Rate Law For Oxidation Research Articles

A comparative study on the oxidation behaviour of NbC-Ni, WC-Co, and high entropy carbide-Ni based cermets

The oxidation behaviour of NbC-13.5(wt%)Ni, NbC-5.7Mo2C-13.3Ni and WC-12Co was compared to that of equimolar quinary (Ti0.2V0.2Nb0.2Ta0.2W0.2)C-10Ni and non-equimolar senary (Ti0.2V0.2Nb0.2Ta0.2Mo0.1W0.1)C-10Ni high entropy carbide (HEC)-Ni cermets. Isothermal oxidation experiments were performed at 700 °C, whereas non-isothermal calorimetric experiments were conducted to determine the onset temperature of oxidation. HEC-based cermets exhibited superior oxidation resistance and a unique oxide layer morphology, which was attributed to the formation of a chemically complex oxide scale mainly consisting of (Ti,V,Nb,Ta,Ni)Ox and Ni(Mo,W)O4. The NbC-based cermets showed the least oxidation resistance and formed a porous oxide layer of Nb2O5 and (Nb,Ni)Ox, whereas WC-Co showed an intermediate oxidation behaviour with respect to the HEC-Ni based cermets. All composites followed a linear oxidation rate law, and the senary HEC cermet demonstrated the slowest oxidation kinetics.

Numerical studies on the propagation of iron dust flames in confinement

Iron powder is a promising alternative fuel owing to its high energy density, non-volatile combustion, and recyclability using green hydrogen. For the safe storage, transportation, and use of iron powder as a reactive energy source, the physical mechanisms behind pressurising dust flames in explosions must be understood. Here, we study the propagation of one-dimensional iron dust flames in confinement using an Euler–Lagrange framework with reflective adiabatic boundary conditions. The gas phase is described by the compressible Navier–Stokes equations, and heterogeneously burning iron particles follow the parabolic rate law for solid-phase oxidation, and diffusion-limited combustion during liquid-phase combustion. We demonstrate that the pressurisation in a closed vessel leads to an increase in unburnt gas density, decelerating flame propagation via a decrease in thermal diffusivity. Flames through dust suspensions at concentrations above the thermodynamic limit are quenched, while for concentrations near the quenching limit, flames are quenched before re-ignition. The trajectories of particles and gas parcels demonstrate that particles are strongly entrained in the gas streams, causing fluctuations in local dust concentrations. The transient evolution of flame speeds in confinement is used to inform a pressure-rise model, which predicts the time to peak pressure in a 20-L vessel with good agreement to experimental measurements. Our insights provide mechanistic understandings about dust flame propagation under pressurising conditions, particularly relevant to the informed design of explosion protection measures.

Comparative studies on oxidation behaviour of cobalt-based D-gun coatings on boiler steel

The protection system against material degradation at elevated temperatures is gaining technological importance. Three cobalt-based coatings were developed on SAE213-T91 boiler steel using the detonation gun method. The oxidation performance of all the specimens was examined for 50 cycles of 1 h heating at 900 °C in a thermal cyclic furnace and 20 min cooling at 25 °C outside the furnace. All the oxidised samples were investigated through SEM/X-ray mapping and an X-ray diffraction analyser. All the specimens obeyed the parabolic rate law of oxidation approximately. The oxidation resistance of Stellite 6 coating was found to be superior among all coated and uncoated specimens due to the growth of protective oxide Cr2O3 and CoCr2O4 spinel in it. The development of Ni, Cr and Co oxides in Stellite 21 coating resulted into its slow oxidation rate, whereas the growth of the non-protective WO3 and CoWO4 phases in WC-Co coating caused its rapid oxidation.

Oxidation behavior of near nanostructured coating developed by the HVOF process

AbstractMicrostructured and near nanostructured Cr3C2–Ni–Cr–B–Si coatings were deposited on Inconel 718 superalloy by high‐velocity oxygen fuel thermal spray technique. The oxidation behavior of microstructured and near nanostructured Cr3C2–Ni–Cr–B–Si coated superalloy at 900°C in air for 50 cycles under cyclic heating and cooling conditions was studied. The kinetics of oxidation of both nanostructured and microstructured coated superalloy was analyzed by weight change measurement. It was noticed that the uncoated and coated alloy obey the parabolic rate law of oxidation. X‐ray diffraction, field‐emission scanning electron microscopy/energy dispersive x‐ray analysis, and elemental mapping techniques were used to analyze the oxidized samples of coated and uncoated samples. In the coated superalloy, iron, silicon, and titanium were oxidized in the inter‐splat region, whereas the splats which consist mainly of Ni and Cr remain unoxidized. Because of the formation of compact and adhesive thin NiO and Cr2O3 scale on the surface of the coating during oxidation, the developed nanostructured coated superalloy shows better oxidation resistance than the microstructured one.

