Hot corrosion behaviors of SrZrO3 ceramic co-doped with Y2O3 and Yb2O3 in molten Na2SO4, V2O5, and Na2SO4 + V2O5 salts mixture

The hot corrosion behaviors of Sr(Y_0.05Yb_0.05Zr_0.9)O_2.95 (SYYZ) ceramic were investigated in Na₂SO₄, V₂O₅, and Na₂SO₄ + V₂O₅ salts mixture, respectively. Na₂SO₄ did not react with SYYZ ceramic at 900, 950 and 1000 °C. m-ZrO₂, YVO₄ and YbVO₄ were the main corrosion products on the SYYZ ceramic surface in V₂O₅ at 800 and 900 °C, whereas Sr₃V₂O₈ and t-ZrO₂ appeared at 1000 °C. In Na₂SO₄ + V₂O₅ salts mixture, the corrosion products were Sr₃V₂O₈ and t-ZrO₂ at 800 and 900 °C on the SYYZ ceramic surface, however, a new phase of SrZrO₃ developed at 1000 °C. The phase transformation and chemical interaction are the primary corrosion mechanisms for degradation of SYYZ ceramic.     

Comparison of hot corrosion resistance of Sm2Zr2O7 and (Sm0.5Sc0.5)2Zr2O7 ceramics in Na2SO4+V2O5 molten salt

Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramics were fabricated by a chemical co-precipitation and calcination method, and their hot corrosion behaviors in Na₂SO₄+V₂O₅ molten salt were investigated. Hot corrosion tests were carried at 700 °C, 800 °C and 900 °C for 4 h, and corroded surfaces were investigated using X-ray diffractometer and scanning electron microscopy. The corrosion products of Sm₂Zr₂O₇ ceramics were composed of SmVO₄ and monoclinic-ZrO₂, while those of (Sm_0.5Sc_0.5)₂Zr₂O₇ ceramic consisted of SmVO₄ and Zr₅Sc₂O₁₃. Considering the fact that Zr₅Sc₂O₁₃ is more desirable than monoclinic-ZrO₂ for thermal barrier coating applications, (Sm_0.5Sc_0.5)₂Zr₂O₇ showed better corrosion resistance to Na₂SO₄+V₂O₅ salt than Sm₂Zr₂O₇. The hot corrosion mechanisms of Sm₂Zr₂O₇ and (Sm_0.5Sc_0.5)₂Zr₂O₇ in Na₂SO₄+ V₂O₅ salt were discussed in detail.     

Microstructure evolution and hot corrosion mechanisms of Ba2REAlO5 (RE = Yb,Er, Dy) exposed to V2O5 + Na2SO4 molten salt

Ba₂REAlO₅ (RE?=?Dy, Er, Yb) powders were synthesized by a solid state reaction method, followed by cold pressing and sintering to produce pellets for hot corrosion tests. When exposed to V₂O₅?+?Na₂SO₄ molten salt at 900?°C and 1000?°C for 4?h and 20?h, REVO₄, Ba₃(VO₄) ₂ and BaAl₂O₄ formed as corrosion products due to chemical interactions between the ceramics and the molten salt, which were temperature and time independent. After the hot corrosion tests at 1000?°C, continuous, dense reaction layers with a thickness of ～80?μm formed on the sample surfaces, which had an effective function on suppressing further penetration of the molten salt. The hot corrosion mechanisms of Ba₂REAlO₅ are proposed based on Lewis acid-base rule, phase diagrams and thermodynamics. From a thermodynamics perspective, the molten salt directly reacting with Ba₂REAlO₅ is difficult compared with Yb₂O₃, Al₂O₃ and BaO.     

Phase and microstructure evolution of Sc2O3–CeO2 –ZrO2ceramics in Na2SO4 + V2O5 molten salts

In this study, the destabilization resistance of Sc₂O₃ and CeO₂ co-stabilized ZrO₂ (SCZ) ceramics was tested in Na₂SO₄ + V₂O₅ molten salts at 750°C–1100 °C. The phase structure and microstructure evolution of the samples during the hot corrosion testing were analyzed with X-ray diffraction (XRD), Raman spectra, scanning electron microscopy (SEM), energy dispersive X-ray spectrum (EDS), and X-ray photoelectron spectroscopy (XPS). Results showed that the destabilization of SCZ ceramics at 750 °C was the result of the chemical reaction with V₂O₅ to produce m-ZrO₂ and CeVO₄, and little ScVO₄ was detected in the Sc₂O₃-rich SCZ ceramics. The primary corrosion products at 900 °C and 1100 °C were CeO₂ and m-ZrO₂ due to the mineralization effect. The Sc₂O₃-rich SCZ ceramics exhibited excellent degradation resistance and phase stability owing to the enhanced bond strength and the decreased size misfit between Zr⁴⁺ and Sc³⁺. The destabilization mechanism of SCZ ceramic under hot corrosion was also discussed.     

