Sulphidation and oxidation of the Ni22Cr10Al1Y alloy in H2/H2S and SO2 atmospheres at high temperatures

The Ni22Cr10Al1Y alloy was exposed in H₂/H₂S gas mixture under the sulphur pressure 10^–3 and 1 Pa as well as in SO₂ at 1173 and 1273 K. At p_s = 1 Pa the sulphidation rate was relatively high and the reaction obeyed the linear rate law. Under these conditions a nickel/nickel sulphide eutectic was formed. At p_s = 10^–3 Pa nickel sulphides became unstable and the sulphidation rate was significantly lower. The reaction obeyed the parabolic rate law. The oxidation rate of the alloy in SO₂ was lower than that in any of the H₂/H₂S atmospheres. The sulphide scales formed during sulphidation in H₂/H₂S had complex microstructures and compositions, with sulphospinel and sulphide phases being present, e.g. NiCr₂S₄, Ni₃S₂, Cr_xS_y. As the temperature increased and the sulphur pressure decreased, these phases were replaced by the chromium-rich sulphide phase. Various oxides formed during oxidation of the alloy in SO₂.     

Investigation of strontium-doped La(Cr,Mn)O3 for solid oxide fuel cells

The (La, Sr) (Cr, Mn)O₃ system was investigated in an effort to develop an interconnect and cathode materials for solid oxide fuel cells. Sintering studies were done in air at temperatures below 1500°C. Significant improvements in densification were observed with substitution of 50 mol% Mn for chromium and a density of 95% theoretical was achieved with the substitution of 70 mol% Mn for chromium in the La(Cr, Mn)O₃ system. Electrical conductivity (d.c.) measurements were made as a function of temperature and oxygen activity. At 1000°C and 1 atm oxygen, the electrical conductivity ranged from 2.2–20 S cm^–1 for LaCr_0.8Mn_0.4O₃ and La_0.9Sr_0.1Cr_0.2Mn_0.8O₃, respectively. All of the compositions showed similar dependence of electrical conductivity on the oxygen activity. Dependence was small at high oxygen activities; as the oxygen activity decreased, a break in electrical conductivity at 10^–12 atm and 1000°C was observed, and then the electrical conductivity decreased asP _O2 ^1/4 . Sintering and electrical conductivity studies indicate that La_0.9Sr_0.1Cr_0.2Mn_0.8O₃ appears to be a candidate for solid oxide fuel cell applications.     

Cyclic hot corrosion of Haynes 230 alloy

Cyclic hot corrosion conducted on Haynes 230 at temperatures of 871 and 1093 C indicated that catastrophic corrosion occurred. The corrosion rate was related to the high content of tungsten and chromium in the alloy. The concept of basic and acid fluxing was applied to explain the dissolution of the protective film of Cr₂O₃ and volatile WO₃ by an Na₂SO₄-rich liquid due to the formation of Na₂CrO₄ and Na₂WO₄. As the basic melts were acidified by continuously consuming oxygen ions, plate-like crystals of Cr₂O₃ were precipitated on the free surface by conversion from Na₂CrO₄. Acid fluxing was achieved by the refractory oxide, WO₃, consuming oxygen ions. The presence of sulphur suppressed the diffusion of chromium outward to form protective Cr₂O₃. Internal chromium-rich sulphide particles were observed. It was suggested that at very lowP _O2, sulphur reacted with chromium to form CrS initially. As oxygen penetrated through the porous layer, the CrS was oxidized internally to Cr₂O₃.     

Phase Composition, Electrical Conductivity, and Stability of ZrO2–Sc2O3–Cr2O3 Solid Electrolytes

The phase composition, electrical conductivity, and structural and electrical stability of ZrO₂–Sc₂O₃–Cr₂O₃ solid electrolytes prepared by solid-state reactions involving three-step firing at 1350, 1850 (vacuum), and 1300°C were studied for compositions along two lines: x(0.91ZrO₂ + 0.09Sc₂O₃)–yCr₂O₃ (I) andx(0.89ZrO₂ + 0.11Sc₂O₃)–yCr₂O₃ (II), x + y = 1, y = 0–0.04. The results indicate that the ternary solid solutions withy= 0.01–0.02 retain a cubic structure in a broad temperature range, down to room temperature. This increases the low-temperature (<600°C) conductivity of the solid electrolytes, especially in system II. In both systems, Cr₂O₃ solubility is about 3 mol %. Stability tests at 900°C for 200 h reduce the conductivity of the solid electrolytes, particularly at the lower Sc₂O₃ content and in the presence of Cr₂O₃. The reduction in conductivity is due to the decomposition of the high-temperature tetragonal phase and the formation of a tetragonal phase with a low stabilizer content.     

