Evaluation of Cyclic Oxidation and Hot Corrosion Behavior of HVOF-Sprayed WC-Co/NiCrAlY Coating
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B. Somasundaram, Ravikiran Kadoli, and M.R. Ramesh
(Submitted January 26, 2014; in revised form April 14, 2014) Corrosion of metallic structural materials at an elevated temperature in complex multicomponent gas environments are potential problems in many fossil energy systems, especially those using coal as a feedstock. Combating these problems involves a number of approaches, one of which is the use of protective coatings. The high velocity oxy fuel (HVOF) process has been used to deposit WC-Co/ NiCrAlY composite powder on two types of Fe-based alloys. Thermocyclic oxidation behavior of coated alloys was investigated in the static air as well as in molten salt (Na2SO4-60%V2O5) environment at 700 °C for 50 cycles. The thermogravimetric technique was used to approximate the kinetics of oxidation. WC-Co/NiCrAlY coatings showed a lower oxidation rate in comparison to uncoated alloys. The oxidation resistance of WC-Co/NiCrAlY coatings can be ascribed to the oxide layer of Al2O3 and Cr2O3 formed on the outermost surface. Coated alloys extend a protective oxide scale composed of oxides of Ni and Cr that are known to impart resistance to the hot corrosion in the molten salt environment.
Keywords
cyclic oxidation, hot corrosion, HVOF thermal spray process, surface analysis
1. Introduction Energy systems present a wide range of situations where components must survive in aggressive high temperature environments. In the coal-fired boiler, the combustion of coal generates very corrosive media, particularly near the superheater tubes, forming highly aggressive ash deposits that contain alkali metals of sodium and potassium, and sulfur. This phenomenon can also be seen in the gas side of power station boilers when the fuel is a residual oil. Ashes formed due to the combustion of such fuels have a high concentration of compounds formed by vanadium, sodium and sulfur, mainly as Na2SO4-V2O5 complex and sodium-vanadates mixtures. Some mixtures of these compounds have low melting points (550 C). These compounds easily liquefy at the operating temperatures of boilers and cause accelerated corrosion (Ref 1, 2). The energy crisis and protection of the environment require that coal-fired power plants have high power and low emissions, which introduces new requirements for boiler pipe materials (Ref 3). The B. Somasundaram, Department of Mechanical Engineering, Reva Institute of Technology and Management, Yelahanka, Bangalore 560 065, India; and Ravikiran Kadoli and M.R. Ramesh, Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore 575 025, India. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
difficulties associated with mechanical properties and workability of highly alloyed materials lead to advances in coating technology where a surface composition for specific requirement can be developed. Coatings provide a way of increasing the limits of use of substrate materials at the upper end of their performance capabilities, by retaining mechanical propertie
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