Evaluation of the behavior of shrouded plasma spray coatings in the platen superheater of coal-fired boilers
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INTRODUCTION
METALLIC components in coal gasification pilot plants are exposed to severe corrosive atmospheres and high temperatures. The corrosive nature of the gaseous environments, which contain oxygen, sulfur, and carbon, may cause rapid material degradation and result in the premature failure of components.[1,2] The most common deposit found on boiler superheaters is sodium vanadyl vanadate, Na2O " V2O4 " 5V2O5, which melts at a relatively low temperature, 550 °C. Above the melting point, this ash material corrodes metals by long-term contact.[3] If the temperature of the metal surface is above the melting point of the salt’s eutectic composition, very corrosive conditions develop with the molten salts fluxing the protective oxide scales or directly dissolving the metal.[4] When considering coal-gasification processes, hot corrosion is expected to be a problem because the gas environment generally has large sulfur activities and low oxygen activities and also contains substantial amounts of salt. At present, methods to minimize the extent of hot corrosion have been identified; however, considerable research effort is needed for a quantitative evaluation of these methods under conditions of interest in the coal-gasification processes.[5] Unacceptable levels of surface degradation of metal containment walls and heat exchanger tubing by a combined erosion-corrosion (E-C) mechanism have been experienced in some boilers, particularly fluidized bed combustors.[6] Increasingly greater demand imposed on materials makes it more difficult or, at the current stage of development, even impossible to combine the different properties required in one single material. Therefore, a composite system of a base material providing the necessary mechanical strength with a protective surface layer different in structure or chemical composition and supplied by a surface treatment can be an optimum choice in combining material properties. Although protective surface treatments BUTA SINGH SIDHU, Assistant Professor, is with the Department of Mechanical Engineering, GZS College of Engineering and Technology, Bathinda (Punjab)-151001, India. Contact e-mail: [email protected] S. PRAKASH, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Roorkee-247667, India. Manuscript submitted May 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
are widely used at low temperature, the use of these at elevated temperature is more recent. Current high-temperature applications are limited largely to the aerospace industry. An enormous challenge exists now to develop and apply these techniques to other high-temperature applications.[7,8] Among the different kinds of coating technologies, plasma spray coating should be one of the most promising. The advance plasma spray techniques have many advantages such as high productivity for coating thick films of more than 100 mm and good applicability for a wide range of coating materials including ceramic powder; further, the process does not cause degradat
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