Electrochemical Reactions in a Pure Na2 SO 4 Melt

Abstract

Cyclic voltammetry and chronopotentiometry were used to study the electrochemical reduction reactions of SO3 gas, Oz-and SO4 s-ions in a Na~SO4 melt at 900~ The reduction reaction of SO~ follows a ce mechanism: SO3 first reacts chemically with SO42-to form $2072-and then proceeds via a one-electron electrochemical reduction reaction to form SO3-. The reduction of peroxide 02-ions forms either O s-or both O 2-and superoxide O2 z-ions. Sulfate ions are subjected to decomposition at either very positive or very negative potentials. At very high positive potentials, sulfate ions decompose to evolve SOs and 02 gases, in addition superoxide ions are also formed. At very negative potentials, sulfate ions decompose to form sulfide and peroxide.In general, the attack of metals by a salt film in an oxidizing gas, which is called "hot corrosion" (1), becomes virulent only when the salt is molten. In this circumstance the alloy is physically separated from the gas phase by the molten salt so that the mechanism of corrosion must involve mainly electrochemical reactions. Obviously, an electrochemical study should be the most promising approach to investigate hot corrosion and might eventually explain the reaction mechanisms.Electrochemical studies have been confined mostly to alkali halide melts where a voluminous literature can be found(2). Sulfate melts, on the other hand, have received little study because of their limited practical importance except for their deleterious effect in the hot corrosion of metals. Na2SO4, because of its high thermodynamic stability, is the constituent most commonly found in the salt film deposited by the combustion of fossil fuels; knowledge about the electrochemical reactions taking place in this melt is essential to the understanding of the Na2SO4-induced hot corrosion mechanism. Electrochemical reduction of SO42-ions has been studied by several investigators. Liu (3) found sulfite and sulfide, but little or no sulfur, as the reduction products of a LisSO4-K2SO4 melt electrolyzed between platinum electrodes a~ 625~ Johnson and Laitinen (4) however reported that a eutectic Li~SO4-Na2SO4-KsSO4 melt electrolyzed at 550~ formed either sulfur or a metallic sulfide. In prolonged cathodic electrolysis of a LiSO4-K2SO4 melt at 625~ Burrows and Hills (5) observed the formation of elementary sulfur, oxide ions, and SOs. Reduction reactions for the dissolved gaseous species O2, SOs, and SO3 in the Li~SO4-K2SO4 eutectic melt were studied using cyclic voltammetry by Burrows and Hills (5). They found no cathodic reduction of dissolved oxygen but there were reduction waves for dissolved SO3 and SO2; nonetheless, no reduction reaction mechanism for SO3 and SO2 was given. Shores and Fang (6) suggested that the reduction of SO3 in a pure Na2SO4 melt at 900~ actually proceeded through the reduction of the pyrosulfate ion (S2OT ~-) which was the reaction product from reacting SO3 with SO42-. The present study concerns the electrochemical reactions of dissolved SOs, oxide ions, and sulfate ions (SO42-) in a pure ...

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