Combating Hot Corrosion Behaviour of Ni-Cr Superalloy for Turbine Applications

Combating Hot Corrosion Behaviour of Ni-Cr Superalloy for Turbine Applications

Abstract Hot corrosion is one of the major degradation mechanisms affecting the performance and life of gas turbine components operating at elevated temperatures. Nickel-Chromium (Ni-Cr) superalloys are extensively used in turbine blades, vanes, and combustion chambers because of their superior mechanical strength, oxidation resistance, and thermal stability. However, under aggressive environments containing molten salts such as sodium sulfate (NaSO) and vanadium pentoxide (VO), these alloys suffer severe hot corrosion damage. The present study investigates the hot corrosion behaviour of Ni-Cr superalloys and evaluates methods to combat degradation through alloy composition optimization, protective coatings, and HVOF thermal treatment techniques. Chromium-rich oxide layers improve oxidation resistance and reduce corrosion penetration. Thermal barrier coatings further enhance durability under cyclic thermal exposure. The study concludes that proper selection of alloying elements and surface engineering techniques significantly improves hot corrosion resistance, increasing operational efficiency and service life.

Keywords: Ni-Cr superalloy, hot corrosion, turbine application, oxidation resistance, thermal barrier coating, high- temperature corrosion.

1. OBJECTIVES OF THE STUDY

   The key objectives of this study are as follows:

   • To study the hot corrosion behaviour of Ni-Cr superalloys used in turbine applications.

   • To analyze the influence of chromium content on oxidation resistance.

   • To investigate the mechanisms responsible for hot corrosion degradation.

   • To evaluate the effectiveness of protective coatings in reducing corrosion.

   • To improve the service life and performance of turbine materials.

2. INTRODUCTION

   Gas turbines are widely used in aerospace, thermal power plants, marine propulsion systems, and industrial power generation due to their high power-to-weight ratio and operational efficiency. Turbine components operate under extremely severe conditions involving high temperature, high pressure, cyclic thermal loading, and corrosive gases. The operating temperature of advanced turbines often exceeds 900°C, creating a challenging environment for structural materials.

   Nickel-based superalloys containing chromium are among the most important materials used in turbine applications because of their exceptional resistance to

   creep, oxidation, and thermal fatigue. Chromium plays a significant role in improving oxidation resistance by forming a stable chromium oxide (CrO) protective layer on the material surface.

   Despite their superior high-temperature properties, Ni- Cr superalloys are vulnerable to hot corrosion in environments containing sulfur, sodium, and vanadium compounds. These contaminants originate from low- quality fuels, marine atmospheres, and industrial combustion gases. Hot corrosion accelerates material degradation by destroying the protective oxide layer, leading to rapid metal loss, crack initiation, and premature component failure.

   The present research focuses on understanding the hot corrosion behaviour of Ni-Cr superalloys and identifying methods to improve corrosion resistance through material selection, alloy modification, and coating technologies.

3. MATERIAL SELECTION

   1. Base Material: Ni-Cr Superalloy

      The selected material for this study is Nickel-Chromium (Ni-Cr) superalloy, chosen for its proven performance in high-temperature environments. The chemical composition and key mechanical properties are presented in Table I and Table II respectively.

      TABLE I. Typical Chemical Composition of Ni-Cr Superalloy

      Element	Percentage (%)
      Nickel (Ni)	70
      Chromium (Cr)	20
      Cobalt (Co)	5
      Aluminum (Al)	2
      Others	2

      TABLE II. Mechanical and Physical Properties of Ni-Cr Superalloy

      Property	Value
      Density	8.2 g/cm³
      Melting Point	1350°C
      Tensile Strength	900 MPa
      Hardness	350 HV
      Thermal Conductivity	11 W/mK
      Oxidation Resistance	Excellent

   2. Hot Corrosion

      Hot corrosion is a form of accelerated oxidation that occurs between 600°C and 900°C in the presence of deposited molten salts. It results from lower melting eutectic salt mixtures and causes severe localized attack on the alloy surface, destroying the protective oxide scale.

      4.2 Coating Process

      Thermal barrier coatings were applied to specimens using the High-Velocity Oxy-Fuel (HVOF) thermal spray technique. The coating system comprised two distinct layers:

      Bond Coat NiCrAlY: A metallic NiCrAlY bond coat was deposited as the first layer to provide adhesion between the substrate and the ceramic top coat, and to offer additional oxidation resistance.

      Ceramic Top Coat YSZ: Yttria Stabilized Zirconia (YSZ) was applied as the ceramic top coat to act as a thermal insulator, reducing the temperature experienced by the metallic substrate.