Effects of sintering additives on hot corrosion behavior of γ-Y2Si2O7 ceramics in Na2SO4+V2O5 molten salt

Dense γ-Y₂Si₂O₇ ceramics were prepared by moulding and sintering of pure γ-Y₂Si₂O₇ powders synthesized by adding various additives. Effects of sintering additives on hot corrosion behavior of γ-Y₂Si₂O₇ ceramics in Na₂SO₄ + V₂O₅ molten salts were systematically investigated. Chemical kinetics of corrosion process was also calculated to illuminate the influence of different additives on chemical stability. Results showed that corrosion reaction started at grain boundary due to the loose microscopic network. When the intercrystalline glass phase was completely etched, the molten salts began to contact Y₂Si₂O₇, forming NaY₉Si₆O₂₆ and YVO₄. Compared with Li₂O and MgO, intercrystalline glass phase formed by Al₂O₃ additive had the most compact microscopic network structure, leading to the best chemical stability. Apparent activation energy for hot corrosion reaction of γ-Y₂Si₂O₇ ceramics in pure Na₂SO₄, Na₂SO₄ + 5 wt% V₂O₅, Na₂SO₄ + 10 wt% V₂O₅, Na₂SO₄ + 15 wt% V₂O₅ molten salts was calculated to be 408.16, 373.60, 310.62, and 249.63 kJ/mol, respectively.     

Corrosion products evolution and hot corrosion mechanisms of REPO4 (RE= Gd,Nd, La) in the presence of V2O5 + Na2SO4 molten salt

REPO₄ (RE = Gd, Nd, La) ceramics with a monazite structure were fabricated by a chemical co-precipitation and calcination method. Hot corrosion tests were carried out in V₂O₅+Na₂SO₄ molten salt at 800 °C, 900 °C and 1000 °C for 2 h and 10 h. The temperature and heat duration had little effect on the type of corrosion products in this study. However, GdPO₄ and REPO₄ (RE = Nd, La) revealed different hot corrosion behavior. Exposed to the molten salt, GdVO₄ and Gd₄(P₂O₇)₃ formed as the corrosion products for the GdPO₄ case, while an RE(P,V)O₄ (RE = Nd, La) solid solution was generated for NdPO₄ and LaPO₄ cases. The formation of the solid solution had less damage to the original microstructure, which benefited the hot corrosion resistance of the ceramics. From the crystallographic characteristics of rare earth phosphates/vanadates and a thermodynamics perspective, the hot corrosion mechanisms of REPO₄ (RE = Gd, Nd, La) are discussed.     

Hot corrosion of V2O5-coated NdMgAl11O19 ceramic in air at 950 °C

NdMgAl₁₁O₁₉ ceramic was prepared by solid-state reaction at 1700 °C for 10 h in air, and exhibited a single phase of magnetoplumbite structure. Reaction between molten V₂O₅ and NdMgAl₁₁O₁₉ was investigated at 950 °C using an X-ray diffractometer, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Molten V₂O₅ reacts with NdMgAl₁₁O₁₉ to form α-Al₂O₃, NdVO₄ and MgAl₂O₄ at 950 °C in air. After hot corrosion at 950 °C for 50 h, α-Al₂O₃ is the main corrosion product. The thickness of the corrosion layer gradually increases with increasing corrosion time from 10 to 50 h.     

Structure,thermal properties and hot corrosion behaviors of Gd2Hf2O7 as a potential thermal barrier coating material

In this work, Gd₂Hf₂O₇ ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd₂Hf₂O₇ ceramics were characterized systematically. Results show that the thermal conductivity of Gd₂Hf₂O₇ ceramics is 1.40 Wm⁻¹K⁻¹ at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd₂Hf₂O₇ ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10⁻⁶ K⁻¹ to 11.3 × 10⁻⁶ K⁻¹ (300–1200 °C). Besides, the hot corrosion behaviors of Gd₂Hf₂O₇ ceramics exposed to V₂O₅ and Na₂SO₄ + V₂O₅ salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd₂Hf₂O₇ ceramics subjected to molten V₂O₅ and Na₂SO₄ + V₂O₅ salts at different temperatures.     

