DEVELOPMENT OF EROSION - CORROSION WEAR RESISTANT COATINGS ON SUPERALLOYS

Abstract

Degradation of materials due to erosion either at room temperature or at elevated temperature is encountered in a large variety of engineering industries. Solid particle erosion is the loss of material that results from repeated impact of small solid particles. In industrial applications, solid particles are produced during the combustion of heavy oils, synthetic fuels, and pulverized coal, and causes erosion of materials. Boilers and other power plant equipments suffer severe erosion and erosion-corrosion problems resulting in substantial losses. Fly ash content of the Indian coal is very high and causes severe erosion and erosion-corrosion of the materials in the power plants, resulting in increased forced shut down and repair. This is one of the major reasons for shortage of power, which further increases the cost of power for industries, thus affecting the overall growth of the country. These facts emphasize the need to develop more erosion and erosion-corrosion resistant coatings to protect the materials (metals and alloys) used in energy generation systems. Currently superalloys are being used to increase the service life of the boilers, especially in the superheater zones of the boilers and new generation ultra and supercritical boilers. Although the superalloys have adequate mechanical strength at elevated temperatures, they often lack resistance to erosion-corrosion environments. Protective coatings are primarily used to restrict the surface damage of the components in practice. Suitable coating techniques also allow for regeneration of parts that have been rendered unusable by erosion. Among the various techniques used for deposition of coatings, plasma spraying is a most flexible and versatile thermal spray process with respect to the sprayed materials. Plasma spraying has been a successful and reliable cost-effective solution for many industrial problems. It allows the spraying of a wide range of high performance materials from superalloys and refractory intermetallic compounds to ceramics with continuously increasing commercial applications. It does not cause deterioration of the substrate alloys, and relatively thick coatings can be formed with high deposition rates.