Oxidation Of Cr Research Articles

Oxidation behavior of Cr–Mn–Si alloyed steel by mold flux melt in high temperatures

The oxidation behavior of a medium carbon Cr–Mn–Si alloyed steel was investigated under a mold flux melt environment exposed to different high temperatures of 1175–1425 °C, and holding time of 5–85 min. The results showed that the thicknesses of oxide layers contained Cr–Mn–Si oxide particles significantly increased from 10 to 183 μm with the increasing of oxidation temperature and holding time. The selective oxidation of Cr, Mn and Si elements in the grain boundaries is caused by the diffusion of free oxygen (O2−) migrating from mold flux to steel matrix, which leads to the growth of oxide layer thicknesses. Moreover, the oxidation rate constant (kp) of the Cr–Mn–Si alloyed steel was revealed under the mold flux environment at the temperatures of 1175–1375 °C. In addition, it was found that the growth kinetics of oxide particles in the oxide layer consists of a rapid growth stage and a slow-steady growth stage, and the growth of oxide particles is co-controlled by the diffusion of O2− within the steel matrix and the competing oxidation reactions involving Fe, Cr, Mn, and Si elements.

An investigation on hot corrosion behavior of Al0.4MnCrCoFeNi high entropy alloy in different molten salts at 750 °C

The hot corrosion behavior of Al0.4MnCrCoFeNi high entropy alloy in different molten salts at 750 °C was investigated. The high entropy alloy was synthesized by vacuum melting and casting. The hot corrosion tests were performed in 75%Na2SO4 + 25%NaCl, 75%Na2SO4 + 20%NaCl + 5%V2O5, 75%Na2SO4 + 5%NaCl + 10%V2O5, and 60%Na2SO4 + 40%V2O5 mixture salts. Microstructural characterizations were carried out using a field emission scanning electron microscope equipped with an X-ray energy dispersive (EDS) analyzer. X-ray diffraction method with CuKα radiation was also used for phase analysis of corrosion products. The results indicated that Al0.4MnCrCoFeNi alloy had relatively good resistance to hot corrosion in 75%Na2SO4 + 25%NaCl molten salt compared to the other salts due to the formation of a protective layer consisting of Mn, Cr, and Al oxides. The addition of V2O5 to the mixture salt promoted the dissolution of the oxide layer in the molten salt and hence decreased the resistance of the alloy to hot corrosion. The mechanisms of hot corrosion were oxidation-sulfidation and fluxing. The least resistance to hot corrosion was observed in 60%Na2SO4 + 40%V2O5 mixture salt because the oxide scale was destroyed seriously, resulting in accelerated propagation of hot corrosion.

An XPS and TEM study of the composition and structure of native oxides on the inner surface of as-received Ni base alloy steam generator tubes

Ni based alloys steam generator (SG) tubes in pressurized water reactors may suffer generalized corrosion and Ni release in primary water. The processes of corrosion and oxidation are strongly dependent on the surface properties. In particular, the chemical composition and structure of the native oxide films present at the surface of the SG tubes are crucial and improving the passive properties of the surface could contribute to reduce the corrosion damage and Ni release. The native oxides on the inner surface of two as-received Ni base alloy SG tubes were investigated using two main techniques, namely X-ray photoemission spectroscopy and transmission electron microscopy. The chemical elements present in the native film for the two tubes were identified and the structure and composition of the native film at the nanoscale was determined. A duplex oxide structure was identified for both samples, but with differences in the chemical composition of the inner and outer oxide layer. Chemical heterogeneities such as alumina, Ti oxides and Cr oxides were also detected; they are considered to play a role in the evolution of the surface exposed to primary water. The two techniques used allowed a multi-scale description of the surface and led to complementary results.

The Direct Alloying of Steel through Silicothermic Self-Reduction of Chromite Ore Utilizing Si-Containing Solid Waste