Hot corrosion behavior of Cr2AlC MAX phase and CoNiCrAlY compounds at 950 °C in presence of Na2SO4+V2O5 molten salts

In this research, a high-purity Cr₂AlC MAX phase sample was prepared via spark plasma sintering (SPS) method with the hot corrosion behavior investigated in the presence of Na₂SO₄+V₂O₅ molten salts at 950 °C. Also, the hot corrosion resistance of this MAX phase was compared with a hot corrosion-resistant SPS-processed CoNiCrAlY sample. The results of the hot corrosion test after 30 h revealed that the MAX phase sample has better hot corrosion resistance compared to CoNiCrAlY sample. According to the results, corrosion kinetics of Cr₂AlC sample followed near-cubic law with diffusion occurring along the grain boundaries. On the other hand, CoNiCrAlY sample followed parabolic kinetics where the diffusion of reactants occurred through the oxide scale. The results indicated that in the Cr₂AlC sample, upon exposure time prolongation, a dense and uniform Cr-rich alumina layer was formed in the surface and Cr₇C₃ phase was created as a sub-layer, while in the CoNiCrAlY sample the oxide layer contained Al₂O₃ and porous spinel oxide phases. In the CoNiCrAlY sample, a considerable volume change and stress occurred during the non-uniform growth of spinel oxide causing the formation of defects such as microcracks which deteriorate its hot corrosion resistance.     

Evolution of Hot Corrosion Behavior of YSZ‐Ta2O5 Composites with Different YSZ/Ta2O5 Ratios

This study compares the hot corrosion performance of yttria‐stabilized zirconia (YSZ)‐Ta₂O₅ composite mixtures (TaYSZ) in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1100°C. The tested compositions vary from pure YSZ (0TaYSZ) to a Ta₂O₅‐dominating (70 wt% Ta₂O₅+ balance YSZ, 70TaYSZ) composite mixture with a gradual increase of Ta₂O₅. 50TaYSZ (50 wt% Ta₂O₅ + balance YSZ) sample forms a highly stable zirconium tantalum oxide. 50TaYSZ is more stable, both thermally and chemically in Na₂SO₄+V₂O₅ media at 1100°C, than other tested YSZ‐Ta₂O₅ (TaYSZ) composites, and shows a good hot corrosion resistance.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Evolution of hot corrosion resistance of conventional CSZ and MoSi2 self-healing thermal barrier coatings in Na2SO4+V2O5 at 950 °C

ZrO₂-based hot corrosion-resistant thermal barrier coatings (TBCs) with MoSi₂+Al₂O₃ have gained increasing attention. In this research, a novel dual-layer TBC (CSZ: ZrO₂-25 wt% CeO₂-2.5 wt% Y₂O₃/MAC: MoSi₂ + Al₂O₃ + CSZ) was developed, and its hot corrosion was compared to a single-layer CSZ. The atmospheric plasma spray (APS) process was utilized to apply CSZ/MAC and CSZ TBCs on NiCrAlY, as a bond coat to nickel-based superalloy (IN738LC). Different investigations, including hot corrosion test, field emission scanning electron microscopy (FESEM/EDS), and X-ray diffraction (XRD) analyses, were used to reveal why the MAC overlayer improves the CSZ hot corrosion behavior. A medium of Na₂SO₄-55 wt% V₂O₅ was used to analyze the hot corrosion; a temperature of 950 °C for 2 h was considered in every single cycle. The results exposed that there is a big difference between the hot corrosion resistance of the dual-layer CSZ/MAC TBC in comparison with the single-layer CSZ. Based on the FESEM analysis, this can be related to the very low diffusion of Na₂SO₄-55 wt% V₂O₅ into the dual-layer TBC where the infiltration of aggressive molten salt was diminished. According to the XRD results, two reasons are leading to the degradation of the aforementioned TBCs: (i) the tetragonal to the monoclinic transformation of ZrO₂ and (ii) the formation of hot corrosion products, i.e., CeVO₄ and YVO₄ crystals.     

Oxidative dehydrogenation of ethane to ethylene over V2O5/Nb2O5 catalysts

V₂O₅/Nb₂O₅ catalysts with various V₂O₅ contents were prepared by impregnation and characterized by various techniques in detail. Oxidative dehydrogenation of ethane was carried out in a fixed bed quartz reactor at 500–600 °C. XPS analysis indicated a clear enrichment of vanadium on the near-surface-region and UV–vis diffuse reflectance spectroscopy revealed the nature of VO_x structures formed. 10 wt.% V₂O₅/Nb₂O₅ catalyst has displayed the best performance (X = 28%, S = 38% at 600 °C) due to enrichment of vanadium in the near-surface-region and formation of optimum amount of monomeric/oligomeric VO_x species.     

