Coking and Dusting of Fe–Ni Alloys in CO–H2–H2O Gas Mixtures

Metal dusting of Fe–Ni alloys was investigated in a CO–H₂–H₂O–Ar gas corresponding to a _C = 19.6 at 650 °C. Thermogravimetric analysis showed that increasing the nickel content in the alloy decreased the initial rate of carbon uptake. A uniform Fe₃C scale formed on pure iron, a layer with mixed structures of Fe₃C, γ and α-Fe developed on ferritic Fe–5Ni, and small amounts of Fe₃C developed at the surface of an austenite layer grown on two-phase (α + γ) Fe–10Ni. At nickel levels above 10%, no carbide appeared. These observations are shown to be broadly consistent with local equilibrium according to the Fe–Ni–C phase diagram. However, the failure of higher nickel austenitic alloys to form the (Fe,Ni)₃C expected at high carbon activities indicates a barrier to nucleation and growth of this phase. Graphite deposition was catalysed by (Fe,Ni)₃C on ferritics and by the metal itself on austenitics. The rates of carbon deposition on Fe–60Ni corresponded to the existence of three parallel and independent paths: the synthesis gas, the Boudouard and the carbon methanation reactions.     

Corrosion Behavior of Y–Al Alloys in a H2/H2S/H2O Gas Mixture at 800–950°C

The corrosion behavior of pure Y and two Y–Al alloys containing 5 and 10 wt.% Al was studied over the temperature range 800–950°C in a H₂/H₂S/H₂O gas mixture. Both alloys had the two-phase structure of Y+Y₂Al. With the exception of Y–10Al, for which a kinetics inversion was observed between 800°C and higher temperatures (T 850°C), the parabolic rate constants generally increased with increasing temperature, but decreased with increasing Al content. The scales formed on pure Y and the Y–Al alloys were single but heterophasic, consisting of mostly Y₂O₃ and minor Y₂O₂S. XRD results showed no evidence of Al₂O₃ and pure sulfides. The formation of Y₂O₃ and Y₂O₂S on Y–10Al at 800°C resulted in a subsurface phase transformation from Y+Y₂Al to YAl₂ and broke the structural integrity of the scale, being responsible for the fast corrosion rate.     

The Corrosion of Titanium Aluminides in H2/H2S/H2O Atmospheres at 800–1000°C

The corrosion behavior of three Ti–Al intermetallics containing 20, 30, and 40 wt.% Al was studied over the temperature range 800–1000°C in a H₂/H₂S/H₂O gas mixture. Ti–20Al and Ti–40Al alloys had the single-phase structure of Ti₃Al and TiAl, respectively, while Ti–30Al was a two-phase mixture of Ti₃Al+TiAl. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but decreased with increasing Al content. The Ti–40Al alloy exhibited the best corrosion resistance among all alloys studied. The scales formed on Ti–Al intermetallics were heterophasic and duplex, consisting of an outer-scale layer of pure -TiO₂ and an inner layer of -TiO₂ with minor amounts of -Al₂O₃ and Ti_l-xS. The amount of -Al₂O₃, which increased with increasing Al content, is responsible for the reduction of the corrosion rates as compared with those of pure Ti oxides.     

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H₂/H₂S gases of 4 Pa sulfur partial pressureat 900°C. Al₂O₃ was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al₂O₃+FeAl₂O₄,and nonreacting Fe₃Mo₂, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl₂O₃ layer further decreased the alloy reaction rate.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

Hydrothermal conversion of Ti-containing minerals in system of Na_2O–Al_2O_3–SiO_2–CaO–TiO_2–H_2O

Titanium has a great effect on the digestion of bauxite in the Bayer process because it reacts readily at high temperatures in alkaline sodium aluminate solution.Under this consideration, the hydrothermal conversion of Ti-containing minerals in the system of Na_2O–Al_2O_3–Si O_2–Ca O–Ti O_2–H_2O with increased temperatures was studied based on the thermodynamic analysis and systematic experiments. The results show that anatase converts to Al_4Ti_2 SiO_(12) at low temperatures(60–120 °C), which is similar to anatase in crystal structure. As the temperature continues to rise, Al4Ti2 Si O12decomposes gradually and converts to Ca_3 Ti Si_2(Al_2Si_(0.5)Ti_(0.5)O_(14) at 200 °C. When the temperature reaches 260 °C, Ca Ti O_3 forms as the most stable titanate species for its hexagonal closest packing with O_2-and Ca_2?. The findings enhance the understanding of titanate scaling in the Bayer process and clarify the mechanism of how additive lime improves the digestion of diaspore.     

