Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Phase Transformations During Li-Insertion into V<Subscript>2</Subscript>O<Subscript>5</Subscript> at Elevated Temperature

Recent interest in developing cathode materials for an elevated temperature operation of Li-ion batteries has motivated researchers to explore the possibility of using layered V₂O₅ as a potential candidate because of its high capacity and cyclic stability. Despite a wide lithiation voltage window of V₂O₅ (between 1.0 V and 4.0 V), compositional fluctuations, metal dissolution, and so on contribute to capacity loss at high temperatures. A first discharge of V₂O₅ to voltages below 2.0 V has been observed to be associated with a series of phase transformations at both room temperature and high temperature and has been characterized here. From structural characterization of harvested electrodes post–first discharge, a new Li-rich phase was observed to be formed at 120°C and the composition was estimated.     

Initial Stages of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Induced Degradation of β-Ni–36Al at 700 °C: I-Intrinsic Behavior

The early-stage scaling behavior of a β-Ni–36Al alloy undergoing Na₂SO₄-deposit-induced degradation at 700 °C was systematically studied using SEM and TEM. After 20 h of exposure in an O₂–1000 ppm SO₂ ambient, the deposit-coated alloy formed a dense but thin Al₂O₃ scale on most areas of the surface; however, large nodules formed locally. Nodule formation occurred where the scale had lost its protective character, with rapid internal oxidation ensuing. The presence of sulfur both in the environment and in the salt played a key role in nodule formation. Removal of SO₂/SO₃ from the gas mixture, or of the Na₂SO₄ deposit from the surface, prevented nodule formation, while removing the sulfur source after nodule formation prevented further nodule growth. The degradation could be linked to the dissolution of reaction products in the Na₂SO₄ deposit and the formation of a low-temperature eutectic liquid. Further, when an Na₂SO₄–48% MgSO₄ deposit was used, the nodule density increased.     

Effect of Carbide Dissolution on Chlorine Induced High Temperature Corrosion of HVOF and HVAF Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-NiCrMoNb Coatings

Highly corrosion- and wear-resistant thermally sprayed chromium carbide (Cr₃C₂)-based cermet coatings are nowadays a potential highly durable solution to allow traditional fluidized bed combustors (FBC) to be operated with ecological waste and biomass fuels. However, the heat input of thermal spray causes carbide dissolution in the metal binder. This results in the formation of carbon saturated metastable phases, which can affect the behavior of the materials during exposure. This study analyses the effect of carbide dissolution in the metal matrix of Cr₃C₂-50NiCrMoNb coatings and its effect on chlorine-induced high-temperature corrosion. Four coatings were thermally sprayed with HVAF and HVOF techniques in order to obtain microstructures with increasing amount of carbide dissolution in the metal matrix. The coatings were heat-treated in an inert argon atmosphere to induce secondary carbide precipitation. As-sprayed and heat-treated self-standing coatings were covered with KCl, and their corrosion resistance was investigated with thermogravimetric analysis (TGA) and ordinary high-temperature corrosion test at 550 °C for 4 and 72 h, respectively. High carbon dissolution in the metal matrix appeared to be detrimental against chlorine-induced high-temperature corrosion. The microstructural changes induced by the heat treatment hindered the corrosion onset in the coatings.     

Type II Hot Corrosion Screening Tests of a Cr<Subscript>2</Subscript>AlC MAX Phase Compound

Low-temperature hot corrosion tests were performed on bulk Cr₂AlC MAX phase compounds for the first time. This material is a known alumina-former with good oxidation and Type I high-temperature hot corrosion resistance. Unlike traditional (Ni,Co)CrAl alumina formers, it contains no Ni or Co that may react with Na₂SO₄ salt deposits needed to form corrosive mixed (Ni,Co)SO₄–Na₂SO₄ eutectic salts active in Type II hot corrosion. Cr₂AlC samples coated with 20K₂SO₄–80Na₂SO₄ salt were exposed to 300 ppm SO₂ at 700 °C for times up to 500 h. Weight change, recession, and cross-sectional microstructures identified some reactivity, but much reduced (<?1/10) compared to a Ni(Co) superalloy baseline material. Layered Al₂O₃/Cr₂O₃ scales were indicated, either separated by or intermixed with some retained salt. However, there was no conclusive indication of salt melting. Accelerated oxidation was proposed to explain the results, and coarse Cr₇C₃ impurities appeared to play a negative role. In contrast, the superalloy exhibited outer Ni(Co) oxide and inner Cr₂O₃ scales, with Cr–S layers at the interfaces. Massive spallation of the corrosion layers occurred repeatedly for the superalloy, but not at all for Cr₂AlC. This indicates some potential for Cr₂AlC as LTHC-resistant coatings for superalloys.     

