Characterization and performance assessment of HVOF sprayed tribaloy coating subjected to cyclic oxidation and hot corrosion conditions

Degradation of Aluminide Coatings Deposited on Nickel Superalloys

Aluminide coating was directly applied onto Ti-48Al-2Cr alloy using electro-spark deposition (ESD) technique. The coating has a thickness of 15-20 μm, a gradient composition, and a metallurgical bonding to the substrate. Cyclic oxidation and hot corrosion tests were carried out at 800°C to evaluate the protective ability of the coating under relatively severe corrosion environments. It was shown that under both test conditions, a uniform and compact external scale mainly composed of α-Al 2 O 3 formed on the coated specimens. This scale exhibited excellent spallation and hot corrosion resistance. While the scale with multi-layered structure formed on Ti-48Al-2Cr under cyclic oxidation condition consisted of TiO 2 and α-Al 2 O 3 , which suffered severe cracking and spallation after extended oxidation. During hot corrosion test, this layer spalled away extensively and dissolved partially into the molten salt. These results showed that aluminide coated Ti-48Al-2Cr could undergo severe oxidation and hot corrosion conditions.

The CoMoCrSi + Cr₃C₂ coating was applied to MDN 420 steel using High-Velocity Oxy-Fuel (HVOF) spraying after subjecting the CoMoCrSi feedstock to high-energy ball milling (HEBM). This process enhanced the formation of hard intermetallic Laves phases and reduced particle size to 60.12 μm. The study focused on the coating’s performance in its as-sprayed condition and after exposure to oxidation and hot corrosion with molten salts at 700 °C for 50 cycles in static air. Characterization methods included X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy Dispersive Spectroscopy (EDS), along with measurements of surface roughness, microhardness, and porosity. The coating exhibited a surface roughness of 2.995 ± 0.25 μm, a thickness of 210.53 ± 15 μm, and a low porosity of 2.160 ± 0.10%. Microhardness increased from 158.6 ± 6 Hv in the substrate to 547.5 ± 15 Hv in the coating, due to the dispersion of Cr₃C₂ particles. The coated samples showed lower parabolic rate constants (Kp) of 0.434 × 10− 15 g2⋅cm− 4⋅s− 1 and 0.330 × 10− 15 g2⋅cm− 4⋅s− 1, indicating improved oxidation and hot corrosion resistance, respectively. XRD analysis revealed that the phases of Co3Mo2Si, Co7Mo6, Co3Mo, and Co2Mo3 were the intermetallic laves phases generated in the CoMoCrSi feedstock through the HEBM process. During oxidation, the formation of dense and adherent Cr₂O₃ and SiO₂ scales provides the primary protection by restricting oxygen diffusion and sealing splat boundaries. While at hot corrosion, with molten salts, leads to the formation of secondary phases such as Na₂MoO₄, NaVO₃, CrMoO₄, and Cr₃(VO₄) ₂ through fluxing reactions with Mo- and Cr-based oxides.

The Cr-Si-Zr (or Y) modified aluminide coatings were produced on Ni-base INC738 by a novel single step pack cementation process and their behavior was evaluated in cyclic oxidation and hot corrosion conditions. A dual layer pack with different composition was applied to produce Cr-Si-Zr or Cr-Si-Y modified aluminide coatings. A mixture of salts 75%Na2SO4-20%NaCl-5%V2O5 at 950°C was used as a hot corrosion environment. The results show that oxidation and corrosion rate of Cr-Si-Zr or Cr-Si-Y modified aluminide coatings produced by dual layer pack process are lower than those of uncoated or simple aluminide coatings. Coatings with higher Al content and uniformly dispersed Cr-Si rich phases show better hot corrosion resistance.

Conventional and gradient CoNiCrAlYSi coatings were produced using high-velocity oxy-fuel (HVOF) and an additional step of diffusional over aluminizing (pack cementation) techniques on the Inconel-738 substrate. Hot corrosion and cyclic oxidation performance of the conventional and the gradient coatings were investigated by exposing samples to a molten salt of Na2SO4–20 % wt. NaVO3 at 880 °C and 1-h exposure at 1100 °C in air. Corrosion and cyclic oxidation rates were determined by measuring the weight gain at regular time intervals. X-ray diffraction, field emission scanning electron microscopy, and electron probe microanalysis techniques were used to characterize the coatings and the thermally grown oxide. Increase in the amount of β aluminum-rich phase and the formation of high-chromium layer beneath the outer aluminum-rich layer and the low surface roughness of gradient coating increased the hot corrosion and oxidation resistance by a factor of 1.7. In addition, the gradient coating showed better rehealing of the thermally formed alumina scale due to its possession of more β phase as a Al reservoir.

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr2O3 and CoNiCrAlY–Cr2O3 coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na2SO4–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10−2 and 1.36 × 10−2 mg/cm2 h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr2O4 spinel on the surface, while as-deposited Cr2O3 in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr2O3 coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr2O3 coating was found to be more protective than the NiCoCrAlY–Cr2O3 coating under hot corrosion condition.

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na2SO4–60% V2O5) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr2O3, NiO and spinel NiCr2O4 has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.

Structure and durability evaluation of blast furnace slag coatings and thermal barrier coatings (TBCs) under high temperature conditions

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 (Na2SO4-25% 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.

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 (Na2SO4-25%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.

Components in energy-producing systems suffer a variety of degradation processes such as oxidation and molten salt-induced corrosion as a consequence of complex multi-component gaseous environment. Coatings provide a composition that will grow the protective scale at high temperatures having long-term stability. Plasma spraying was used to deposit CoCrAlY + WC-Co composite coatings on turbine alloys of Hastelloy X and AISI 321. The thermocyclic oxidation behavior of coated alloys was investigated in static air and in molten salt (Na2SO4-60%V2O5) environment at 700 °C. The thermogravimetric technique was used to approximate the kinetics of oxidation in 50 cycles, each cycle consisting of heating and cooling. X-ray diffraction and SEM/EDAX techniques are used to characterize the oxide scale formed. Coated alloys showed a lower corrosion rate as compared to uncoated alloys. The coatings subjected to oxidation and hot corrosion showed slow scale growth kinetics. Preferential oxidation of Co, Cr, W and its spinel blocks the transport of oxygen and corrosive species into the coating by providing a barrier, thereby making the oxidation rate to reach steady state. As compared to the substrate alloys, coatings show better hot corrosion resistance.

Investigation of oxidation and hot corrosion behavior of molybdenum coatings produced by high-velocity oxy-fuel coating method

Oxidation, sulfidation and hot corrosion of intermetallic compound Fe 3Al at 605°C and 800°C

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.

The excellent combination of high-temperature strength and lightweight properties makes titanium-base alloys attractive for high-temperature applications in aircraft engines. However, more hot corrosion of titanium alloys is a life-limiting factor, particularly when aircraft fly at low altitudes across the sea. In the present paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures. The hot corrosion studies were carried out by determining weight loss at different temperatures and in salts of pure Na2SO4, 90% Na2SO4+10% NaCl and 90% Na2SO4+5% NaCl+5% V2O5. Subsequently, the rate constants were evaluated. The depth of attack due to hot corrosion was compared with oxidation data. Finally, the degradation mechanism of the titanium alloy that leads to degradation of mechanical properties in aggressive environments has been discussed and suitable coatings suggested to enhance the operational life of engines by effectively preventing both oxidation and hot corrosion.