Comparative evaluation of hot corrosion resistance of nanostructured AlCrN and TiAlN coatings on cobalt-based superalloy
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Sandeep Sahub) and Prabhat Chand Yadavc) Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India (Received 8 December 2017; accepted 23 February 2018)
Molten vanadate-induced hot corrosion is the major cause of failure of superalloys which are generally used at higher temperatures (such as in internal combustion engines, gas turbines, high temperature tooling and dies, and petrochemical industries and marines). This effect can be minimized by applying thermally stable coatings over the superalloy. In this aspect, the current work investigates the effect of nanostructured aluminum chromium nitride (AlCrN) and titanium aluminum nitride (TiAlN) coatings on the hot corrosion behavior of Co-based superalloy, Superco-605, in an aggressive environment of Na2SO4–60% V2O5 (ratio by weight) at 700 °C up to 80 cycles. Each cycle consisted of 1 h heating at 700 °C followed by 20 min cooling in an ambient temperature. Hot corrosion kinetics was studied using the thermogravimetric technique and found to follow the parabolic rate law. The corrosion surface morphology and phases formed during hot corrosion were studied using field emission scanning electron microscopy equipped with energy dispersive spectroscopy and X-ray diffraction techniques. It was found that AlCrN coating had a better hot corrosion resistance than TiAlN coating.
I. INTRODUCTION
Nickel (Ni), cobalt (Co), and iron (Fe)-based superalloys have increased the efficiency of gas turbines due to their property of retaining the strength, even at elevated temperatures.1 However, these materials show accelerated deposition of oxide layers when used in air/fuel environment contaminated with molten salts at elevated temperatures (generally between 700 and 925 °C). This form of attack is generally termed as ‘hot corrosion’2–8 to differentiate it from the traditional low-temperature corrosion. The temperature range, in which hot corrosion takes place, depends on a number of factors such as salt chemistry, gas constituents and their velocity, alloy composition and its fabrication condition, and thermal cycles as well as on the erosion of the alloy during corrosion attack. Hot corrosion has been observed in various marine and industrial applications such as coal gasifiers, internal combustion engines, exhaust systems, petrochemical equipment, boilers and gas turbine engines, etc.9,10 This form of attack, unlike oxidation, consumes the material at a very fast rate, which decreases
Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected], [email protected] b) e-mail: [email protected] c) e-mail: [email protected] DOI: 10.1557/jmr.2018.53
their load-bearing capability and ultimately results in the failure of the components. Ni, Co, and Fe-based superalloys were developed for use in the high temperature applications with emphasis to retain the mechanical properties up to a great extent of their melting temperature. However, their hot corrosion resistance propert
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