Influence of ion implantation on the microstructure of oxide scales formed on a 20Cr/25Ni/Nb-stabilized stainless steel in carbon dioxide at 825° C

Specimens of a stainless steel (20%Cr, 25%Ni stabilized with niobium and also containing 0.9% Mn and 0.6% Si) implanted with lanthanum to a dose of 10¹⁷ ion cm^–2 , together with unimplanted specimens, have been oxidized in carbon dioxide at 825° C for times up to 9735 h. Transverse sections through the oxide scales formed on the respective specimens have been studied by analytical electron microscopy. After this exposure the scale on the unimplanted 20/25/Nb stainless steel consists of an outer, large-grained, spinel layer, a middle fine-grained Cr₂O₃ layer and an inner, discontinuous silicon rich, niobium and chromium bearing, amorphous layer. The main effects of the lanthanum implantation are to improve oxidation resistance and maintain scale adherence during thermal cycling. The microstructural changes in the scale formed on the lanthanum implanted 20/25/Nb steel include finer Cr₂O₃ oxide grains and an intermediate region between the outer spinel and inner Cr₂O₃ layers comprised of both oxides. The lanthanum concentrates in this region and appears to act as a marker due to its low diffusivity. Mechanisms of scale development on the 20/25/Nb stainless Red and the influence of lanthanum implantation are discussed.     

Optical absorption and EPR studies of borate glasses with PbCrO4 and Pb2CrO5 microcrystals

The optical absorption (in the 400–1000 nm region) and electron paramagnetic resonance (EPR) spectra of 50PbO-20Cr₂O₃-(30-y)B₂O₃-yAl₂O₃ glasses and those crystallized by suitable thermal treatments have been studied. The results are applied to account for the crystallization process, which revealed the crystalline products due to PbCrO₄ and Pb₂CrO₅, and crystallization yield was found to be drastically enhanced (by as much as 85 vol%) with Al₂O₃ additives. The addition of Al₂O₃ in the present glasses and/or the thermal treatments induce the optical absorption intensity in the visible region at the expense of the near infrared region and reflect a gradual decrease in the EPR linewidth, H, at g2. The chromium in these cases most likely exists in a thermodynamic equilibrium between Cr³⁺ and Cr⁶⁺ oxidation states and the Al₂O₃ favours the Cr³⁺Cr⁶⁺ nucleation transformation to accord with the above observations. In addition, the crystalline PbCrO₄ and Pb₂CrO₅ comprise CrO ₄ ^2– groups with the Cr⁶⁺ oxidation state of chromium. This presents a phenomenological correlation for a local symmetry similarity of the CrO ₄ ^2– groups in glass and the microcrystalline products and observation of an enhanced crystallization yield due to Pb₂CrO₅ crystallites in the glasses using Al₂O₃ additives. The chromium occupying the tetragonal CrO ₄ ^2– anion sites migrates rather easily from the glass (subject to the thermal treatment) to the growing nucleation sites of PbCrO₄/Pb₂CrO₅, whereas those Cr³⁺ usually occupying the octahedral sites, which are associated with characteristically large stabilization energy, hardly play a direct role in the crystallization.     

YbCl3–Li6P6O18–H2O System

The formation of solid phases and soluble complexes in the YbCl₃–Li₆P₆O₁₈–H₂O system was studied. Raising the Li₆P₆O₁₈ concentration from 0.01 to 0.05 mol/l at a fixed YbCl₃ concentration of 0.05 mol/l leads to Yb₂P₆O₁₈ · 16H₂O precipitation (solubility product, 7.2 × 10^–10). If Li₆P₆O₁₈ is present in excess, ytterbium cyclohexaphosphate dissolves owing to the formation of soluble complexes, primarily [YbP₆O₁₈]^3– and [Yb(P₆O₁₈)₂]^9–. The mole fractions and stability constants of ytterbium cyclohexaphosphate complexes were determined.     