Organosilicon materials generate copious amounts of Si-containing solid waste during production, leading to severe environmental pollution and substantial resource squandering. In pursuit of the resource utilization of Si-containing solid waste, this study conducted experimental research on the direct alloying of molten steel through the silicothermic self-reduction of chromite ore using Si-containing solid waste as a reducing agent. Additionally, thermodynamic analysis was performed, employing the thermodynamic calculation software FactSage 8.2 (Thermfact Ltd., Montreal, QC, Canada and GTT-Technologies, Aachen, Germany), to examine the equilibrium reactions of the silicothermic reduction of chromite ore and the variations in the thermodynamic equilibrium compositions of slag and metal phases. The results indicate a reduction sequence for the reducible components in chromite ore as Fe2O3 → Cr2O3. The introduction of CaO and Al2O3 into the silicothermic self-reduction compacts altered the forms of Fe and Cr oxides in equilibrium, significantly reducing the standard Gibbs free energy (ΔG0) of the silicothermic reduction reaction. The initial slag melting point decreased from 1700 °C without the addition of CaO and Al2O3 to 1500 °C with the addition of CaO and Al2O3. Correspondingly, the slag viscosity at 1600 °C decreased from 134.1 Pa·s without CaO and Al2O3 addition to 1.81 Pa·s with CaO and Al2O3 addition. The addition of CaO and Al2O3 accelerated the reduction of Cr oxide in chromite ore and enhanced the recovery of Cr, consistent with the thermodynamic calculation results. In the process of steelmaking through the direct alloying of chromite ore silicothermic self-reduction compacts, the final recovery rate of Cr increased from 86.4% without CaO and Al2O3 addition to 95.4% with CaO and Al2O3 addition.

Investigation on the synthesis mechanism and size control of (Ti,W,Mo,Cr)(C,N) solid solution powder

Complex interfaces in cermets limit its application as cutting or heat-work die materials. Interface modification of hard particles by composition optimization is proved to be an effective strategy for high-performance Ti(C,N)-based cermets. Herein, we prepared (Ti,W,Mo,Cr)(C,N) solid solution powder through carbothermal reduction nitridation method. The phase and microstructure were characterized to discover the synthesis mechanism. Oxides of Mo, W, Ti, and Cr participated in the reaction successively, MoO2 transformed to M2X (X is C and/or N, M is W, Mo and/or Cr) directly, both W2C and W formed simultaneously as the reduction products of WO3, TiO2 followed TiO2 → TinO2n-1 → (Ti, M)(C,N) (M is W, Mo and/or Cr), and Cr diffused into (Ti, M)(C,N) and M2X phases directly from its oxide. Finally, the solid solution powder with composition (Ti0.83W0.07Mo0.09Cr0.01)(C0.62N0.38) in the particle size of 0.73 μm with ∼0.14 wt% oxygen and ∼ 0.43 wt% free carbon was obtained at 1750 °C for 2 h. Our findings provide insight into developing advanced cermets with better comprehensive properties.

Influence of scanning speed on microstructure and corrosion resistance of Fe-based amorphous coatings by high-speed laser cladding

Fe-based amorphous coatings were prepared on the surface of the AISI 1020 steel pipe by high-speed laser cladding technique. This study is aimed to investigate the influence of scanning speed on the microstructure, hardness, amorphous content and corrosion resistance offered by the Fe-based amorphous coatings. The results showed that, the coating obtained at 50 mm/s exhibited a distinct three-layer hardness distribution, with the second layer displaying the highest hardness. This phenomenon was primarily ascribed to amorphous strengthening, fine-grain strengthening, and martensitic strengthening. The bottom region of the coatings contains budding crystals, with prominent aggregation of elemental Fe. Away from the budding regions, a transition from columnar to dendritic crystals was observed. The amorphous content of the coatings exhibited an initial rise followed by a decline with increasing scanning speed. Among these, the amorphous phase content of 130 mm/s coating was up to 95 %. Differential Scanning Calorimetry (DSC) test results confirmed the presence of amorphous phases in the bottom and middle regions of the 50 mm/s coating. Electrochemical experiments underscored the exceptional corrosion resistance of the 130 mm/s coating, evidenced by its maximum corrosion potential (−0.471 V), minimal corrosion current density (2.7 × 10−6 A/cm2), and highest polarization resistance value (22,149 Ω·cm2). This superior performance was attributed to the presence of the amorphous phase and the Cr oxide layer formed on the surface, providing robust protection to the substrate.

High-Temperature Oxidation Resistance and Molten Salt Corrosion Study of YSZ, CeYSZ, and YSZ/CeYSZ Thermal Barrier Coatings by Atmospheric Plasma Spraying

