Corrosion and Degradation of Silica Bricks in Glass Furnaces
Mechanisms, Structural Evolution, and Practical Protection Strategies
Silica bricks are widely used in the upper structures of glass tank furnaces—particularly crowns and superstructures—due to their excellent high-temperature performance, high refractoriness under load, and good thermal stability. However, under prolonged high-temperature operation and complex furnace atmospheres, silica bricks are inevitably subjected to chemical corrosion and structural degradation, which directly affect furnace campaign life, energy efficiency, and glass quality.
As modern glass manufacturing continues to move toward higher operating temperatures and increased productivity, a systematic understanding of silica brick corrosion mechanisms and corresponding preventive measures has become increasingly important.
1. Chemical Corrosion Behavior of Silica Bricks
Silica bricks exhibit relatively poor resistance to alkaline oxides. In glass furnaces, the primary corrosive agents are alkali metal oxides, commonly expressed as R₂O (Na₂O, K₂O, etc.), originating from batch materials, volatilized alkali vapors, and gas-phase circulation within the furnace.
When large amounts of R₂O contact the silica brick surface, the melting point of the surface layer decreases sharply, leading to the formation of low-melting glassy phases. Under extreme conditions, stalactite-like molten droplets may appear; however, such phenomena are rarely observed during stable furnace operation.
Alkali components can also diffuse into the brick interior through open pores. Compared with clay-based refractories, the penetration depth in silica bricks is significantly shallower. During the initial stage of corrosion, R₂O dissolves the surface layer and forms a thin, low-melting altered zone enriched in SiO₂. Due to its high viscosity, this glassy phase tends to seal pores and hinder further inward diffusion of alkali ions, thereby slowing subsequent corrosion. Only when localized overheating occurs—such as direct flame impingement on the crown—does the protective glassy layer get removed, allowing corrosion to progress further.
2. Phase Transformation and Structural Degradation
After alkali corrosion, the surface of silica bricks often appears white and smooth, with a clearly defined altered layer. Apart from SiO₂-based crystalline phases, few other crystals are present. The diffusion of Na₂O promotes the growth and recrystallization of tridymite, which plays a critical role in the altered zone of silica refractories.
With prolonged contact with glassy phases, tridymite may grow into columnar structures within newly formed glass phases during replacement reactions. Near the highest-temperature regions, the inner surface of silica bricks is dominated by cristobalite crystals. While the theoretical transformation temperature of tridymite to cristobalite is approximately 1470 °C, the presence of R₂O can reduce this temperature to around 1260 °C.
Quartz begins transforming into tridymite at about 870 °C, allowing local temperature conditions to be inferred from observed polymorphic transformations. Both recrystallization and polymorphic phase transitions weaken intergranular bonding and may cause uneven expansion and contraction, leading to spalling, loosening, or structural failure.
3. Typical Layered Structure in High-Temperature Zones
In the high-temperature zone of the glass tank melting area, silica bricks subjected to long-term alkali attack and thermal exposure typically develop a clearly defined layered structure. From the hot face toward the cold face, this structure consists of an extremely thin, high-viscosity glassy surface layer that provides temporary protection to the brick; beneath it lies a white, dense cristobalite crystalline layer; further inward is a light-green cristobalite layer, whose coloration results from a higher FeO content; this is followed by a gray filtration layer in which the content of tridymite is higher than in the original brick, while the proportion of cristobalite is relatively reduced; the innermost zone is a light-yellow, largely unaltered original silica brick layer. This distinct stratification clearly reflects the combined long-term effects of chemical corrosion, mineral phase transformations, and thermal gradients.
4. Joint Corrosion and Localized Severe Damage
Silica bricks exhibit very poor resistance to liquid R₂O phases. Alkali liquids preferentially attack the bonding phases within the brick, leading to binder loss and subsequent loosening and detachment of aggregates.
If excessive joints exist due to improper furnace construction or heating-up procedures, R₂O vapor can penetrate into brick joints. At lower temperatures, alkali vapors condense into highly concentrated liquid phases at approximately 1400 °C, rapidly corroding the bricks and forming cavities. Ventilation or cooling further accelerates condensation and corrosion, resulting in severe structural damage.
In practice, the most severe corrosion typically occurs at the upper one-third to one-half of the silica brick height, where both condensation and high temperatures coexist. Although surface flame gaps may appear small, large internal cavities often develop beneath the surface.
5. Engineering Countermeasures and Operational Recommendations
Effective measures to mitigate silica brick corrosion include:
Minimizing joint width and quantity by using large-format crown bricks;
Applying proper crown insulation when furnace temperatures are below 1600 °C to prevent alkali condensation in joints;
Optimizing flame direction and combustion control to avoid direct impingement on crown areas;
Maintaining stable batch composition and furnace operation to limit alkali volatilization.
Field experience confirms that large-format crown bricks combined with appropriate insulation not only reduce fuel consumption but also effectively protect the crown structure and extend service life.
Under normal operating conditions (furnace temperatures below 1600 °C), stone formation from silica brick crowns is rare. Since silica bricks consist primarily of SiO₂, they readily dissolve and homogenize into the glass melt. However, under abnormally high temperatures, molten silica brick material may flow downward, corrode underlying fused-cast refractories, and enter the glass melt, forming refractory stones.
As furnace operating temperatures continue to rise, higher-quality silica bricks are required. Premium-grade silica bricks typically contain ≥97% SiO₂, with Al₂O₃ content below 0.3% and total impurities under 0.5%. Their refractoriness under load is approximately 30–40 °C higher than that of conventional silica bricks, enabling furnace operating temperatures to increase by 20–30 °C while maintaining long-term stability and efficiency.