Hot Corrosion Mechanism of Steels Exposed to Heavy Metal Chlorides and Sulphates in SO<Subscript>2</Subscript> Environment

Copper flash smelting produces flue dust containing SO₂-rich exhaust gas, causing corrosion problems in the heat recovery boiler of the gas train. In order to understand the corrosion behaviour of boiler steels, conditions of the boiler were simulated in the laboratory. Corrosion damage occurred as chlorine reacted with steel surfaces forming chlorides which deplete the steels from their alloying metals. At the scale/dust deposit interface, where the highest sulphur partial pressures prevail, a sulphate layer covered the mixed oxide and chloride scale. Molten sulphate deposit reduced the metal loss of AISI 304 steel by preventing chromium chloride diffusion away from the steel surface. The Cr₂O₃ scales were quite stable under molten sulphates. The dust deposit melted partially due to the presence of ZnCl₂ in the deposit which initiated corrosion damage by producing a molten salt layer on the steel surfaces bringing it in contact with aggressive compounds in the copper smelter flue dust.     

Corrosion Behavior and Microhardness of Ni-P-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano-composite Coatings on Magnesium Alloy

In the present work, nano-composites of Ni-P-SiO₂-Al₂O₃ were coated on AZ91HP magnesium alloy. The surface morphology of the nano-composite coating was studied by field emission scanning electron microscopy (FESEM). The amount of SiO₂ in the coating was determined by energy-dispersive analysis of x-ray (EDX), and the crystalline structure of the coating was examined by x-ray diffractometer (XRD). All the experiments concerning the corrosion behavior of the coating carried out in 3.5 wt.% NaCl solution and evaluated by electrochemical impedance spectroscopy (EIS) and polarization technique. The results showed that an incorporation of SiO₂ and Al₂O₃ in Ni-P coating at the SiO₂ concentration of 10 g/Land 14 g/LAl₂O₃ led to the lowest corrosion rate (i _corr = 1.3 µA/cm²), the most positive E _corr and maximum microhardness (496 VH). Furthermore, Ni-P-SiO₂-Al₂O₃ nano-composite coating possesses less porosity than that in Ni-P coating, resulting in improving corrosion resistance.     

Oxidation Kinetics of UZr<Subscript>2.3</Subscript> and U<Subscript>2</Subscript>Ti Alloys in Dry Air

The isothermal oxidation behavior of UZr_2.3 and U₂Ti alloys in dry air was studied by thermogravimetric technique in the temperature ranges of 773–848 and 548–623 K, respectively. The oxidation products were found to be U₃O₈ and ZrO₂ for UZr_2.3 and U₃O₈ and TiO₂ for U₂Ti by X-ray diffraction analysis. The oxidation kinetics were found to best fit into linear rate law for both the alloys in the temperature range of investigation and the lowest value of α (0.11 for UZr_2.3 and 0.12 for U₂Ti), which corresponds to the fraction of completed reaction. The linear rate constants (k) were evaluated from the oxidation data. The activation energies of oxidation reactions were calculated using the Arrhenius equation and found to be 161 kJ mol^?1 for UZr_2.3 and 88 kJ mol^?1 for U₂Ti.     