Five thermal barrier coatings (TBCs), namely TBC-1 (YSZ), TBC-2 (CeYSZ), TBC-3 (YSZ:CeYSZ = 1:2), TBC-4 (YSZ:CeYSZ = 1:1), and TBC-5 (YSZ:CeYSZ = 2:1), were fabricated using the atmospheric plasma spraying (APS) method. Their oxidation behaviors at 1100 °C and corrosion resistance to molten salts (V2O5 + Na2SO4) at 900 °C were examined. After 100 h of oxidation, the thermally grown oxide layer (TGO) for YSZ primarily contained Cr and Ni oxides with significant internal fractures, presenting a continuous band-like Al2O3. In dual-ceramic configurations, an increase in CeYSZ thickness led to a rise in Al content and reduced Cr and Ni in TGO, with the surface fracture morphing into an internal porosity. Following salt corrosion, YSZ revealed rod-like YVO4 and m-ZrO2 as corrosion products, whereas CeYSZ displayed chain-structured CeO2, CeYO4, and YVO4 combined with m-ZrO2. YSZ coatings underwent notable phase transitions with evident densification, forming a corrosion layer of approximately 10 μm. Conversely, CeYSZ showed a limited phase change, retaining porosity without a distinguishable corrosion layer. As CeYSZ thickness increased from 100 μm to 200 μm in the dual-ceramic structure, salt penetration reduced. Evidently, the dense structure of CeYSZ heightened diffusion resistance against oxygen and corrosive salts, rendering superior oxidation and corrosion resistance over YSZ. By optimizing the thickness ratio between CeYSZ and YSZ, whilst retaining total ceramic layer thickness, the dual-ceramic TBC’s resistance to high-temperature oxidation and corrosion can be enhanced.

A Continuous-Wave EPR Investigation into the Photochemical Transformations of the Chromium(I) Carbonyl Complex [Cr(CO)4bis(diphenylphosphino)]+ and Reactivity with 1-hexene.

Chromium complexes containing a bis(diphenylphosphino) ligand have attracted significant interest over many years due to their potential as active catalysts for ethylene oligomerisation when combined with suitable co-catalysts such as triethylaluminium (TEA) or methylaluminoxane (MAO). While there has been considerable attention devoted to the possible reaction intermediates and the nature of the Cr oxidation states involved, the potential UV photoactivity of the Cr(I) complexes has so far been overlooked. Therefore, to explore the photoinduced transformations of bis(diphenylphosphino) stabilized Cr(I) complexes, we used continuous-wave (CW) EPR to study the effects of UV radiation on a cationic [Cr(CO)4(dppp)]+[Al(OC(CF3)3)4]- complex (1), where dppp represents the 1,3 bis-(diphenylphosphino)propane ligand, Ph2P(C3H6)PPh2. Our preliminary investigations into the photochemistry of this complex revealed that [Cr(CO)4(dppp)]+ (1) can be readily photo-converted into an intermediate mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ complex (2) and eventually into a trans-[Cr(CO)2(dppp)2]+ complex (3) in solution at room temperature under UV-A light. Here, we show that the intermediate species (2) involved in this transformation can be identified by EPR at much lower temperature (140 K) and at a specific wavelength (highlighting the wavelength dependency of the reaction). In addition, small amounts of a 'piano-stool'-type complex, namely [Cr(CO)2(dppp-η6-arene)]+ (4), can also be formed during the photoconversion of [Cr(CO)4(dppp)]+ using UV-A light. There was no evidence for the formation of the [Cr(L-bis-η6-arene)]+ complex (5) in these UV irradiation experiments. For the first time, we also evidence the formation of a 1-hexene coordinated [Cr(CO)3(dppp)(1-hexene)]+ complex (6) following UV irradiation of [Cr(CO)4(dppp)]+ in the presence of 1-hexene; this result demonstrates the unprecedented opportunity for exploiting light activation during Cr-driven olefin oligomerisation catalysis, instead of expensive, difficult-to-handle, and hazardous chemical activators.

Understanding a novel form of intergranular corrosion of stainless steel 316L exposed to molten LiCl-Li2O-Li

Stainless steel 316 L exposed to LiCl-Li2O molten salt containing Li metal for 500 and 1000 h experiences deep intergranular attack and bulk void formation. This degradation is due to a synergistic corrosion mechanism, different than other forms of molten salt corrosion, where Cr and Mn oxides and carbides form along grain boundaries and are dissolved by Li metal. Synchrotron transmission X-ray microscopy 3D imaging is used to show corrosion morphology and Cr enrichment and depletion in a novel format that has not previously been used for bulk corrosion samples.

