In this study, HVOF sprayed NiCr alloy coatings on A213 TP347H boiler steel are evaluated by severe corrosion tests in a high-temperature molten salt environment. XRD and FE-SEM/EDS analysis results are presented and discussed and correlated with corrosion kinetics.

In the present work, commercially available NiCr and NiCr-TiC powder blends were deposited on boiler steel substrates by HVOF spraying. To evaluate coating performance, bare and coated steel samples were placed in the superheater zone of a coal fired boiler for 15 cycles. Weight change and thickness loss were measured and the results were used to establish erosion-corrosion kinetics. XRD and SEM/EDS techniques were used to analyze the microstructure and phase composition of as-sprayed and eroded-corroded specimens. The improvement in erosion-corrosion resistance provided by the coating may be attributed to the formation of nickel and chromium oxides and spinels.

Detonation-gun thermal spray technique was used to deposit commercial available Ni-20Cr and WC-Co coatings on ASTM A213 TP347H boiler steel. The coated specimens were subjected to air environment at 900°C in laboratory furnace to determine high temperature oxidation resistance. Mass change data was recorded to formulate oxidation kinetics of both the coated specimens. The exposed specimens were assessed by field emission scanning electron microscopy and energy dispersive spectroscopy (FE-SEM/EDS) and XRD analysis. The primary focus of the study is attractive oxidation resistance of Detonation gun sprayed Ni-20 Cr coating over the WC-Co coating on the said steel in the air environment.

In this study, detonation spraying was used to deposit commercially available Ni-20Cr and WC-Co powders on SA213-T22 boiler steel. Coated and uncoated specimens were subjected to 50 thermal cycles in a molten salt boiler environment 900 °C in order to evaluate their hot corrosion behavior. Mass change measurements were made at the end of each cycle to assess corrosion kinetics and XRD and SEM/EDS were used to characterize corrosion products. An analysis of the reaction kinetics and the formation of oxide scales is provided in the paper.

Nickel-chromium alloys have been used as coatings to deal with oxidation environments at high temperature. The present work is a comparative study of HVOF and cold sprayed Ni-20Cr coating on a boiler steel (SAE 213-T22) in a molten salt environment of Na 2 SO 4 -60%V 2 O 5 at 900°C under cyclic conditions. The weight change technique was used to establish the kinetics of corrosion. X-ray diffraction, surface and cross-sectional FE-SEM/EDS techniques were used to analyse the corrosion products. The hot corrosion resistance of both the coatings was better than the uncoated steel. This may be attributed to the formation of oxides and spinels of nickel and chromium in the coated steels. These oxides might have blocked the pores and splat boundaries, and acted as diffusion barriers to the inward diffusion of corroding species. Based upon the overall results and subsequent analysis of hot-corrosion data the cold spray process may be recommended as a better choice for the deposition of the Ni-20Cr coating on Mo-containing T22 steel in comparison with the HVOF spray process for hot corrosion protection.

HVOF thermal spray technique was used to deposit Ni-20Cr coating on a ASTM-SA213-T22 boiler steel. The corrosion behaviour was investigated for the uncoated and HVOF spray Ni-20Cr coated boiler steel in a molten salt environment (Na 2 SO 4 -60%V 2 O 5 ) at 900°C for 50 cycles. Each cycle consisted of 1 hour heating in the silicon carbide tube furnace followed by 20 min cooling in air. Mass change technique was used to approximate the kinetics of high temperature corrosion. The uncoated sample suffered intensive spallation along with a significant mass gain as compared to the coated sample. The exposed specimens were characterized by X-ray diffraction [XRD] and scanning electron microscopy/energy dispersive spectroscopy [SEM/EDS]. It was observed that HVOF sprayed Ni-20Cr coating was suitable to provide high temperature corrosion resistance to the given steel in the said(salt) environment.

In this study, HVOF thermal spray technique was used to deposit Ni-20Cr coating on a commonly used boiler steel ASTM-SA213-T22.The specimens with and without coating were subjected to cyclic oxidation testing at an elevated temperature of 900°C to ascertain usefulness of the coating for 50 cycles. Each cycle consisted of 1 hour heating in a silicon carbide tube furnace followed by 20 min cooling in air. Mass change technique was used to approximate the kinetics of oxidation. The uncoated sample suffered intensive spallation along with a significant overall mass gain. The oxidation rate was found to reduce appreciably after the deposition of the coating by 91%. The exposed specimens were characterized by X-ray diffraction [XRD] and scanning electron microscopy/energy dispersive spectroscopy [SEM/EDS]. It was observed that HVOF-sprayed Ni-20Cr coating was suitable to provide oxidation resistance to the given steel in the air environment.

