Content area
Full text
1. Introduction
Hot corrosion degradation of metals and alloys has been recognized as a serious problem. It can be defined as degradation of metallic components in the presence of salt and gases which react with each other in high temperature environment. Hot corrosion has been classified into two types, firstly which takes place above 900°C and the second which occurs between 700°C-750°C (Jones, 1992; Khanna, 2002; Goyal et al., 2016; Srikanth et al., 2003). Hot corrosion is a mechanism, in which fuel after burning, forms compounds like vanadium, sulfur which gets deposit on the components of boiler, e.g. water walls, supper heaters, re-heaters, rotary air pre heater and economizer (Fagoaga et al., 1998). The degradation of material occurs when these molten salts dissolve protective oxide layer that naturally form on materials during boiler operation (Sidhu et al., 2006f). Vanadium mainly present in fuel oil, it reacts with boiler steel between (593°C-816°C) temperature but it can be controlled by limiting excess air into the boiler (Sidhu et al., 2006d). Fly ash particles having chlorine content gets deposit on the surface of component and increase corrosion rate (Rapp, 2002). To stand in extremely complex environments iron, nickel and cobalt based steel are used for manufacturing of components used in aggressive environments of gas turbines, steam boilers etc. (Sidhu et al., 2005). Many techniques have been used in recent past to protect these parts from corrosion. Among all the techniques, thermal spray coating has been considered as best technology to protect the components. Coatings provide a way of extending the limits of materials use and their performance capabilities against wear and corrosion (Grabke et al., 1995). Coating is a layer of material, deposited artificially on the surface of an object, with aim of obtaining required technical or decorative properties (Sidky and Hocking, 1999). Thermal spray coating is produced by heating the feedstock material in a hot gaseous medium or chamber called as gun and simultaneously projecting it with high velocity on a prepared surface (Burakowski and Wierzchon, 1998). Coatings are typically designed in such a way that the selective oxidation of an element occurs to form a protective oxide scale such as Cr2O3, Al2O3 or SiO2 on revelation to reactive environments...