Molybdenum effects on the stability of passive films unraveled at the nanometer and atomic scales

Data recently obtained on model FeCrNi(Mo), 316 L stainless steel, and FeCrNiCo(Mo) passivated surfaces by advanced surface analysis and density functional theory modeling are comprehensively discussed to unravel the multiple effects that molybdenum might have at the nanometer and atomic scales to enhance the stability of passive films. The key role played on corrosion protection by the compositional and structural nanoscale defects of the passive film that originate from the pre-passivation mechanisms of the surface is considered. It is shown how Mo, enriched together with Cr in the nanometer-thick passive film, can combine several effects to enhance the resistance to Cl--induced passivity breakdown. Enriched as Mo(VI) species in the outer exchange layer of the passive film, Mo impedes the deep penetration of Cl- ions and limits their access to the inner barrier layer. Dispersed as Mo(IV) at the interface with the inner layer, Mo protects against the entry of Cl- ions into the defect sites of the Cr(III) oxide barrier. Present as Mo(IV + δ) in the Fe-rich compositional nanoscale defects self-generated by the local failure of Cr supply upon initial formation of the barrier layer, Mo enhances the selective dissolution of iron and its replacement by chromium and molybdenum. By impeding the formation of O vacancies, Mo also increases the resistance against chloride entry in the oxide matrix, thereby curing these the Fe-rich weak sites against Cl--induced passivity breakdown.

Структура и свойства покрытий SiO2 – Cr2O3, полученных методом импульсного магнетронного распыления на керамической основе ZrO2

The technology for SiO2 – Cr2O3 coatings on a ceramic Y-TZP (ZrO2 stabilized by Y2O3) zirconia base has been proposed. The coating was produced in three stages: 1) formation of SiO2 adhesion layer by application of 5-amino-propyltriethoxysilane alcohol solution to the ceramic base surface followed by heat treatment at 450 ± 5 °C for 30 min. The next step was application of Cr at purity of 99.9 % by pulse magnetron sputtering; 3) diffusion oxidation of Cr, applied on the ceramic base, to Cr2O3 in the muffle furnace at 450 ± 5 °C for 30 min. Dependence of physical-mechanical characteristics of coating depending on preparatory and finishing operations (abrasive blasting, polishing) has been investigated. Samples with submicron coating with thickness of 150 ± 20 nm, having nanostructured lamellar structure, possessing open porosity value of 1.3 %, microhardness 2000 HV, roughness Ra 0.32 – 0.63, friction coefficient 0.175 and increased by 184 % wear resistance to abrasion loadings in comparison with pure zirconia ceramics were obtained.

Effect of molybdenum on the oxidation resistance of FeCrAl alloy in lower temperature (400 °C) and higher temperature (1200 °C) steam environments

The effect of 0, 1, 3 wt% Mo addition on the corrosion behavior of Fe21Cr5.5Al was studied through steam tests at 400 °C and 1200 °C. At 400 °C, Mo addition leads to non-protective Fe-Mo oxide growth on the surface, compromising protective uniform Cr oxide growth and subsequent Al2O3 layers as in the Mo-free specimen. At 1200 °C, the presence of Mo causes the spallation of the surface oxide scale. However, a thin alumina surface film was present. Current results show that Mo addition is detrimental to the corrosion resistance of FeCrAl alloy under both operational and accidental temperature scenarios.

New insight into the interaction between maleic anhydride grafted polypropylene and aluminum foil converted with trivalent chromate: An experimental and DFT study

The good adhesion between layers of aluminum (Al)/polymer composite is necessary for the safety of lithium-ion batteries. The Al foil was converted with environmentally friendly trivalent chromate [Cr(III)] to enhance its adhesive strength with the inner adhesive layer. The morphology, composition and adhesive strength with maleic anhydride grafted polypropylene (mPP) of different Cr-converted Al foils were studied. The Cr conversion coating mainly consists of metal oxides and hydroxides and the interfacial adhesion strength is enhanced by 93 % through Cr(III) conversion treatment. The effect of Cr conversion on the acid-base characteristics on the Al was determined through contact angle measurement. The results reveal that Cr conversion increases the acidity of the Al surface. The bonding mechanism at the interface was further studied fundamentally. To study the reaction at the interface mPP/Al foils, a coating-evaporation technique was utilized to fabricate a nano-thick mPP layer on the Al foils. The morphology and thickness of the mPP coating on the substrate were analyzed using atomic force microscopy and the interfacial reaction was characterized through attenuated total reflection-Fourier Transform Infrared Spectroscopy. The bonding mechanism at the molecular level was further investigated by quantum chemical calculations based on density function theory. The results of theoretical calculations further indicate that the strength of the interfacial interaction between mPP and Cr oxides and hydroxides is stronger than that between mPP and Al oxides and hydroxides.

