Outer Oxide Layer Research Articles

Enhanced electrothermal efficiency of flexible graphene fabric Joule heaters with the aid of graphene oxide

Flexible fabric Joule heaters reflect a broad prospect due to their wearable substrate, low cost and easily compatibility to garment. In this paper, graphene-based fabric Joule heaters were fabricated with spraying coating method, the as-prepared fabrics possessed bilayer structure corresponding to the inner graphene/polyurethane layer and the outer graphene oxide layer respectively. The morphology and electrothermal performance of the fabric Joule heaters were accomplished by scanning electron microscopy and infrared camera. The outer graphene oxide could dramatically promote the electrothermal efficiency of the fabric Joule heaters with the steady-state temperature (162.6 °C) and the maximum heating rate (8.4 °C/s) under 10 V applied voltage, which was much higher than the heaters without GO layer with the steady-state temperature 70.1 °C and the maximum heating rate 2.61 °C/s. Therefore, the existence of GO layer was first demonstrated to observably enhance the electrothermal efficiency of flexible fabric Joule heaters.

Effects of W Addition on the Electrochemical Behaviour and Passive Film Properties of Fe-Based Amorphous Alloys in Acetic Acid Solution

The electrochemical behaviour and passive film properties of Fe–Cr–Mo–W–C–B–Y amorphous alloys in acetic acid solution were investigated. The potentiodynamic polarisation and Nyquist curves demonstrated that W addition significantly enhanced the corrosion resistance. Mott–Schottky plots and angle-resolved X-ray photoelectron spectra indicated that passive films with different W contents exhibited dipolar (p–n) semiconducting characteristics separated by flat-band potentials. The outer and inner oxide layers of the passive films were modified by reducing the acceptor and donor densities. Moreover, W addition favoured the formation of a thicker and more stable passive film to inhibit the dissolution of alloy elements.

Effect of Laser Shock Peening on Residual Stress, Microstructure and Hot Corrosion Behavior of Damage-Tolerant TC21 Titanium Alloy

The effect of laser shock peening (LSP) on the residual stress, microstructure and hot corrosion behavior of damage-tolerant TC21 alloy was investigated. The equilateral scanning pattern was adopted to obtain a large overlapping area for LSP. The hot corrosion experiments in a salt mixture composed of 75 wt.% Na2SO4 + 25 wt.% NaCl at 700 and 800 °C for a total exposure of 25 h were conducted. The results demonstrated that LSP caused a high compressive residual stress with a significant increase in micro-hardness and microstructural changes in the form of crystal defects, i.e., high density dislocations. The crystal defects promoted the outward diffusion of Al, Cr and Ti to form protective mixed oxides (Al, Cr, Ti)O2. These oxides improved the adhesion of outer oxide layer and hindered the inward diffusion of O, S and Cl, which was beneficial to improve hot corrosion resistance. The mass gains of LSP-treated specimens after hot corrosion at 700 and 800 °C were, respectively, 3.72 and 4.78 mg/cm2, which were much lower than those of untreated specimens (5.65 and 6.81 mg/cm2). Therefore, LSP leads to an improvement in resistance to hot corrosion of TC21 alloy.

Electrochemical study on the corrosion behaviors of 316 SS in HITEC molten salt at different temperatures

Corrosion of 316 SS in HITEC molten salt was investigated at 450 °C, 600 °C, and 680 °C by electrochemical methods including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. Corrosion products were investigated by XRD and SEM equipped with EDS. Corrosion of 316 SS in HITEC molten salt exhibited active corrosion characteristics at these temperatures. 316 SS exhibited resistance to corrosion in HITEC molten salt at 450 °C, suffered severe corrosion at 680 °C, and demonstrated discontinuously increasing corrosion rate from 600 °C to 680 °C. A thin layer of Cr2O3 and Fe-based oxides formed at 450 °C. At 600 °C and 680 °C three distinct oxide layers could be distinguished with a loose outer Fe-based oxide layer. After corrosion at 600 °C and 680 °C for 200 h, Na2O and K2O were detected in the HITEC molten salts, which caused the decomposition of the protective Cr2O3 layer and enhanced corrosion of 316 SS. EIS results show that the corrosion processes of 316 SS in molten salt at 450 °C and 600 °C are controlled by outward diffusion of metal ions and oxygen vacancies, while corrosion mechanism at 680 °C change to was inward diffusion of oxygen ions. The change in corrosion mechanism became more prominent when corrosion time was prolonged to 197 h.

