Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.     

Breakdown of the protective oxide on 11 % Cr steel at high temperature in the presence of water vapor and oxygen,the influence of chromium vaporization

Abstract

We report on the effect of water vapor and oxygen on the oxidation of a ferritic/martensitic 11 % Cr steel (CrMoV11 1). The influence of pH₂O, exposure time, gas velocity and temperature was investigated. The samples were exposed to dry O₂, O₂+10 or 40 % H₂O for up to 336 hours. Total pressure was 1 atm (1.02 × 10⁵ Pa). The gas velocity was between 0.05 and 10 cm/s while temperature was in the range 450–700°C. The samples are investigated by thermogravimetry, GI-XRD, SEM/EDX, GDOES, FIB and TEM/EDX. Oxidation is strongly affected by the vaporization of CrO₂(OH)₂ in H₂O/O₂ environment. The mechanism of vaporization of CrO₂(OH)₂ from a Cr₂O₃ surface is modelled by DFT calculations. In the absence of chromium vaporization the alloy forms a protective oxide consisting of a corundum-type solid solution (Fe_1–xCr_x)₂O₃. The vaporization of chromium tends to deplete the oxide in chromium. In some cases the oxide remains protective in spite of chromium depletion while in other cases there is a transition to breakaway oxidation. In the latter case a thick layered scale forms, consisting of an outer hematite part and an inner iron-chromium spinel. Oxidation behavior in an O₂+H₂O environment is to a large extent determined by the ability of the metallic substrate to supply the oxide with chromium by diffusion in order to compensate for the losses by vaporization. The corrosivity of the environment increases with the concentration of water vapor and oxygen, with the gas velocity and with temperature.     

Kinetic and metallographic study of oxidation at high temperature of cast Ni 25Cr alloy in water vapour rich air

Abstract

The high temperature oxidation behaviour of a Ni–25 wt-%Cr alloy in air enriched with water vapour (180 mbars H₂O) was studied at 1000, 1100, 1200 and 1300°C. The oxidised samples were characterised by X-ray diffraction, electron microscopy and wavelength dispersion spectroscopy. The obtained data were compared to the ones earlier obtained for the same alloy oxidised in dry air. Water vapour globally induced at all temperatures a decrease of the parabolic constant Kp and an increase in the chromia volatilisation constant Kv. The oxide scales do not present morphologic difference between the two atmospheres. After oxidation in humidified air the scale thickness is thinner and the Cr depleted depth is lower than in dry air.     

Electrochemical behaviour of LiM y Mn2−y O4 (M = Cu, Cr; 0 ≤y ≤ 0.4)

Spinel lithium manganese oxide, LiMn₂O₄, is beset with problems of capacity fade upon repeated cycling. The loss in capacity upon cycling is attributable to Jahn-Teller distortion and manganese dissolution in the electrolyte in the charged state. One way to circumvent this capacity fade is to introduce other 3d bdtransition metal ions in the LiMn₂O₄ lattice. In this paper, we report on the effect of partial substitution of manganese in the LiMn₂O₄ phase with copper (II) and chromium (III) ions. It has been shown that the higher octahedral stabilization energy of trivalent chromium imparts greater structural stability to chromium-doped LiMn₂O₄ spinels. Both copper and chromium reduce the capacity of the spinel in the 4 V region. In terms of its good reversible capacity and ability to sustain cycling with minimal capacity fade, LiCr_0.1Mn_1.9O₄ may be considered as a potential cathode material for lithium rechargeable cells.     

