V2O5 Salt Research Articles

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.

High-Temperature Oxidation and Corrosion Behaviour of APS CoCrAlY + Cr3C2–NiCr Composite Coating

An approach of chrome carbide-based composite coating entailing of hard phase (Cr3C2–NiCr) and matrix-CoCrAlY was tailored to encounter the severe environments such as elevated temperature oxidation, corrosion and erosion in gas turbine application. CoCrAlY + Cr3C2–NiCr coating was deposited by atmospheric plasma spray process. Oxidation and hot corrosion tests were conducted under cyclic conditions in Na2SO4–V2O5 salt environment at 700 °C. The oxidation and hot corrosion behaviour base metal and coated samples were evaluated by thermogravimetric method. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction techniques were used to characterize the tested samples in terms of elemental and phase analysis. Microhardness of the composite coating is 318 ± 26 HV and the adhesion strength is found to be 11.5 ± 2.5 MPa. The uncoated alloys exhibited limited weight gain for the initial oxidation cycles followed by linear weight gain curve with the increasing in thermocycles. The CoCrAlY + Cr3C2–NiCr coating experienced good corrosion resistance as compared to substrates- MDN 321 and Superni 76 at 700 °C. The coatings subjected to oxidation and hot corrosion showed slow-scale growth kinetics with parabolic rate constant (Kp) value of 0.4 and 0.053 10–10 g2 cm−4 s−1, respectively. The higher Cr content in the coating resulted in the growth of Cr2O3 and its spinel CoCr2O4 as a strong phase is beneficial for oxidation and corrosion resistance.

Hot corrosion behavior of NdYbZr2O7 exposed to V2O5 and Na2SO4 + V2O5 molten salts

In order to evaluate the application prospects of NdYbZr2O7 as a novel TBC material, NdYbZr2O7 ceramic was synthesized via a solid-state reaction sintering method, and its hot corrosion behavior exposed to V2O5 and Na2SO4 + V2O5 molten salts at 900 °C, 1000 °C, and 1100 °C was comparatively investigated. For the V2O5 salt, the primary corrosion products were granular (Nd,Yb)VO4 as well as cube-like m-ZrO2. The corrosion layer consisted of two distinct layers, one of which was Zr-rich layer and another was V-rich layer. In the case of Na2SO4 + V2O5, NaVO3, as an intermediate product, played an important role in dissolving the NdYbZr2O7 ceramic. Herein, the (Nd,Yb)VO4 exhibited a rod/plate-like morphology, which could be attributed to the synergistic effect of low driving force and low nucleation rate. Since the molten salt infiltration rate was superior to the pore filling rate throughout the hot corrosion, the thickness of corrosion layer increased with the rise of temperature. The hot corrosion mechanisms of NdYbZr2O7 ceramic in various molten salts were discussed based on the phase diagram, Lewis acid-base rule and chemical thermodynamics. On this basis, the NdYbZr2O7 coating was prepared by atmospheric plasma spray (APS) and it exhibits a higher corrosion resistance compared to YSZ coating.

Corrosion behavior of wire arc sprayed Ni-based coatings in extreme environment

The current study represents the corrosion behavior of wire arc sprayed Ni-based (Ni-5Al and Ni-20Cr) coatings on commonly used boiler tube steels (T11 and Gr A1). A cyclic hot corrosion behavior of the samples has been carried out at 900 °C for 50 cycles with the application of Na2SO4-60%V2O5 salt. The coatings offered excellent corrosion resistance in comparison to bare steel. The surface and cross sectional analysis of coated samples before and after corrosion has been carried out using SEM/EDS and XRD analysis. It was observed that Ni-20Cr coating exhibited better corrosion resistance for both T11 and Gr A1 steel than a Ni-5Al coating. The Ni-20Cr coating has shown 87% and 75% better corrosion resistance in terms of weight change in comparison to bare T11 and Gr A1 boiler steel, while Ni-5Al offered 86% and 73% better corrosion resistance to bare steels respectively. This may be due to the formation of protective Cr2O3 oxide in the top oxide layer, lower porosity of coatings and high micro hardness.

