Studies on V2O5 induced hot corrosion

Nickel samples coated with a monitored thickness of vanadium pentoxide were oxidized at 900°C under various oxygen pressures. The microstructures of the thin film grown on nickel were studied by X-ray diffraction, A E S, M E B and energy dispersive X-ray analyses. A nickel vanadium oxide is assumed to be located in the short-circuits of the growing oxide NiO leading to a rapid diffusion of nickel ions unless a vanadate precipitates. This mechanism allows us to forecast a way of protection against V₂O₅ induced hot corrosion. Thus, AISI 310 coated with magnesium is much less attacked by V₂O₅, because magnesium and vanadium react to form solid Mg₃V₂O₈.     

Influence of Cr content on high-temperature oxidation behaviour of arc sprayed Ni–Cr coatings

In this study, the high-temperature oxidation behaviour of arc-sprayed Ni–Cr coatings with high Cr contents of 30, 45 and 50 at.% was investigated in comparison with reference AISI 1020 steel. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy were utilised to characterise the oxide scales. The oxidation resistance of the steel substrates was found to be enhanced after the application of the Ni–Cr coatings since the oxidation kinetics followed the parabolic law. In addition, the oxidation rate of Ni–50Cr coating was 56.5% lower than that of Ni–30Cr coating, indicating that the oxidation performance of coatings was improved with increasing Cr content. The oxide layers of Ni–Cr coating were found to be a double layer structure protecting the substrate from severely oxidation, which composed of a top layer of NiO and a basal layer of Cr₂O₃ and NiCr₂O₄. The surface of Ni–30Cr coating contained lots of multi-angle NiO crystals, while the surface of Ni–50Cr coating contained a dense Cr₂O₃ structure, suggesting that the growth of NiO crystals was limited due to the large amount of Cr-rich oxides.     

Phase Separation Derived Core/Shell Structured Cu11V6O26/V2O5 Microspheres: First Synthesis and Excellent Lithium‐Ion Anode Performance with Outstanding Capacity Self‐Restoration

Novel amorphous vanadium oxide coated copper vanadium oxide (Cu₁₁V₆O₂₆/V₂O₅) microspheres with 3D hierarchical architecture have been successfully prepared via a microwave‐assisted solution method and subsequent annealing induced phase separation process. Pure Cu₁₁V₆O₂₆ microspheres without V₂O₅ coating are also obtained by an H₂O₂ solution dissolving treatment. When evaluated as an anode material for lithium‐ion batteries (LIBs), the as‐synthesized hybrid exhibits large reversible capacity, excellent rate capability, and outstanding capacity self‐recovery. Under the condition of high current density of 1 A g^?1, the 3D hierarchical Cu₁₁V₆O₂₆/V₂O₅ hybrid maintains a reversible capacity of ≈1110 mA h g^?1. Combined electrochemical analysis and high‐resolution transmission electron microscopy observation during cycling reveals that the amorphous V₂O₅ coating plays an important role on enhancing the electrochemical performances and capacity self‐recovery, which provides an active amorphous protective layer and abundant grain interfaces for efficient inserting and extracting of Li‐ion. As a result, this new copper vanadium oxide hybrid is proposed as a promising anode material for LIBs.     

Corrosion of thermal barrier coatings by vanadium and sulfur compounds

Abstract

Hot corrosion studies of two plasma-sprayed coatings, yttria-stabilized zirconia and calcium silicate, were undertaken in order to compare the performance of these materials for use as thermal barrier coatings in high-temperature combustion environments. The coatings were tested in contact with vanadium pentoxide at 1,000°C and, also, under conditions in which they were exposed to sulfur-containing compounds at 900°C or 1,000°C. The samples were subsequently characterized by scanning electron microscopy and X-ray diffraction analysis to identify the effects of these tests on the microstructure and composition. The results indicate that reactions with V₂O₅ lead to a disruptive phase transformation in zirconia that rapidly degrades the coating. For calcium silicate, the reactions with V₂O₅ appear to be more limited and less disruptive so that the coating is much more slowly degraded by the vanadium compounds. Exposure to SO_x and sulfate salts at high temperature caused rapid degradation of the calcium silicate coatings through a reaction involving the formation of CaSO₄. Under similar conditions, the yttria-stabilized zirconia coatings experienced much less attack.     