Effect of H2S on High Temperature Corrosion of Fe–Ni–Cr Alloys in Carburizing/Oxidizing Environments

This investigation involves the corrosion behavior of two Fe–Ni–Cr alloys containing different Si content at 1050?°C in carburizing-oxidizing environments (typical of ethylene pyrolysis) with varied concentration of H₂S. High-Si containing alloy could form thinner but less uniform oxide scale than low-Si alloy after pre-oxidation due to the barrier effect of continuous SiO₂ at interface of scale/substrate. Pre-oxidized alloy showed a better resistance to carburization/sulfidation attacks than the bare alloy in absence of pre-oxidation. It was found that carburization and sulfidation of the Fe–Ni–Cr alloys could be prevented in the environment with a ratio of $ P_{{{\text{H}}_{ 2} {\text{S}}}} /P_{{{\text{H}}_{ 2} }} $ at 1.7?×?10^?5. When the sulfur partial pressure was lower than this value, oxides were found to be converted to porous and non-protective carbides. When the sulfur potentials were increased, manganese or chromium sulfide on outer layer and internal sulfide stringers mixed with silicon oxide in substrate could be formed. Under high sulfur partial pressures, spallation of outer sulfide or oxide scale was observed on high-Si alloy due to less stability of oxide layer formed at surface which was converted to sulfide faster than on low-Si alloy.     

Effect of pretreatment on electrochemical etching behavior of Al foil in HCl–H2SO4

The aluminum foil for high voltage aluminum electrolytic capacitor was immersed in 0.5 mol/L H₃PO₄ or 0.125 mol/L NaOH solution at 40 °C for different time and then DC electro-etched in 1 mol/L HCl+2.5 mol/L H₂SO₄ electrolyte at 80 °C. The pitting potential and self corrosion potential of Al foil were measured with polarization curves (PC). The potentiostatic current—time curve was recorded and the surface and cross section images of etched Al foil were observed with SEM. The electrochemical impedance spectroscopy (EIS) of etched Al foil and potential transient curves (PTC) during initial etching stage were measured. The results show the chemical pretreatments can activate Al foil surface, facilitate the absorption, diffusion and migration of Cl^? onto the Al foil during etching, and improve the initiation rate of meta-stable pits and density of stable pits and tunnels, leading to much increase in the real surface area and special capacitance of etched Al foil.     

Oxidation of Minor Elements from an Iron–Nickel–Chromium–Cobalt–Phosphorus Alloy in 17.3% CO2–H2 Gas Mixtures at 700–1000 °C

Fe–Ni–Cr–Co–P alloys were exposed to 17.3% CO₂–H₂ gas mixtures to investigate the oxidation of minor elements in metallic alloys in the early solar system. Reaction temperatures varied between 700 and 1000 °C. Gas-phase equilibrium was attained at 800, 900, and 1000 °C, yielding H₂–H₂O–CO–CO₂ gas mixtures. Experiments at 700 and 750 °C did not achieve gas-phase equilibrium and were performed in H₂–CO₂ gas mixtures. Reaction timescales varied from 1 to 742 h. The experimental samples were characterized using optical microscopy, electron microprobe analysis, wavelength-dispersive-spectroscopy X-ray elemental mapping, and X-ray diffraction. In all experiments Cr experiences internal oxidation to produce inclusions of chromite (FeCr₂O₄) and eskolaite (Cr₂O₃) and surface layers of Cr-bearing magnetite [(Fe,Cr)₃O₄]. At 900 and 1000 °C, P is lost from the alloy via diffusion and sublimation from the metal surface. Analysis of P zoning profiles in the remnant metal cores allows for the determination of the P diffusion coefficient in the bulk metal, which is constant, and the internally oxidized layer, which is shown to vary linearly with distance from the metal surface. At 800 and 900 °C, P oxidizes to form a surface layer of graftonite [Fe₃(PO₄)₂] while at 700 and 750 °C P forms inclusions of the phosphide-mineral schreibersite [(Fe,Ni)₃P].     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

φ–pH diagram of As–N–Na–H2O system for arsenic removal during alkaline pressure oxidation leaching of lead anode slime

