Hot corrosion behaviour of a Ni + CrAlYSiN composite coating in Na2SO4–25 wt.% NaCl melt

Characterisations of HVOF sprayed NiCrBSi coatings on Ni- and Fe-based superalloys and evaluation of cyclic oxidation behaviour of some Ni-based superalloys in molten salt environment

An investigation on hot corrosion behavior of Al0.4MnCrCoFeNi high entropy alloy in different molten salts at 750 °C

The present research work investigates the air oxidation and hot corrosion behavior of bare and CO2 laser welded Fe-based superalloy A-286 in air and in simulated aero engine molten salt (Na2SO4–7.5%NaVO3–5%NaCl) environment at 700 °C for 50 cycles. Spalling of oxide scales is observed in laser weldment in molten salt environment during hot corrosion study which is not observed in air-oxidized weldments at the end of 50 cycles. Corrosion kinetics of the weldments is obtained using the thermogravimetric technique. Scanning electron microscope/energy-dispersive spectroscopy and X-ray diffraction method are used to characterize the surface morphology and phase identification of the oxidized and hot-corroded weldments. Also, cross-sectional investigation is performed on the hot-corroded weldments to analyze the oxide scale thickness and distribution of alloying elements by optical/SEM microscopic and X-ray mapping analysis. The results show that higher corrosion rate is observed in the base metal compared to the weld zone and weldment. The grain refinement in the fusion zone facilitates the formation of Cr2O3 resulting in improved corrosion resistance in the fusion zone.

High-temperature oxidation and hot corrosion behaviors of the NiCr–CrAl coating on a nickel-based superalloy

In order to improve the hot corrosion resistance of conventional YSZ TBC system (YSZ/CoNiCrAlY/Inconel 601), an overlay Al{sub 2}O{sub 3} was sprayed on the surface of TBC samples by high velocity oxy-fuel (HVOF) spray techniques. The TBC preparation in Japan was based on our technical requirement by plasma spray. Bond coat CoNiCrAlY and the YSZ was produced by low-pressure plasma spray and air plasma spray respectively. Hot corrosion tests were carried out on the TBC with and without Al{sub 2}O{sub 3} coating in molten salts mixtures (Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5}) at 950 C for 10h. The microstructures of TBC and overlay before and after exposure were examined by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD). It has been found that TBC reacted with V{sub 2}O{sub 5} to form YVO{sub 4}. A substantial amount of M-phase was formed due to the leaching of Y{sub 2}O{sub 3} from YSZ. Al{sub 2}O{sub 3} overlay coating sprayed by HVOF was dense, continues and adherent to the TBC even after exposure to the molten salts. As a result, overlay Al{sub 2}O{sub 3} coating can prevent the YSZ from the attack by molten salts containing vanadium and arrest the penetration of salts into the YSZ along porous and cracks in the YSZ TBC. Accordingly, the amount of M-phase formed in TBC with Al{sub 2}O{sub 3} overlay was significantly lower than that in conventional YSZ TBC system. In the next period, the hot corrosion tests of TBC with EB-PVD Al{sub 2}O{sub 3} coating under Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5} will be again performed at 950 C. However before hot corrosion tests, the post-annealing will be carried at 1273K for 1h in order to transform the as-sputtered {gamma}-Al{sub 2}O{sub 3} overlay to crystalline {alpha}-Al{sub 2}O{sub 3} overlay. In addition, the effect of coating thickness on corrosion resistance and the mechanisms of cracking of EB-PVD alumina layer during hot corrosion will be also investigated.

