Ceramics-Silikáty 47 (4) 162-168 (2003)

THE BEHAVIOUR OF MOLYBDENUM ELECTRODE IN SULPHATE-REFINED GLASS MELT

J. Matěj, R. Kocourová, A. Langrová, V. Čierná

The extent of corrosion and the composition of both condensed and gaseous reaction products on the molybdenum electrode in a model glass melt with an approximate composition of 74 SiO₂, 10 CaO, 16 Na₂O and 0.5 SO₃ (wt.%) were investigated in terms of the electrode potential and loading by alternating current at 1400°C. At the rest corrosion potential, probably a liquid layer of Mo₂S₃ is formed on the electrode. In the course of the corrosion process, molybdenum is probably introduced into the glasss melt as Mo(III). No development of SO₂ was established under these conditions. An intimation of passivation and appearance of oxygen compounds of molybdenum begin at a potential increased by 200 mV. The passivation is interfered with by development of SO₂ on the electrode, which starts already at a potential raised by 300 mV, and is probably originated by electrochemical oxidation of sulphate anions. The corrosion rate thus increases again with increasing potential. The oxidation of molybdenum stops when the potential has been reduced by about 200 mV. The depolarizing reaction was suppressed at the same time. Beginning of the penetration of silicon into the electrode was recorded at potentials decreased by 400 mV. Further reduction of the potential resulted in the formation of MoSix layers and in the liberation of hydrogen, which, however, was rapidly oxidized by the glass melt. The corrosion rate is virtually unaffected by alternating current with a density of 1 A/cm². No electrochemical separation of gas on the electrode takes place at loadings of up to 5 A/cm². The increased formation of bubbles in glass melt in the neighbourhood of the electrode is a result of overheating of the glass melt by the liberated heat.

Keywords: Molybdenum electrode, Glass melt, Sulphate, Passivation