Oxidation Of Cr Research Articles

Early corrosion behavior of 35CrMo steel for high-strength bolt in simulated shallow and deep sea environments

Early corrosion behavior of 35CrMo low-alloy steel for high-strength bolt in the simulated shallow seawater and deep-sea environment was studied using mass loss measurement, electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), Raman spectrometer, X-ray photoelectron spectrometer (XPS) and 3D microscope. The results show that the corrosion of 35CrMo steel initiates mainly at the steel matrix around the inclusions of CaO–MgO–Al2O3, (Ca, Mn)S and their mixed composite. The corrosion rate in the simulated deep-sea environment is lower than that in the shallow seawater, which is declined with immersion time. The corrosion products formed in the two environments are composed of γ-FeOOH, α-FeOOH, Fe3O4 and little Cr(III) oxide or/and hydroxide, but the rust layer produced in the deep-sea environment is thin and uniform. The steel has pits in a shallow dish shape and tends to develop uniform corrosion in the deep-sea environment.

Effect of rare earth La2O3 on the microstructure and corrosion resistance of CoCrFeNiMoSi high entropy alloys

This study systematically investigated the effect of incorporating trace amounts of La2O3 into the CoCrFeNiMoSi high-entropy alloy (HEA) system on its corrosion resistance in neutral chloride ionic salt solutions. The CoCrFeNiMoSi HEA coatings and CoCrFeNiMoSi-0.5 % La2O3 HEA coatings were prepared using the laser melting technique. The microstructure, phase composition, and electrochemical corrosion performance were characterized using X-ray diffraction, scanning electron microscopy, electrochemical workstation, and X-ray photoelectron spectroscopy. The experimental results revealed that the microstructure primarily comprised face-centered cubic structure (FCC), body-centered cubic structure (BCC), and Si-rich intermetallic compounds. Notably, the addition of La2O3 facilitated the formation of the FCC phase. Electrochemical tests demonstrated that the CoCrFeNiMoSi-0.5 % La2O3 HEA coating exhibited an elevated corrosion potential and a reduced self-corrosion current density when compared to the CoCrFeNiMoSi HEA coating. The X-ray photoelectron spectroscopy (XPS) data revealed a notable increase in the Cr content and oxide content of the passive film when La2O3 was present in the HEA. This resulted in the formation of a more protective passivated film. Consequently, the potential utilization of rare earth lanthanum oxide to enhance the corrosion resistance of HEA coatings is promising.

Model development for oxidation and degradation behavior of accident tolerant Cr coating on Zr alloy cladding under high temperature steam atmosphere

The oxidation and degradation mechanisms of Cr coating on Zr alloy cladding under high temperature steam atmosphere are summarized. Six basic phenomena are considered for model development, including: oxidation of Cr, growth of ZrCr2, formation of Zr(Cr) solid solution, outward diffusion of Zr, reduction of Cr2O3, and volatilization of Cr2O3. The mathematical analysis model is established and validated, demonstrating its good capacity to predict the increase and subsequent decrease in the thickness of Cr2O3 scale, as well as the thickness changes in other individual layers. The developed model is applied in the analysis of the structure evolution of Cr coating under steady state, transient heating conditions and postulated accident scenarios.

Oxidation resistance of Cr-modified MoSi2 composites at high temperature

Alloying is an important measure to improve the oxidation resistance of MoSi2. Cr-modified MoSi2 composites were prepared by hot pressing sintering in this study, and their oxidation behaviors at different temperatures were examined by XRD, SEM and EDS. The results show that the MoSi2 matrix forms CrSi2 compound with the increase of Cr content, which is conductive to the formation of surface oxide scale. Besides, the microstructure study shows that the modified element Cr promotes the formation of SiO2 protective scale at 500 °C–900 °C. When oxidized at 1300 °C, Cr2O3 in the form of Cr oxidation dissolved in MoSi2 is dispersed in the SiO2 oxide scale. The high temperature oxidation resistance of MoSi2 composites is improved by adding Cr modified element.