Synthesis of hexagonal Al2−x Cr x O3 nanodisks by a thermal decomposition of mesoporous Cr4+:Al2O3 powders

Monodispersed hexagonal Al_2−x Cr _x O₃ nanodisks are synthesized through a reactive doping of Cr⁶⁺ cations in a hydrogenated mesoporous AlO(OH)·αH₂O powder followed by annealing at 1,200 °C in air. The reaction was carried out by a drop wise addition of an aqueous Cr⁶⁺ solution (0.5–1.0 M) to AlO(OH)·αH₂O, at room temperature. Al_2−x Cr _x O₃ nanostructure formation was controlled by the nucleation and growth from the intermediate amorphous mesoporous Cr⁴⁺:Al₂O₃ composites in the temperature range 400–1,000 °C. The nanodisks of ∼50 nm diameter and thickness of ∼16 nm is observed in the sample with x of 0.2 and similar nanodisks with a low dimension is observed at a higher value of x of 1.6 (after 2 h of heating at 1,200 °C). The Cr³⁺ ↔ Al³⁺ substitution, x ≤ 1.2, inhibits grain growth in small crystallites. The crystallites in x = 0.2 composition have 43 nm diameter while it is 15 nm in those with x = 1.2 composition.     

Interaction of Si3N4 with Cr

The interaction of Si₃N₄ with chromium was investigated under an argon or nitrogen atmosphere at temperatures from 773–1573 K. Reaction rates were determined by thermogravimetric analysis, and reaction products were examined by X-ray diffraction. Cr₂N, Cr₃Si, Cr₅Si₃ and CrSi were produced under argon, while reaction products under nitrogen were Cr₂N, CrN and Cr₃Si. Possible reactions between Si₃N₄ and chromium have been discussed thermodynamically. The initial rate obeyed a linear rate law. When chromium particles were coated with the reaction layer, the rate was described by a parabolic rate law. The reaction mechanism and the rate-determining step have been proposed.     

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.     

Physical and spectroscopic properties of multi-component Na2O–PbO–Bi2O3–SiO2 glass ceramics with Cr2O3 as nucleating agent

Transparent glass ceramics, synthesized from melt quenching followed by heat treatment, of the composition 10Na₂O–30PbO–10Bi₂O₃–(50 − x)SiO₂:xCr₂O₃ (mol%), where 0 ⩽ x ⩽ 0.5, were characterized with XRD, DTA, SEM and EDS. Physical and spectroscopic studies, viz., optical absorption, electron paramagnetic resonance (EPR), FTIR and Raman were investigated. The characterization of the host glass ceramic has revealed that the formation of a major phase of sodium silicate along with two minor phases such as lead silicate and bismuth oxide. By integrating Cr₂O₃ to the host glass additional crystal phases viz., NaCrO₂, Na₂Cr₂O₇ and Pb(CrO₄) which are the complexes of Cr³⁺ and Cr⁶⁺ ions were also developed. As the concentration of nucleating agent is increased, a part of the Cr⁶⁺ ions is found to reduce in to Cr³⁺ ions. Spectroscopic studies have revealed that with an increase in the concentration of Cr₂O₃ from 0.1 to 0.5 mol%, there is a gradual increase in the intensity of vibrational modes of various asymmetric structural units of silicate, bismuthate and chromate in the glass ceramic network at the expense of symmetrical structural units. The analysis of the results of these studies has indicated that in the samples containing higher concentration of Cr₂O₃, chromium ions exists predominantly in Cr³⁺ state and occupy the octahedral positions in glass ceramic matrix and such glass ceramic samples are suitable for lasing action.     

Metastable phase relation and phase equilibria in the Cr2O3-Fe2O3 system

In order to ascertain the metastable phase relation in the Cr₂O₃-Fe₂O₃ system, the existing phases were investigated by X-ray analysis using samples obtained by heating the coprecipitated powders for 1 h at 600–1000°C. There was a metastable two-phase region of Cr₂O₃-rich (CC) and Fe₂O₃-rich (FC) phases below about 940°C. Equilibrium state of 1:1 composition at 600–900°C was considered to be a single phase of the corundum solid solution. The metastable two-phase CC + FC region was suggested to appear probably due to the compositional inhomogeneity in the coprecipitated powders.     