In order to illustrate the thermodynamic characteristics of arsenic during alkaline pressure oxidation leaching process of lead anode slime (NaNO₃ as oxidant; NaOH as alkaline reagent), the φ–pH diagrams of As–Na–H₂O, N–H₂O, As–N–Na–H₂O systems at ionic mass concentration of 0.1 mol/kg and temperatures of 298, 373, 423 and 473 K were established according to thermodynamic calculation. The results show that the existence forms of arsenic are associated with pH value, which mainly exists in the forms of H₃AsO₄, H₂AsO₄^?, HAsO₄^2?, and As₂O₃ in lower pH region, while it mainly exists in the form of when pH>11.14. High alkali concentration and high temperature are advantageous to the arsenic leaching. The alkaline pressure oxidation leaching experiments display that the tendency of arsenic leaching rate confirms the thermodynamic analysis results obtained from the φ–pH diagrams of As–N–Na–H₂O system, and the highest leaching rate of arsenic reaches 95.85% at 453 K.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.     

Kinetics of high temperature deformation of lamellar Ti48Al–2Nb–2Cr

Based on microstructural observations, the deformation resistance of lamellar Ti48Al–2Nb–2Cr in the temperature range between 1000 and 1200 K is expressed by treating the material as a composite of regions with lamellar and globular structure deforming independently of each other. For stresses below 1000 MPa the dislocation velocity is lower in the lamellar regions compared to the globular ones due to a larger athermal hardening component caused by interfaces (lamella boundaries in the lamellar structure and (sub)grain boundaries in the globular structure). Combining deformation kinetics and structural evolution allows the modelling of the maximum deformation resistance and the subsequent softening resulting from the increase in globular volume fraction. The model is applied to predict the creep life.     

Thermodynamic and structural characterization of the Mg–Li–N–H hydrogen storage system

The Mg–Li–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. Some of thermodynamic and structural properties for this system are characterized here. Pressure-composition isotherms are measured and presented in this paper for absorption–desorption at 220, 200 and 180 °C. Powder X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis were carried out for samples at various degrees of hydrogenation. These results provide information about the structural changes during absorption/desorption. The mixture of (2LiNH₂ + MgH₂) partially converts to (Mg(NH₂)₂ + 2LiH) when heated at 220 °C and 100 bar of hydrogen without undergoing desorption. Based on two distinct parts which appear in all of the pressure-composition isotherms (180–220 °C), two reactions taking place isothermally in hydrogen absorption/desorption are proposed for the material starting with (2LiNH₂ + MgH₂) or (Mg(NH₂)₂ + 2LiH). These reactions include a single solid-phase reaction, corresponding to the sloping region for hydrogen weight percent (Hwt%) smaller than 1.5%, and a multiple-phase reaction, corresponding to a plateau region for Hwt.% > 1.5 in the isotherms. During hydrogen absorption/desorption, the single-solid-phase reaction corresponds to the forming/consuming of NH₂ which is bonded to Li and the multiple-solid-phase reaction corresponds to forming/consuming Mg(NH₂)₂ and LiH. A mechanism for the sorption reactions has been proposed.     

Preparation of low-oxygen-containing Ti–48Al–2Cr–2Nb alloy powder by direct reduction of oxides

A high-purity Ti–48Al–2Nb–2Cr alloy powder with an oxygen content as low as 0.0572 wt.% and a particle size of <150 μm was produced from a mixture of TiO₂, Al₂O₃, Nb₂O₅, and Cr₂O₃ powders through reduction with magnesium and deoxidation with calcium. The phase and composition of the products were analyzed. The final product mainly included γ-TiAl and minor α₂-Ti₃Al phases, and Ti, Al, Cr, and Nb were homogenously distributed in the powder with a mole ratio of 49.73:43.51:2.05:1.98. The reduction and deoxidation mechanisms were investigated by thermodynamic modeling using the HSC Chemistry software and Pandat software based on the Ti alloy database.     