Effect of Rhenium Addition on Hot Corrosion Resistance of Ni-Based Single Crystal Superalloys

Hot corrosion behavior of two Ni-based single crystal superalloys containing 0 Re and 2 wt pct Re in molten sodium sulfate (Na2SO4) at 900 °C in static air has been investigated using the “deposit recoat” method. It was shown that the incubation period of the Re-containing alloy was much longer than that of the Re-free alloy. It was because Re increased the stability of the chromium oxide (Cr2O3) scale, promoted the formation of nickel titanate (NiTiO3), and suppressed the diffusion of Ni. In addition, the overall corrosion kinetics of the Re-containing alloy followed a multistage parabolic law, compared with the Re-free alloy. This could be because Re increased the activity of chromium (Cr) and titanium (Ti), which resulted in the periodically opening and closure of the cracks in the outer oxide layer.

Fabrication and characterization of NiCoCrAlY coating deposited on nickel-based superalloy substrates

NiCoCrAlY coatings were prepared on GH3128 nickel-based superalloy substrates using radio-frequency magnetron sputter deposition. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy measurements showed different particle growth in the thin films after annealing the as-deposited coatings at various heat treatment conditions. In addition, X-ray diffraction data revealed that the annealed coatings showed in common only one main peak, which was assigned to theγ′-Ni3Al phase. Cross-sectional SEM analysis showed that NiCoCrAlY coating after heat treatment at 1100 °C in air for 3 h had five layers, including the oxide layer, outer layer, middle layer, substrate, and interdiffusion layer. Elemental line scanning confirmed that the oxide layer contained Al and O. The electrical resistance of the insulating films at high temperature was measured over the range of 400 °C–800 °C. Our results revealed that the use of non-oxidizing gas during pre-heat treatment contributed to improving the electrical insulation behavior of the films.

Stress corrosion cracking behavior of 310S in supercritical water with different oxygen concentrations

The effect of dissolved oxygen (DO) on the stress corrosion cracking (SCC) of 310S in supercritical water was investigated using slow-strain-rate tensile tests. The tensile properties, fracture morphology, and distribution of the chemical composition of the oxide were analyzed to evaluate the SCC susceptibility of 310S. The results showed that the rupture elongation decreased significantly as the degree of DO increased. A brittle fracture mode was observed on the fracture surface, and only intergranular cracking was observed on the surface of the gauge section, regardless of the DO. Cracks were widely distributed on the gauge surface near the fracture surface. Oxides were observed in the cracks with two-layered structures, i.e., a Cr-rich inner oxide layer and an Fe-rich outer oxide layer.

Dry storage of spent nuclear fuel and high active waste in Germany—Current situation and technical aspects on inventories integrity for a prolonged storage time

Abstract Licenses for the storage of spent nuclear fuel (SNF) and vitrified highly active waste in casks under dry conditions are limited to 40 years and have to be renewed for prolonged storage periods. If such a license renewal has to be expected since as in accordance with the new site selection procedure a final repository for spent fuel in Germany will not be available before the year 2050. For transport and possible unloading and loading in new casks for final storage, the integrity and the maintenance of the geometry of the cask's inventory is essential because the SNF rod cladding and the cladding of the vitrified highly active waste are stipulated as a barrier in the storage concept. For SNF, the cladding integrity is ensured currently by limiting the hoop stress and hoop strain as well as the maximum temperature to certain values for a 40-year storage period. For a prolonged storage period, other cladding degradation mechanisms such as inner and outer oxide layer formation, hydrogen pick up, irradiation damages in cladding material crystal structure, helium production from alpha decay, and long-term fission gas release may become leading effects driving degradation mechanisms that have to be discussed.

