ABSTRACT
Specimens ........................................................................................................ 144 12.3.2 Electrochemical Impedance Spectroscopy .......................................... 147 12.4 Conclusion ....................................................................................................... 149 Keywords ................................................................................................................. 149 References ................................................................................................................ 149
12.1 INTRODUCTION
Hot corrosion is a serious problem in power generation equipment, gas turbines for ships and aircraft, and in other energy conversion and chemical process systems. No alloy is immune to hot corrosion attack indefinitely. Super alloys have been developed for high temperature applications. However, these alloys may not be able to meet both the high temperature strength requirements and high temperature corrosion resistance simultaneously for longer life. So, protective coatings are used to counter the latter. Due to continuously rising cost of the materials as well as increased material requirements, the coating techniques have been given more importance in recent times. Coatings can add value to products up to ten times the cost of the coating [1]. The necessity to preserve good mechanical properties of alloys at elevated temperature under the highly oxidizing and corrosive conditions led to the development of coating materials [2, 3]. The composition and structure of the coatings are determined by the role that, they play in various material systems and performance environments and may vary from one system to another according to the requirements. In the service environment, the coatings are expected to form protective oxides and carbides such as TiO2, Al2O3,
Cr2O3, SiC, and WC. So, the coatings are designed to serve as a reservoir for the elements forming or contributing to form these surface oxides and carbides. Nickel-based alloy coatings show good high temperature wear and corrosion resistance. They have good wear resistance after adding W and Mo elements to the alloy [4]. Nickel-based coatings are used in applications when wear resistance combined with oxidation or hot corrosion resistance is required. Nickel-based self-fluxing alloys are mainly used in the chemical industry, petrol industry, glass mould industry, hot working punches, fan blades, and mud purging elements in cement factories. Their advantages are especially related to coating large-sized components such as piston rods, earth working machines, and so on [5].