Hot Corrosion Behavior of BaLa2Ti3O10 Thermal Barrier Ceramics in V2O5 and Na2SO4 + V2O5 Molten Salts

BaLa2Ti3O10 ceramics for thermal barrier coating (TBC) applications were fabricated, and exposed to V2O5 and Na2SO4 + V2O5 molten salts at 900 °C to investigate the hot corrosion behavior. After 4 h corrosion tests, the main reaction products resulting from V2O5 salt corrosion were LaVO4, TiO2, and Ba3V4O13, whereas those due to Na2SO4 + V2O5 corrosion consisted of LaVO4, TiO2, BaSO4 and some Ba3V4O13. The structures of reaction layers on the surfaces depended on the corrosion medium. In V2O5 salt, the layer was dense and had a thickness of 8–10 μm. While in Na2SO4 + V2O5 salt, it had a ~15 μm porous structure and a dense, thin band at the bottom. Beneath the dense layer or the band, no obvious molten salt was found. The mechanisms by which the reaction layer forms were discussed.

Hot Corrosion Study of 9Cr–1Mo Boiler Steel Exposed to Different Molten Salt Mixtures

The present study is an attempt to observe the hot corrosion behavior of P91 boiler steel when exposed to different molten salt environments at higher temperature. The hot corrosion behavior was observed for temperature range 550–850 °C under the application of air, 40% Na2SO4 + 60% V2O5 (salt 1), 40% K2SO4 + 60% NaCl (salt 2), 50% Na2SO4 + 50% NaCl (salt 3), 40% Na2SO4 + 40% K2SO4 + 10% NaCl + 10% KCl (salt 4). Thermogravimetric study results in parabolic oxidation kinetics when the specimen was exposed to temperature 550 °C and 650 °C, whereas multistage oxidation kinetics was observed at temperatures above 650 °C. An acceleration in the oxidation process was observed with the increase in the exposure temperature, also causing an increase in the oxide scale thickness. Specimen exposed to salt mixtures containing more Na2SO4 and NaCl (salt 2 and salt 3) experienced severe corrosion attack at 750 °C and 850 °C. Thick, non-adherent, and porous magnetite and hematite layers were observed on the outer scale of the specimen which resulted in higher weight gain of the alloy. The corrosion rate of P91 alloy exposed to different salt mixtures was in the order: salt 3 > salt 2 > salt 1 > salt 4 > air.

Cyclic Hot Corrosion Failure Behaviors of EB-PVD TBC Systems in the Presence of Sulfate and Vanadate Molten Salts

The cold gas dynamic spray (CGDS) method has been considered a promising technology to produce a metallic bond coat for thermal barrier coating (TBC) systems. In this study, CoNiCrAlY bond coats produced by CGDS method were coated with yttria-stabilized zirconia (YSZ) by electron beam physical vapor deposition (EB-PVD). TBCs were subjected to 50 wt % V2O5 and 50 wt % Na2SO4 molten hot corrosion salt combinations at 1000 °C. In the case of YSZ top coat on TBCs, the reaction between Na2SO4, V2O5, and Y2O3 salts generates YVO4 crystals, and these structures cause the transformation of tetragonal ZrO2 to monoclinic ZrO2. This situation occurs under operating conditions that lead to TBC failure. Hot corrosion behavior and the related failure mechanisms of TBC systems were investigated and discussed using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, and X-ray diffractometer (XRD).

Effects of Pt and Si on the low temperature hot corrosion of aluminide coatings exposed to Na2SO4-60 mol% V2O5 salt

Hot corrosion of pack (zero/medium) Pt-modified aluminide and slurry (zero/low/medium) Pt-modified silicon-aluminide coatings exposed to a Na2SO4-60 mol%V2O5 deposit at 700 °C was evaluated using the deposit recoat technique. Thickness loss metrology was utilized to study corrosion penetration into the coatings. The corrosion products and remaining coatings were characterized by means of SEM, EDX, XRD and EPMA. The aluminide coating and medium Pt-modified silicon-aluminide coating showed the worst and the best corrosion resistance, respectively. Enhanced hot corrosion resistance of the Pt-modified silicon-aluminide was attributed to microstructure of the coating with a two-phase PtAl2-NiAl outer layer and formation of refractory silicides.