Investigations of oxidation behavior and establishment of life-cycle model during the steam oxidation of Cr-coated Zry-4 at 1200 °C

The isothermal steam oxidation behaviors of Cr-coated and non-coated Zry-4 cladding at 1200 °C have been investigated systematically. The oxidation of Cr coating is divided into three stages based on the different oxidation kinetics. First is protective stage where a thick Cr2O3 layer is gradually formed on the outer surface. Second is transition stage where the coating failure occurs because ZrO2 short circuit paths accelerates the reduction of Cr2O3. The O2- of Cr2O3 is transferred inward and dissolves in the substrate as α-Zr(O). The last is the non-protective stage where the Cr coating is oxidized again and substrate is oxidized as well. The oxidation of substrate presents three periods based on the different oxidation rate, which are accelerated period, normal period and abnormal period. Intermetallic precipitates derived from inward-diffusion of Cr make the oxidation kinetics be logarithmic law in the abnormal period by restraining the oxygen diffusion and t-ZrO2 → m-ZrO2 transformation. In this study, we find that the Cr coatings has important effect in restraining Zry-4 oxidation even after Cr has been completely consumed.

Effect of grain refinement on high temperature steam oxidation of an FeCrAl alloy

FeCrAl alloys are promising candidates to replace Zr alloys as fuel cladding materials in nuclear light-water reactors. Grain refinement has been indicated to improve irradiation resistance. To enhance corrosion resistance as well, the effects of grain refinement on steam corrosion behavior were investigated in this work. Samples of Kanthal D alloy (Fe-21Cr-5Al) with two different grain sizes (coarse-grained and ultrafine-grained) were exposed to steam at 1200 °C for 2 hrs. Results indicate improved steam corrosion resistance in ultrafine-grained Kanthal D with formation of a thinner protective Al oxide layer and the presence of a thin underlying Cr oxide layer.

Redox regulation of layered Fe-Cr hydroxides during Cr(VI) reduction by sulfate green rust: The overlooked electron conductivity and reactivity of Fe(III)-Cr(III) residues

In this study, unique layered Fe(III)-Cr(III) oxides were formed during the Cr(VI) removal by sulfate green rust (GRSO4). Solid state characterization confirmed that the hexagonal plate and layered structure of GRSO4 remained after fast reduction of Cr(VI). Then, sodium dithionite was further employed to reduce the Fe(III) to Fe(II) in these layered Fe(III)-Cr(III) oxides, enabling stable Fe(II)/Fe(III) cycle in the hexagonal plates. As the Fe(II) content increased in the layered Fe(III)-Cr(III) oxides, there was no significant Fe(II) and Cr(III) release into the solution and the GR-like plate remained intact, indicating that this hexagonal layer Fe-Cr oxides has stable structure during the reduction of Fe(III). Interestingly, better crystallinity was observed at higher Fe(III) reduction rate, and the layered metal oxide structure was confirmed with a basal spacing of 0.935 nm when ∼ 100 % of Fe(III) was reduced to Fe(II). Multiple treatment cycles experiment demonstrated that Cr(III) loading on the layered Fe(III)-Cr(III) oxides could be increased in the following 6 runs of Fe(II)/Fe(III) redox cycle. This study provides new evidence to confirm the electron conductivity and redox reactivity of the Fe(III)-Cr(III) residues under ambient condition, which might help to formulate novel strategies for elimination of Cr(VI) or related heavy metals.