Oxidation of nickel-based alloys in dry and water vapour containing air

Abstract

Ni-based alloys are widely used in power generation and their oxidation behaviour in the uncoated state is of interest, especially the impact of water vapour in the air on the formation of a protective underlying alumina scale. High-temperature X-ray diffraction was applied to investigate in situ the oxide scale formation in the initial state on the alumina formers CM247DS and CMSX4 in comparison to the chromia formers IN792 and SCA425+. Post-oxidation analysis was performed by field emission scanning electron microscopy. The samples were oxidised for 100 h at 950°C in dry air and in air with 20% relative moisture in the high temperature device on the X-ray diffractometer. In dry air, CM247 and CMSX-4 form α-Al₂O₃ from the beginning simultaneously with spinels and nickel oxide. The alumina scale underlies the spinels which spall partially on cooling. When adding water vapour, the same oxides were formed simultaneously resulting in a comparable oxide scale. IN792 forms in dry air mainly NiAl₂O₄ and transitionally CrO₂ under laid by an alumina scale. With water vapour, Cr₂O₃ forms and the underlying alumina scale shows a non coherent dendritic structure. SCA425+ forms in dry air Cr₂O₃ and Cr containing mixed oxides and with water vapour a more coherent alumina scale than IN792.     

Microhardness studies on as-grown faces of NaClO3 and NaBrO3 crystals

Single crystals of NaClO₃ and NaBrO₃ are grown from their aqueous solutions at a constant temperature of 35°C by slow evaporation by using good quality seed crystals. Systematic microhardness studies are made on as-grown faces of these crystals at various loads. Typical cracks are observed at the corners of the impressions in NaClO₃ whereas in addition to the cracks at the corners microcracks also appeared in NaBrO₃ crystals around the impressions. The impressions formed in NaBrO₃ are not very clear as in NaClO₃, a possible mechanism for it is discussed. The work hardening index number(n) for both these crystals is around 1.6 suggesting that these are moderately harder samples. The hardness studies point out that NaBrO₃ is harder than NaClO₃ (ΔH ≈ 100 kg/mm ²,this could be due to strong inter ionic forces acting between Na-Br in NaBrO₃ crystals. Using Gilman’s empirical relation, hardness values are calculated from the values of elastic constants (C ₄₄) and are found to be close to the experimental results.     

Role of nickel in the oxidation of Fe–Cr–Ni alloys in air–water vapour atmospheres

Abstract

A series of experimental austenitic alloys has been produced in which the nickel content ranges from 14 to 43%, with constant levels of 20%Cr, 1%Mn and 0.5%Si. A combination of isothermal, discontinuous and cyclic oxidation testing has been used to elucidate the performance in dry air and in air with 10%, 45% or 62% water vapour at 700°C and 1000°C. Evaluation was by means of thermogravimetry, surface analysis with glow discharge optical emission spectroscopy and scanning electron microscopy.

Nickel is shown to have several roles: it accelerates the kinetics of chromia formation yet suppresses chromia spallation at 700°C. At 1000°C, it strongly decreases the breakaway oxidation and spalling associated with iron oxide formation. This effect is particularly marked in environments containing water vapour, where the material loss may be decreased 10-fold by an increase in the nickel content. Results correlate to thermodynamic and kinetic data which show nickel to increase the chromium activity and diffusivity in the alloy.     

On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr₂O₄ is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr₂O₃ particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.     

Effects of water vapour on the oxidation of a nickel-base 625 alloy between 900 and 1,100 °C

The effect of water vapour was studied on a nickel-based SY 625 alloy oxidized at 900, 1000 and 1100 °C under dry and wet conditions. It appears that H₂O has little effect on the oxidation rate and scale composition after 48 h. The outer scale is composed of chromia Cr₂O₃. At 900 and 1,000 °C, NbNi₄ and Ni₃Mo intermetallics are found at the oxide/alloy interface. At 1,100 °C, the scale is composed of an outer chromia scale and an internal CrNbO₄ subscale. At this temperature the oxide scale morphology differs between dry and wet conditions. Under dry conditions the oxide scale appears to be compact but the external part of the scale partially spalled of during cooling. The oxide scales formed under wet conditions show porosities spread inside the scale and the chromia grain size is smaller. At 1,100 °C scale spallation is observed under dry conditions due void accumulation in the middle part of the scale. Under wet conditions the uniform distribution of the porosities inside the scale leads to a better scale adherence.     