Effects of La<Subscript>2</Subscript>O<Subscript>3</Subscript> on Mechanical Properties and Corrosion Resistance of H62 Brass

In this article, the effects of lanthanum oxide (La₂O₃) on the microstructure and mechanical properties of H62 brass were investigated by using the universal testing machine, Brinell hardness tester, optical microscope, and scanning electron microscope (SEM). Immersion corrosion and electrochemical measurements were carried out to identify the influence of La₂O₃ on the corrosion behavior of the H62 brass. The phase constitution, microstructure, and phase composition of the H62 brass were analyzed by x-ray diffraction, SEM, and energy-dispersive spectrometer, respectively. The results show that the microstructure of α phase changes from dendrite grains to equiaxed grains, and the content and distribution of β phase are improved significantly. When the La₂O₃ content reaches 0.8 wt.%, the H62 brass obtains favorable comprehensive mechanical properties and the strength and hardness decrease but elongation increases, which is conducive to plastic processing. In addition, under the optimum amount of 0.8 wt.% La₂O₃ content, the corrosion rate of immersion corrosion attains the minimum values: As 12.6 g m^?2 h^?1, it decreases by 24%; as the corrosion potential changes from ?1.1327 V to ?0.328 V, it increases by 70.9%; and as the corrosion current density decreases from ?2.833 mA mm^?2 to ?3.28 mA mm^?2 corrosion, it decreases by 15.78%, when compared with H62 brass.     

Fabrication by Electrophoretic Deposition of Nano-Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> and Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>@SiO<Subscript>2</Subscript> 3D Structure on Carbon Fibers as Supercapacitor Materials

Core–shell nanostructured magnetic Fe₃O₄@SiO₂ with particle size ranging from 3 nm to 40 nm has been synthesized via a facile precipitation method. Tetraethyl orthosilicate was employed as surfactant to prepare core–shell structures from Fe₃O₄ nanoparticles synthesized from pomegranate peel extract using a green method. X-ray diffraction analysis, Fourier-transform infrared and ultraviolet–visible (UV–Vis) spectroscopies, transmission electron microscopy, and scanning electron microscopy with energy-dispersive spectroscopy were employed to characterize the samples. The prepared Fe₃O₄ nanoparticles were approximately 12 nm in size, and the thickness of the SiO₂ shell was?~?4 nm. Evaluation of the magnetic properties indicated lower saturation magnetization for Fe₃O₄@SiO₂ powder (~?11.26 emu/g) compared with Fe₃O₄ powder (~?13.30 emu/g), supporting successful wrapping of the Fe₃O₄ nanoparticles by SiO₂. As-prepared powders were deposited on carbon fibers (CFs) using electrophoretic deposition and their electrochemical behavior investigated. The rectangular-shaped cyclic voltagrams of Fe₃O₄@CF and Fe₃O₄@C@CF samples indicated electrochemical double-layer capacitor (EDLC) behavior. The higher specific capacitance of 477 F/g for Fe₃O₄@C@CF (at scan rate of 0.05 V/s in the potential range of ??1.13 to 0.45 V) compared with 205 F/g for Fe₃O₄@CF (at the same scan rate in the potential range of?~???1.04 to 0.24 V) makes the former a superior candidate for use in energy storage applications.     

High-Temperature Corrosion Behavior of Different Regions of Weldment of 2.25Cr-1Mo Steel in SO<Subscript>2</Subscript> + O<Subscript>2</Subscript> Atmosphere

This paper investigates the corrosion behavior of different regions of weldment of 2.25Cr-1Mo steel exposed in mixed oxidation and sulfidation (SO₂ + O₂) environment up to 500 h at 773 K. Microstructural investigation and characterization of oxide scales are done using SEM, TEM, and XRD. The obtained results infer that heat-affected zone corrodes faster than both base and weld metal. The reaction kinetics follows a parabolic growth rate for all regions. The higher corrosion rate of heat-affected zone is attributed to the formation of Cr₂₃C₆ secondary precipitates leading to depletion of protective inner scale of the Cr-rich oxide during welding.     

High-Temperature Exposure Studies of HVOF-Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25(NiCr)/(WC-Co) Coating

In this research, development of Cr₃C₂-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr₃C₂-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr₃C₂-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na₂SO₄-82%Fe₂(SO₄)₃ molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr₃C₂-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.     