Solubility,reactivity and nucleation effect of Cr2O3 in the CaO-MgO-Al2O3-SiO2 glassy system

The effect of Cr₂O₃ on some anorthite-diopside glass-ceramics has been investigated up to amounts of 5 mol%. The solubility in the glassy compositions analysed is total for the oxide, but for amounts higher than 0.5 mol%, an insoluble spinel form, MgCr₂O₄, precipitates. Ultraviolet-visible spectroscopy has proved to be the most sensitive technique to the presence of Cr(III) in a crystalline spinel site, followed by X-ray diffraction and scanning electron microscopy observations. Electron spin resonance and X-ray photoelectron spectroscopy techniques excluded any oxidation state, other than Cr³⁺. The influence of the transition cation on glass nucleation is that of an increasing bulk effect with chromium, and thus chromium-spinel, content. The magnesium content affects spinel formation, while heat treatments up to 1100 °C do not. The spinel formation influences the anorthite-diopside ratio in the glass-ceramic, with a large favour towards the pyroxene.     

Oxidation-reduction equilibria of vanadium in CaO-SiO2, CaO-Al2O3-SiO2 and CaO-MgO-SiO2 melts

A series of redox studies of vanadium have been carried out in CaO-SiO₂, CaO-MgO-SiO₂ and CaO-Al₂O₃-SiO₂ melts/slags equilibrated over oxygen partial pressures (pO₂) range 10^–2–10^–9 atm at 1600°C. V₂O₅ level was varied from 1–5 mol%. Three different melt basicities and alumina contents were investigated. Magnesia content was varied between 3.5 and 4.9 wt%. A newly developed analytical technique based upon electron paramagnetic resonance (EPR) spectroscopy was successfully applied to these melts. Two redox equilibria corresponding to V³⁺/V⁴⁺ and V⁴⁺/V⁵⁺ pairs followed the O-type redox reaction over the oxygen partial pressure range investigated. Higher oxidation states of vanadium were stabilized with increasing basicity of slags. Two redox pairs coexisted within oxygen pressure region 10^–4–10^–6 atm. However, redox ratios did not indicate clear trends with increasing V₂O₅ content in CaO-SiO₂-V₂O₅ system. In CaO-SiO₂-Al₂O₃-V₂O₅ slags, a slight increase in redox ratios (V³⁺/V⁴⁺) was obvious when alumina quantity was changed from 3.22 to 5.44% at a basicity ratio 1.3, indicating an increase in slag acidity. CaO-MgO-SiO₂-V₂O₅ slags showed a sharp decrease in redox ratios (V⁴⁺/V⁵⁺) between 10^–2–10^–6 atm with addition of 3.5 wt% MgO, due to increasing free oxygen ions in slags.     

Early stages of oxidation of yttrium-implanted Ni-20% Cr

The effect of ion-implanted yttrium on oxidation at elevated temperatures (500 to 1000° C) has been studied in Ni-Cr alloys using high-voltage electron microscopy (HVEM) ofin situ oxidized specimens, electron microscopy of scales oxidizedex situ, and Rutherford back scattering. The presence of yttrium enhances nucleation and growth of Cr₂O₃ at the alloy surface. Although the radiation damage induced by the ion implantation accelerates oxide nucleation and the initial rate of oxidation at 500° C, the growth of a mature oxide scale is slowed down, and the main influences seem to come from the chemical characteristics of the implanted yttrium. A Cr₂O₃ layer forms first on the yttrium-implanted alloys. Outward diffusion of Ni²⁺ cations through this layer forms an outer NiO scale. The initial growth process on unimplanted alloys is opposite. Here, NiO is the predominant initial oxide. As it thickens, a porous spinel and Cr₂O₃-enriched layer is formed between alloy and NiO. The inner oxide layer on yttrium-implanted alloys is fully dense and contains more than 50% Cr₂O₃. On the unimplanted alloy, the inner spinel layer is porous and contains a lesser enrichment in Cr₂O₃. The porous spinel delays formation of a protective Cr₂O₃ layer and gives poor scale adherence. The oxide growth mechanisms are discussed in the light of TEM results.     