Performance of air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 alloy subjected to high temperature corrosion environment

Thermal barrier coating plays a vital role in protecting materials’ surfaces from high-temperature environment conditions. This work compares the demeanour of uncoated and air plasma sprayed Cr3C2–25NiCr and NiCrMoNb coated X8CrNiMoVNb16–13 substrates subjected to air oxidation and molten salt (Na2SO4 + 60%V2O5) environment condition at 900 °C for 50 cycles. Coating characteristics have been analyzed through microstructure, thickness, porosity, hardness, and bond strength. SEM, EDS and XRD analysis were used to analyze corrosion’s product at the end of the 50th cycle. Coating microstructures showed a uniform laminar structure that is adherent and denser with a coating thickness of 150 ± 20 μm and porosity less than 3.5%. The Microhardness of both the coated substrates were higher than that of the bare substrate. Cr3C2–25NiCr and NiCrMoNb coating bond strength was 38.9 MPa and 42.5 MPa. Thermogravimetric analysis showed the parabolic rate law of oxidation for all the substrates in both environments. In the molten salt environment, all the substrates exhibited higher weight gain compared to the air oxidation environment. In both environmental conditions, the uncoated X8CrNiMoVNb16–13 alloy exhibited higher weight gain than the coated substrates. The formation of Cr2O3, NiO and spinel oxide NiCr2O4 offers good resistance to corrosion to all the substrates in both the environmental condition. However, the presence of Mo and Nb significantly accelerated the corrosion of the substrate, thereby increasing the weight of the NiCrMoNb substrate. It is observed that Cr3C2–25NiCr and NiCrMoNb coating over the X8CrNiMoVNb16–13 substrate significantly protected the substrate against the hot corrosion than the bare alloy exposed to similar environmental conditions.

Bivalent copper oligopyrocatecholate as a novel heterogeneous catalyst for the oxidative degradation of mercaptan in caustic solution: Synthesis, characterization, and kinetic study

A novel catalyst based on bivalent copper oligopyrocatecholate was first successfully synthesized and dispersed in a polymer matrix for oxidative degradation of mercaptan in aqueous caustic solution. X-ray diffraction analysis has demonstrated that the synthesized catalyst was a crystalline phase with a minimum amorphous component. Mechanism analysis and kinetic experiments were conducted to investigate the kinetic mechanism of the reaction of isopropyl mercaptan oxidation catalyzed by copper oligopyrocatecholate dispersed in a polymer matrix. The influences of temperature, initial concentrations of reactants, and catalytic surface area on the reaction rate were studied to obtain the rate expression of intrinsic kinetics. The research results showed that the subsequent electron-transfer step was the rate-limiting step of the reaction. Additionally, the mercaptan oxidation rate in caustic solution was inversely proportional to the first power of the alkali concentration. The apparent activation energy was approximately 27.71 ± 1.12 kJ/mol. Importantly, this rate law for mercaptan oxidation can be used to design industrial reactors for the light oil sweetening process.

Long-term oxidation behaviour and thermal stability of heat-resistant stainless steel claddings deposited on AISI 316 stainless steel by the GTAW process

