Degradation Behaviour of HR-120 Alloy Exposed to CH4–CO–H2–H2O Mixed-Gas Environments at 950 °C

The HR-120 alloy is a candidate structural material for heat-exchanger reactors of steam methane reforming. Consequently, the behaviour at high-temperature of this alloy in oxidizing/carburizing atmosphere is of considerable interest for industrial applications. In this study, the behaviour of HR-120 alloy was evaluated in Ar, CH₄, CO, H₂, H₂O oxidant/carburizing gas mixtures at 1,223 K, with or without pre-oxidation. In the former case, the as-grown scale layer consisted of inner Cr₂O₃ layer and an outer MnCr₂O₄ spinel layer. This scale structure, which is completely transformed into carbide layer in Ar–CH₄ atmosphere, exhibits an excellent stability in CH₄, CO, H₂, H₂O gas mixture. However, the oxidation of particles rich in Nb promoted cracking and spalling of protective scale layer, resulting in exposure of substrate metal.     

Effect of Plasma Nitriding and Nitrocarburizing on HVOF-Sprayed Stainless Steel Coatings

In this work, the effects of plasma nitriding (PN) and nitrocarburizing on HVOF-sprayed stainless steel nitride layers were investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steels were plasma-nitrided and nitrocarburized using a N₂ + H₂ gas mixture and the gas mixture containing C₂H₂, respectively, at 550 °C. The results showed that the PN and nitrocarburizing produced a relatively thick nitrided layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF-sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C₂H₂ (plasma nitrocarburizing process) was added. The PN and nitrocarburizing resulted in not only an increase of the surface hardness, but also improvement of the load bearing capacity of the HVOF-sprayed stainless steel coatings because of the formation of CrN, Fe₃N, and Fe₄N phases. Also, the plasma-nitrocarburized HVOF-sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr₂₃C₆ on the surface.     

Effect of H2O content in air on the carburizing behavior of charcoal gas

Conventionally, charcoal gas is produced by reacting air with hot charcoal. The constituents of such charcoal gas are N2, CO, and CO₂. Because no H₂ exists in this kind of charcoal gas, its carburizing rate to steels is relatively slow, compared with atmospheres containing CO and H2. A simple, but effective, method to raise the carburizing rate of the charcoal gas has been found through this study; that is, the air used for generating charcoal gas is humidified with some water vapor before passing through the hot charcoal layer. In this way, the carbon potential and carburizing rate of the charcoal gas can be raised markedly due to the formation of H₂. For example, when the air is humidified with 7.30% H₂O, the carbon potential increases by about three times, and the carburizing rate increases by about four times, compared with the charcoal gas generated from dried air and charcoal.     

The Effects of KCl, K2SO4 and K2CO3 on the High Temperature Corrosion of a 304-Type Austenitic Stainless Steel

The oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel was investigated at 500 and 600 °C in 5% O₂ + 40% H₂O. Prior to exposure the samples were sprayed with KCl, K₂CO₃ or K₂SO₄, the amount of salt corresponding to 1.35 ??mol K⁺/cm². For reference, salt-free samples were exposed in 5% O₂ + 40% H₂O and in 5% O₂ (N₂ was used as carrier gas). The oxidized samples were analyzed with SEM/EDX, XRD, IC and FIB. KCl and K₂CO₃ strongly accelerate the corrosion of 304L while K₂SO₄ has little influence on the corrosion rate and on the morphology of the corroded surface. KCl and K₂CO₃ react with the chromium-rich oxide on the sample surface, forming K₂CrO₄. The resulting chromium depletion of the protective oxide causes rapid oxidation and the formation of a thick duplex scale consisting of an outer hematite layer and a inner layer made up of FeCrNi spinel-type oxide. The differences in the corrosivity of the three salts are directly connected to their ability to form chromate on the surface and, hence, to the relative stability of the corresponding leaving groups (HCl, CO₂ and SO₃).     