Hot pressing 0f Y2O3-stabilized ZrO2 with Cr2O3 additions

The sinterbility of Y₂O₃-stabilized zirconia with Cr₂O₃ additive was studied by a hot pressing technique using graphite and alumina dies; the dependence of the density on temperature, pressure and time was measured. Using graphite dies, it was found that the addition of Cr₂O₃ to Y₂O₃-stabilized zirconia was effective as an at for densification due to formation of chromium at higher temperatures. Addition of Cr₂O₃ inhibited the grain growth of Y₂O₃-stability zirconia. The solubility of Cr₂O₃ in ZrO₂ was found to be 0.7 mol% at 1450° C. The results could be explained in relation to the phase relations of the system ZrO₂-Y₂O₃-Cr₂O₃.     

Investigation of microstructure and mechanical properties of multi-layer Cr/Cr2O3 coatings

Single and multi-layer Cr/Cr₂O₃ coatings were deposited by reactive magnetron sputtering with the total thickness of 7 μm on steel substrates. X-ray diffraction analysis showed that single and multi-layer Cr/Cr₂O₃ coatings have different preferred crystal orientations. Columnar microstructure was detected by transmission electron microscopy both in metal chromium and ceramic chromium oxide layers. Grain size increased with the coating thickness. The value of single and multi-layer coating's fracture toughness is between 4 and 6 MPa·m^1/2 measured with the Berkovich tip indentation, and it is between 2.8 and 3.9 MPa·m^1/2 when measured with the Vickers indenter. The adhesion is about 192.1 and 246.7 J/m² for single and multi-layer coatings, respectively.     

An internal synthesis method for mullite-chromium carbide composite in a vacuum system

In situ formation of chromium carbide in a mullite matrix through reaction of Cr₂O₃, SiC and A₂O₃ has been studied. Three different chromium compounds, Cr₃Si, Cr₃C₂, Cr₇C₃, and mullite were formed. In a vacuum environment, the Cr₃Si particles formed first and were retained below 1550 °C, while the Cr₇C₃ phase was only dominant above 1600 °C. The Cr₃C₂ phase was the dominant dispersed phase at temperatures of 1450–1500 °C. In an argon environment, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450 to 1550 °C. The mullite phase formed concurrently through the diffusion of the SiO₂ phase into the Al₂O₃. SiO₂ was the product of the reaction between Cr₂O₃ and SiC. The composite hot-pressed at 1450 °C in vacuum gave a flexural strength and fracture toughness of up to 457 MPa and 4.1 MPa m^1/2, respectively.     

The toughening behaviours of Cr3C2 particulate-reinforced Al2O3 composites

Enhancement of the fracture toughness of Al₂O₃ was achieved by the additions of 10–40 vol% Cr₃C₂ particulates through a hot-pressing process. The dependence of Cr₃C₂ particle size (0.5, 1.5 and 7.5 m) on the toughening effect was investigated. The maximum fracture toughness of composites could be improved to 5.9, 7.6 and 8.0 MPa m^1/2 for fine, medium and coarse Cr₃C₂ particle reinforced composites, respectively. Both the quantitative analysis of toughening contributions and experimental observations are extensively discussed in terms of Cr₃C₂ particle size, microcracks, as well as thermal residual stress between Al₂O₃ and Cr₃C₂.     

Ion-exchange equilibria between (Mn,Co)O solid solution and (Mn,Co) Cr2O4 and (Mn,Co) Al2O4 spinel solid solutions at 1100° C

The compositions of the (Mn,Co)O solid solution with rock salt structure in equilibrium with (Mn,Co)Cr₂O₄ and (Mn,Co)Al₂O₄ spinel solid solutions have been determined by X-ray diffraction measurements at 1100° C and an oxygen partial pressure of 10^–10 atm. The ion exchange equilibria are quantitatively analysed, using values for activities in the (Mn,Co)O solid solution available in the literature, in order to obtain activities in the spinel solid solutions. The MnAl₂O₄-CoAl₂O₄ solid solution exhibits negative deviations from Raoult's law, consistent with the estimated cation disorder in the solid solution, while the MnCr₂O₄-CoCr₂O₄ solid solution shows slightly positive deviations. The difference in the Gibbs free energy of formation of the two pure chromites and aluminates derived from the results of this study are in good agreement with recent results obtained from solid oxide galvanic cells and gas-equilibrium techniques.On leave from the Institute for Metals Research, Polish Academy of Sciences, 25 Reymonta St, 30-059 Krakow, Poland.