Characterization of detonation gun thermal spray coatings on SA213T91 in simulated boiler environment

The Ni-Cr and Stellite-21 coated SA213T91 boiler steel deposited by D-gun thermal spray process on the boiler steel were oxidized in the presence of air and Na2SO4-60% V2O5 salt mixture at 900˚C for 100 cycles. One cycle consists 1 hour heating in silicon carbide tube furnace and 20 minute cooling in ambient air. The kinetics of oxidation shows that the D-gun spray coating s technique is better than other coating techniques. The measurements of mass change kinetics corrosion by SEM/EDS, XRD and X-Ray techniques. The Oxidation and hot corrosion mechanisms were sustained by analyzing the results from various techniques. The results of the study have been discussed w.r.t. thermo gravimetric structure.

Enhancement of hot corrosion resistance of thermal barrier coatings by using nanostructured Gd2Zr2O7 coating

Improvement of hot corrosion resistance in Na2SO4-V2O5 environments is one of the main goals in development of thermal barrier coatings (TBCs). In this study, nano-Gd2Zr2O7 (nano-GZ) powder was synthesized by co-precipitation method and then deposited with atmospheric plasma spray (APS) technique on Ni-based superalloy (IN738LC) substrates as a top coat with a CoNiCrAlY bond coat. Hot corrosion behavior of nano-GZ coated samples were studied and compared with those of CoNiCrAlY/conventional YSZ coatings. Hot corrosion resistance of the samples was investigated in a cyclic mode in which after each 4 h exposure to Na2SO4–55 wt% V2O5 salts at 950 °C. The results indicated that nano-GZ coatings have better hot corrosion resistance compared to conventional YSZ coatings. It was found that micro-cracks and pores played a main role in the molten salts infiltration into the TBCs during hot corrosion process. In the nano-GZ coating with nano zones structure which surrounded by fully molten parts could fill the aforementioned defects and could act as barrier for the corrosive molten salts penetration into the TBC. The main factor in destruction of the investigated coatings is the formation and growth of needle-like YVO4 and GdVO4 crystals, in both classes of the coatings.

Sulfate‐vanadate hot corrosion of neodymium cerate/yttria stabilized zirconia composite coating

AbstractNeodymium cerate (Nd2Ce2O7) was laboratory synthesized by solid‐state reactions of neodymia and ceria and spray dried to get the feedstock for plasma spraying. Hot corrosion behavior of air plasma sprayed 10% Nd2Ce2O7/yttria stabilized zirconia (YSZ) topcoat was studied in the presence of V2O5 and Na2SO4 salts at 950°C for 10‐60 hours. It was observed that due to the presence of relatively higher basic compounds Nd2O3 and CeO2 (form Nd2Ce2O7) than Y2O3 in YSZ changed the reaction dynamics during hot corrosion and the major part of corrosive (NaVO3) was consumed by Nd of Nd2Ce2O7, making NdVO4. The Y2O3 of YSZ contributed in the corrosive reaction partially, relieving majority tetragonal zirconia (t‐ZrO2) in topcoat (YSZ). No remarkable cracking was developed in the topcoat even after 60 hours of accelerated hot corrosion, which was attributed to the retention of in‐relief t‐ZrO2 in YSZ.

Evaluation of the hot corrosion behavior of Inconel 625 coatings on the Inconel 738 substrate by laser and TIG cladding techniques

The present study investigated the effect of TIG and laser cladding of Inconel 625 coating over an Inconel 738 substrate on its hot corrosion resistance. The hot corrosion test was performed by exposing the specimens to Na2SO4–60 wt%V2O5 salt at 900 °C for 36 h. Microstructural, elemental, and phase characterization was performed using a field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometry (EDS) detector and an X-ray diffractometer (XRD). The microstructural evaluation did not show any crack or pore in the laser-cladded specimen, but revealed a series of cracks at the surface and coating-substrate interface of the TIG-cladded specimen. The phase formed in the coated and uncoated specimens was found to be ϒ-Ni. The results were also indicative of the formation of chromium oxide, nickel oxide, NiCr2O4 spinel, and NiCO3 after the corrosion test. The corrosion-affected areas in the laser-cladded specimen, TIG-cladded specimen, and uncoated specimen had a thickness of approximately 40 ± 5 μm, 80 ± 10 μm, and 130 ± 10 μm, respectively. The results demonstrated the superior hot corrosion resistance of the laser-cladded specimen, as compared to its TIG-cladded and uncoated counterparts.