Effect of oxide interactions on chromium speciation transformation during simulated municipal solid waste incineration

Chromium released during municipal solid waste incineration (MSWI) is toxic and carcinogenic. The removal of chromium from simulated MSWI flue gas by four sorbents (CaO, bamboo charcoal (BC), powdered activated carbon (PAC), and Al2O3) and the effects of four oxides (SiO2, Al2O3, Fe2O3, and CaO) on chromium speciation transformation were investigated. The results showed that the removal rates of total Cr by the four sorbents were Al2O3 < CaO < PAC < BC, while the removal rates of Cr(VI) by the four sorbents were Al2O3 < PAC < BC < CaO. CaO had a strong oxidizing effect on Cr(III), while BC and PAC had a better-reducing effect on Cr(VI). SiO2 was better for the reduction of Na2CrO4 and K2CrO4 above 1000°C due to its strong acidity, and the addition of CaO significantly inhibited the reduction of Cr(VI). MgCrO4 decomposed above 700°C to form MgCr2O4, and the reaction between MgCrO4 and oxides also existed in the form of a more stable trivalent spinel. Furthermore, when investigating the effect of oxides on the oxidation of Cr(III) in CrCl3, it was discovered that CaO promoted the conversion of Cr(III) to Cr(VI), while the presence of chlorine caused chromium to exist in the form of Cr(V), and increasing the content of CaO and extending the heating time facilitated the oxidation of Cr(III). In addition, silicate, aluminate, and ferrite were generated after the addition of SiO2, Al2O3, and Fe2O3, which reduced the alkalinity of CaO and had an important role in inhibiting the oxidation of Cr(III). The acidic oxides can not only promote the reduction of Cr(VI) but also have an inhibitory effect on the oxidation of Cr(III) ascribed to alkali metals/alkaline earth metals, and the proportion of acidic oxides can be increased moderately to reduce the generation of harmful substances in the hazardous solid waste heat treatment.

Effect of Predeformation on the High‐Temperature Oxidation Behavior of 436 Stainless Steel

The effect of a 30% predeformation treatment on the high‐temperature oxidation behavior of 436 stainless steel is investigated by oxidizing it at 900 °C for 100 h in air. After 48 h of oxidation, the undeformed sample primarily exhibits bilayer structures consisting of Cr‐rich oxide and outer Fe oxide, while the predeformed sample predominantly displays a single‐layer structure composed of Cr oxide. The oxidation mechanisms of predeformed and undeformed samples are clarified from the point of recrystallization through oxidation kinetic curves, surface and section micrographs, X‐ray diffraction (XRD), electron probe microanalyzer (EPMA), scanning electron microscope (SEM), energy‐dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), and line scanning characterization methods. The results show that after 1 h of oxidation, the grain size of the undeformed sample is 34.6 μm, and the grain size of the predeformed sample is 16.0 μm. This reduces the critical content required for the generation of protective Cr oxides and provides more diffusion channels for Cr elements, resulting in the formation of a greater amount of Cr oxides. Cr oxides can inhibit the formation of Fe and Mn oxides, thus delaying the onset of breakaway oxidation and improving its resistance to oxidation.

Selective oxidation and nickel enrichment hinders the repassivation kinetics of multi-principal element alloy surfaces

Robust and sustained corrosion resistance in multi-principal element alloys (MPEA) requires rapid repassivation, i.e. regrowth of the passive layer once it is damaged or destroyed at the surface. In this study, we show that the repassivation of Al0.1CrCoFeNi MPEA in 0.6 M NaCl solution is hindered at pH of ∼ 2.4 - 6.8 due to the formation of a Ni-enriched subsurface layer as a result of selective oxidation and dissolution of several principal elements, which can be fully restored at pH of ∼ 14 from the oxidation of all principal elements. Specifically, surface characterization via X-ray photoelectron spectroscopy (XPS), high-angle annular dark-field (HAADF) imaging in scanning transmission electron microscopy (STEM), and atom probe tomography (APT), are coupled with density functional theory (DFT) calculations to determine surface composition, oxidation state, and electron work function to uncover the structural origin of the pH-dependent repassivation mechanisms. It was found that selective oxidation of Cr, Co, and Fe in the acidic to neutral solutions altered the surface composition to be significantly enriched in Ni as compared to the bulk. Once the original passive film is destroyed locally by either pitting or tribocorrosion, this altered surface composition exhibited a much poorer repassivation capability due to the increased electron work function and reduced surface reactivity at higher Ni concentration. These understandings could shed light on the future compositional design of non-equiatomic MPEAs towards sustained repassivation and corrosion resistance over a wide pH range.

