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Electrochemical aspects of D.C. electroslag remelting Beynon, Gordon Thomas


It is predicted from the known ionic properties of the slags used in electroslag melting, that the D.C. melting process should be accompanied by Faradaic reactions on the slag/ingot and slag/electrode interfaces. In the present work we have determined the magnitude of the overpotentials resulting from concentration polarization at these Interfaces, for the case of pure iron in contact with CaF₂ + Al₂O₃ and CaF₂ + CaO slags. This has been carried out using a galvanos tatic pulsing technique in an electrolytic cell. The polarization overpotential existing on an electrode in an operating ESR unit has been measured essentially by the same technique. It was found that the potentials observed on melting ESR electrodes agree well with the results from the electrolytic cell. The primary anodic process is postulated to be the corrosion of iron, leading to an Fe²⁺ - saturated slag layer on the anode surface at sufficiently high current densities. The cathodic process is suggested to be the Faradaic reduction of Al³⁺ or Ca²⁺, to produce a concentration of [Al][sub Fe] or (Ca)[sub slag] in the cathode interface region. The concentration polarization behaviour of other pure electrode materials (Cr, Ni, Co) was also investigated in CaF₂ + Al₂O₃ slags. For these materials, it has been postulated that the primary anodic process is the corrosion of the metal which again leads to interface saturation by the appropriate metal ions. The concentration polarization behaviour of iron alloy electrodes (Fe-Cr, Fe-C) was also investigated. It was found that the more easily oxidizable alloying elements are preferentially removed from these alloys when they are anodically polarized. At very high current densities both the anodic and cathodic processes may convert to arcs, leading to process instability. The chemical and thermal phenomena associated with D.C. ESR were studied by making ingots from pure electrode materials and iron-alloy electrode materials. Chemical effects which include metal oxidation and possible alloy loss were explained in terms of the Faradaic reaction mechanisms proposed in the electrolytic cell studies. Thermal effects were also explained on the same basis. It was concluded that electrochemical reaction products were responsible for the formation of large numbers of small oxide inclusions in the pure metals. In the case of alloy materials, electrochemically produced oxidation resulted in significant losses of alloy components during melting.

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