In the present study, the microstructure, oxidation behaviour, and thermal stability of the ER310 and ER446 weld overlay claddings deposited on AISI 316 stainless steel by the gas tungsten arc welding (GTAW) process were investigated. In this regard, the chemical composition, microstructure, and phase composition of the weld metals were characterized by optical microscopy (OM), field-emission scanning electron microscopy (FESEM-EDS), and X-ray diffraction (XRD) analysis. The oxidation behaviour and the bonding state of the clad plates were evaluated and compared after 1500 h of oxidation at 1000 °C using the isothermal oxidation and shear strength tests. Cross-sectional microstructural observations revealed that the substrates and weld overlay claddings were well bonded, and dense and uniform cladding layers without pores, cracks, or discontinuities were achieved. The weld metal produced by ER310 filler wire exhibited a completely austenitic microstructure, while the weld overlay produced by ER446 filler wire showed a ferritic-martensitic two-phase microstructure. The shear strength test results indicated that the bond strength of the ER446/AISI 316 and ER310/AISI 316 interfaces in the as-welded and oxidized conditions was higher than the minimum value (140 MPa) stated in the ASTM A264 standard. It was found that the oxidation process did not significantly reduce the bond strength of the interfaces. The weight gain data showed that both weld overlay claddings follow the parabolic rate law of oxidation, improving the oxidation behaviour of the AISI 316. The ER446 weld overlay had a lower parabolic rate constant (k p ) than the ER310 weld overlay. The SEM images showed that the oxide scales formed on the weld overlay claddings mainly consisted of three layers; Si-rich oxide at the innermost layer, Cr-rich oxide in the middle layer, and spinel oxide at the outer part of the scale. The oxidation behaviour of the clad samples was explained by the diffusion coefficients of elements, thermal stresses, and growth stresses, which are all associated with the matrix structure. • Heat-resistant stainless steel claddings were deposited on the surface of AISI 316 stainless steel using the GTAW process. • Thermal stability and oxidation behavior of the claddings after 1500 h of isothermal oxidation at 1000 °C were investigated. • The ER446 overlay displayed a ferritic-martensitic, while the ER310 overlay exhibited a fully austenitic microstructure. • The clad plates in the as-welded and oxidized conditions had a strength higher than the minimum value stated in ASTM A264. • Both ER446 and ER310 weld overlay claddings were improved the oxidation behaviour of the substrate.

Diverse oxidation behaviors of metal carbide solutions in high-temperature water vapor

Carbide solid solutions show diverse oxidation behaviors at 1000–1400 °C in water vapor. (Ti,Nb,Ta,Hf,Zr)C and (Ti,Nb,Ta,Hf)C exhibit parabolic or near-parabolic oxidation kinetics, while the oxidations of (Ti,Nb,Ta) and (Ti,Nb)C are mainly controlled by linear oxidation kinetics. The oxidation kinetics of (Ti,Nb,Ta,Hf,Zr)C shows a tendency of transition from parabolic to linear oxidation rate law with increasing temperature. On the contrary, a tendency of transition from linear to parabolic rate law with increasing temperature was observed in (Ti,Nb)C, which exhibits the highest oxidation resistance at investigated temperatures. The presence of dense Nb-doped TiO2 oxidation layer is responsible for the high oxidation resistance.

Investigation of the High-Temperature Oxidation Behavior of the Al0.5CoCrFeNi High Entropy Alloy

As a new class of materials, High Entropy Alloys (HEAs), have been recently used for various usages such as high-temperature applications. High entropy alloys have such unique properties as high strength, high-temperature phase stability, good fracture resistance, and excellent softening resistance at high temperatures. Studies on the oxidation behavior of as-cast high entropy alloys are, however, currently limited. The oxidation behavior of Al0.5CrCoFeNi HEA was investigated at three different temperatures of 800, 900, and 1000°C, and in the air. The results showed that the oxidation kinetics of this alloy followed the parabolic rate law. However, at 1000°C, two-step oxidation kinetics was found: in the first step, it followed the almost linear oxidation rate law (n=0.97); in the second one, the predominant mechanism followed the parabolic rate law (n=0.56). Analysis of the morphology and phase composition of the oxide layers was done using the Scanning Electron Microscopy (SEM) and X-ray diffraction. At all temperatures, the results showed that the oxide layers consisting of Cr2O3/Al2O3 and spinel of Ni, Co, and Fe were formed in the Al depleted zone.