Corrosion properties of plasma-sprayed Al2O3-TiO2 coatings on Ti metal

Three different oxides of CrO₂-TiO₂, Al₂O₃ and Al₂O₃-TiO₂ were plasma-sprayed on Ti substrate to evaluate the crystal structure and the corrosion properties of the coatings. No phase change of the coatings after corrosion test in 0.5 M H₂SO₄ solution at 25°C was found regardless of the presence of the NiCoCrAlY bond layer. Electrochemical measurements and SEM results revealed that the single coatings without the bond layer were always effective against corrosion resistance due to lower current density within the passive region. Pitting corrosion of the surface was observed for the Al₂O₃ coating. It can be concluded that the Al₂O₃-TiO₂ coating without the bond layer may be the best oxide among the oxides investigated due to low porosity (5.4%), smooth surface roughness (4.5 μm), low current density (6.3 × 10^‒8 A/cm²) in the passive region, low corrosion potential (E_corr, ‒0.55 V) and no pitting corrosion.     

Corrosion reactions in molten sulphates

The redox potential in sulphate melts is controlled by the partial pressures of oxygen and SO₃ in equilibrium with the melt. The rate for the reduction of oxygen is limited by the solubility. Higher rates are observed for the reduction of SO₃ which is identified as the principal oxidizing species. Equilibrium potentials for reversible metal-metal ion couples for the principal components of superalloys are negative relative to the redox potential for the melt. The anodic oxidation of the metal therefore proceeds irreversibly in conjunction with the cathodic reduction of SO₃ An oxide layer is formed on the surface of the metal when the metal ion concentration at the surface, combined with the oxide ion concentration in the melt, which is related to the partial pressure of SO₃, exceeds the solubility limit for the oxide. The corrosion behaviour will depend on the mass transport processes through this oxide layer. Temperature gradients through the molten sulphate, and oxide ion concentration gradients established as a consequence of the corrosion reaction, may influence the morphology of the corrosion product. The gradient between the oxygen chemical potential at the outer surface of the molten sulphate, defined by the oxygen partial pressure in the gas and the oxygen chemical potential at the metal/metal oxide interface, defined by the dissociation equilibrium for the metal oxide, combined with the transport of SO₃ through the molten sulphate, increases the sulphur chemical potential at the metal surface and leads to the formation of the metal sulphide. Stress in the protective oxide layer caused by the growth of the sulphide phase at the interface between the metal and the oxide will eventually fracture the oxide and cause accelerated corrosion.     

Synthesis and characterization of layered and scrolled amine-templated vanadium oxides

In order to fully understand the formation mechanism, structure, and role of structural curvature of vanadium oxide nanotubes (VONTs), two isostructural materials (one planar and the other curved)—ethylenediammonium (enH₂) intercalated V₇O₁₆ and vanadium oxide nanourchins (VONUs)—were synthesized and characterized via X-ray diffraction (XRD), electron microscopy, thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy, and magnetic measurements. The synthesis route to (enH₂)V₇O₁₆ is developed using vanadium pentoxide as a starting material and employing pH control. VONUs are synthesized using n-dodecylamine as an amine template for the first time. We demonstrate that the structure of the vanadium oxide layer in these compounds is similar to that of VONTs and their magnetic properties all fit to the same model including the temperature-independent, Curie–Weiss, and spin ½ antiferromagnetic dimer contributions. The vanadium oxidation state in tubular structures appears to be higher than in planar compounds such as (eH₂)V₇O₁₆ and BaV₇O₁₆. The role of the template and vanadium reduction in the formation of nanotubes is discussed.     

