Isothermal oxidation of HVAF-sprayed Ni-based chromia, alumina and mixed-oxide scale forming coatings in ambient air

The power generation industry has been progressively shifting towards higher operating steam temperatures and pressures to increase efficiency and reduce CO2 emissions. However, higher operating temperatures lead to more aggressive oxidation of the boiler components. A promising route to improve the durability of degradation -prone components is through deployment of high-performance coatings. In the present work, four Ni-based coatings- Ni21Cr, Ni5A1, Ni21Cr9Mo, and Ni21Cr7AllY- thermally sprayed by the high-velocity air fuel (HVAF) technique on boiler steel (16Mo3) substrates were investigated. The isothermal oxidation behavior of the coatings was studied in ambient air environment at 600 degrees C for different time intervals i.e. 1, 5, 10, 24, 48, 96, and 168 h. The oxidation behavior of the as-sprayed and polished coatings was compared. The protective alpha-Al2O3 was not detected on the exposed alumina-forming NiAI coating. On the other hand, Cr2O3 along with a small amount of NiO were the main oxidation products on the surface of the NiCr and NiCrMo coatings, and were found to be relatively less protective. The mixed-oxide scale forming NiCrAIY coatings showed the best oxidation resistance due to the formation of a thin and slow-growing Al2O3 scale along with Ni(ALCr)(2)O-4 and Cr2O3. The polished coatings were found to significantly reduce the oxidation rate in each case as the protective scale forming elements were more uniformly supplied to the surface oxide scale by removing the surface asperities. (C) 2017 Elsevier B.V. All rights reserved.