Influence of Water Vapor on Oxide Scale Morphology of Incoloy800HT at 850°C

This paper aims to report the influence of water vapor on thermal oxide scale grown on incoloy800HT at 850°C. Alloy was prepared in coupons with a surface finishing up to 1200 SiC abrasive paper. Oxidization was performed in tubular furnace at 850°C during 50 hours. The oxidizing gases were varied as a dry oxygen gas and a wet oxygen gas. Thermal oxide morphology was characterized by scanning electron microscopy (SEM). Oxide phases were identified by X-ray diffraction (XRD) and Raman spectroscopy techniques. The oxide multilayers were revealed in all the oxidized samples. Oxide spallation was obviously detected on the samples oxidized under a dry oxygen gas, whereas, the spallation was not detected on the samples oxidized under a wet oxygen gas. Moreover, by water vapor mixing gas, the alloy surface presented a finer oxide. XRD and Raman spectroscopy provided the coincident oxide identification results. The corundum oxide of (Fe,Cr)2O3and the spinel oxide of (Fe,Cr)3O4were identified as a typical thermal oxide, however, the oxides were different in stoichiometry. The existence of water vapor promoted a Cr2O3corundum oxide, whereas, a Fe3O4spinel oxide was hindered from the outer oxide layer. Hence, water vapor not only clearly influenced on oxide scale morphology but also affected on stoichiometry of (Fe,Cr)2O3and (Fe,Cr)3O4solid solution.

Control of the spontaneous formation of oxide overlayers on GaP nanowires grown by physical vapor deposition

Growth of gallium phosphide nanowires by vapor deposition of simple thermally evaporated inorganic precursors is generally accompanied by unintentional formation of thick oxide coating, which may compromise the optical and electrical properties of the nanowires. Controlling and eliminating this outer layer during thermal evaporation growth of GaP nanowires represents a barrier to simple and scalable preparation of this technologically important material. In this article, we systematically investigated the role of different parameters (temperature, hydrogen flow rate, and starting Ga/P ratio) in the synthesis of GaP nanowires, and mapped out the conditions for the growth of oxide-layer-free nanowires. Increase in temperature, hydrogen flow, and phosphorus concentration led to diminished oxide layer thickness and improved nanowire morphology. Long and straight nanowires with the near perfect stoichiometry and complete absence of oxide outer layer were obtained for 1050 °C, 100 sccm hydrogen flow rate, and Ga/P flux ratio of 0.5. In contrast to other reports, this work emphasizes the importance of introducing hydrogen flow and excess phosphorus, which provide for reducing environment and reduced rate of the reaction of Ga with O in the growth process, respectively. The ability to control dielectric medium around GaP NWs by controlling the formation of oxide overlayer was demonstrated by Raman spectroscopy. The results of this work demonstrate a full control of the multi-parameter space in the simple, inexpensive, and scalable synthesis of GaP NWs, and may provide a guideline for rational improvement of the growth conditions for other types of semiconductor nanowires.

Plasma-Sprayed LSM Protective Coating on Metallic Interconnect of SOFC

In this study, a (La0.8Sr0.2)0.98MnO3 protective layer was prepared on the C276, Crofer22 APU, SUS304, and SUS430 alloys by the atmospheric plasma spraying technique (APS). The oxidation behavior and electrical property of these metal alloys have been investigated isothermally at 800 °C in air for up to 300 h. Results showed that the ferritic steels transform into MnCr2O4 spinels and a Cr2O3 layer during isothermal oxidation. The C276 alloy formed NiCr2O4 and FeCr2O4 layers; these are protective and act as an effective barrier against chromium migration into the outer oxide layer, and the alloy demonstrated good oxidation resistance and a reasonable match to the coefficient of thermal expansion of the substrate and a low-oxide scale area-specific resistance. The ASR effects on the formation of oxide scale have been investigated, and the ASR of coated samples was below 0.024 Ω·cm2. It has good electrical conductivity for SOFC in long-term use.

Fretting wear behavior of zirconium alloy in B-Li water at 300 °C

The tangential fretting wear of three kinds of zirconium alloys tube mated with 304 stainless steel (SS) plate was investigated. The tests were conducted in an autoclave containing 300 °C pressurized B-Li water for tube-on-plate contact configuration. The worn surfaces were examined with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and 3D microscopy. The cross-section of wear scar was examined with transmission electron microscope (TEM). The results indicated that the dominant wear mechanism of zirconium alloys in this test condition was delamination and oxidation. The oxide layer on the fretted area consists of outer oxide layer composed of iron oxide and zirconium oxide and inner oxide layer composed of zirconium oxide.

