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Silica-alumina fibers can withstand various chemical erosions.

       
Post:2024-03-05Hit:61

(1) Fluorides and oxides. Undoubtedly, oxygen is the most corrosive to silica-alumina fibers. Below 100°C, fluorine and water react with silica-alumina fibers, causing significant structural damage to the fibers. Even at low concentrations, hydrofluoric acid reacts first with alumina, forming AlF, AlF H2O, leading to severe structural damage. Hydrofluoric acid is most likely to react with SiO2. In low-temperature environments (below 980°C), fluorine-rich environments may promote low-temperature recrystallization, leading to minor structural damage to the fibers. At temperatures where extensive recrystallization occurs, nitrogen may react on the fiber surface, causing changes in fiber structure and the formation of a thin hard shell layer on the fiber surface. Fluorine reacts with all silica-alumina materials, including mullite. After reacting with fluorine, the reaction products often gasify, making it difficult to determine the cause of silica-alumina fiber destruction.

(2) Vanadium and other heavy metals. Vanadium and other heavy metals present in fuel oil can corrode silica-alumina fibers during combustion. This corrosive substance comes from vanadium pentoxide (V2O3), which is solid at room temperature but melts at approximately 690°C. This molten slag adsorbs into the pore structure of the fibers and reacts with the silica-alumina in the fibers. After the reaction, a hardened shell will form on the hot side of the fibers. After a period of time, the hardened shell will peel off from the unreacted fiber layer of the fiber blanket, and the chemical reaction will continue on the exposed fresh fiber surface. The rate of reaction depends on factors such as heavy metal concentration, time, material porosity, and temperature. There is no definitive concentration threshold that determines whether a reaction occurs. Typically, long-term exposure to high concentrations of heavy metals shortens the life of refractory materials. Vanadium pentoxide reacts with fibers at temperatures above 690°C, but the reaction rate is influenced by the above factors. As the chemical reaction continues, regular replacement of hot surface materials is necessary. Of particular note is that vanadium present in alkaline compounds can form highly corrosive slag at very low temperatures, leading to faster degradation of silica-alumina fibers. Typically, fuels with high metal contents also contain alkalis.

(3) Sulfur and sulfuric acid. Silica-alumina fibers have good resistance to sulfuric acid erosion, but in very few cases, some minor chemical reactions may occur, with the typical reaction products being aluminum sulfate (Al2(SiO4)3) or aluminum sulfate hydrate. Generally, such reactions do not result in fiber destruction. In fact, the corrosion of metal fasteners requires special attention. Asphalt coatings or stainless steel films should be used to protect metal fasteners, or ensure that the temperature outside the metal fasteners is higher than the dew point temperature of sulfuric acid, generally 121~177°C. In the presence of iron (Fe) and sulfuric acid (H2SO4), silica-alumina fibers will produce iron-aluminum-silicon composite sulfate, which will “dissolve” fasteners and furnace linings, turning fasteners into gray rock-like substances. Typically, fibers turn yellow after contact with sulfur, and it is generally believed that the deposition of sulfur on fiber surfaces affects the crystallization process of fibers at around 980°C.

The reaction between sulfur and silica-alumina fibers is generally distributed discontinuously on the surface of the fiber lining, and this reaction is believed to result in the formation of a uniformly powdered layer on the fiber lining surface. Under mechanical forces (such as vibration) or air flow flushing, the fiber lining hot surface refractory material will gradually peel off. The loss of refractory material from the hot surface will expose new surfaces, which are easily corroded further, leading to the repetition of the aforementioned reaction process. Only after long-term use will these reactions cause substantial damage to the furnace lining as a whole. For layered fiber linings, it may be necessary to periodically replace the fiber blanket on the hot surface. If fiber components are used, consider spraying a layer of ceramic coating material on the fiber surface.

(4) Anti-corrosion. Alkaline corrosion of silica-alumina fibers is believed to depend mainly on two factors: time and temperature. Alkali metals react with fibers to form low-melting compounds, causing fibers to shrink or sinter, ultimately leading to the destruction of fiber linings. Chemicals such as V2O3 and SO3 can accelerate this destructive behavior, leading to earlier damage to the furnace lining. Alkali corrosion generally occurs on the hot surface of the fiber lining, where it forms a hardened shell or “slag” layer after the reaction. As the reaction progresses, the surface layer of the fibers is destroyed and becomes ineffective, and a new surface is exposed, which then undergoes continuous chemical erosion.

(5) Other acid erosion. Silica-alumina fibers are generally considered to have good resistance to hydrochloric acid (HCl), acetic acid (CH3COOH), and nitric acid. At low temperatures, they can resist sulfuric acid erosion. However, when aluminum phosphate forms at high temperatures, the fibers will undergo significant shrinkage. Under conditions where aluminum phosphate forms at temperatures above 540°C, it is advisable to avoid contact between silica-alumina fibers and phosphoric acid.

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Silica-alumina fibers can withstand various chemical erosions.
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