Role of Alloyed Niobium on the Isothermal Oxidation of Fe-40Ni-24Cr Alloy

Fe-40Ni-24Cr alloy is the heat-resistant alloy usually use at high temperature service. The alloying elements normally added to the alloy to enhance the resistance to severe oxidation during service. The Fe-40Ni-24Cr alloy was experienced a high temperature oxidation at 700 °C for 500 h exposure time in laboratory air. The discontinuous isothermal oxidation test was carried out at different time intervals to record the weight change of the oxidized alloy throughout the experiment. The oxidized Fe-40Ni-24Cr alloy was characterized in terms of oxidation kinetics, phase analysis by using x-ray diffraction (XRD) technique and oxide surface morphology determination by using scanning electron microscope (SEM) equipped with energy dispersive x-ray (EDX) spectrometer. The oxidation kinetics of oxidized Fe-40Ni-24Cr alloy was exhibited a weight gain trend. The calculation of oxidation rate law was proved that this oxidized Fe-40Ni-24Cr alloy was obeyed parabolic rate law indicating the growth of oxide scales was followed a diffusion-controlled mechanism. The phase analysis of the oxidized Fe-40Ni-24Cr alloy recorded the formation of several oxide phases consists of Cr-rich, Fe-rich, Ti-rich, Nb-rich and spinel oxides structure. These oxides were served as a protective barrier between base metal and environment. The surface morphology of oxide scale after different exposure displayed a continuous oxide layer formed on the alloy surface with evidence of overgrown Nb-rich oxide particle at discrete area on the alloy surface starting from 150 h exposure time.

Creep rupture life assessment of superheater tube with oxide formation in power plant

AbstractThis paper proposes a method for predicting the creep rupture life of superheater tubes, considering the continuous increasing metal temperature and stress as a result of oxide formation. The steam‐side oxide growth was assessed by the parabolic oxidation rate law, and the temperature profiles of superheater tubes was calculated combining the 3D computational fluid dynamics simulation and the 1D analytical models. Based on the estimated temperature and maximum elastic hoop stress, the parametric extrapolation technique was utilized to assess the creep rupture life. The validity of this method was confirmed by measuring the oxide thickness and the steam temperature in the field. The results found that T91 alloy in the steam outlet region and T23 alloy at the connection point was susceptible to creep rupture failure, which had the greatest cumulative damage and thickest oxide scale. The cumulative damage of superheater tubes varied significantly with tube count. Results suggested that replacement of the particular T91 alloy tubing section having the minimum creep rupture life with a better heat‐resistant steel would avoid mechanical failure by creep for the subject boiler system.

Oxidation kinetics of nanocrystalline Al thin films

Purpose This paper aims to study the oxidation kinetics of the nanocrystalline Al ultrathin films. The influence of structure and composition evolution during thermal oxidation will be observed. The reason for the change in the oxidation activation energy on increasing the oxidation temperature will be discussed. Design/methodology/approach Al thin films are deposited on the silicon wafers as substrates by vacuumed thermal evaporation under the base pressure of 2 × 10−4 Pa, where the substrates are not heated. A crystalline quartz sensor is used to monitor the film thickness. The film thickness varies in the range from 30 to 100 nm. To keep the silicon substrate from oxidation during thermal oxidation of the Al film, a 50-nm gold film was deposited on the back side of silicon substrate. Isothermal oxidation studies of the Al film were carried out in air to assess the oxidation kinetics at 400-600°C. Findings The activation energy is positive and low for the low temperature oxidation, but it becomes apparently negative at higher temperatures. The oxide grains are nano-sized, and γ-Al2O3 crystals are formed at above 500°C. In light of the model by Davies, the grain boundary diffusion is believed to be the reason for the logarithmic oxidation rate rule. The negative activation energy at higher temperatures is apparent, which comes from the decline of diffusion paths due to the formation of the γ-Al2O3 crystals. Originality/value It is found that the oxidation kinetics of nanocrystalline Al thin films in air at 400-600°C follows the logarithmic law, and this logarithmic oxidation rate law is related to the grain boundary diffusion. The negative activation energies in the higher temperature range can be attributed to the formation of γ-Al2O3 crystal.

