Isothermal Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coatings: Effect of Surface Treatment (original) (raw)
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Isothermal oxidation behavior of HVAF-sprayed Ni and NiCr coatings in H 2 -H 2 O environment
Surface and Coatings Technology, 2017
The formation of a protective chromia scale on stainless steels is known to be suppressed by the presence of water vapor in reducing conditions. Thermal spray coatings present a promising approach to improve the durability of steels by transferring the first line of oxidation attack from the bulk steel to the coating. In the present work, isothermal oxidation behavior of Ni and NiCr coatings deposited by High-Velocity Air Fuel (HVAF) process on 304L stainless steel was investigated at 600°C for 168h. Ar-10%H 2-20%H 2 O was selected as the oxidation environment to study the oxidation behavior of the coatings in a low pO 2 environment containing H 2 and H 2 O. BIB/SEM, EDS, and XRD techniques were used to characterize the as-sprayed coatings and to investigate the oxidation mechanisms in the coated samples. Results showed that both Ni and NiCr coatings imparted oxidation protection to the 304L substrate. The chromia-forming 304L steel presented a duplex but non-protective oxide scale comprising of an outer Fe 3 O 4 layer on an inner (Fe,Cr) 3 O 4-spinel oxide. In contrast, the NiCr coating presented superior oxidation behavior due to the formation of a continuous, thin, and slow-growing Cr 2 O 3 scale. The Ni coating, too, protected the substrate owing to limited nucleation and growth of the deleterious NiO scale in the low-oxygen-activity environment.
Surface & Coatings Technology, 2017
The power generation industry has been progressively shifting towards higher operating steam temperatures and pressures to increase efficiency and reduce CO 2 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, Ni5Al, Ni21Cr9Mo, and Ni21Cr7Al1Y-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°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 α-Al 2 O 3 was not detected on the exposed alumina-forming NiAl coating. On the other hand, Cr 2 O 3 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 NiCrAlY coatings showed the best oxidation resistance due to the formation of a thin and slow-growing Al 2 O 3 scale along with Ni(Al,Cr) 2 O 4 and Cr 2 O 3. The polished coatings were found to significantly reduce the oxidation rate in each case as the protective scaleforming elements were more uniformly supplied to the surface oxide scale by removing the surface asperities.
Effect of water vapor on the oxidation behavior of HVAF‐sprayed NiCr and NiCrAlY coatings
Materials and Corrosion, 2018
Isothermal oxidation behavior of NiCr and NiCrAlY coatings deposited onto low alloy 16Mo3 steel by high‐velocity air fuel (HVAF) process was investigated in 5% O2 + 20% H2O + N2 at 600 °C for 168 h. Whereas NiCrAlY showed lower mass gain compared to NiCr, both coatings succeeded in maintaining the integrity with the substrate during the exposure without any breakaway oxidation. A thin Cr‐rich oxide scale (Cr2O3) formed on NiCr, and a thin mixed oxide scale (Al2O3 with NiCr2O4) formed on NiCrAlY significantly increasing the oxidation protection in the presence of water vapor.
Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coating in H2–H2O Environment
Oxidation of Metals, 2016
Isothermal oxidation behavior of an HVAF-sprayed NiCoCrAlY coating on AISI 304L was studied in an Ar-10 %H 2-20 %H 2 O environment at 600°C. Techniques such as BIB/SEM, EDS, and XRD were used to comprehensively characterize the coating and the coating/substrate interface to investigate the oxidation mechanisms. Results were also compared with those obtained from an uncoated AISI 304L substrate. The alumina-forming NiCoCrAlY coating was found to exhibit superior oxidation behavior due to the formation of a slow-growing and protective Al 2 O 3 scale, while the chromia-forming bare 304L substrate lost its protective capability due to the formation of a duplex [Fe 3 O 4 on (Fe,Cr) 3 O 4 spinel oxide] corrosion product layer.
Role of Coating in Improving High Temperature Oxidation of Steel
Oxidation is the major degradation mechanism of failure for various components operating at high temperature. Protective coatings are used to improve the oxidation resistance of such component. In the present investigation, Al2O3 and Ni-20Cr coatings have been deposited on SAE431 boiler steel by Detonation Gun Spraying Method. The oxidation performance of Al2O3 and Ni-20Cr coated as well as uncoated SAE-431 steel has been evaluated in air under cyclic conditions at an elevated temperatures of 8000C. Al2O3 coating on SAE431 boiler steel has shown approximately 26% improvement in the oxidation resistance of SAE431 steel whereas Ni-20Cr coating on SAE431 boiler steel has indicated about 21% improvement in the oxidation resistance as compared to the uncoated SAE431 boiler steel.