Hot corrosion resistance of air plasma sprayed ceramic Sm2SrAl2O7 (SSA) thermal barrier coatings in simulated gas turbine environments

Samarium strontium aluminate (Sm₂SrAl₂O₇-SSA) and Yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs) were developed on NiCrAlY bond coated Inconel 718 superalloy substrate using air plasma spray process. The hot corrosion study was conducted in simulated gas turbine environments (molten mixtures of 50?wt% Na₂SO₄ + 50?wt% V₂O₅ and 90?wt% Na₂SO₄ + 5?wt% V₂O₅ + 5?wt% NaCl) for two different temperatures of 700 and 900?°C. A developed SSA TBCs showed about 8% and 22% lower lifetime at 700 and 900?°C, respectively than YSZ TBCs in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ (vanadate). The hot corrosion life of SSA TBCs being found about 13% and 39% lower than YSZ TBCs in 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl (chloride) at 700 and 900?°C, respectively. X-ray diffraction results showed the formation of SmVO₄, SrV₂O₆, and SrSO₄ as a major hot corrosion product in 50?wt% Na₂SO₄ +?50?wt% V₂O₅ and 90?wt% Na₂SO₄ +?5?wt% V₂O₅ +?5?wt% NaCl environments respectively for SSA TBCs. Similarly, YSZ TBCs also showed YVO₄ as hot corrosion product in vanadate and chloride environments. Both the TBCs suffer a more severe hot corrosion attack in chloride environment at 900?°C. The leaching of Sr²⁺ and Y³⁺ ions from SSA and YSZ respectively play a vital role in the destabilization of coating in vanadate and chloride environments at 700 and 900?°C. In both SSA and YSZ TBCs, the leaching of ion has significantly low influence as compared to attack by chloride ions at the bond coat-top coat interface in the presence of chloride environment. The hot corrosion resistance of SSA TBCs was improved three times higher in the presence of MgO and NiO inhibitor in vanadate environment at 900?°C mainly due to the formation of a stable Ni₃V₂O₈ phase at the surface.     

Evolution of hot corrosion resistance of YSZ,Gd2Zr2O7, and Gd2Zr2O7 + YSZ composite thermal barrier coatings in Na2SO4 + V2O5 at 1050 °C

This paper compares the hot corrosion performance of yttria stabilized zirconia (YSZ), Gd₂Zr₂O₇, and YSZ + Gd₂Zr₂O₇ composite coatings in the presence of molten mixture of Na₂SO₄ + V₂O₅ at 1050 °C. These YSZ and rare earth zirconate coatings were prepared by atmospheric plasma spray (APS). Chemical interaction is found to be the major corrosive mechanism for the deterioration of these coatings. Characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) indicate that in the case of YSZ, the reaction between NaVO₃ and Y₂O₃ produces YVO₄ and leads to the transformation of tetragonal ZrO₂ to monoclinic ZrO₂. For the Gd₂Zr₂O₇ + YSZ composite coating, by the formation of GdVO₄, the amount of YVO₄ formed on the YSZ + Gd₂Zr₂O₇ composite coating is significantly reduced. Molten salt also reacts with Gd₂Zr₂O₇ to form GdVO₄. Under a temperature of 1050 °C, Gd₂Zr₂O₇ based coatings are more stable, both thermally and chemically, than YSZ, and exhibit a better hot corrosion resistance.     

High velocity oxygen fuel sprayed Cr3C2-NiCr coatings against Na2SO4 hot corrosion at different temperatures

Cr₃C₂-NiCr/NiCrAlY coating was prepared by high velocity oxygen fuel (HVOF) spraying. The microstructure and the Na₂SO₄ hot corrosion behavior at different temperatures of the coating were investigated. The Na₂SO₄ hot corrosion mechanism of Cr₃C₂-NiCr coating was also discussed. The results showed that HVOF Cr₃C₂-NiCr coating was relatively dense and mainly composed of Cr₃C₂, NiCr and a small amount of Cr₇C₃ three phases. The dense Cr₂O₃ layer was formed on the surface of Cr₃C₂-NiCr coating after Na₂SO₄ corrosion at 750 °C to further prevent corrosion. The coating had produced the obvious longitudinal crack with the increase of hot corrosion temperature up to 900 °C. The corrosion mechanism of Cr₃C₂-NiCr coating against the Na₂SO₄ salt at high-temperature was as follows: firstly, the protective oxidizing film was formed at 750 °C, then the protective oxide film dissolved at the interface between the coating and Na₂SO₄ salt with the hot corrosion temperature increasing up to 900 °C, and subsequently the dissolved anions and cations could migrate to the interface between molten salt and air and the loose and unprotected oxides were regenerated, thereby exacerbating the failure of the coating.     

