A comparative study of hot corrosion resistance of HVOF sprayed NiCrBSi and Stellite-6 coated Ni-based superalloy at 900 °C

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Abstract

A comparative study was carried out to evaluate the hot corrosion resistance of NiCrBSi and Stellite-6 coated nickel-based superalloy Superni 600 (15.5Cr–10Fe–0.5Mn–0.2C–balance Ni) in the molten salt environment of Na2SO4–60%V2O5 salt mixture at 900 °C under cyclic conditions. Hot corrosion experiments were performed for 50 cycles, each cycle consisting of 1 h heating in the laboratory tube furnace followed by 20 min cooling in the open air. The thermogravimetric technique was used to establish the kinetics of corrosion. The morphology, phase composition and element concentration of the corrosion products were detected using the combined techniques of X-ray diffractometry (XRD), scanning electron microscopy/energy-dispersive analysis (SEM/EDAX) and electron probe micro analyzer (EPMA). The hot corrosion resistance of NiCrBSi coating has been found to be better than that of Stellite-6 coating. The hot corrosion resistance of both the coatings has been attributed to the formation of oxides of chromium, silicon and nickel along with spinels of nickel–chromium and cobalt–chromium. These oxides seal/plug the pores and splat boundaries, and act as diffusion barriers to the inward diffusion of corroding species.

Introduction

Due to depletion of high-grade fuels and for economic reasons, residual fuel oils are often used in boilers, refinery furnaces and gas turbines. Residual fuel oil contains sodium, vanadium and sulphur as impurities, which form compounds such as Na2SO4 (melting point 884 °C), V2O5 (melting point 670 °C), and complex vanadates by reactions in the combustion systems [1], [2], [3]. These compounds, known as ash, deposit on the surface of materials and induce accelerated oxidation (hot corrosion). Moreover, the vanadium compounds are also good oxidation catalysts and allow oxygen and other gases in the combustion atmosphere to diffuse rapidly to the metal surface and cause further oxidation [4].

A number of countermeasures are currently in use or under investigation to combat the hot corrosion such as using inhibitors, controlling the process parameters, designing suitable industrial alloys and depositing the protective coatings. The inhibitors like MgO, CeO2, CaO, MnO2, etc. have been successfully used to decrease the extent of hot corrosion in the most aggressive environment of Na2SO4–60% V2O5 at 900 °C [5], [6]. However, their use in actual industrial environment is limited due to the practical problems of injecting them along with the fuel in the combustion chamber. Further, controlling the various process parameters of the boiler and gas turbine such as air/fuel ratio, temperature, pressure, etc., can also be useful to some extent to combat the hot corrosion, but these parameters can be controlled only within certain limits.

The corrosion resistance of the superalloys can be improved by adding fair amounts of Al and Cr, and small amounts of Y, Zr and Hf [7]. However, these elements (such as Al, Cr) can be added only up to a limited extent as their higher concentrations adversely affects the mechanical properties of the alloys [8], [9]. The use of protective coatings is the most practical, reliable and economically viable method to resist the hot corrosion. These composite materials perform better under extreme conditions, the base material provides the required mechanical strength and the coatings protect them against wear, erosion or corrosion [10], [11], [12].

The composition and structure of the coatings are determined by the role that they have to play in the various material systems and service environments [13]. During service, the coatings are expected to form slowly growing protective oxides which should not allow the corrosive species to diffuse into the coating and should also have resistance to cracking and spallation under mechanical and thermal stresses induced during operation of the components. Further, these coating should also serve as a reservoir for the elements expected to form protective oxides [13], [14]. Therefore, the identification/development of suitable coatings is of great interest for higher temperature applications in boilers and gas turbines. The present investigation is in continuation to an earlier publication of the author [15] and is an attempt to evaluate the hot corrosion behaviour of NiCrBSi and Stellite-6 coated nickel-based superalloy, for applications of these coatings on the hot section components of boilers and gas turbines.

Section snippets

Deposition and characterisation of the coatings

The nickel-based superalloy Superni 600 (15.5Cr–10Fe–0.5Mn–0.2C–balance Ni) was used as a substrate material. The specimens with dimensions of approximately 20 mm × 15 mm × 5 mm were ground with SiC papers down to 180 grit and subsequently grit blasted with alumina powders (Grit 45) before spraying of the coatings by HVOF Process. Commercially available NiCrBSi and Stellite-6 powders were used as feedstock alloys and the details are given in Table 1.

The coatings were sprayed at M/S Metallizing

Corrosion kinetics

The weight gain plots for the hot corroded bare and NiCrBSi and Stellite-6 coated Superni 600 are shown in Fig. 1. It can be inferred from the plots that the weight gain values for the coated Superni 600 are smaller than those for bare Superni 600. The NiCrBSi coating provides relatively higher protection than that provided by Stellite-6 coating. Both the coatings deposited on Superni 600 follow the parabolic behaviour up to the total 50 cycles of study as can be inferred from the square of

Discussions

In general, both the coatings under study showed protective behaviour in the given molten salt environment at 900 °C under cyclic conditions and performed better than the bare superalloy. The results of the present investigation show that due to selective oxidation property of chromium and silicon, Cr2O3 and SiO2 formed along the boundaries of nickel-rich and cobalt-rich splats, and in pores have blocked the passages and enabled the coatings to develop barriers against the penetration and

Conclusions

  • 1.

    The HVOF sprayed NiCrBSi and Stellite-6 coatings improve the hot corrosion resistance of the Superni 600 in the given conditions. The hot corrosion resistance of the coatings has been attributed to the formation of oxides of silicon and chromium and spinels of cobalt–chromium and nickel–chromium at the surface of the coatings, and at the splat boundaries.

  • 2.

    The oxide scale has preferentially formed at the splat boundaries due to oxidation of the active elements of the coatings. The Ni- and Co-rich

References (22)

  • N. Eliaz et al.

    Eng. Fail. Anal.

    (2002)
  • M.J. Pomeroy

    Mater. Design

    (2005)
  • P.S. Liu et al.

    Corros. Sci.

    (2001)
  • M.G. Hocking

    Surf. Coat. Technol.

    (1993)
  • T.S. Sidhu et al.

    Surf. Coat. Technol.

    (2006)
  • T.S. Sidhu et al.

    Thin Solid Films

    515

    (2006)
  • Q.M. Wang et al.

    Surf. Coat. Technol.

    (2004)
  • B.S. Sidhu et al.

    J. Mater. Process. Technol.

    (2006)
  • W.T. Reid

    External Corrosion and Deposits—Boilers and Gas Turbines

    (1971)
  • P.A. Alexander et al.
  • K.L. Luthra et al.

    J. Electrochem. Soc.

    (1982)
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