Reactive Condensation of Cr Vapor on Aluminosilicates Containing Alkaline Oxides

The research presented herein is part of a larger project delving into the mechanisms and pathways for reactive condensation of Chromium (Cr) vapors in high-temperature (>500o C) environments. The overall goal of this research project is to improve fundamental understanding of reactive condensation pathways for chromium (Cr) vapors generated in high-temperature systems such as exhaust manifolds, steam turbines, boilers, or solid oxide fuel cells (SOFCs). Reactive evaporation of Cr from stainless steels commonly used in these systems is well-documented; however, the interactions between volatilized Cr species and surrounding interfaces during complex and dynamic system exposures is poorly understood. Understanding the interactions between volatilized Cr species and downstream components during operation is critical for improving system performance, environmental health, and safety as some condensed Cr forms toxic hexavalent Cr(VI) species. The focus of this study is on the effects of alkaline oxides present within the substrate material on reactive condensation mechanisms of Cr vapors. Studying the reaction mechanisms between Cr vapors and ceramic surfaces has direct application to SOFCs as these materials are widely used in the field. For example, alumina-silica glass-ceramics are used as sealants in SOFCs. These materials belong to the SiO2–Al2O3 system and contain different compositions of oxides including CaO, Na2O, MgO, K2O, B2O3, Y2O3, and BaO. Another sealing material used in SOFCs is mica, a class of silicate minerals that forms into distinct layers. Two types of mica commonly used in SOFC applications are muscovite (potassium aluminum silicate hydroxide fluoride, KAl2(AlSi3O10)(F,OH)2), and phlogopite (potassium magnesium aluminum silicate hydroxide, KMg3(AlSi3O10)(OH)2). The specific research objective is to explore the effects of alkaline oxide additives in aluminosilicates on Cr collection and speciation. To accomplish this objective, an experimental platform was designed to assess interactions between samples of high-temperature steel alloys or Cr oxide (Cr2O3) powder with ceramic insulating materials. The chemical composition of the insulating materials used in this study range from pure silica (SiO2) and alumina (Al2O3) to combinations thereof with and without inclusions of alkaline earth metals (e.g., Ca, Mg). Preliminary results show three- to four-fold increases in Cr(VI) formation on silica fibers with weight percentages of ~30 and ~40 percent alkaline oxides (CaO and MgO), respectively, after exposure to volatile Cr species, relative to low-alkaline silica. Figure 1 presents an overview of the experimental concept, Cr(VI) test kit, and preliminary results for dependency on alkaline oxide content. Figure 1

Is Cr(III) re-oxidation occurring in Cr-contaminated soils after remediation: Meta-analysis and machine learning prediction

Whether Cr(III) in Cr(III)-containing sites formed after Cr(VI) reduction and stabilization remediation are re-oxidized and pose toxicity risks again has been a growing concern. In this study, 1030 data were collected to perform a meta-analysis to clarify the effects of various factors (oxidant type, soil and Cr(III) solid compound properties, aging conditions, and testing methods) on Cr(III) oxidation. We observed that the soil properties of clay, pH ≥ 8, the lower CEC capacity, easily reducible Mn content, and Cr(III) content, and the higher Eh value and Fe content can promote the re-oxidation of Cr(III). Publication bias and sensitivity analyses confirmed the stability and reliability of the meta-analysis. Subsequently, we used five machine learning algorithms to construct and optimize the models. The prediction results of the RF model (RMSE <1.36, R2 >0.71) with good algorithm performance showed that after ten years of remediation, the extractable Cr(VI) concentration in the soil was 0.0087 mg/L, indicating a negligible secondary pollution risk of Cr(III) re-oxidation. This study provides theoretical support for subsequent risk management and control after Cr(VI) soil remediation and provides a solution for the quantitative prediction of Cr(III) re-oxidation.

Development of Novel Coating on Metallic Bipolar Plates for Proton Exchange Membrane Water Electrolyzer Application