      4.3 Hot Corrosion Test

      The hot corrosion test was conducted in a muffle furnace under controlled conditions. Samples were coated with a salt mixture of NaSO and VO and exposed cyclically. Post-exposure analysis was carried out using Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) to assess microstructural changes and phase transformations. Test conditions are given in Table III.

      TABLE III. Hot Corrosion Test Conditions

      Parameter	Value
      Temperature	900°C
      Exposure Time	50 hours
      Salt Mixture	NaSO + VO
      Analysis	SEM and XRD

   3. Chemical Reactions

   The principal chemical reactions occurring during hot corrosion are as follows:

   Formation of chromium oxide: 2Cr + 3O 2CrO Formation of nickel oxide: 2Ni + O 2NiO Sulfidation reaction: NaSO + Ni NiS + NaO + O The protective oxide scale becomes unstable due to sulfur penetration into the alloy substrate, accelerating

   overall corrosion damage.

4. EXPERIMENTAL METHODOLOGY

   4.1 Sample Preparation

   Ni-Cr alloy specimens were cut into standard dimensions suitable for high-temperature exposure testing. Surface polishing was performed using successive grades of emery paper to achieve a uniform surface finish. Samples were cleaned in acetoneto remove surface contaminants, dried, and initial weight was recorded using a precision analytical balance.

5. RESULTS AND DISCUSSION

   1. Weight Gain Behaviour

      The uncoated Ni-Cr alloy exhibited significantly higher weight gain throughout the exposure period due to continuous oxidation and scale formation. In contrast, coated specimens demonstrated markedly lower weight gain, confirming the effectiveness of the thermal barrier coating as both a thermal and chemical barrier against aggressive salt attack.

      Key observations from the weight gain analysis:

      • Uncoated sample: Higher corrosion rate due to direct salt-alloy contact.

      • Coated sample: Substantially lower weight gain and reduced corrosion penetration.

      • Higher chromium content alloys: Demonstrated improved oxidation resistance.

   2. Oxide Layer Formation

      A stable, adherent chromium oxide (CrO) layer formed on the alloy surface at elevated temperatures. This oxide

      scale acted as a diffusion barrier, significantly reducing the rate of oxygen and sulfur penetration into the substrate. SEM analysis confirmed dense oxide layer formation in higher-chromium specimens compared to baseline alloys.

   3. Effect of Thermal Barrier Coating

      The application of TBC through the HVOF process resulted in substantial improvement in hot corrosion resistance. The YSZ top coat effectively reduced the thermal gradient across the alloy surface, while the NiCrAlY bond coat provided supplementary oxidation protection. Thermal barrier coatings significantly reduced:

      • Oxidation rate and oxide scale spallation tendency.

      • Sulfidation attack through molten salt infiltration.

      • Thermal fatigue damage during cyclic heat exposure.

   4. Failure Mechanism

   Analysis of failed uncoated specimens revealed a multi- stage degradation sequence. In the initial stages, the protective CrO scale was destabilized by the molten salt mixture. Subsequent sulfur and vanadium penetration disrupted scale continuity leading to accelerated metal loss. The primary failure mechanisms identified were:

   • Oxide scale spallation due to thermal cycling stresses.

   • Crack propagation at the oxide-metal interface.

   • Molten salt penetration dissolving the protective scale.

   • Sulfur attack converting protective oxides to non- protective sulfides.

6. CONCLUSION

The present study investigated the hot corrosion behaviour of Ni-Cr superalloys under turbine operating conditions involving aggressive molten salt environments at 900°C. The following conclusions are drawn:

1. The uncoated Ni-Cr superalloy exhibited significant weight gain and accelerated degradation due to the destabilization of the protective oxide scale in the presence of NaSO and VO salt mixtures.

2. Chromium-rich oxide layers (CrO) formed on the alloy surface provided effective oxidation protection by acting as a diffusion barrier against oxygen and sulfur ingress.

3. Application of thermal barrier coatings using the HVOF technique with NiCrAlY bond coat and YSZ ceramic top coat significantly reduced the oxidation rate, sulfidation attack, and thermal fatigue damage.

4. Failure analysis confirmed that oxide scale spallation, crack propagation, and molten salt penetration were the primary mechanisms responsible for accelerated degradation in uncoated alloys.

5. Optimized alloy composition combined with HVOF- applied diffusion and thermal barrier coatings significantly enhanced hot corrosion resistance, improving operational efficiency and extending the service life of turbine components.

ACKNOWLEDGMENT

The present investigation has been carried out in the framework of the research cooperation program at RRIT Bengaluru, To Combat Hot Corrosion and Erosion Degradations of Superalloys for Turbine Applications under the guidance of Rakshith Kumar P, Assistant Professor, Department of Mechanical Engineering, RRIT Bengaluru. The authors gratefully acknowledge the support and facilities provided by the Department of Mechanical Engineering, RRIT Bengaluru.

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