PurposeMolten sulphate-vanadate induced hot corrosion is the main reason of failure of boiler tubes used at high temperatures in thermal power plants. The hot corrosion can be encountered by applying thermal spray coatings on the alloy steels. In this perspective, this paper aims to attempt to investigate the effect of carbon nanotubes reinforcement on Cr2O3 composite coatings on hot corrosion behaviour of ASTM-SA213-T22 steel in a corrosive environment of Na2SO4 – 60%V2O5 at 900°C for 50 cycles.Design/methodology/approachThe coatings have been deposited with high velocity oxy fuel process. The samples were exposed to hot corrosion in a Silicon tube furnace at 900°C for 50 cycles. The kinetics of corrosion behaviour were analysed by the weight gain measurements after each cycle. Corrosion products were analysed with X-ray diffraction, scanning electron microscopy, energy dispersive and cross-sectional analysis techniques.FindingsDuring investigations, the carbon nanotubes (CNT) reinforced Cr2O3 composite coatings on T22 steel were found to provide better corrosion resistance in the molten salt environment at 900°C. The coatings showed lower weight gain along with formation of protective oxide scales during the experiment. Improvement in protection against hot corrosion was observed with increase in CNT content in the coating matrix.Research limitations/implicationsThe addition of CNT has resulted in reduction in porosity by filling the voids in chromium oxide coating, with interlocking of particle and has blocked the penetration of corroding species to enhance the corrosion resistance of the composite coatings. The corrosion rate was found to be decreasing with increase in CNT content in coating matrix.Originality/valueIt must be mentioned here that high temperature corrosion behaviour of thermally sprayed CNT-Cr2O3 composite coatings has never been studied, and it is not available in the literature. Hence, present investigation can provide valuable information for application of CNT-reinforced coatings in high temperature fuel combustion environments.

Effect of pre-oxidation treatment on the hot corrosion behavior of pack-cemented aluminide coatings on the K438 alloy in salt mixture

No alloy is immune to hot corrosion attack indefinitely. Coatings can extend the lives of substrate materials used at higher temperatures in corrosive environments by forming protective oxides layers that are reasonably effective for long-term applications. This article is concerned with studying the performance of high-velocity oxyfuel (HVOF) sprayed NiCrBSi, Cr3C2−NiCr, Ni−20Cr, and Stellite-6 coatings on a nickel-base superalloy at 900 °C in the molten salt (Na2SO4-60% V2O5) environment under cyclic oxidation conditions. The thermogravimetric technique was used to establish kinetics of corrosion. Optical microscope, x-ray diffraction, scanning electron microscopy/electron dispersive analysis by x-ray (SEM/EDAX), and electron probe microanalysis (EPMA) techniques were used to characterize the as-sprayed coatings and corrosion products. The bare superalloy suffered somewhat accelerated corrosion in the given environmental conditions. whereas hot corrosion resistance of all the coated superalloys was found to be better. Among the coating studied, Ni−20Cr coated superalloy imparted maximum hot corrosion resistance, whereas Stellite-6 coated indicated minimum resistance. The hot corrosion resistance of all the coatings may be attributed to the formation of oxides and spinels of nickel, chromium, or cobalt.

Ni-20Cr coatings were obtained on the boiler tube steels through a plasma spray process. Ni-Cr-Al-Y was used as a bond coat before the application of Ni-Cr coatings. The coating was characterized and the hot corrosion behavior was evaluated after exposure to molten salt (Na2SO4-60%V2O5) at 900°C under cyclic conditions. The coating was very effective in decreasing the hot corrosion rate in molten salt at 900°C in the case of the ASTM-SA210-Grade A1 and ASTM-SA213-T-11 types of steels. The coating was found to be less effective in the case of the ASTM-SA213-T-22 type of steel. The uncoated ASTM-SA213-T-22 type of steel had shown very poor resistance to hot corrosion in a molten salt environment and also indicated the spalling of oxide scale.

Cr3C2–NiCr coatings were sprayed on SAE–213–T22 and SAE–347H boiler steels by detonation gun spray process. High temperature corrosion studies were conducted on the uncoated as well as detonation gun sprayed specimens in a molten salt environment at 700°C under cyclic conditions. The molten salt comprising Na2SO4–82%Fe2(SO4)3 was applied on the specimens as a deposit. The weight change technique was used to establish kinetics of the corrosion. X-ray diffraction, field emission scanning electron microscopy/energy dispersive spectroscopy and X-ray mapping techniques were used to analyse the corrosion products. SAE–213–T22 steel suffered an accelerated corrosion in the form of intense spalling of its oxide scale, whereas SAE–347H steel exhibited only marginal spallation in the form of fine powder. The Cr3C2–NiCr coating showed good adherence to the boiler steels during the exposure with no tendency for spallation of its oxide scale. The coating was found to be successful in developing resistance against hot corrosion in the molten salt environment, which may be attributed to the formation of oxides of nickel and chromium.