In this investigation, Ni-20Cr alloy powder was deposited on SA 516 boiler steel by cold spraying. The oxidation kinetics of both coated and uncoated samples, evaluated in molten salt, followed a parabolic rate. The rate constant of the Ni-20Cr coated steel was much lower than that of the bare boiler steel. X-ray diffraction, SEM-EDX, and X-ray mapping were used to analyze the oxidation products of the coated and uncoated boiler steel. The uncoated steel exhibited intense spalling and peeling of its oxide scale, which may be due to the formation of Fe 2 O 3 oxides. The Ni-20Cr coating reduced the weight gain of the steel by more than 87% possibly due to the formation of nickel and chromium oxides.

The aim of this study is to investigate the role of zirconium (Zr) in HVOF coatings in order to formulate a high-temperature oxidation-resistant composition. Ni-20Cr coatings with and without 1% Zr additions were deposited on 347H boiler steel by HVOF spraying and their oxidation behavior was assessed at a temperature of 900 °C under cyclic conditions. Test specimens were also subjected to actual boiler environments to ascertain their erosion-corrosion behavior. Weight change measurements were used to determine high-temperature oxidation kinetics as well as erosion-corrosion resistance. The specimens were examined after each thermal cycle to observe physical changes and spallation in their oxide scales. It was found that the addition of Zr in the Ni-20Cr coating reduced the oxidation rate but had little effect on erosion-corrosion resistance.

High velocity oxy fuel thermal spray (HVOF) processing is one of the proven thermal spray technologies to develop high temperature oxidation and corrosion resistant coatings for various high temperature applications such as thermal power plants, aero turbines, and energy conversion systems. In the present investigation Nickel based coatings, viz. Ni-5Al, NiCrAl, and NiCrAlY-0.4wt%CeO 2 were sprayed on Fe based superalloy (Superfer 800) by HVOF spray process as the chosen superalloy finds extensive use steam boilers, furnace equipment, heat exchangers and piping in chemical industry The degradation behaviour of the coated superalloy was studied in the aggressive environment of 40%Na 2 SO 4 -60%V 2 O 5 at 900°C, using thermogravimetric technique, to understand the protective nature of the coatings under cyclic conditions. The coatings were found to be effective in increasing the resistance to the degradation in the given environment. Among the coatings investigated, NiCrAlY-0.4wt%CeO 2 coating was found to be more protective on the superalloy in the given aggressive environment. The corrosion mechanisms were elucidated through the X-ray diffraction (XRD) analysis and microstructural characterization techniques such as FE-SEM/EDAX analysis of the corroded products formed at high temperature.

Detonation-gun spray technology is a novel coating deposition process which is capable of achieving very high gas and particle velocities approaching 4-5 times the speed of sound. This process provides the possibility of producing high hardness coatings with significant adherence strength. In the present study, this technique has been used to deposit WC-Co coatings on T22 boiler steel. Investigations on the oxidation performance of detonation-sprayed WC-Co coatings in air and in molten salt Na 2 SO 4 -82Fe 2 (SO 4 ) 3 at 700°C under cyclic conditions have been carried out. The thermogravimetric technique was used to establish the kinetics of corrosion. The uncoated boiler steel suffered a catastrophic corrosion in the form of intense spalling of its oxide scale during air, as well as, molten salt induced oxidation. The WC-Co coated specimens showed lesser overall weight gains in comparison to their uncoated counterparts in both the environments. The oxidation kinetics for the coated specimens followed nearly the parabolic rate law. The overall weight gain has been found to be higher in the case of air oxidation as compared to that in salt environment for all the cases. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) techniques were used to analyse the corrosion products, which indicated the formation of W and O as main elements in the oxide scales of the coated steel in both the environments.

A study was carried out to determine the role of HVOF-spray Ni-20Cr coatings to enhance high temperature oxidation resistance of T22 and 347H boiler steels. Ni-20Cr coatings were deposited on the boiler steels. Subsequently high temperature oxidation behavior of these coatings was investigated by laboratory tests at an elevated temperature of 9000C under cyclic conditions. These coatings were also subjected to a boiler environment at an elevated temperature of 700ºC under cyclic conditions to ascertain their erosion-corrosion behaviour. Thermogravimetric data was taken after each cycle to formulate the oxidation, as well as, erosion-corrosion kinetics. The specimens were also physically examined after each cycle to notice any macrographic physical changes, spallation tendency of their oxide scales etc. The exposed specimens were examined using scanning electron microscopy and Energy Dispersive Spectroscopy (SEM/EDS) analysis. It was observed that oxidation rate was subsequently low in the coated substrates as compare to uncoated steels. It was observed that Ni-20Cr coating on 347H boiler substrate showed the maximum oxidation resistance. Moreover, both the steels showed better resistance to erosion-corrosion in actual boiler conditions after the application of Ni-20Cr coating.