Magnetoresistive properties of Ni-doped La<Subscript>0.7</Subscript>Sr<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganites

La_0.7Sr_0.3Mn_1?x Ni _x O₃ (x = 0, 0.025, 0.050 and 0.075) ceramics were prepared by the conventional solid-state reaction method. The partial substitution of Mn by Ni²⁺ leads to a decrease in cell volume as well as a structural transition from the rhombohedral to the orthorhombic structure. Ni²⁺ doping increases the electrical resistivity, decreases the semiconductor–metal transition temperature (T _ms) and relatively enhances the room temperature magnetoresistance (MR), especially in x = 0.025 and around T _ms. With respect to conduction mechanism, the small polaron hopping (SPH) and the variable range hopping (VRH) models were used to examine conduction in the semiconducting region.     

Experimental Study and Thermodynamic Re-optimization of the FeO-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> System

Phase equilibria and thermodynamic data in the FeO-Fe₂O₃-SiO₂ system were critically reviewed. New experiments were undertaken to resolve discrepancies found in previous data. The liquid oxide/slag phase was described using the modified quasichemical model. New optimized parameters of the thermodynamic models for the Gibbs energies of slag and other phases in the selected system were obtained. The new parameters reproduce all available phase equilibria and thermodynamic data within the experimental error limits from 298 K (25 °C) to above the liquidus temperatures at all compositions and oxygen partial pressures from metal saturation to 1 atm of O₂. This study was carried out as part of the development of a self-consistent thermodynamic database for the Al-Ca-Cu-Fe-Mg-Si-O-S multi-component system.     

Hydrogen storage properties of as-cast and annealed low-Co La<Subscript>1.8</Subscript>Ti<Subscript>0.2</Subscript>MgNi<Subscript>8.9</Subscript>Co<Subscript>0.1</Subscript> alloys

Low-Co La_1.8Ti_0.2MgNi_8.9Co_0.1 alloys were prepared by magnetic levitation melting followed by annealing treatment. The effect of annealing on the hydrogen storage properties of the alloys was investigated systematically by X-ray diffraction (XRD), pressure-composition isotherm (PCI), and electrochemical measurements. The results show that all samples contain LaNi₅ and LaMg₂Ni₉ phases. LaCo₅ phase appears at 1,000 °C. The enthalpy change of all hydrides is close to ?30.6 kJ·mol^?1 H₂ of LaNi₅ compound. Annealing not only increases hydrogen capacity and improves cycling stability but also decreases plateau pressure at 800 and 900 °C. After annealing, the contraction of cell volume and the increase of hydride stability cause the high rate dischargeability to reduce slightly. The optimum alloy is found to be one annealed at 900 °C, with its hydrogen capacity reaching up to 1.53 wt%, and discharge capacity remaining 225.1 mAh·g^?1 after 140 charge–discharge cycles.     

Viscosity and Structure of CaO-SiO<Subscript>2</Subscript>-P<Subscript>2</Subscript>O<Subscript>5</Subscript>-Fe<Subscript>t</Subscript>O System with Varying P<Subscript>2</Subscript>O<Subscript>5</Subscript> and FeO Content

A rotary viscosimeter and Raman spectrum were employed to measure the viscosity and structural information of the CaO-SiO₂-P₂O₅-Fe_tO system at 1673 K. The experimental data have been compared with the calculated results using different viscosity models. It shows that the National Physical Laboratory (NPL) and Pal models fit the CaO-SiO₂-P₂O₅-FeO_t system better. With the P₂O₅ content increasing from 5% to 14%, the viscosity increases from 0.12 Pa s to 0.27 Pa s. With the FeO content increasing from 30% to 40%, the viscosity decreases from 0.21 Pa s to 0.12 Pa s. Increasing FeO content makes the complicated molten melts become simple, and increasing P₂O₅ content will complicate the molten melts. The linear relation between viscosity and structure parameter Q(Si + P) was obtained by regression analysis. The calculated viscosity by using the optimized NPL and Pal model are almost identical with the fitted values.     