Hot corrosion behaviour of nickel-based superalloy irradiated with a laser beam

In order to improve the hot corrosion resistance of conventional YSZ TBC system, the overlay of Al{sub 2}O{sub 3} coating was deposited on the TBC by EB-PVD techniques. Hot corrosion tests were carried out on the TBC with and without Al{sub 2}O{sub 3} coating in molten salts mixtures (Na{sub 2}SO{sub 4} + 5%V{sub 2}O{sub 5}) at 950 C for 10h. The microstructures of TBC and overlay before and after exposure were examined by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectrometer (EDX) and X-ray diffraction (XRD). It has been found that TBC will react with V{sub 2}O{sub 5} to form YVO{sub 4}. A substantial amount of M-phase was formed due to the leaching of Y{sub 2}O{sub 3} from YSZ. Al{sub 2}O{sub 3} overlay coating deposited by EB-PVD was dense, continues and adherent to the TBC. As a result, overlay Al{sub 2}O{sub 3} coating can prevent the YSZ from the attack by molten salts containing vanadium and arrest the penetration of salts into the YSZ along porous and cracks in the YSZ TBC, although there were some cracks in overlay Al{sub 2}O{sub 3} coating and at the interface between alumina and zirconia formed during hot corrosion tests due to the presence of tensile stress in the alumina coating. In the next reporting period, we will study the mechanisms of cracking of the overlay Al{sub 2}O{sub 3} layer and finish the hot corrosion tests of TBC with Al{sub 2}O{sub 3} coating deposited by high velocity oxy-fuel (HVOF) technique. The hot corrosion test of TBC with EB-PVD deposited Al{sub 2}O{sub 3} coating will be again performed. However before hot corrosion tests, a post-annealing will be carried out in vacuum (residual pressure 10{sup -3} Pa) at 1273K for 1h in order to transform the as-sputtered Al{sub 2}O{sub 3} overlay to crystalline {alpha}-Al{sub 2}O{sub 3} overlay.

The present work aims to investigate the hot corrosion resistance of HVOF (high velocity oxy fuel) sprayed CNT (Carbon nanotubes) reinforced Cr2O3 coated ASTM-SA213-T22 (T22) steel at 800 °C temperature in molten salt environment. The coatings have been deposited with high velocity oxy fuel process. The samples were exposed to hot corrosion in a Silicon tube furnace at 800 °C for 50 cycles. The kinetics of corrosion behaviour were analysed by the weight gain measurements after each cycle. Corrosion products were analysed with x-ray diffraction, scanning electron microscopy, energy dispersive and x-ray analysis techniques. During investigations, the CNT reinforced Cr2O3 composite coatings on T22 steel were found to provide better corrosion resistance in the molten salt environment at 800 °C. The coatings showed lower weight gain along with formation of protective oxide scales during the experiment. Improvement in protection against hot corrosion was observed with increase in CNT content in the coating matrix. The addition of CNT has resulted in reduction in porosity by filling the voids in chromium oxide coating, with interlocking of particle and has blocked the penetration of corroding species to enhance the corrosion resistance of the composite coatings. The corrosion rate was found to be decreasing with increase in CNT content in coating matrix.

Hot-corrosion behavior of Cr2O3-CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700°C

In order to improve the hot corrosion resistance of DZ125 alloy, Ce-Y modified aluminum coatings were prepared on DZ125 alloy by pack cementation process at 950 °C for 2 h. The microstructure, phase constitution and formation mechanism of the coatings were investigated. The hot corrosion behaviors of DZ125 alloy and the coatings in molten salt environment of 25%K2SO4+75%Na2SO4 (mass fraction) at 900 °C were studied. Results show that the obtained Al-Ce-Y coatings were mainly composed of Al3Ni2, Al3Ni and Cr7Ni3, with a thickness of about 120 μm. After hot corrosion test, DZ125 alloy suffered catastrophic hot corrosion and serious internal oxidation and internal sulfidation arose. Two layers of corrosion products formed on surfaces of DZ125 alloy, including the outer layer consisting of Cr2O3 and NiCr2O4, and the inner layer of Al2O3, Ni3S2 and Ni-base solid solution. After being coated with Al-Ce-Y coating, the hot corrosion resistance of DZ125 alloy is improved notably, due to the formation of a dense scale mainly consisting of Al-rich Al2O3 in the coating layer.