Rapid reactive synthesis of TiAl3 intermetallics by thermal explosion and its oxidation resistance at high temperature

Porous TiAl3 intermetallics were synthesized from Ti-75 at.% Al elemental powder mixtures using an energy-saving and rapid reactive method of thermal explosion (TE). The results demonstrated that the actual temperature of the compact climbed rapidly from 673 °C to 1036 °C within 24 s, indicating that an obvious TE reaction occurred during sintering process. The video graphs suggested that the TE in Ti–Al system behaved instant occurrence and overall heating whether from axial or radial direction. The silver wires and NaCl particles that pressed on the surface of the sample disappeared due to the heavy heat released during TE reaction. Only pure TiAl3 phases were synthesized in TE products and the open porosity of 55.4% was easy to obtain. After high-temperature treatment at 1000 °C, large amounts of sintering-neck formed and then improved the compressive strength of porous TiAl3 materials. Moreover, the mass gain curve of porous TiAl3 intermetallics oxidized at 650 °C for 120 h exhibited the parabolic oxidation rate law. XPS analysis confirmed that the strong O 1s peak was 531.4 eV which was the typical binding energy of Al2O3. Therefore, the excellent oxidation resistance of porous TiAl3 foams would be considered as good candidate materials for prolonging the service life at high temperatures.

Development of a rate law for arsenite oxidation by manganese oxides

Abstract Arsenite (As(III)), may be rapidly oxidized to arsenate (As(V)) by manganese oxides over a wide range of pH. Interestingly, pseudo-first order rate constants for Mn(IV) oxides (MnO2) reported in the literature differ by orders of magnitude, mainly due to the fact that experimental conditions have not been normalized and surface passivation may hinder the reaction rate. As a result, a formal kinetic rate law for the oxidation of As(III) by MnO2 does not exist. In this study, a comprehensive rate law that describes the abiotic oxidation of As(III) by vernadite (δ-MnO2) was developed by systematically varying pH and the concentrations of δ-MnO2 and As(III). Experiments were performed with excess δ-MnO2 compared to As(III) in buffered solutions with an ionic strength typical of freshwater environments to reflect the geochemical conditions of most environments. The half-life of the pseudo-first order reaction is less than three minutes in all experiments. Results show the initial reaction rate is proportional to the concentrations of δ-MnO2 and As(III) and independent of pH in the pH range 4–9. However, below pH 6 and above pH 8 the pseudo-first order rate constant is slightly lower due to the protonation of the surface δ-MnO2 and the deprotonation of As(III). Overall, the reaction follows a first order rate law with respect to As(III) and effective surface sites of δ-MnO2 and zeroth order with respect to pH. The overall second order rate constant, k, was calculated to be 0.36 ± 0.11 L h−1 m−2. This rate constant is comparable to the vast majority of rate constants derived from previously published data when using the effective surface sites of MnO2 as reactive species only, indicating the most influential factors of As(III) oxidation by MnO2 are constrained in the developed rate law. Based on the present experiments, the oxidation of As(III) by MnO2 is proposed to proceed as one two-electron transfer reaction between As(III) and Mn(IV) to eventually form As(V) and Mn2+ species. Two steps of one-electron transfer, however, may be possible though it is likely that both Mn(IV) and Mn(III) species are ultimately reduced to Mn2+. Comproportionation of Mn2+ and Mn(IV) to Mn(III) does not seem to be important on the time scale of these experiments.

Hot Corrosion Demeanour of Alloy 80A Weldments Fabricated Through Tungsten Inert Gas Welding Technique

In the current study, alloy 80A weldments were produced through tungsten inert gas welding (TIGW) and pulsed current tungsten inert gas welding (PCTIGW) technique using 263 filler wire. Corrosion study was conducted on weldments in both air oxidation environment and molten salt environment under 50 cyclic condition at 900 °C. The results of thermogravimetric analysis showed that corrosion behaviour of weldments obeyed the parabolic rate law of oxidation. Higher weight gain was observed in TIGW under molten salt condition. Less weight was observed in PCTIGW under air oxidation condition. It is inferred that substrates subjected to molten salt environment showed higher weight gain than its respective substrates under air oxidation condition. The existence of Cr2O3, NiO and spinal oxide NiCr2O4 offered good corrosion resistance to substrates. Thus, it was deduced that PCTIG weldments provided good corrosion resistance at high temperature than its respective TIG weldments.