Colouring process for ferritic stainless steel alloy in NaNO3-KNO3 eutectic melt

The colouring process of the ferritic stainless steel alloy (15.03 Cr) was carried out in pure NaNO₃-KNO₃ eutectic melt and in presence of Na₂O₂, NaCl and their mixtures at different temperatures (673–873 K) under open-circuit and galvanostatic anodic polarization conditions. Coloured oxide films can be formed and the colours greatly depend on the thickness which in turn depends on the composition of the molten bath and its temperature. The more attractive, bright, adherent and uniform coloured films can be formed at temperatures of 673, 723 and 773 K in the nitrate melt containing Na₂O₂ and mixtures of 0.5 M NaCl with Na₂O₂ additions. The corrosion tests, carried out on the coloured oxide films in 0.1 M HCl solution, show that the corrosion resistance of the coloured films greatly depends on the previous operating conditions of the colouring process such as composition of molten bath and its temperature.     

Cyclic Oxidation and Hot Corrosion Behavior of Plasma-Sprayed CoCrAlY + WC-Co Coating on Turbine Alloys

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.     

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

Microstructure plays a predominant role in determining material behaviour. Increasing microstructure uniformity has long been considered a fruitful means of improving thermal, chemical and mechanical properties of the materials. High velocity oxy-fuel (HVOF) is one of the emerging technologies among the thermal spraying techniques, for producing uniform and dense coatings, having high hardness and good adhesion values. In this study, HVOF technique was used to deposit NiCrBSi coatings, approximately 250-300 μm thick, on the Ni- and Fe-based superalloys for hot corrosion applications. The coatings were characterised in relation to coating thickness, porosity, microhardness and microstructure. The hot corrosion behaviour of the coatings deposited on nickel-based superalloys after exposure to molten salt (Na₂SO₄-60% V₂O₅) at 900 °C under cyclic conditions was also studied. The techniques used in the present investigation include X-ray diffraction, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX) and electron probe microanalysis (EPMA). The thermogravimetric technique was used to establish kinetics of corrosion. The structure of the as sprayed NiCrBSi coating mainly consisted of γ-nickel solid solution containing small fraction of Cr₇C₃ and Ni₃B phases. Very weak peaks of NiCr₂O₄ spinel oxides were also formed during spraying of the coatings. Some porosity (less than 1.4%) and inclusions were observed in the structure of the coatings. Coating microhardness values were found to be in the range of 750-930 Hv (Vickers Hardness) on different substrates. The NiCrBSi coating was found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. The hot corrosion resistance imparted by NiCrBSi coatings may be attributed to the formation of oxides of silicon, chromium, nickel and spinels of nickel and chromium.     

Characterization and hot corrosion performance of LVPS and HVOF-CoNiCrAlYSi coatings

The CoNiCrAlYSi coatings were produced by using low vacuum plasma spray (LVPS) and high velocity oxy-fuel (HVOF) techniques on the Inconel-738. Hot corrosion behavior and microstructure characterization was investigated by exposing the sample to a molten film of Na₂SO₄-20? wt NaVO₃ at 880 °C for up to 560 h. The hot corrosion rate was determined by measuring the weight gain of the specimens at regular intervals for a duration of 20 h. The result of weight change measurements showed better hot corrosion resistance for HVOF-CoNiCrAlYSi coatings. This was attributed to the α-Al₂O₃ nucleation during the HVOF coating process and replacement of non-protective oxide due to a fluxing mechanism. It was also observed that the non-protective and porous oxides such as (Co,Ni)Al₂O₄ and (Co,Ni)Cr₂O₄ were formed on the both types of coatings due to large β-depleted zone after long exposure time of hot corrosion testing.     