Dual-Layer Oxidation-Protective Plasma-Sprayed SiC-ZrB<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Carbon Nanotube Coating on Graphite

Graphite is used in high-temperature gas-cooled reactors because of its outstanding irradiation performance and corrosion resistance. To restrict its high-temperature (>873 K) oxidation, atmospheric-plasma-sprayed SiC-ZrB₂-Al₂O₃-carbon nanotube (CNT) dual-layer coating was deposited on graphite substrate in this work. The effect of each layer was isolated by processing each component of the coating via spark plasma sintering followed by isothermal kinetic studies. Based on isothermal analysis and the presence of high residual thermal stress in the oxide scale, degradation appeared to be more severe in composites reinforced with CNTs. To avoid the complexity of analysis of composites, the high-temperature activation energy for oxidation was calculated for the single-phase materials only, yielding values of 11.8, 20.5, 43.5, and 4.5 kJ/mol for graphite, SiC, ZrB₂, and CNT, respectively, with increased thermal stability for ZrB₂ and SiC. These results were then used to evaluate the oxidation rate for the composites analytically. This study has broad implications for wider use of dual-layer (SiC-ZrB₂/Al₂O₃) coatings for protecting graphite crucibles even at temperatures above 1073 K.     

Phase Formation of the Yttrium Aluminates in the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiC System

Phase formation sequence of the yttrium aluminates in the Y₂O₃-Al₂O₃-SiC ternary system as temperature increases were investigated via x-ray diffraction (XRD). Results showed that YAM (monoclinic), YAP (perovskite) and YAG (garnet) were the yttrium aluminates presented in the solid-state reacted samples at a fixed Al₂O₃:SiC ratio of 1:1. Formation of the yttrium aluminates depended on the temperature. The YAM, YAP and YAG started to form below 1150 °C, at 1300 °C, and at 1450 °C, respectively. Accordingly, two behavior phase diagrams of the Y₂O₃-Al₂O₃-SiC ternary system were recognized, one is in the temperature range of 1150-1300 °C and the other is in 1300-1450 °C, respectively. Thereafter, the phase equilibrium was reached in the temperature range of 1450-1700 °C. Effects of SiC on the phase formation processes in the ternary system were discussed.     

Elevated Temperature Solid Particle Erosion Performance of Plasma-Sprayed Co-based Composite Coatings with Additions of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>

In this paper, investigation into solid particle erosion behavior of atmospheric plasma-sprayed composite coating of CoCrAlY reinforced with Al₂O₃ and CeO₂ oxides on Superni 76 at elevated temperature of 600 °C is presented. Alumina particles are used as erodent at two impact angles of 30° and 90°. The microstructure, porosity, hardness, toughness and adhesion properties of the as-sprayed coatings are studied. The effects of temperature and phase transformation in the coatings during erosion process are analyzed using XRD and EDS techniques. Optical profilometer is used for accurate elucidation of erosion volume loss. CoCrAlY/CeO₂ coating showed better erosion resistance with a volume loss of about 50% of what was observed in case of CoCrAlY/Al₂O₃/YSZ coating. Lower erosion loss is observed at 90° as compared to 30° impact angle. The erosion mechanism evaluated using SEM micrograph revealed that the coatings experienced ductile fracture exhibiting severe deformation with unusual oxide cracks. Reinforced metal oxides provide shielding effect for erodent impact, enabling better erosion resistance. The oxidation of the coating due to high-temperature exposure reforms erosion process into oxidation-modified erosion process.     

Electrical resistivity and thermal electromotive force of Ni<Subscript>75</Subscript>V<Subscript>25</Subscript>, Ni<Subscript>72</Subscript>V<Subscript>28</Subscript>, and Ni<Subscript>67</Subscript>V<Subscript>33</Subscript> alloys at high temperatures

The temperature dependences of the electrical resistivity and thermal electromotive force (thermal e.m.f.) of the Ni–25 at % V, Ni–28 at % V, and Ni–33 at % V alloys in a temperature range of 300–1600 K have been reported; the dependences have been measured during slow heating and cooling of quenched and annealed samples. It has been shown that, near the order–disorder phase-transformation temperature, the temperature dependences of the electrical resistivity of the Ni₇₅V₂₅ and Ni₆₇V₃₃ alloys demonstrate a kink (second-order phase transition) and a jump (first-order phase transition), respectively. The behavior of the experimental dependences is discussed in terms of the band Mott s–d scattering model.