Advances in bone preservation: Identifying possible collagen preservation using sulfur speciation mapping

The complexity and heterogeneity of bone chemistry makes it difficult to discern information on physiological and taphonomic processes stored within the bone matrix. Analysis of archaeological and palaeontological bone becomes more difficult because in many cases the most pivotal specimens are too scientifically valuable for destructive analysis. This problem is further escalated by the fact that the heterogeneity of the bone may cause small “pockets” of preservation that can be missed during sampling. Therefore, a non-destructive technique that can spatially resolve such heterogeneity within the bone is needed. Here we use microfocus, non-destructive synchrotron-based X-Ray Fluorescence (XRF) imaging and X-ray Absorption Spectroscopy (XAS) to map the organic constituents within extant and fossil bovid bones. XAS analysis of sulfur allowed organic sulfur (within collagen as methionine) to be distinguished from inorganic sulfate (within bone apatite). Mapping and quantification of organic sulfur within the samples were made by setting the beam to the methionine resonance, allowing for the detection, distribution and quantification of collagen present by using organic sulfur as an internal marker. Results show organic sulfur to be distributed in small “pockets” throughout the bone matrix in both extant and fossil specimens. Significant loss of organic sulfur was seen in specimens between 100 ka and 650 ka with little organic sulfur preservation persisting after this date. Comparison of residual organic sulfur concentrations as a function of sample age revealed a second order rate law for organic sulfur oxidation (k ≈ 1 × 10−5 y−1) within bone. These results show that non-destructive, synchrotron-based XRF mapping of organic sulfur is a useful tool for not only calculating rates of collagen degradation through time, but also identifying areas of potential collagen preservation for other paleobiological applications such as proteomics and stable isotope analyses.

Experimental determination of pyrite and molybdenite oxidation kinetics at nanomolar oxygen concentrations

Constraining the rate at which sulfide minerals undergo oxidative weathering at low atmospheric O2 is crucial for understanding the evolution of the Archean and Proterozoic biosphere when O2 was a trace atmospheric gas. However, recent studies attempting to constrain sulfide oxidation rates, atmospheric O2 sinks, and trace metal delivery to seawater under Archean conditions are limited by the need to extrapolate from experimental pyrite oxidation kinetics determined at much higher O2 levels. Extrapolation of those data sets to Archean levels of O2 (<10−5 present atmospheric level or PAL prior to 2.4 Ga) leads to more than an order of magnitude uncertainty in sulfide mineral oxidation rates, hampering efforts to quantify oxidative weathering under early Earth conditions. To quantify sulfide oxidation kinetics at low pO2, we conducted aqueous pyrite and molybdenite oxidation experiments at ∼2–1200 nM dissolved O2 and pH values 1.83, 5.08, and 8.58. Our experimental approach used LUMOS O2 sensors to extend the pO2 range explored by oxidation experiments down to 10−5 PAL pO2, the limit of the sensors, which is up to three orders of magnitude lower than the pO2 range explored in previous work. From these experiments, we use 28 independent rate measurements to derive a new rate law for the oxidation of pyrite as a function of pO2: rpyrite=10-8.83(±0.27)O20.50±0.04H+-0.25±0.02, Where rpyrite is the rate of oxidation in mol/m2 sec, and the activities of dissolved [O2] and [H+] are in (mol/L). Our results most closely match the previous rate law presented by Williamson and Rimstidt (1994), but indicate a stronger pH dependence than previous studies. We also present the first kinetic rate law for molybdenite oxidation at low O2, based on 13 independent rate measurements: rmolybdenite=10-8.3(±2.3)O20.5±0.3, Where rmolybdenite is the rate of oxidation in mol/m2 sec, and the activity of dissolved [O2] is in (mol/L). We find molybdenite oxidation to be nearly as rapid as pyrite oxidation even at low concentrations of dissolved O2 (equivalent to <10−5 PAL pO2), in contrast to previous work which argued for a threshold effect for molybdenite oxidation. Both pyrite and molybdenite oxidation kinetics exhibit a constant half-order dependence on dissolved O2 down to nanomolar levels of O2. We show that this behavior is best explained by a reaction mechanism in which O2 undergoes dissociative adsorption to the sulfide mineral surface. This mechanism helps to resolve major uncertainties regarding the reaction mechanism of O2 with pyrite and molybdenite mineral surfaces and provides a strong theoretical basis for the robust extrapolation of present results to higher and lower O2 concentrations.