Effects of iron content in Ni-Cr-xFe alloys and immersion time on the oxide films formed in a simulated PWR water environment

The iron content in Ni-Cr-xFe (x = 0–9 at.%) alloys strongly affected the properties of oxide films after 978 h of immersion in the simulated PWR primary water environment at 310 °C. Increasing the iron content in the alloys increased the amount of iron-bearing polyhedral spinel oxide particles in the outer oxide layer and increased the local oxidation penetrations into the alloy matrix from the chromium-rich inner oxide layer. The effects of iron content in the alloys on the oxide film properties after 500 h of immersion were less significant than those after 978 h. Iron content increased, and chromium content decreased, in the outer oxide layer with increasing iron content in the alloys. Increasing the immersion time facilitated the formation of the local oxidation penetrations along the matrix/film interface and the nickel-bearing spinel oxides in the outer oxide layer.

Optimisation of micro-arc oxidation electrolyte for fabrication of antibacterial coating on titanium

This study has been carried out to optimise the silver (Ag) content of the coating synthesised on commercially pure titanium (Cp–Ti, Grade 4) for biomedical applications by micro-arc oxidation (MAO) process. The MAO process has been conducted in electrolytes containing silver acetate (AgC2H3O2) at different concentrations between 0 and 0·002 mol L−1. When compared to the base electrolyte, coatings synthesised in ≥0·001 mol L−1 AgC2H3O2 added electrolytes exhibited an antibacterial efficiency of 99·98% against Staphylococcus aureus (S. aureus). Detailed examination revealed that the presence of 0·001 mol L−1 AgC2H3O2 in the electrolyte resulted in incorporation of 1·14 wt-% Ag into fabricated coating consisting mainly of outer hydroxyapatite (HA) and inner titanium oxide (TiO2) layers. In comparison to the Ag-free coating, 1·14 wt-% Ag in the coating lowered the proliferation of SAOS-2 cells, which still tended to grow at a relatively low rate with increasing culturing time.

Preparation of layered α-Al2O3/TiO2 composite coating by pack cementation process and subsequent thermochemical treatment

An Al-Ti coating deposited by pack cementation technique was subjected to the thermochemical treatment. The Al-Ti coating consisted of an outer AlTi-rich layer and an inner transition layer. After the direct oxidation treatment, the coating surface was covered by many particle agglomerations and the scale was porous. A 10μm thick oxide layer, which consisted of an outer TiO2 dominated layer and an inner α-Al2O3 dominated layer, had grown on top of the coating. The multilayer structure of this coating was “TiO2/α-Al2O3/transition layer”.In order to prepare a better α-Al2O3/TiO2 composite coating, nitridation pretreatment was added before oxidation treatment. The surface of resulting coating was compact and had no obvious particle agglomerations. The outer oxide layer, with a thickness of approximately 4μm, exhibited a bilayer structure with an outer α-Al2O3 dominated layer and an inner TiO2 dominated layer. The multilayer structure of this coating was “α-Al2O3/TiO2/transition layer”. Beneath the oxide scale, a thin and continuous nitride layer was present, slowing down oxygen transport into the coating significantly. The oxidation behavior of Al-Ti coating changed with the nitridation pretreatment, and a better α-Al2O3/TiO2 composite coating was obtained.

Early oxidation mechanism of austenitic stainless steel TP347H in supercritical water

The initial corrosion behaviour of austenitic TP347H exposed to supercritical water was investigated. The early oxidation can be categorized into three stages (rapid oxidation, transitory, and diffusion-controlled stages). Higher temperature resulted in a transformation towards the subsequent stage earlier and less oxidation resistance. Cr6+ species (likely CrO2(OH)2) were only generated on the surface in the very early phase, and then they gradually disappeared as the exposure time increased. Compared with a complete outer Fe-rich oxide layer, the formation priority of a continuous inner Cr-rich layer was observed at 540°C within 35h. A corresponding early oxidation process was determined.