The present work evaluates the oxidation and hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed WC-NiCrFeSiB coating deposited on Ni-based superalloy (Superni 75) and Fe-based superalloy (Superfer 800H). The coated as well as uncoated specimens were exposed to air and molten salt (Na 2 SO 4 -25%NaCl) environment at 800 °C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. The corrosion products were characterized using the combined techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro analyser (EPMA). The WC-NiCrFeSiB coating provides necessary resistance against oxidation and hot corrosion to both the nickel and iron based superalloys in the given environmental conditions at 800 °C. The oxides of active elements of the coatings, formed in the surface scale as well as at the boundaries of nickel and tungsten rich splats, have contributed for the oxidation and hot corrosion resistance of WC-NiCrFeSiB coatings, as these oxides act as barriers for the diffusion/penetration of the corrosive species through the coatings.

Stellite-6 coatings were deposited on two Ni-base superalloys namely Superni 75 and Superni 600 by shrouded plasma spray process and the oxidation behaviour of these coatings has been investigated in air and in aggressive environment of Na 2 SO 4 - 60%V 2 O 5 . Accelerated oxidation tests were conducted in a silicon carbide tube furnace under cyclic conditions at an elevated temperature of 900°C for 50 cycles. Each cycle consisted of one hour heating and 20 minutes cooling in air. Thermogravimetric technique was used to approximate the oxidation kinetics. The corrosion scales were characterized by XRD, SEM/EDAX and EPMA analyses. It has been observed that coatings followed the parabolic rate law of oxidation, although minor deviations were observed. Overall weight gain conceived in air was just 51% of that in the aggressive environment in case of coated Superni 75, whereas in case of coated Superni 600, the cumulative weight gain in air was found to be one-third of that in the Na 2 SO 4 -60% V2O5 atmospheres. Further the coating showed very good adherence to both of the substrates during the course of air oxidation, whereas some cracks were observed near the edges of the specimens during the molten salt induced oxidation. XRD analysis revealed the presence of phase like CoO, CoCr 2 O 4 and Cr 2 O 3 in the oxide scales, which are reported to be protective oxides. The XRD results were further supported by SEM/EDAX and EPMA.

Cr 3 C 2 -NiCr, NiCr, WC-Co and Stellite-6 alloy coatings were sprayed on ASTM SA213-T11 steel using the HVOF process. Liquid petroleum gas (LPG) was used as the fuel gas. Hot corrosion studies were conducted on the uncoated as well as HVOF sprayed specimens after exposure to molten salt at 9000C under cyclic conditions. The thermo-gravimetric technique was used to establish the kinetics of corrosion. XRD, SEM/EDAX and EPMA techniques were used to analyse the corrosion products. All these overlay coatings showed a better resistance to hot corrosion as compared to that of uncoated steel. NiCr Coating was found to be most protective followed by the Cr 3 C 2 -NiCr coating. WC-Co coating was least effective to protect the substrate steel. It is concluded that the formation of Cr 2 O 3 , NiO, NiCr 2 O 4 , and CoO in the coatings may contribute to the development of a better hot corrosion resistance. The uncoated steel suffered corrosion in the form of intense spalling and peeling of the scale, which may be due to the formation of unprotective Fe 2 O 3 oxide scale.

In this study, high velocity-oxy fuel (HVOF) technique was used to deposit Cr 3 C 2 -NiCr coating on the Ni-base superalloys for their hot corrosion applications. The coatings were characterised with regard to coating thickness, porosity, microhardness and microstructure. The hot corrosion behaviours of the bare and Cr 3 C 2 -NiCr coated superalloys were studied after exposure to molten salt (Na 2 SO 4 -60%V 2 O 5 ) at 900°C under cyclic conditions. Optical microscopy, XRD, SEM/EDAX and EPMA techniques were used to characterise the coatings. The thermogravimetric technique was used to establish kinetics of corrosion. The structure of the as sprayed Cr 3 C 2 -NiCr coating mainly consisted of γ-nickel solid solution with very low intensity peaks of Cr 7 C 3 and Cr 2 O 3 phases. Some porosity (less than 1.5%), inclusions, unmelted and semi-melted powder particles were observed in the structure of the coatings. Coating microhardness values were found to be in the range of 850-900 Hv (Vickers hardness). The Cr 3 C 2 - NiCr coating was resistant to hot corrosion in the given molten salt environment at 900°C. The hot corrosion resistance imparted by Cr 3 C 2 -NiCr coatings may be attributed to the formation of oxides of nickel, chromium, and spinels of nickel and chromium.