Hot corrosion behavior of YSZ/ZrW2O8 composites exposed to V2O5 at 700 °C and 850 °C in air

The hot corrosion behavior of YSZ/ZrW₂O₈ composites as a promising thermal barrier coating system exposed to V₂O₅ at 700 °C and 850 °C was investigated in order to better understand the influence of the incorporated ZrW₂O₈ with isotropic negative thermal expansion performance on the corrosion resistance. Results indicate that the ZrW₂O₈ incorporation could retard the degradation of YSZ from V₂O₅ attack and the corrosion process is significantly related to the inclusion content and the temperature. The corrosion resistance could be determined by the incorporation content, while the reaction products are only temperature dependent. At 700 °C, ZrV₂O₇, YVO₄ and m-ZrO₂ were the main corrosion products, while ZrW₂O₈ recrystallized under the acidic environment provided by V₂O₅. At 850 °C, ZrW₂O₈ decomposed and only WO₃, YVO₄ and m-ZrO₂ could be detected as final corrosion products. The corrosion mechanisms of YSZ/ZrW₂O₈ composites at 700 °C and 850 °C were discussed based on the phase diagrams and Lewis acid-base rule as well as the volume compensation of the positive and negative expansion ceramics.     

The influence of laser treatment on hot corrosion behavior of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

In this study, the effect of laser glazing on the hot corrosion behavior of nanostructured thermal barrier coatings (TBCs) was investigated. To this end, the hot corrosion test of plasma-sprayed and laser-glazed thermal barrier coatings conducted against 45 wt.% Na₂SO₄ + 55 wt.% V₂O₅ molten salt at 910 °C for 30 h in open air atmosphere. The results obtained from hot corrosion test showed that the reaction between Y₂O₃ and the corrosive salt produced YVO₄, leached Y₂O₃ from YSZ and led to the progressive destabilization transformation of YSZ from tetragonal to the monoclinic phase. The lifetimes of the plasma-sprayed TBCs were enhanced approximately twofold by laser glazing. Reducing the reactive specific surface area of the dense glazed layer with the molten salts and improving the stress accommodation through network cracks produced by laser glazing were the main enhancement mechanisms accounting for TBC life extension.     

Effect of molten V2O5 salt on the corrosion behavior of micro- and nano-structured thermal sprayed SYSZ and YSZ coatings

The corrosion resistance of micro-and nano-structured scandia and yttria codoped zirconia (nano-4 mol%SYSZ and micro-8.6SYSZ) and yttria doped zirconia (4YSZ) in the presence of molten vanadium oxide were investigated. To this end, duplex TBCs (thermal barrier coatings), composed of a bond coat (NiCrAlY) and a top coat (4SYSZ or 4YSZ), were deposited on the IN738LC Ni-based supper-alloy by atmospheric plasma spraying (APS). The corrosion studies of plasma sprayed TBCs were conducted in 25 mg V₂O₅ molten salt at 910 °C for different times. The nanostructured coating, as compared to its micro-structured counterpart, in spite of a further reaction with the V₂O₅ salt, showed a higher degradation resistance during the corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones. Finally, the corrosion resistance and degradation mechanism of SYSZ and YSZ coatings were compared with the presence of molten NaVO₃ and V₂O₅ salt, respectively.     

Preparation of plasma sprayed nanostructured GdPO4 thermal barrier coating and its hot corrosion behavior in molten salts

Nanostructured GdPO₄ coatings, designed as the outer layer of double-ceramic-layer thermal barrier coatings (DCL-TBCs), were produced by air plasma spraying (APS). The coatings have close chemical composition to that of the agglomerated particles used for thermal spray. Nanozones with porous structure are embedded in the coating microstructure, having a percentage of ~30%. Hot corrosion tests of the coatings were carried out in V₂O₅ and Na₂SO₄+V₂O₅ salts at 900 °C for 4 h. Results indicate that dense reaction layers, consisting of GdVO₄ and Gd₄(P₂O₇)₃, form on the coating surfaces, which could suppress further penetration of the molten salts. In the V₂O₅ molten salt, the reaction layer is thicker and less molten salt trace could be found beneath the layer.