Interaction between chromite and Mn(II/IV) under anoxic, oxic and anoxic-oxic conditions: Dissolution, oxidation and pH dependence

Chromite oxidative dissolution has been recognized as an important process leading to elevated Cr(VI) in soil and groundwater. Under natural conditions, direct oxidation of Cr(III) by O2 is very unfavorable, and a critical determinant of Cr(VI) generation in soil and groundwater is the interaction between chromite and Mn(II) or Mn(III/IV) oxides. Here, the effects of Mn(II) or Mn(IV) on the oxidative dissolution of chromite were investigated at pH values of 5, 7 and 9 during anoxic, oxic and anoxic-oxic processes. The results showed that the direct oxidation of Cr(III) by O2 was slow in aqueous-phase system, while the Mn oxides in chromite could oxidize dissolved Cr(III). The added Mn(II) can be catalytically oxidized to MnOOH on the chromite surface only under alkaline oxidation conditions, and the catalytic efficiency is slow, which has less effect on chromite oxidative dissolution. Compared with the direct oxidation of O2 and catalytic oxidation of Mn(II), the synthesized biogenic Mn oxides drove the oxidative dissolution of chromite to release more Cr(VI) and were the main threat to the long-term stability of chromite in the environment. Overall, both acidic and alkaline environments are favorable to the catalytic oxidation of chromite by O2, Mn(II) and δ-MnO2, while neutral conditions are favorable to the long-term stability of chromite. These above processes may occur in soils and sediments with redox fluctuations (e.g., rice paddies, river floodplains, wetlands, and peatlands), and the presence of Mn(II) and Mn(III/IV) may play an important role in the oxidation and mobilization of Cr(III), leading to elevated Cr(VI) levels in soils and groundwater.

Kinetics of the Temperature-Dependent eaq - and ⋅OH Radical Reactions with Cr(III) Ions in Aqueous Solutions.

The reactivity of chromium(III) species with the major oxidizing and reducing radiolysis products of water was investigated in aqueous solutions at temperatures up to 150 °C. The reaction between the hydrated electron (eaq - ) and Cr(III) species showed a positive temperature dependence over this temperature range. The reaction was also studied in pH 2.5 and 3.5 solutions for the first time. This work also studied the reaction between acidic Cr(III) species and the hydroxyl radical (⋅OH). It was found that Cr3+ did not react significantly with the ⋅OH radical, but the first hydrolysis species, Cr(OH)2+ , did with a rate coefficient of k= (7.2±0.3)×108 M-1 s-1 at 25 °C. The oxidation of Cr(OH)2+ by the ⋅OH radical formed an absorbing product species that ultimately oxidized to give Cr(VI). These newly measured reaction rates allow for the development of improved models of aqueous chromium speciation for the effective remediation of liquid high-level nuclear waste via vitrification processes.

Effects of He and Ni ion irradiation on the ZrO2/Cr composite coating on Zr1Nb alloy

ZrO2/Cr composite coating has been irradiated by 2.8 MeV He and 38 MeV Ni ions at 350 °C. Similar extents of grain-growth and partial healing of microcracks have been identified in the ZrO2 layer of both irradiated samples. The monoclinic to tetragonal transformation of ZrO2 and the formation of an amorphous (Zr,Cr)O phase at the ZrO2/Cr interface is identified in the Ni-irradiated sample. Large amounts of dislocation loops and small cavities are observed in the Cr layer while significantly less loops are found in the ZrO2 layer in both samples. The irradiation processes also cause significant growth of surface Cr oxides.