RF sputtering of ZnO films in Ar and Ar–H2 gas mixtures: Role of H incorporation in developing transparent conductive coatings

Fundamental and applied investigation of ZnO has been recently experiencing a renaissance due to its prospective use in various technological domains and, in particular, as transparent conductive oxide (TCO). In this respect, the present work aims to study the structural and physical properties of ZnO thin films deposited by RF sputtering in pure Ar and Ar–H₂ plasmas, at various concentrations (0–50%). The plasma chemical species were followed in function of the different gas mixture settings by optical emission spectroscopy (OES). X-ray photoelectron spectroscopy (XPS) and ATR-FTIR (Attenuated Total Reflection Fourier-Transformed Infrared) spectroscopy were used to study the bulk and surface chemical composition of the films, X-ray Diffraction (XRD) analysis allowed lattice structure and grain size determination while samples morphology was checked with a scanning electron microscope (SEM). The films were also characterized for their electrical and optical properties.The introduction of hydrogen in the plasma phase strongly affected the structural, chemical and physical properties of the films. In particular a pronounced change in the film electrical behavior was observed which become conductive when H is added in the gas mixture ([H₂] > 6%). The film transparency was on the other hand maintained. By combining XPS, ATR-FTIR and OES data we could correlate the established conductivity and its variations with intentional hydrogen incorporation in the crystal structure in the form of hydroxide species.     

The Influence of H,W, H/E,H 3/E 2, Structure and Chemical Composition on the Resistance of Ti–B–(N), Mo–B–(N), Cr–B–(N), and Zr–B–(N) Coatings to Cyclic Impact Loading

Protection of Metals and Physical Chemistry of Surfaces - Coatings based on transition metal borides (Ti, Mo, Cr, Zr) were obtained by magnetron sputtering of ceramic targets in Ar and Ar–15%...     

Corrosion and Electrochemical Behavior of Ni–P Coatings in 0.5 M H2SO4

An increase in the phosphorus content in Ni–P coatings (from 6.6 to 13.4%) is shown to be accompanied by amorphization of their structure and a decrease in the anodic dissolution rate (at low overvoltages) by two–three orders of magnitude, whereas the corrosion rate is decreased to a lesser degree. Polarization curves of the coatings measured in 0.5 M H₂SO₄ comprise two linear segments and no passive range. In the first segment, the b _a coefficient is shown to be anomalously high (0.4–0.5 V for amorphous alloys), and the dissolution is controlled by transient volume diffusion of nickel and accompanied by surface accumulation of phosphorus. Being added as a stabilizer, thiourea reduces the corrosion resistance of the coatings and increases the dissolution currents, which can be to a considerable degree eliminated by increasing the phosphorus content in the coatings. The adverse effect of Pb incorporation into the coatings manifests itself as a substantial acceleration of their anodic dissolution.     

Chemical and physical sputtering of aluminium and gold samples using Ar–H2 DC-glow discharges

We present a series of sputtering experiments on aluminium samples performed with an Ar–H₂ DC-glow discharge at varying Ar–H₂ gas-composition, driven at a discharge voltage of –300 V and a pressure of 0.2 mbar, in conjunction with measurements of the corresponding ion-energy distributions of the ions bombarding the discharge cathode (Ar⁺, Ar²⁺, ArH⁺, H₂⁺ and H₃⁺). Similar measurements on gold samples, which have been published, have shown that the Au-sputtering efficiency of an Ar–H₂ glow discharge as a function of gas-composition could be adequately described by the corresponding change in the measured ion-energy distributions, under the assumption of a purely physical sputtering process. The experiments presented here show that this is not the case for aluminium (effectively Al₂O₃). In this case, a measured optimal gas-composition of 80% H₂ was found for Al-sputtering, while the energy-distributions suggest an optimum at 20% (as for gold). This clearly suggests that hydrogen-enhanced chemical sputtering is taking place.     

Effect of heat treatment on mechanical and wear properties of Zn–40Al–2Cu–2Si alloy

In order to determine the effect of heat treatment on the mechanical and wear properties of Zn–40Al– 2Cu–2Si alloy, different heat treatments including homogenization followed by air-cooling (H1), homogenization followed by furnace-cooling (H2), stabilization (T5) and quench–aging (T6 and T7) were applied. The effects of these heat treatments on the mechanical and tribological properties of the alloy were studied by metallography and, mechanical and wear tests in comparison with SAE 65 bronze. The wear tests were performed using a block on cylinder type test apparatus. The hardness, tensile strength and compressive strength of the alloy increase by the application of H1 and T6 heat treatments, and all the heat treatments except T6, increase its elongation to fracture. H1, T5 and T6 heat treatments cause a reduction in friction coefficient and wear volume of the alloy. However, this alloy exhibits the lowest friction coefficient and wear volume after T6 heat treatment. Therefore, T6 heat treatment appears to be the best process for the lubricated tribological applications of this alloy at a pressure of 14 MPa. However, Zn–40Al–2Cu–2Si alloy in the as-cast and heat-treated conditions shows lower wear loss or higher wear resistance than the bronze.