Hot corrosion behavior of plasma sprayed 8YSZ-alumina- CNT composite coating in Na2SO4–60% V2O5 molten salt environment

Hot corrosion (HC) behavior of air plasma sprayed 8% yttria stabilized zirconia (8YSZ) -alumina (YA) composite thermal barrier coating has been evaluated in the present work. Three variants of YA coatings were produced, two by reinforcing 1%, 3% multiwall carbon nanotube (MCNT) and one coating was obtained without any MCNT reinforcement. The creep strength enhanced ferritic P91 steel was used as substrate material for coating. Na2SO4 and V2O5 salts mixed in 2:3 ratio was applied to the coating and subjected to 800 °C for 120 h in an electric furnace. The microstructure and phase characterization of the coating was carried out before and after HC using scanning electron microscope (SEM) and X-ray diffraction (XRD) respectively. Energy dispersive spectroscopy (EDS) integrated with the SEM was used for elemental analysis of the coating. The investigation reveals improved isothermal HC behavior of 1% MCNT reinforced coating. The dominating effect of the HC was recognized as depletion of yttria leading to destabilization of 8YSZ. The formation of YVO4 was the corrosion product containing the depleted Y2O3 of YSZ. The 1% and 3% MCNT reinforced coating exhibited monoclinic phase percentage of around 9% and 34% respectively. Nanoindentation was carried out along the cross-section before and after the isothermal HC. The Youngs modulus after HC increased by 46%, 42%, 12.5% and 38% for 8Y, 8YA, 8YA1C and 8YA3C coating, respectively. Weibull modulus of Youngs modulus of bond coats was used to identify the efficiency of top coat in retarding the infiltration of molten salt. The bond coat of 8YA1C coating exhibited lowest modulus value (m = 8.55) indicated non-uniform infiltration of detrimental species. The 1% MCNT reinforced thermal barrier coating (TBC) system was more resistant to degradation than the conventional 8YSZ and YA composite coatings.

HOT CORROSION OF CERIA-YTTRIA STABILIZED ZIRCONIA PLASMA SPRAYED THERMAL BARRIER COATING BY EUTECTIC VANDIUM PENTAOXIDE-SODIUM SULFATE

The high temperature corrosion behavior of thermal barrier coating (TBC) systemconsisting of IN-738 LC superalloy substrate, air plasma sprayed Ni24.5Cr6Al0.4Y (wt%)bond coat and air plasma sprayed ZrO2-20 wt% ceria-3.6 wt% yttria (CYSZ) ceramic coatwere characterized. The upper surfaces of CYSZ covered with 30 mg/cm2 , mixed 45 wt%Na2SO4-55 wt% V2O5 salt were exposed at different temperatures from 800 to 1000 oC andinteraction times from 1 up to 8 h. The upper surface plan view of the coatings wereidentified for topography, roughness, chemical composition, phases and reaction productsusing scanning electron microscopy, energy dispersive spectroscopy, talysurf, and X-raydiffraction. XRD analyses of the plasma sprayed coatings after hot corrosion confirmed thephase transformation of nontransformable tetragonal (t') into monoclinic phase, presence ofYVO4 and CeVO4 products. Analysis of the hot corrosion CYSZ coating confirmed theformation of high volume fraction of YVO4, with low volume fractions of CeOV4 and CeO2.The formation of these compounds were combined with formation of monoclinic phase (m)from transformation of nontransformable tetragonal phase (t').

Hot Corrosion Behaviour of Refractory and Rare Earth Oxide Reinforced CoCrAlY APS Coatings at 700 °C

This paper investigates cyclic hot corrosion of plasma sprayed CoCrAlY + Al2O3 + YSZ (C1) and CoCrAlY + CeO2 (C2) composite coatings on MDN 321 and Superni 76 substrates in molten salt (Na2SO4-60%V2O5) environment exposed to 700 °C. Weight change technique is used to evaluate the corrosion performance. Both C1 and C2 coatings showed better corrosion resistance than uncoated alloy. Both the coatings showed linear weight gain during the initial cycles and parabolic weight gain nature with subsequent hot corrosion cycles. The parabolic rate constant (Kp) of C1 and C2 coating was observed to be in the range 0.29–0.32 × 10−10 g2 cm−4 s−1 and 1.0–1.13 × 10−10 g2 cm−4 s−1 respectively. In C1 coating, the globular and continuously packed structure on the corroded surface having CoO, Cr2O3, CoCr2O4 and CoAl2O4 spinel oxides provided superior hot corrosion resistance. While in case of C2 coating, the outward growth of CeVO4 irregular crystals as a corrosion product of CeO2 and V2O5 salt deteriorated the oxide scales resulting in higher corrosion rate.