Considering the demand for a more environmentally friendly system, renewable and sustainable energy sources, such as wind and solar power are regarded as a promising substitute for fossil fuels in the generation of electricity. Since these energy sources exhibit intermittency and unpredictability, there is a need to develop a highly efficient and rapidly responsive energy conversion system for the purpose of storing this energy as a chemical fuel. Due to their ability to manage the variability of intermittent energy sources, proton exchange membrane electrolyzer cells (PEMECs) are garnering considerable interest. However, their broad commercial adoption is hindered by their relatively modest performance and elevated costs. PEM electrolysis is a promising technology for generating substantial quantities of hydrogen from surplus electricity generated by renewable sources. Nonetheless, the expense of the electrolyzer stack arises primarily from the metallic bipolar plates, because it possesses heavy weight and expensive precious metal coatings. Significant efforts have been performed on the development of titanium-based bipolar plates, but it has high base material and fabrication cost. In recent years, there has been a substantial focus on stainless steel bipolar plates for PEM electrolyzer application. Stainless steel can serve as an alternative, but it requires a coating to enhance its anti-corrosion properties. Electroplated chromium coatings exhibit favorable mechanical characteristics and find extensive application in precision mechanical components such as molds and surgical instruments. Their outstanding wear resistance, high hardness, impressive corrosion resistance, and glossy finish make them appropriate for many other applications, including both decorative and functional coatings. As a result, chromium electroplating has emerged as one of the most extensively employed techniques within the electroplating sector. The dense pack chromium layer is prepared on a 316L stainless steel substrate to enhance the anti-corrosion property and electronic conductivity.Herein, the role of gold striking underlayer, and thick chromium coating is studied for preparing the anti-corrosive stainless steel bipolar plates. First, a gold striking film as an underlayer on the stainless-steel substrate is electroplated to passivate the chromium-based oxides. Further, a thick and dense chromium layer is deposited on the gold underlayer by electroplating. The electroplating bath is operated using platinized titanium felt as an anode and 316L stainless steel as a working electrode without the separation of anodic and cathodic compartments. The electroplating solution is consisting of Cr (VI) oxide and H2SO4 (pH = 2). The nanocrystalline chromium coating with a thickness of approximately 2 µm is bright and uniform. Furthermore, gold as a protective layer of thickness approximately 1 µm is coated by electroplating on the chromium surface. The Cr and Au/Cr coatings are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron microscopy (XPS) analysis. Subsequently, the protective layer-coated substrates were analyzed in a simulated PEM electrolyzer environment, and these coatings facilitated promising protective properties on the stainless-steel substrate. The corrosion testing was carried out in 0.5 M H2SO4 + 2 ppm HF solution at 70 0C purged with oxygen. The stable passive film and high corrosion resistance were obtained by gold striking underlayer and chromizing the 316L stainless steel substrate. As a result, the potentiodynamic polarization analysis showed the corrosion current density of 0.84 µA/cm2 for the Au/Cr-Au underlayer sample in a simulated operating environment. In contrast, the absence of the gold striking underlayer between Cr and stainless steel causes the pitting corrosion. The Au/Cr coating is tested in an acidic corrosion media for a long duration of 24 hours and it showed lower degradation as compared to without gold striking underlayer and without Cr film. The findings presented in this study illustrate the feasibility of employing stainless steel as the foundational material for the bipolar plate of a PEM electrolyzer.

Revisiting Open Circuit Corrosion of Cr in Unpurified Molten FLiNaK Salts with a Multi-Modal Approach

