Combating Corrosion Degradation of Turbine Materials Using HVOF Sprayed 25% (Cr3C2-25(Ni20Cr)) + NiCrAlY Coating

High velocity oxy fuel process (HVOF) is an advanced coating process for thermal spraying of coatings on to components used in turbines. HVOF process is a thermal spray coating method and is widely used to apply wear, erosion, and corrosion protective coatings to the components used in industrial turbines. 25% (Cr3C2-25(Ni20Cr)) + NiCrAlY based coatings have been sprayed on to three turbine materials, namely, Ti-31, Superco-605, and MDN-121. Coated and uncoated substrates were subjected to hot corrosion study under cyclic conditions. Each cycle consisted of 1 hour heating at 800°C followed by 20 minutes air cooling. Gravimetric measurements were done after each cycle and a plot of weight gain as a function of number of cycles is drawn. Parabolic rate constants were estimated for the understanding of corrosion behaviour. It was observed that coated Ti-31 and MDN-121 were more resistant compared to the uncoated ones. Uncoated superco-605 was undergoing sputtering during corrosion study and hence comparison between coated and uncoated superco-605 was difficult. The cross-sectional analysis of the corroded, coated samples indicated the presence of a thin layer of chromium oxide scale on the top of the coating and it imparted better corrosion resistance. Parabolic rate constants also indicated that coating is more beneficial to Ti-31 than to MDN-121. 1. Introduction Superalloys are extensively used in gas turbine engines, particularly in combustion zone which is subjected to high temperature and long periods of working time [1–3]. Hot corrosion has become a major degradation mechanism (solid salt deposit on the component Type I hot corrosion) in turbines which are operated with low grade fossil fuels [4–6]. To protect the components from degradation and to prolong their life, coatings are commonly used on the blades and vanes of gas turbines [7]. Use of mechanical blend carbide alloy powder as a coating material is highly promising [8]. A cermet system like Cr3C2-NiCr is a strong contender for coating applications requiring higher wear, abrasion, and wear resistance. Cr3C2 imparts wear, abrasion, and wear resistance and NiCr acts as a metal binder providing necessary coating toughness. It also has good thermal conductivity and hence helps in reducing cooling requirements. Alloy, NiCrAlY, is used in coating applications for imparting oxidation resistance and it reduces thermal mismatch between coating and the substrate [9, 10]. The alloy, NiCrAlY, is used as a bond coat in thermal barrier systems (TBCs) to minimize the mismatch in the coefficient of thermal