Field-Assisted Reaction at Molybdenum-Molten Silicate Glass Interface

Effects of Additives such as Fe₂O₃, NiO, Cr₂O₃ and MnO₂

Abstract

The fundamental electrochemical behavior of molybdenum was investigated in a commercially important glass (Na₂O-CaO-SiO₂) with various additives such as Fe₂O₃, NiO, Cr₂O₃ and MnO₂ by means of linear potential sweep voltammetry at 1400°C. The molybdenum-glass interface after polarization was also analysed by EPMA.
The cathodic current in the initial applied potential region was larger in the additive-containing glass than in the glass with no additive. The additives were reduced predominantly to the metallic state at the initial potential but reduced together with silicate anions at higher potentials and made alloys with Si and Mo.
The additives were more or less effective to promote oxidation of molybdenum. Fe³⁺ ions oxidized molybdenum to MoO₂ by the reaction 2Fe₂O₃+Mo=4FeO+MoO₂ (ΔG=-35.2kcal/mol). Ni²⁺ ions showed marked oxidizing power by the reactions 2NiO+Mo=2Ni+MoO₂ (ΔG=-26.4kcal/mol) and 3NiO+Mo=3Ni+MoO₃ (ΔG=-10.6kcal/mol). Cr⁶⁺ and Mn³⁺ ions should also have marked oxidizing power but the respective redox equilibrium in the soda-lime-silica glass seemed to be shifted to the lower valence states. Therefore, the oxidation of molybdenum by Cr and Mn ions was depressed.