Corrosion and mitigation of environmental degradation of structural materials in high-temperature Gen IV molten salt reactors, are critical issues. However, any corrosion studies have historically been forensic post-test examinations and materials development has occurred mainly by trial and error until recently. Meanwhile, a fundamental understanding can only be achieved by recognizing how the corrosion system of model materials in molten fluoride salts evolves as a function of time. A number of in-situ spectroscopy techniques to identify corrosion product compositions in molten salts have been suggested in recent years. Raman, high-temperature UV-Vis absorption spectroscopy, and X-ray absorption spectroscopy are a few of them [1,2]. These techniques' major limitations include the fact that they are mainly qualitative, and complex hardware requiring a beam focusing inside the glove box is needed. At the same time, electrochemical techniques have been presented which can provide the information on behavior of both metal and its corrosion product in the continuously evolving corrosion system in molten salts. However, use of single isolated methods renders it difficult to obtain mechanistic information on instantaneous, time-dependent processes and phenomena during corrosion. These are needs gaps opportunities. In this work we present a detailed electrochemical analysis of the corrosion of polycrystalline chromium at 600°C in FLiNaK over 50 h including an electrochemical analysis of in-situ near-instantaneous corrosion rates corroborated with other methods.A variety of corroborating methods including (1) in-situ electrochemical diagnostic techniques, (2) static immersion exposure with gravimetric and SEM analysis, (3) electrochemical impedance spectroscopy, (4) XRD, (5) ICP-OES, was utilized to study the corrosion behavior in unpurified FLiNaK at 600°C. The focus was to evaluate the viability of using an external non-reactive electrode to measure the concentration of corrosion products via redox process analysis in order to determine the corrosion rate during the process of OCP corrosion of Cr in FLiNaK. This work reports the possibility of using of electrochemical techniques such as CV for the instantaneous determination of Cr corrosion rates when Cr is exposed to a 600oC FLiNaK salts for 50 h. It was found that the concentration of dissolved corrosion products, in the more likely possible forms of CrF3 - and CrF6 3- , might be calculated by established analytical approaches to CV analysis such as Berzins-Delahay (soluble/insoluble) and Randles-Ševcik equations (soluble/soluble). The calculated values showed the definite concentration of Cr(II)/Cr(III) corrosion products at the distance of the external electrode (due to the static corrosion conditions). These finding supported by XRD and ICP-OES. A semi-infinite diffusion model was employed in this study to adjust the calculated concentrations assuming a stagnant solution and diffusion based one-dimensional concentration diffusion profile in order to calculated the Cr(III) concentration at a fixed position in the Cr(III) concentration gradient. It was shown that the semi-infinite diffusion model helps correctly describe this profile between 10 and 50 h, based on the geometry of the working electrode and corrosion cell. Based on these results the OCP corrosion rate was interpreted to switch over 50 h from initially charge transfer control Cr(II) oxidation at early times with likely HF and Cr(III) reduction, followed by mass transport control regulated by a salt film and then possible stifling due to depletion of HF and Cr(III) solubility limits.The results of half-cell redox potentials and peak currents pertaining to oxidation/reduction of Cr corrosion products were correlated to the concentrations of particular Cr species and compared with the results of mass loss experiment and ex-situ ICP-OES of post-exposed FLiNaK. The mass loss of the Cr(II)/Cr(III) metal that was approximated by spatially integrating the concentration profiles at a selected time, and the result of Cr mass loss separately obtained from gravimetric measurements showed a reasonable agreement. Furthermore, the rate of accumulation of Cr ions in the residual FLiNaK salt after 50 h of exposure determined by ICP-OES, correlated with the electrochemical CV and EIS analysis. In addition to this, the oxidation and reduction peak potentials pertaining to Cr/CrF3 - and CrF3 -/CrF6 3- redox reactions (measured on Pt wire) were compared with the OCP (measured on Cr) to provide insights on the predominant anodic and cathodic reactions. Acknowledgments This research was supported as part of the fundamental understanding of transport under reactor extremes (FUTURE), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Utilization of the Malvern-Panalytical Empyrean diffractometer was supported by Nanoscale Materials Characterization Facility with National Science Foundation under award CHE-2102156. References Y. Liu et al., (2020) Corrosion Science. 169 108636. https://doi.org/10.1016/j.corsci.2020.108636.S. Fayfar, et al., (2022). https://doi.org/10.26434/chemrxiv-2022-3nspm.

The Effects of Trace H2S Levels on the Structure of Surface Scales in CO2 Corrosion

The CO2 corrosion of carbon steel is very strongly influenced by the formation of surface scales of corrosion products. Hence, the structure and composition of those scales has been the subject of much research. For oxygen-free brines in contact with gaseous CO2 at partial pressures up to ~50 bar, and temperatures up to ~120 °C, the formed scales are generally found to consist of a mixture of Fe3C (cementite) and FeCO3 (siderite). In simulated oil and gas production brines, the siderite incorporates Ca and Mg, while DeMarco et al [1] identified the scales that form over short times at ambient temperature as a mixture of Fe2(OH2)CO3 and Fe2O2CO3.Williams and colleagues [2-7] carried out detailed studies of the scales formed in solutions of moderate chloride content (0.5 – 1 M), saturated with CO2 at ~ 1 bar, pH adjusted to ~ 6.5 and at temperatures of ~ 80 °C. For the solutions containing only sodium chloride, siderite (FeCO3) was the major phase in the scale, though chukanovite (Fe2(OH)2CO3) was also present in some cases. With the addition of magnesium chloride, both siderite and chukanovite were always found in the scale, while there was no evidence of magnesium incorporation until the bulk concentration of MgCl2 was ~ 0.1 M. Considering the effect of low-level Cr and Mo alloying, it was found in the base sodium chloride solutions that mixed siderite/chukanovite scales were formed, but the scale became thinner and the chukanovite-to-siderite ratio decreased with alloy content. Even for alloys with 3.5 wt% Cr there was no evidence of discrete Cr (hydr)oxide layers within the scale. However, the addition of trace levels of H2S (∼ 0.5 μM) produced more significant changes. First, only chukanovite and FeS (mackinawite) were now found in the scale, with no siderite. Second, for the unalloyed steel and for alloys with 1 wt% Cr and up to 0.7% Mo, the scales were highly porous. Finally, for the 3.5wt% Cr alloy, a non-porous, protective layer was formed, about 200–600 nm thick.In this paper, we utilize ab-initio techniques to investigate the effects of very low level H2S on carbonate scale formation. In the past DFT has been used to understand the various aspects of corrosion and the design of corrosion inhibitors for various conditions [9-11]. We have used similar approach for studying the H2S adsorption on siderite and chukanovite surfaces, considering the experimental observation by Hassan et al [2], that trace amounts of H2S result in chukanovite over siderite. Initial studies show that there is a 0.7 eV higher interaction energy (more thermodynamically favorable) for H2S adsorption on chukanovite compared to siderite. This may imply the possibility that H2S could encourage chukanovite formation over siderite formation in aqueous environments.We further investigate this possibility using DFT to understand the mechanism that leads to formation of chukanovite. We aim to use implicit solvation in order to bring the effect of water in the vicinity. We further use the cluster approach within DFT, to explain the effect that alloying has on the scale formation. We take a bare Fe cluster and replace one of its atoms with an alloying element and study the electronic structure in the presence of H2S and CO2. The potential energy surface gets modified for a H2S—Fe interaction in the presence of Ni. We expect that this work would help us to have a deeper understanding of how alloying can impact or rather initiate certain kind of scale formation over others.