Hot corrosion studies of HVOF sprayed carbide and metallic powder coatings on alloy 80A at 900 °C

The current work compares the high temperature corrosion behavior of alloy 80 A coated with carbide (Cr3C2-25NiCr) and Metallic (NiCrMoNb) powders through high velocity oxy-fuel (HVOF) process, under 50 cyclic condition at 900 °C. The corrosion study was conducted in the air oxidation environment and molten salt environment. The coating thickness, hardness, and porosity were assessed along the cross section direction. The results of thermogravimetry analysis showed that the substrates subjected to air oxidation environment showed a lesser mass gain, in comparison to its molten salt environment substrates (uncoated and coated substrate) and it trails the parabolic rate law of oxidation. The uncoated substrate had shown excellent corrosion resistance, whereas poor corrosion resistance was observed in the metallic powder coated substrate in the molten salt (Na2SO4 + 60% V2O5) environment. The existence of MoO3 phase reduced the corrosion resistance of the metallic powder coated substrate. The existence of NiCr2O4 and Cr2O3 phases observed on Cr3C2-25NiCr coating provided good resistance to hot corrosion in the molten salt environment.

Improving arsenopyrite oxidation rate laws: implications for arsenic mobilization during aquifer storage and recovery (ASR).

Aquifer storage and recovery (ASR) and aquifer recharge (AR) provide technical solutions to address water supply deficits and growing future water demands. Unfortunately, the mobilization of naturally present arsenic due to ASR/AR operations has undermined its application on a larger scale. Predicting arsenic mobility in the subsurface during ASR/AR is further complicated by site-specific factors, including the arsenic mobilization mechanisms, groundwater flow conditions, and multi-phase geochemical interactions. In order to ensure safe and sustainable ASR/AR operation, a better understanding of these factors is needed. The current study thus aims to better characterize and model arsenic remobilization at ASR/AR sites by compiling and analyzing available kinetic data on arsenic mobilization from arsenopyrite under different aqueous conditions. More robust and widely applicable rate laws are developed for geochemical conditions relevant to ASR/AR. Sensitivity analysis of these new rate laws gives further insight into the controlling geochemical factors for arsenic mobilization. When improved rate laws are incorporated as the inputs for reactive transport modeling, arsenic mobilization in ASR/AR operations can be predicted with an improved accuracy. The outcomes will be used to guide groundwater monitoring and specify ASR/AR operational parameters, including water pretreatment requirements prior to injection.

High-temperature corrosion behaviour of air plasma sprayed Cr3C2-25NiCr and NiCrMoNb powder coating on alloy 80A at 900 °C

The air plasma spray process is extensively adopted by many industries because of versatility, cost-effectiveness and good quality of the coatings produced. The present study compares the high-temperature corrosion behavior of air plasma sprayed Cr3C2-25NiCr and NiCrMoNb powders coatings on alloy 80A (Ni80TiAl). The corrosion study was carried out by air and molten salt environment of Na2SO4+60%V2O5 under cyclic conditions for 50cycles at 900°C. Thermogravimetric analysis was used to analyze the hot corrosion (air and molten salt environments) of uncoated and coated alloy 80A. Microstructure, porosity, thickness, and hardness of the coatings were evaluated in the cross section of the coatings. XRD and SEM/EDS analysis were carried out to evaluate the corrosion product at the end of the 50th cycle. The microstructure shows the coatings are laminar structure with uniform, adherent and dense microstructure with porosity less than 4% and thickness of 150±10μm. The microhardness of both coatings found to be higher than the substrate. Thermogravimetric analysis shows the air oxidation both coated and uncoated alloy 80A obey the parabolic rate law of oxidation. Uncoated alloy 80A shows the very less weight gain compared to other coated substrates. NiCrMoNb coated with molten salt environment substrates shows the higher weight gain. Also, molten salt environment shows higher mass gain compared to the corresponding air oxidation environment substrates. Presence of NiO, Cr2O3, and spinel oxide NiCr2O4 provides the excellent corrosion resistance in the air oxidation environment both coated and uncoated substrates. The high Mo element content in NiCrMoNb coating is causing more weight gain at 900°C in molten salt (Na2SO4+60%V2O5) environment by forming a volatile MoO3 oxide. The results conclude that NiCrMoNb coating may be used as a protective coating in air oxidation environment and the Cr3C2-25NiCr coating may be used as a protective coating in the molten salt environment.