Comparison of microstructure and phase transformation for nanolayered CrN/AlN and TiN/AlN coatings at elevated temperatures in air environment

CrN/AlN and TiN/AlN multilayer coatings with modulation period of 4 nm and thickness ratio equal to 1.0 were manufactured by RF magnetron sputtering. Both films were annealed at temperatures of 800 °C in air for 1 h and then for an additional 9 h. Both coatings in as-deposited and after heat treatment were evaluated with a transmission electron microscope (TEM) equipped with EDS. After heat treatment at 800 °C for 1 h, a thick oxide layer around 260 nm was formed on the surface of the TiN/AlN coating. The oxide layer which formed on the coating was composed of three different regimes, including Al-enriched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO₂ layer 30-80 nm thick above the nitride coating and in between, mixed nano-crystalline Al₂O₃ and TiO₂ films. In comparison, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating. This amorphous or nanocrystalline oxide layer identified by EDS was a metal-deficient oxide, in which Al₂O₃ and Cr₂O₃ were mixed together forming a solid solution. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures.     

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

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.     

L'oxyde de vanadium amorphe hydrate

Hydrated amorphous vanadium pentoxide obtained by quenching of molten V₂O₅ in water is compared to amorphous V₂O₅ previously obtained by splat cooling. The first one contains some strongly bonded water which prevents crystallisation up to 300°C. Short range order around V⁴⁺ ions seems to be the same in both samples and in orthorhombic V₂O₅. Disorder in the amorphous phases leads to a localization of the charge carriers, the electron mobility being about ten times smaller than in the crystalline oxide.     

Microstructure and microhardness of vanadium oxides in the range VO<Subscript>0.57</Subscript>-VO<Subscript>1.29</Subscript>

X-ray diffraction and optical microscopy data are presented which demonstrate that substoichiometric vanadium oxide (VO_0.57-VO_0.97) consists of a cubic phase with the B1 structure (sp. gr. Fm \(\bar 3\) m) and an ordered monoclinic phase of composition V₁₄O₆ (sp. gr. C2/m). The content of the latter phase decreases with increasing oxygen content. The superstoichiometric vanadium oxide VO_1.29 is shown to contain trace amounts of V₅₂O₆₄. Vickers microhardness data for nonstoichiometric vanadium oxides in the range VO_0.57-VO_1.29 show that, with increasing oxygen content, their H _V has a tendency to decrease, from 18 to 12 GPa. Their microhardness is shown for the first time to have a maximum near the stoichiometric composition VO_1.00.     

Interface control of atomic layer deposited oxide coatings by filtered cathodic arc deposited sublayers for improved corrosion protection

Sublayers grown with filtered cathodic arc deposition (FCAD) were added under atomic layer deposited (ALD) oxide coatings for interface control and improved corrosion protection of low alloy steel. The FCAD sublayer was either Ta:O or Cr:O–Ta:O nanolaminate, and the ALD layer was Al₂O₃–Ta₂O₅ nanolaminate, Al_xTa_yO_z mixture or graded mixture. The total thicknesses of the FCAD/ALD duplex coatings were between 65 and 120 nm. Thorough analysis of the coatings was conducted to gain insight into the influence of the FCAD sublayer on the overall coating performance. Similar characteristics as with single FCAD and ALD coatings on steel were found in the morphology and composition of the duplex coatings. However, the FCAD process allowed better control of the interface with the steel by reducing the native oxide and preventing its regrowth during the initial stages of the ALD process. Residual hydrocarbon impurities were buried in the interface between the FCAD layer and steel. This enabled growth of ALD layers with improved electrochemical sealing properties, inhibiting the development of localized corrosion by pitting during immersion in acidic NaCl and enhancing durability in neutral salt spray testing.     

