equation 1.154). Lucey s theory thus rejects the conventional large cathode small anode relationship that is invoked to explain localised attack, and this concept of an electronically conducting membrane has also been used by Evans to explain localised attack on steel due to a discontinuous film of magnetite. 

Magnetite (FejOJ has been in use since the 1970s as a cathodic protection anode, although its use as anode material has been known for some time . 

Each cement box is provided with an anode made of magnetite, arid a lid and exit pipe for the chlorine. The iron box forms the cathode. 

Sodium chlorate is produced by the electrolysis of a sodium chloride solution in an electrolyzer without a diaphragm, having an iron cathode and a magnetite or graphite anode. For the manufacture of potassium chlorate either the sodium chloride solution is used, or a mixed solution of both sodium and potassium chloride. The chlorate solution obtained will finally be converted with potassium chloride into potassium chlorate, which is separated by crystallisation while the mother liquor is returned to the electrolytic process. The potassium chloride solution will not be electrolyzed directly as potassium chlorate is sparingly soluble and the potassium chloride entrained in the chlorate crystals is not easy to remove. 

The electrolysis is carried out in a set of steel tanks arranged in cascades (for a 9000 to 12 000 A load the tank measures 3 000.2 000. 1 400 mm). They are lined with acid resisting bricks. Suspended in each tank are 144 anodes and 28 steel sheets acting as cathodes arranged in parallel rows. The magnetite electrodes shaped as narrow, hollow tiles closed at the bottom (120.50. ... 

Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heat-exchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe304), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5... 

Corrosion potentials of hematite, magnetite, maghemite, and roasted gold ore indicated that they can act as a cathode and could potentially or at least temporarily promote the dissolution of gold in galvanic coupling. 

For atmospheric wet-dry corrosion cycles, we have the well-known Evans model [93], in which the wet corrosion couples the anodic iron dissolution with the cathodic ferric oxyhydroxide reduction into magnetite ... 

For one mole of Fe, the volume of FeOOH (red rust) is 21.3cm-3 and the volume of Fe304 (magnetite) is 14.9 cm 3. This involves a 30% reduction in volume, which allows the electrolyte to penetrate more easily through the corrosion products and reach the deeply-buried chloride ions. In addition, the negatively-charged chloride ion migrates due to the influence of the electric field, from the cathode (artefact) toward the anode. This speeds up the rate of chloride ion removal and decreases the time for conservation. 

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