MOCVD-grown homoepitaxial YBa2Cu3Ox thin films and its multilayers

YBa₂Cu₃O _x (YBCO) films, Zn-doped YBCO (YBCO : Zn) films, and their bilayers have been epitaxially grown on SrTiO₃(100) and single-crystal YBCO(001) substrates by metalorganic chemical vapour deposition. The YBCO(001) films homoepitaxially grown on YBCO(001) substrates have flat surfaces on an atomic scale, and interfaces free from crystalline defects. We can systematically reduce the superconducting transition temperature (T _c) of YBCO : Zn films from 90 K to 37 K by increasing Zn concentration. The bilayers have a sharp distribution of Zn as evaluated fromT _c measurements of the upper YBCO films and depth profiles of secondary ion mass spectrometer, suggesting the possibility to form the homoepitaxial SNS (S, superconductor; N, normal metal) junction operatable between 40 K and 90 K.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

Novel method for doping nano TiO2 photocatalysts by chemical vapour deposition

Highly active photocatalytic Fe-doped nano TiO₂ was successfully synthesised by chemical vapour deposition (CVD) method using FeCl₃ as Fe source. CVD was carried out by evaporating FeCl₃ at 350°C in nitrogen flow during 30–90?min. The amount of Fe incorporated into TiO₂ framework is adjusted by the amount of FeCl₃ used and the evaporation time. The obtained sample was characterised by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), energy dispersive X-ray spectroscopy (EDS), UV-Vis, Fourier transform-infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Photocatalytic activities of the samples were tested in photocatalytic decomposition of 2-propanol in liquid phase using visible light instead of UV light irradiation. Non-doped TiO₂ and high Fe loading TiO₂ samples showed very low photocatalytic activity, whereas the low Fe loading TiO₂ sample exhibited high photocatalytic activity under visible light. The high photocatalytic activity of this sample was rationalised by the existence of defects (Ti–OH groups) as the active sites.     

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

Abstract

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O₂ and Ar–H₂–H₂O mixtures. It was found that in the low-pO₂ gases the oxide scales exhibited higher growth rates than in the high-pO₂ gases. The scales formed in the low-pO₂ gases showed substantially better adherence during cooling, than scales formed in the high-pO₂ gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O₂-H₂O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H₂O/O₂-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H₂O- or H₂-molecules or due to OH^? ions, cannot be derived with certainty from the present results.     

Characterization of Al rich oxide layers on austenitic stainless steels by fluorescence spectroscopy

Abstract

An Al rich oxide passivation technique has been developed to improve the corrosion resistance of the stainless steel to ozone added ultra-pure water. Fluorescence spectroscopy was applied to the study on the selective oxidation of aluminum containing austenitic stainless steel in low oxygen atmosphere at 1,353K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of Al resulted in the formation of thin oxide layers. The frequency shifts of fluorescence spectra show that the compressive stresses exist in the oxide layers as a result of the difference of the thermal expansion coefficients between substrate steels and α-Al₂O₃, and depends on the thickness of the oxide layers. It is confirmed that pure α-Al₂O₃ protective layers grown on the stainless steels, which remain stable and attached to the stainless steels in ozone added ultra-pure water. These act as a diffusion barrier and protect the stainless steels from the metal dissolution.     

On the vertical stacking in semiconducting WSe2 bilayers

The vertical stacking in semiconducting WSe₂ bilayers grown by physical vapour transport was studied using atomic resolution annular dark field imaging. Our results show that the most common geometry was consistent with AA′, the most stable configuration. However in some areas AB alignment was also observed, as expected due to the small energy difference between AA′ and AB. Additionally, two different rotational stacking orientations were observed, with rotation angles of 12 and 21°. These different vertical WSe₂ bilayers could provide a means of engineering electronic band structure for specific optoelectronic properties.     