Thermal oxidation of InP with V<Subscript>2</Subscript>O<Subscript>5</Subscript> + PbO nanolayers of different compositions

Data are presented on the thermal oxidation of (V₂O₅ + PbO)/InP structures which demonstrate that the combined effect of the oxides deposited by magnetron sputtering does not follow the additivity rule and that this behavior is due to the formation of a quasi-liquid phase (maximum) and lead vanadate (minimum) (IR spectroscopy, Auger electron spectroscopy). The effective activation energy for the oxidation of the (V₂O₅ + PbO)/InP structures is shown to systematically decrease with increasing initial vanadium oxide content. The oxidation of the structures follows a partially catalytic mechanism, with V₂O₅ acting as a catalyst (oxidation kinetics, IR spectroscopy, ultrasoft X-ray emission spectroscopy).     

Characterization of ceramic plasma-sprayed coatings, and interaction studies between U-Zr fuel and ceramic coated interface at an elevated temperature

Candidate coating materials for re-usable metallic nuclear fuel crucibles, HfN, TiC, ZrC, and Y₂O₃, were plasma-sprayed onto niobium substrates. The coating microstructure and the thermal cycling behavior were characterized, and U-Zr melt interaction studies carried out. The Y₂O₃ coating layer had a uniform thickness and was well consolidated with a few small pores scattered throughout. While the HfN coating was not well consolidated with a considerable amount of porosity, but showed somewhat uniform thickness. Thermal cycling tests on the HfN, TiC, ZrC, and Y₂O₃ coatings showed good cycling characteristics with no interconnected cracks forming even after 20 cycles. Interaction studies done on the coated samples by dipping into a U-20wt.%Zr melt indicated that HfN and Y₂O₃ did not form significant reaction layers between the melt and the coating while the TiC and the ZrC coatings were significantly degraded. Y₂O₃ exhibited the most promising performance among HfN, TiC, ZrC, and Y₂O₃ coatings.     

Aqueous Metal Oxide Inks for Modifying Electrode Work Function in Polymer Solar Cells

A method to prepare aqueous metal oxide inks for tuning the work function (WF) of electrodes is demonstrated. Thin films prepared from the metal oxide ink based on vanadium oxide (V₂O₅) nanoparticles are found to increase the WF of an indium‐tin‐oxide (ITO) electrode. ITO substrates modified with V₂O₅ films are applied as a hole selective layer (HSL) in polymer solar cells (PSCs) using a poly(3‐hexylthiophene) and [6,6]‐phenyl‐C61 butyric acid methyl ester blend as a photoactive layer. The PSCs prepared with V₂O₅‐modified ITO show better device performance, achieving a power conversion efficiency of 3.6%, demonstrating 15% enhancement compared to conventional ITO/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT‐PSS) based devices. Furthermore, ITO/V₂O₅‐modified devices exhibit better ambient stability with 60% improvement in device lifetime than those using PEDOT:PSS as an HSL. This solution‐processable and highly stable WF‐modifying metal oxide film can be a potential alternative material for engineering interfaces in optoelectronic devices.     

The effect of SO3 on vanadate-induced hot corrosion

Unrefined fuel oils may contain considerable amounts ofboth sulphur and vanadium. Despite this, exposure to combustion gases seldom yields deposits which consist ofboth vanadates and sulphates. Calculations for the sodium sulphate/vanadate system show that this is due to thermodynamics ofthe system. Sodium sulphate cannot exist in equilibrium with fused vanadate unless the sulphur trioxide pressure in the ambient atmosphere is high or the vanadium to sodium ratio in the deposits is less than one.

Thermogravitmetric studies which delineate the conditions for simultaneous sulphate- and vanadate-induced corrosion at 650 to 8OO°C have been performed. NiCrAlY coatings with K-sodium vanadyl vanadate deposits and the deposit alone have been exposed to oxygen containing 4% sulphur dioxide at 650 to 8OO°C. The results generally confirm the calculations from available thermodynamic data, but the solubility ofsulphur oxide in fused sodium vanadate is higher than expected from the literature values. The ' corrosion mechanism changes from initial vanadate-induced to essentially sulphate-induced hot corrosion when the sodium trioxide pressure is high enough that sodium sulphate may be formed.