High-temperature corrosion combustion products

When refractory linings are intended to contain moderate to high temperature environments having products of combustion or reaction containing compounds of sodium, lithium, potassium, vanadium and titanium and bromides, fluorides, chlorides, sulfides, phosphates along with the usual CO2, CO, H2, and O2, extreme care must be taken in their design. In these highly corrosive atmospheres, refractories perform differently than they do in clean environments. 

V.A. Izhevskyi, L.A. Genova, A.H.A. Bressiani, and J.C. Bressiani, Liquid Phase Sintered SiC. Processing and Transformation Controlled Microstructure Tailoring, MatRes., 3(4), 131-38 (2000). V.A. Lavrenko, D.J. Baxter, A.D. Panasyuk, and M. Desmanion-Brut, High-Temperature Corrosion of AIN-Based Composite Ceramic in Air and in Combustion Products of Commercial Fuel. 1. Corrosion of Ceramic Composites in the AIN-SiC System in Air and in Combustion Products of Kerosene and Diesel Fuel, Powder Metallurgy and Meta Ceramics, 43(3-4), 179-86 (2004). 

Because PTFE resins decompose slowly, they may be heated to a high temperature. The toxicity of the pyrolysis products warrants care where exposure of personnel is likely to occur (120). Above 230°C decomposition rates become measurable (0.0001% per hour). Small amounts of toxic perfiuoroisobutylene have been isolated at 400°C and above free fluorine has never been found. Above 690°C the decomposition products bum but do not support combustion if the heat is removed. Combustion products consist primarily of carbon dioxide, carbon tetrafluoride, and small quantities of toxic and corrosive hydrogen fluoride. The PTFE resins are nonflammable and do not propagate flame. 

The furnace is constmcted with a steel shell lined with high temperature refractory (see Refractories). Refractory type and thickness are deterrnined by the particular need. Where combustion products include corrosive gases such as sulfur dioxide or hydrogen chloride, furnace shell temperatures are maintained above about 150—180°C to prevent condensation and corrosion on the inside carbon steel surfaces. Where corrosive gases are not present, insulation is sized to maintain a shell temperature below 60°C to protect personnel. 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

---