Preparation of porous oxide layers by oxygen bubble templated anodic deposition followed by galvanic displacement

Galvanic displacement reactions between porous PbO2, prepared by oxygen bubble templated anodic deposition, and the low-valent cations Mn2+, Co2+ and Sn2+, have been studied using electrochemical methods, SEM-EDS and XPS. These reactions occur at open circuit by immersion of PbO2 coated electrodes in aqueous solutions of the cations and lead to the formation of outer oxide layers onto porous PbO2, whose composition and chemical nature have been assessed by XPS. The growth of the outer oxide layers has been studied by combining Evans’ diagrams with SEM-EDS and XPS results, as a function of experimental variables, such as the chemical nature of the cations, their concentration and the solution temperature. The combination of oxygen bubble templated anodic deposition and galvanic displacement provides a route for the preparation of porous low-conductivity electroactive oxides not attainable by direct electrodeposition. Deposition of Co3O4 thin layers by galvanic displacement results in a strong enhancement of the activity of porous PbO2 electrodes in the oxygen evolution reaction.

Anodization of Al(100), Al(111) and Al Alloy 6063 studied in situ with X-ray reflectivity and electrochemical impedance spectroscopy

We present results from the anodization of single crystal Al(100) and Al(111) surfaces and the aluminum alloy AA 6063 studied in situ by X-ray reflectivity and electrochemical impedance spectroscopy. We observe that the anodic oxide layer grows linearly with the anodization potential and that the thicknesses are similar for all samples. However, the thicknesses obtained from X-ray reflectivity are higher than that obtained from electrochemical impedance spectroscopy. We attribute the higher thicknesses to an outer porous oxide layer, which is not detected by electrochemical impedance spectroscopy. Both, electrochemical impedance spectroscopy and X-ray reflectivity suggests that a more heterogeneous and rough oxide is formed on AA 6063 due to the influence of the alloying elements and intermetallic particles during the growth.

Microstructural evolution and oxidation resistance of T92 boiler tube steel upon long-term supercritical steam test

High-chromium heat-resistant steels are widely used as key materials to improve the condition of steam pressure and temperature in the boiler tube system of the supercritical power plants. Material-related failures in boiler system have occurred mainly due to internal oxidation and corrosion. For the stable maintenance of boilers, determination of the corrosion mechanism, microstructural evolution and data collection pertaining to the oxidation is essential. In this study, the supercritical steam test was carried out on T92 steel at the temperatures of 600/650/700°C for 10,000/15,000/20,000h. The matrix of steels and the oxide scale layers were investigated after steam treatment using scanning electron microscopy, back-scattered electron microscopy and electron backscattered diffraction. The oxide scale layers from the outer surface to the matrix consisted of: i) an outer layer of nanoscale iron oxide (Fe2O3); ii) inner layer of microscale crystals of Fe3O4, and iii) a mixture of chromium oxide and chromite (Cr2O3 and FeCr2O4) adjacent to the matrix.

Nano-Channels Early Formation Investigation on Stainless Steel 316Ti after Immersion in Molten Pb-Bi

Development of fuel cladding and structural materials in Pb-Bi environment, especially at high temperature, is a critical issue for the deployment of LFR (Lead alloy-cooled Fast Reactor) and ADS (Accelerator Driven Transmutation System). This is because of the corrosive characteristic of Pb-Bi to metals as constituent materials of fuel cladding and structural of the reactors. Corrosion test of a high-chromium austenitic stainless steel i.e. SS316Ti in molten Pb-Bi at 550 oC has been carried out for about 300 hours continuously. The characterization using SEM-EDS (Scanning Electron Microscopy - Energy Dispersive X-ray Spectroscopy) showed that an iron oxide as the outer layer and a chromium oxide as the inner layer on the surface of the specimen were formed which in general have protected the steel specimen from corrosion and dissolution attack of Pb-Bi. However, small amount of Pb could penetrate into the iron oxide layer through ultra-narrow channels. Atomic Force Microscopy (AFM) with Electrostatic Force Microscopy (EFM) mode was employed to investigate the phenomena of the nano-channels early formation. The results of the nano-scale investigation showed clearly the formation of the channels at the early time of corrosion as the way of Pb to penetrate into iron oxide outer layer.