Hot corrosion and mechanical properties of nanostructured Al2O3/CSZ composite TBCs

The purpose of this research is to improve the quality and efficiency of yttria-stabilised zirconia (YSZ) thermal barrier coating (TBC) through replacing CSZ. Three types of top coats including conventional YSZ, CSZ and nanostructured Al2O3/CSZ composite were applied, by thermal spray technique, on IN738LC base metal and the results were compared to one another. In the present investigation, hot corrosion behaviours of the coatings were tested in a furnace containing a mixed environment of Na2SO4 and V2O5 salts at 1050°C. Nanoindentation tests were used to evaluate mechanical properties of the coatings. The experimental results confirmed that nanostructured Al2O3/CSZ composite coating had much hot corrosion resistance than those of conventional TBCs. The transformation of tetragonal zirconia to monoclinic ZrO2 and formation of YVO4 and CeVO4 crystals as hot corrosion products led to the degradation of the mentioned TBCs. Microstructural analysis indicated that the penetration of molten salt was much less in the nanostructured Al2O3/CSZ composite coating where the infiltration of both oxygen and aggressive molten salt was reduced. Mechanical results showed that the superior mechanical properties of nanostructured Al2O3/CSZ composite coating may be attributed to its structural stability.

Studies on thermally grown oxide as an interface between plasma-sprayed coatings and a nickel-based superalloy substrate

A thermally grown oxide layer formed by hot corrosion was investigated as an interface between plasma-sprayed coatings and a nickel-based superalloy substrate. The hot corrosion mechanism of NiCr–Cr2O3 and Al2O3–40wt% TiO2 (A40T) plasma coated Inconel 617 was evaluated. The experiments were carried out at 1000°C using a combination of Na2SO4, NaCl, and V2O5 salts to simulate the conditions of a gas turbine in a marine environment. The hot corrosion results revealed the spallation and dissolution of oxides upon prolonged exposure. Optical images and scanning electron micrographs of the exposed samples revealed the formation of oxide scale and provided details of its morphology in NiCr–Cr2O3 coated samples. Microstructure characterization of A40T coatings demonstrated a thermally grown oxide (TGO) layer at 1000°C. Increasing the thickness of the TGO layer decreased the corrosion resistance. The elemental analysis and image mapping revealed the migration of active elements from the substrate and coatings toward the corrosive environment.

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 4h. 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.

Preparation and hot corrosion behavior of plasma sprayed nanostructured Gd2Zr2O7-LaPO4 thermal barrier coatings

Nanostructured 30 mol% LaPO4 doped Gd2Zr2O7 (Gd2Zr2O7-LaPO4) thermal barrier coatings (TBCs) were prepared by air plasma spraying (APS). The chemical composition of the coating is close to that of the agglomerated particles used for thermal spray. Nanozones were obviously found in the coating microstructure. Hot corrosion tests were conducted in V2O5 and Na2SO4+V2O5 salts at 900 °C to study the corrosion resistance of the coatings. Results indicated that the coating exhibited excellent corrosion resistance in both salts, but it performed better in V2O5 salt. After 4 h corrosion tests, a continuous, dense reaction layer formed on the coating surfaces, effectively arresting further penetration of the molten salt. By comparison, Na2SO4+V2O5 salt corrosion caused thinner reaction layer than V2O5 corrosion, and some NaVO3 trace was found in the inner regions of the coating. Furthermore, the nanozones in the coating have positive function on reducing the molten salts infiltration.

The role of the surface morphology and segmented cracks on the damage forms of laser re-melted thermal barrier coatings in presence of a molten salt (Na2SO4 + V2O5)

Hot corrosion behaviour of yttria-stabilized zirconia thermal barrier coatings (TBCs) with various segmented cracks and surface morphologies, obtained using ultrasonic-assisted laser re-melting, was investigated during exposure to Na2SO4+V2O5 salt at 1100°C. It is demonstrated, that the damage types of TBCs were significantly affected by segmented cracks and surface morphology. Four main damage mechanisms: (i) chemical reaction, (ii) Coefficient thermal mismatch stress, (iii) oxidation of bond coating and (iv) intergranular corrosion, based on the observed transport behaviour of oxygen and molten salt, stress-tolerance and self-healing of various segmented cracks, were discussed to account for the failure behaviour.