STUDIES ON HOT CORROSION BEHAVIOUR OF CONVENTIONAL AND NANOSTRUCTURED COATINGS

Abstract

Huge amounts of material wastage occur due to high temperature oxidation and erosion of boiler tubes in steam generating system of coal fired boiler and power generation industry, resulting in tube wall thinning and premature failure. Ni-based, Co-based, and Fe-based super alloys have been developed to enhance their high temperature strength and oxidation resistance in high temperature applications. Since these alloys are very costly, a composite system comprising of base material (steel) and wear/corrosion resistant protective surface layer has been proposed as cost effective and most favorable choice of material for combining both mechanical and corrosion resistance properties. Among different approaches employed to ensure longer service life of the components used at elevated temperature, wear and corrosion resistant coatings are widely used to provide protection. It is well known fact that a single material can not provide a combination of both mechanical and corrosion resistance properties. Thermal spray process is widely used to produce wear and corrosion-resistant coatings on the components used in hot section of power generation industry and coal fired boiler as reported in the literature. Fe based high-chromium and nickel containing alloys are extensively used to fabricate corrosion and wear resistant coatings. These FeCr-base coatings are deposited by different thermal spray techniques such as plasma spray process, detonation gun process, high velocity arc spray process (HVAS), and high velocity oxy-fuel process (HVOF). Among all these thermal spray processes, HVOF & HVAS techniques are usually employed due to its high efficiency of deposition and less applications difficulties. HVOF process produces dense coating with higher bond strength and lower oxide content. Hot corrosion may be defined as an accelerated corrosion, resulting from the presence of salt contaminants such as Na2SO4, Fe2(SO4)3, NaCl, and V2O5 that combine to form molten deposits, which damage the protective surface oxides. At higher temperatures, deposits of Na2SO4 are molten (m.p. 884°C) and can cause accelerated attack on boiler steel, the attack is commonly called as hot corrosion. For example, alloys used in gas turbines in aircraft, marine power plants, land-based power generators, boilers, internal combustion engines, fluidized bed combustion and industrial waste incinerators undergo hot corrosion. During hot corrosion, a porous non-protective oxide scale is formed at the surface and sulphides in the substrate. This form of corrosion, unlike oxidation, can consume the material at an unpredictably rapid rate. Consequently, the load-carrying ability of the components reduces quickly, leading eventually to catastrophic failure. For example, boiler steels used for high temperature applications could not meet the requirements of both high-temperature strength and high-temperature erosion– ii corrosion resistance, simultaneously. Therefore, thermal spray coatings deposited on 310S alloy substrate provided a significant contribution for combating high temperature oxidation and hot corrosion. The viable countermeasures against the oxidation and hot corrosion of alloys used in high temperature applications constitute the use of protective coatings. The oxidation and hot corrosion studies of Cr3C2-NiCr, FeCr- based conventional and nanostructured coatings developed by HVOF and HVAS spray are limited in the literature. Therefore, oxidation and hot corrosion behavior of austenitic stainless steel designated as 310S as per the manufacturer’s specifications, has been investigated with and without the application of thermal spray coatings (Cr3C2-NiCr, FeCr- based coatings) in air, Na2SO4-60%V2O5, Na2SO4–82% Fe2(SO4)3, at 700 & 900 oC, and in actual coal fired boiler environment under cyclic conditions. These alloy substrate (310S) are developed for the high temperature applications such as boilers and gas turbine parts, heat exchangers and piping in chemical industries and high temperature furnace parts. A thorough investigation on the behaviour of Cr3C2-NiCr, FeCr- based conventional and nanostructured coatings in different environment is very essential to choose the suitable coating and substrate for precluding the oxidation and hot corrosion problems manifested in the gas turbine, boiler etc. The results of the present research work are critically analyzed and discussed in light of the existing literature to propose an insight in to the corrosion mechanisms in both coated and bare specimens. The behavior of these coatings in different degrading environments will be helpful in choosing the suitable coating for the hot section components of the thermal power plant.