Effect of pretreatment on the interfacial bonding strength between modified polyolefin and aluminum foil treated with trivalent chromium chemical conversion: Experimental, quantum chemical calculations and molecular dynamics simulation

Good adhesive strength and joint durability are necessary for aluminum (Al)/polymer laminated film for lithium-ion battery packages. A pretreatment method involving alkaline degreasing followed by acid pickling was employed to treat the Al foil surface to facilitate the deposition of trivalent chromium [Cr(III)] conversion coating on the surface. The adhesive strength between propylene-based elastomer grafted glycidyl methacrylate (GMA-g-PBE) and Cr(III)-converted Al foil pretreated with 0.5 M HF pickling was 51% higher than Cr(III)-converted Al foil without pretreatment. Moreover, the effect of different pretreatments on the interfacial bond strength of GMA-g-PBE/Cr(III)-converted Al foil was studied from macroscopic to microscopic perspectives. The results indicate that the pretreatment of 0.5 M HF pickling results in complete coverage of the Cr(III) compounds and the formation of a thicker Cr(III) conversion coating on the Al foil compared to other pretreatments. A higher content of Cr(III) compounds results in an increased surface free energy of Cr(III)-converted Al foil, improving the wettability of adhesive melt on the substrate and enhancing the adhesion strength. Moreover, quantum chemical calculations and molecular dynamics simulations were used to investigate the interface bonding mechanism, and the results indicate that GMA in the adhesive exhibits a higher binding energy with Cr(III) oxides and hydroxides compared to Al oxides and hydroxides.

Dissolved organic carbon diminishes manganese oxide-driven oxidation of chromium

Oxidation of chromium (Cr)-bearing minerals by manganese (Mn) oxides is viewed as the dominant mechanism controlling geogenic production of Cr(VI) and its contamination of groundwater. This process may be modulated by other chemical constituents found in the natural environment, but such confounding factors have not been quantified. Here, we evaluated the mechanism of Cr(III) oxidation by mixed-valence Mn oxide in the presence of citric and gallic acids, two natural organic matter (NOM) constituents commonly found in the soil environment. Incubation experiments showed that each organic acid enhanced solubilization of Cr(III) and Mn over controls without organic addition but increasing organic acid concentration decreased production of Cr(VI), with approximately 8.5 times less Cr(VI) produced in the citric acid than gallic acid experiments. X-ray absorption spectroscopy showed that negligible Cr(VI) was present in solid-phase reaction products, regardless of treatment. Geochemical modeling revealed that in the citric acid experiments, unprotonated Cr(III)-citrate was the dominant organo-metallic complex in solution, while (CrOH)2+ distribution positively correlated with concentrations of Cr(VI) produced. Collectively, these results illustrate how NOM can modify expected chemical pathways driving Cr cycling, and such mechanistic information should be better integrated into models predicting Cr redox dynamics and availability in the environment.

Cross-sectional transmission electron microscopy studies of Si coated heat-treated stainless steel with improved optical absorption for solar thermal applications

Heat-treatment of stainless steel substrate is reported to improve absorptance due to an increase in roughness along with the formation of surface oxides. Depositing a thin Si layer on stainless steel prior to the heat-treatment results in the formation of islands of nano-micrometer sizes with gaps between them which helps in improving absorptance. Cross-sectional Transmission Electron Microscopy studies are required to understand the development of the surface structures and improved optical properties. The cross-sectional bright- and dark-field Transmission Electron Microscopy images showed the presence of a bright Si-rich region where small nanocrystalline islands were embedded. On top of this layer, larger nanocrystalline protrusions were observed. Energy Dispersive X-ray analysis and Selected Area Diffraction data showed that the substrate elements diffuse out through the Si layer forming smaller islands of various Fe-Cr-Mn-based silicides in the Si layer and islands of Fe, Cr, and Mn-based silicides and oxides at the surface. The High-Resolution Transmission Electron Microscopy images showed that the islands are nanocrystalline in nature and are embedded in a Si-rich amorphous region. The absence of O in the Si-rich layer in SAD data was observed. Also, from the top layer EDX data, Fe peak intensity was found to be higher than Cr peak intensity. These observations indicate that the presence of the Si layer helps in reducing the inward diffusion of O into the substrate, thus preventing internal oxidation. Also, this Si layer prevents the Cr from forming a continuous Cr oxide layer and instead forms mixed oxide/silicide islands which enhance optical properties. Similar studies in the heat-treated substrate without a Si layer showed the presence of sharp-angled nanocrystalline structures of Fe-Cr-based oxides and smooth-angled nanocrystalline structures of Fe-Mn-based oxides on the surface.