Investigation of Element Effect on High-Temperature Oxidation of HVOF NiCoCrAlX Coatings
Coatings
MCrAlX (M: Ni or Co or both, X: minor elements) coatings have been used widely to protect hot components in gas turbines against oxidation and heat corrosion at high temperatures. Understanding the influence of the X-elements on oxidation behavior is important in the design of durable MCrAlX coatings. In this study, NiCoCrAlX coatings doped with Y + Ru and Ce, respectively, were deposited on an Inconel-792 substrate using high velocity oxygen fuel (HVOF). The samples were subjected to isothermal oxidation tests in laboratory air at 900, 1000, and 1100 • C and a cyclic oxidation test between 100 and 1100 • C with a 1-h dwell time at 1100 • C. It was observed that the coating with Ce showed a much higher oxidation rate than the coating with Y + Ru under both isothermal and cyclic oxidation tests. In addition, the Y + Ru-doped coating showed significantly lower β phase depletion due to interdiffusion between the coating and the substrate, resulting from the addition of Ru. Simulation results using a moving phase boundary model and an established oxidation-diffusion model showed that Ru stabilized β grains, which reduced β-depletion of the coating due to substrate interdiffusion. This paper, combining experiment and simulation results, presents a comprehensive study of the influence of Ce and Ru on oxidation behavior, including an investigation of the microstructure evolution in the coating surface and the coating-substrate interface influenced by oxidation time.
Materials Science and Engineering: A, 1999
The microstructural evolution and oxidation behavior of nanocrystalline 316-stainless steel coatings produced by high-velocity oxygen fuel spraying is described. Stainless steel powders with a particle size in the range of 45 -11 mm were mechanically milled for 10 h in liquid nitrogen to produce powders with a nanocrystalline grain size of 21 98 nm and an aspect ratio of 1.68. The cryomilled powders were subsequently sprayed onto a stainless steel substrate by high-velocity oxygen fuel spraying. The resultant coating exhibited a superior microhardness, despite an increased porosity, over that of the conventional coating sprayed with the same parameters. Transmission electron microscopy performed on the cross-sections of the nanocrystalline coating revealed the splat formation with a thickness ranging from 40 to 400 nm. Various oxide phases (Cr 2 O 3 , FeO, Fe 2 O 3 and g-Fe 2 O 3 ) in the stainless steel matrix were identified using selected area diffraction. This observation suggests that in-flight oxidation may have occurred during spraying and/or during splat formation.
Effects of surface finish on the initial oxidation of HVAF-sprayed NiCoCrAlY coatings
Surface & Coatings Technology, 2019
Oxide scale formed on HVAF-sprayed NiCoCrAlY coatings and the effect of surface treatment were investigated by a multi-approach study combining photo-stimulated luminescence, microstructural observation and mass gain. The initial oxidation behavior of as-sprayed, polished and shot-peened coatings at 1000 ⁰C is studied. Both polished and shot-peened coatings exhibited superior performance due to rapid formation of α-Al 2 O 3 fully covering the coating and suppressing growth of transient alumina, assisted by high density of α-Al 2 O 3 nuclei on surface treatment induced defects. Moreover, the early development of a twolayer alumina scale consisting of an inward-grown inner α-Al 2 O 3 layer and an outer layer transformed from outward-grown transient alumina resulted in higher oxide growth rate of the as-sprayed coating.
High-Temperature Corrosion of HVAF-Sprayed Ni-Based Coatings for Boiler Applications
Oxidation of Metals, 2019
The present study investigates the initial corrosion behaviour of HVAF-sprayed NiCr, NiAl and NiCrAlY coatings in two different environments, O 2 + H 2 O and O 2 + H 2 O + KCl at 600 °C for up to 168 h in order to evaluate the possibility of utilizing such coatings in biomass-and waste-fired boilers. SEM/EDX analysis showed that all coatings displayed a protective behaviour in O 2 + H 2 O. Upon addition of KCl (O 2 + H 2 O + KCl), the corrosion behaviour of the NiCr coating drastically changed as it formed a thick oxide layer and displayed major chlorine diffusion down to the substrate. The NiCrAlY coating displayed a significantly better corrosion resistance with only minor oxide formation. The NiAl coating exhibited a protective behaviour similar to when exposed in the absence of KCl indicating that a thin protective oxide has formed on the coating surface. The performance of the NiAl and NiCrAlY coatings is promising for future studies with long-term exposures in more corrosive environments such as in a biomass-and waste-fired boiler.
Metallurgical and Materials Transactions A, 2010
The high-velocity oxy-fuel (HVOF) spray technique was used to deposit Ni-20Cr coating on a commonly used boiler steel ASTM A213 347H. The specimens with and without coating were exposed to the super heater zone of a thermal power plant boiler at a temperature of 973 K (700°C) under cyclic conditions to ascertain their erosion-corrosion (E-C) behavior. High-temperature oxidation behavior of the specimens was also evaluated under cyclic thermal loading conditions at an elevated temperature of 1173 K (900°C). Mass change data and thickness loss were measured to formulate the kinetics of E-C/oxidation for the specimens. The exposed specimens were characterized by X-ray diffraction (XRD) and field emission-scanning electron microscopy/energy dispersive spectroscopy (FE-SEM/EDS). The uncoated steel suffered higher E-C in comparison with its coated counterpart in terms of mass loss as well as thickness loss. It was observed that overall mass loss was reduced by 31 pct and thickness loss by 44 pct after the application of the coating. The possible formation of Cr 2 O 3 phase in the coated substrate may be suggested to contribute to better E-C behavior. During air oxidation exposures, the coating was found to be intact with only marginal spallation of its oxide scales, which is an indicator of good adhesion between the coating and substrate steel. The air oxidation mass change data indicated that the coating enhanced the oxidation resistance of the steel by 85 pct.