Corrosion of Chromium in Glass Melts

Abstract

The corrosion of pure chromium was studied in four molten glasses, using both scanning electron microscopy and electrochemical methods to characterize the metal/glass interactions. It is shown that direct immersion of chromium into glass does not allow obtaining a protective oxide scale, even if the glass contains oxidizing species such as Fe III , Zn II , etc. In these conditions, the corrosion mechanisms vary with temperature and glass composition. When the metal is oxidized in hot air preliminary to glass diving, the passive state is reached and related to the presence of a Cr 2 O 3 continuous layer at the metal/glass interface. It is maintained up to a temperature called "depassivation temperature." This temperature is close to 1160°C in a borosilicate glass and is shifted to the higher values when the glass is enriched into oxidizing species ͑Fe III ͒. Coupling both SEM and electrochemical data shows that the Cr 2 O 3 layer continuity is ensured by microgalvanic couples occurring between parts where the oxide scale is in contact with the metal substrate and other parts where the oxide scale is peeled off. When the glass temperature is higher than the depassivation temperature, a chromium boride CrB layer develops at the metal/glass interface. Most of the alloys and superalloys used in the glass-making industry are chromia-forming materials.1-3 Generally, when a given material is used for a specific application, the process is never changed because it is known that a variation of one parameter can drastically change the metal behavior in molten glass, particularly its corrosion resistance. 4 One of the most important parameters is the glass temperature. Increasing the glass temperature increases its fluidity and the making of the glass becomes easier. On the contrary, the mechanical properties of the alloy are decreased as well as its corrosion resistance. 5-7Many works concerning the study of the pure metals behavior in glass melts have been realized since the late 1950s. Ni, 2,21 Fe, Co, and Cr, 2,22 which are proposed to be used in the glass-making industry as raw materials or coatings on ceramics. The results of these studies indicate that the behavior of pure metals immersed in molten glasses ͑in terms of corrosion rates and corrosion layers͒ varies with glass composition, melt temperature, and the metal potential ͑in the case of use of the material as a fusion electrode͒. Di Martino et al. 2 have shown that the glass working temperature increase can induce important changes in the metal behavior versus glass corrosion. As an example, these authors have shown that when pure chromium rods are dived in a C-type borosilicate molten glass, there is a corrosion mechanism modification when the glass temperature is increased.Most of the alloys in contact with molten glasses that are used in the glass industry are chromia-forming materials. As a consequence, the knowledge of pure chromium corrosion in glass melts will provide useful information concerning the comprehension of chromiaforming alloys in molten gla...

A new anode material for oxygen evolution in molten oxide electrolysis

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