Vibratory polishing effects on passivity of 304L stainless steel surfaces

ABSTRACT Surface spectroscopy analysis and electrochemistry were applied to study the effects of surface preparation by vibratory polishing on the passivity of stainless steel 304L surfaces. Compared to grinding by traditional mechanical polishing, vibratory polishing promotes the enrichment of Cr(III) oxide and hydroxide species in the duplex chemical structure of the surface native oxide film by enhancing selective Fe oxidation and dissolution. As a result, spontaneous passivity, tested in aggressive sulphuric acid electrolyte, is enhanced. However, in the absence of enrichment in Mo(IV) and Mo(VI) species in the passive film, the Cr enrichment does not enhance passivity upon anodic polarisation nor increase the resistance to Cl-induced passivity breakdown and initiation of localised corrosion in accelerated testing conditions. The results provide comprehensive insight into the mechanisms underlying the passivity enhancement of stainless steel by surface engineering.

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.

Redox Behavior of Chromium in the Reduction, Coagulation, and Biotic Filtration (RCbF) Drinking Water Treatment—A Pilot Study

The chromium (Cr) limit values are currently tightened to 25 μg L−1 (EU), 5 μg L−1 (Germany), and possibly 10 μg L−1 Cr(VI) (California). The combined process of chemical reduction, coagulation, and biotic filtration (RCbF) efficiently removes Cr(VI) in drinking water. In this study, redox-active substances (O2, NO3−, Fe2+, MnO2) were investigated concerning their effect on the RCbF process. The experiments were performed at two-stage pilot waterworks for biological iron and manganese removal. O2 or NO3− as oxidants affected the RCbF process, neither by consumption of the reductant Fe(II) nor by re-oxidation of already formed Cr(III) in the supernatant of the filter bed. However, the oxidation of Cr(III) by O2 to Cr(VI) with MnO2 as a mediator was identified as potential risk for Cr breakthrough. Up to one third of the initial Cr(III) concentration was oxidized to Cr(VI) in the second filter bed within a contact time of only 5 min. The kinetically relevant mechanism seemed to be the formation of Cr(III)Fe(III)-hydroxides and not the reduction of Cr(VI) by Fe(II). Further, the mixing of Cr(VI) containing raw water with Fe(II) containing groundwater was determined as a chemical-free alternative for the RCbF process, depending on the resulting Fe(II) concentration after mixing.

Preparation and properties of nitride multilayer coating modified stainless steel as bipolar plates for proton exchange membrane fuel cells

Nitride multilayer coating is prepared on 316L stainless steel (SS) by employing multi-arc ion planting to enhance the corrosion resistance, interfacial electrical conductivity, and hydrophobicity. The electrochemical results show that the coating can effectively inhibit the interface reactions. The icorr of the coated 316L SS was 0.04 μA cm−2, which was significantly lower than that of the substrate (54.7 μA cm−2). The protection efficiency (Pi) of the multilayer nitride coating was 99.93%. After 9 hours of PP, the current density of the coated 316L SS stabilized at 0.085 μA cm−2. The typical ICR value of the coated 316L SS slightly increased from 8.6 mΩ cm2 to 12.6 mΩ cm2. The XPS results demonstrated that the significantly increased ICRs (114.7–200.4 mΩ cm2) of the 316L SS substrate were mainly attributed to the increased proportion of Cr oxide, while the oxide content of the coating was still relatively small. The change in surface morphology and current transient events are further discussed.