Effect of Co-Mo catalyst preparation and CH4/H2 flow on carbon nanotube synthesis

Abstract

Supported Co-Mo catalysts with a given ratio of metals were prepared from polyoxomolybdate Mo₁₂O₂₈(μ₂-OH)₁₂{Со(H₂O)₃}₄ using impregnation and combustion methods. Effects of the type of catalyst and the ratio and flow of methane and hydrogen gases on the structure of carbon nanotubes (CNTs) synthesized by catalytic chemical vapor deposition (CCVD) method were studied using transmission electron microscopy and Raman spectroscopy. The catalyst prepared by combustion method yielded mainly individualized CNTs, while the CNTs were highly entangled or bundled when impregnation method was used. In both cases, addition of hydrogen to methane led to reduction of the CNT yield. The samples synthesized using two different catalysts and the same CH₄/H₂ ratio and flow of gases were tested in electrochemical capacitors. A higher specific surface area of the CNTs grown over impregnation-prepared catalyst caused a better performance at scan rates from 2 to 1000?mV/s.     

X-ray diffraction studies of NbTe<Subscript>2</Subscript> single crystal

NbTe₂ is a member of transition metal dichalcogenide (TMDC) group. Single crystals of niobium ditelluride (NbTe₂) have been grown by a chemical vapour transport technique using iodine as transporting agent. The composition of the grown crystals was confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and remaining structural characterization was also accomplished by X-ray diffraction (XRD) studies. Lattice parameters, volume and X-ray density have been carried out for the grown crystals. The particle size for a number of reflections has been calculated using Scherrer’s formula.     

Transport properties of MoSe x Te2−x (0 ≤x ≤ 2) single crystals

The layer type MoSe _x Te_2−x (0 ≤x ≤ 2) have been grown in single crystalline form by chemical vapour transport technique using bromine as the transporting agent. The electrical resistivity and Hall mobility perpendicular to thec-axis of the crystals were measured at room temperature. The variation of the Seeback coefficient with temperature was also investigated.     

Crystal growth and structural analysis of zirconium sulphoselenide single crystals

A series of zirconium sulphoselenide (ZrS _x Se_3−x , where x = 0, 0·5, 1, 1·5, 2, 2·5, 3) single crystals have been grown by chemical vapour transport technique using iodine as a transporting agent. The optimum condition for the growth of these crystals is given. The stoichiometry of the grown crystals were confirmed on the basis of energy dispersive analysis by X-ray (EDAX) and the structural characterization was accomplished by X-ray diffraction (XRD) studies. The crystals are found to possess monoclinic structure. The lattice parameters, volume, particle size and X-ray density have been carried out for these crystals. The effect of sulphur proportion on the lattice parameter, unit cell volume and X-ray density in the series of ZrS _x Se_3−x single crystals have been studied and found to decrease in all these parameters with rise in sulphur proportion. The grown crystals were examined under optical zoom microscope for their surface topography study. Hall effect measurements were carried out on grown crystals at room temperature. The negative value of Hall coefficient implies that these crystals are n-type in nature. The conductivity is found to decrease with increase of sulphur content in the ZrS _x Se_3−x series. The electrical resistivity parallel to c-axis as well as perpendicular to c-axis have been carried out in the temperature range 303–423 K. The results obtained are discussed in detail.     

Use of oxygen isotope to study the transport mechanism during high temperature oxide scale growth

Abstract

The examination of high temperature (HT) oxide scale growth mechanisms was performed using secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS), in conjunction with ¹⁶O₂/¹⁸O₂ HT oxidation experiments. Cr₂O₃, NiO, ZrO₂ and Al₂O₃ were studied because they constitute excellent representative thermally grown oxide scales: they grow by cationic diffusion (Cr₂O₃, NiO), anionic diffusion (ZrO₂) or mixed anionic-cationic diffusion (Al₂O₃). The oxidation tests were performed first in ¹⁶O₂ and subsequently in ¹⁸O₂ at several temperatures (600–1000°C for NiO, 600°C for ZrO₂, 1000°C for Cr₂O₃ and 1100°C for Al₂O₃). The oxygen isotope distribution observed by SIMS and SNMS profiles are discussed and related with the HT oxidation mechanisms proposed in the literature.