Cyclic and Isothermal Oxidation Behavior of a Directionally Solidified Ni-Al-Ti-Co-Cr Superalloy

Nickel based superalloys are widely used in such applications as gas turbine blades due to their excellent mechanical properties at elevated temperatures. Oxidation is one of the life limiting factors at such temperatures since it accelerates failure mechanisms, e.g. fatigue and creep. In this project, oxidation behavior of Ni-3Al-4.8Ti-9.5Co-14Cr (wt.%) superalloy was investigated under various testing conditions. The alloy was investment-cast with preferential crystal growth direction [100]. Isothermal oxidation tests were carried out at 800°C for 150h. Cyclic oxidation tests were then conducted between 100-800°C for 150 one-hour cycles. An additional one-hour cycle was tested to replicate real-life working condition of the alloy, which consisted of heating the samples up to 800°C followed by slow cooling to 400°C. During the testing, samples were periodically taken from the furnace and weighted with an accuracy of 10-4 g to calculate the weight gain and to analyze the microstructure of formed oxide. Scanning electron microscopy (SEM) combined with energy-dispersive spectroscopy (EDS) and X-ray diffraction were used to study the microstructure of oxidation products. The results reveal that the highest amount of weight gain is obtained in isothermal testing, while the lowest in cyclic. Diffusion of Ti and Ni to the free surface results in the early-stage formation of needle-shaped oxide phases followed by severe oxidation of Cr and Al. The layer of chromium oxide is continuous while aluminum oxide is blocky at the deepest position from the free surface. Cracks form at the interface of carbide particles and gamma phase near the free surface of cyclic oxidation samples due to concentration gradient of Ni and Al. Precipitate-free zone are observed near the free surface of all the samples at the end of the tests, but internal oxidation does not occur significantly in all the samples. It is found that temperature fluctuation increases oxide spallation, which causes crack formation. At the same time, in isothermal testing, dense, crack-free layer of oxide form on the surface of superalloy.

NaH2PO4 synergizes with organic matter to stabilize chromium in tannery sludge

Heavy metal stabilization is an effective method to treat chromium in tannery sludge. Here we show that mainly investigated NaH2PO4 (MSP) and organic matter (OM) to stabilize chromium in tannery sludge. The experimental investigation revealed that the addition of montmorillonite (MMT) and MSP samples showed a significant increase in the percentage of reducible and oxidizable Cr in the former compared to the samples with the addition of MMT. This is attributed to the formation of Cr–O bond, which allows the MSP to undergo an inner-sphere complexation reaction with the metal oxide of Cr via ligand exchange. Significantly, the MSP moiety adsorbs on the surface of OM through monodentate, which increases the adsorption sites of OM for Cr6+ and promotes the reduction of Cr6+ to Cr3+. Moreover, PO43− reacts with Cr3+ to produce CrPO4 precipitation, thus reducing the free Cr3+ content. Finally, DFT calculations confirmed that a ternary system is formed between PO43−, OM, and Cr, and the binding energy is negative, which indicated that PO43− could co-stabilize Cr with OM.