High temperature corrosion of alloy Haynes 556 in carburizing/oxidizing environments

The iron based alloy Haynes 556 has been recently used in hydrocarbon and carbonaceous environments due to its excellent resistance to carburization. This alloy outperforms stainless steels and some of the best commercial carburization‐resistant nickel‐based alloys. This paper is concerned with the behavior of alloy Haynes 556 in high temperature carburizing environments containing trace amounts of oxygen. Thermal cyclic exposures were conducted in 2% and 10% CH₄/H₂ gas mixtures at 800, 900, 1000, and 1100°C for 10 cycles, 50 h each. Carbon activities, oxygen partial pressures, and stabilities of oxides and carbides were used to identify the role played by the reaction products in providing protection. Thermodynamic analyses, weight changes, and microstructural characterization were correlated with environmental parameters and alloy composition to elucidate the causes of its marked resistance to carburization. The results indicate protective character in both gas mixtures under all exposure conditions except the most aggressive, namely 1100°C in 10% CH₄/H₂ gas mixture. Below 1000°C, the formation of Cr, Al, and Si oxides along with Cr carbides provides the primary means of protection. Catastrophic failure at 1100°C was manifested by extensive fracture and crack development within the outer substrate surface in the 10% CH₄/H₂ gas mixture resulting in a dramatic increase in weight gain. This has been attributed to the increased carbon pick‐up, coupled with the loss of the protective outer scales.     

Synthesis of Porous Silicon Nitride-Boron Nitride Composites by Gel-Casting and PIP

The Si₃N₄-BN composites were prepared by gel-casting and precursor infiltration and pyrolysis route using borazine as the precursor. The composition, mechanical, microstructural, and dielectric properties of the composites were investigated. The composites are composed of h-BN, α-Si₃N₄, and β-Si₃N₄. The typical density, porosity, flexural strength, elastic modulus, and fracture toughness of the composites are 2.21 g/cm³, 17.8%, 185.59 MPa, 69.13 GPa, and 2.47 MPa m^1/2, respectively. The dielectric constant and loss tangent of the composites are 4.507-4.635 and 1.06 × 10^?3-1.97 × 10^?3 at the frequency of 7-18 GHz. Desirable properties of the composites have been achieved.     

The influence of carbonaceous surface impurities on excrescence initiation in the rimming steel/CO2/CO system

The influence of surface carbon contamination in determining the sites of excrescence growth has been studied, for the reaction of rimming steel in carbon dioxide containing 1·5%CO + 1000 ppm H₂O + 10 ppm CH₄ at 17·25 × 10⁶ N m^?2 and 450°C. Patterns visible in the oxide layer in the early stages of oxidation can be attributed to hydrocarbon impurities remaining on surfaces not rigorously cleaned. The impurity can accumulate at specific areas on the surface and can then accelerate oxidation such that excrescences start growing early in the normally protective stage. Surface cleaning procedures and their limitations for corrosion studies are discussed.     

High-Temperature Corrosion of Aluminized and Chromized Fe–25.8 %Cr–19.5 %Ni Alloys in N2/H2S/H2O-Mixed Gases

Alloys of Fe–25.8 %Cr–19.5 %Ni (SUS310 stainless steel) were either chromized or aluminized via pack cementation, and corroded at 800 °C for 100 h in 1 atm of (0.9448 atm of N₂ + 0.031 atm of H₂O + 0.0242 atm of H₂S)-mixed gases. The chromized layer consisted primarily of Cr_1.36Fe_0.52 and some Cr₂₃C₆. Its corrosion resulted in the formation of Cr₂S₃ and some FeS and Fe₅Ni₄S₈. The aluminized coating consisted primarily of FeAl. Its corrosion resulted in the formation of α-Al₂O₃, Al₂S₃, and Cr₂S₃. Aluminizing was more effective than chromizing in increasing the corrosion resistance of the substrate, due mainly to the formation of α-Al₂O₃.     

Improving the Physicochemical Properties of Fe–25Cr Ferritic Steel for SOFC Interconnects via Y-Implantation and Y2O3-Deposition

The present study investigated the role of the reactive-element effect (REE) in improving the corrosion resistance, chromium vaporization rate, and electrical conductivity of the Fe–25Cr ferritic steel modified either by means of yttrium implantation or chemical deposition of yttrium oxide from metaloorganic compound vapors. The corrosion kinetics of the Fe–25Cr steel, both pure and modified, were determined under isothermal conditions in air and an Ar–H₂–H₂O gas mixture at 1,073 K. A significant improvement in corrosion resistance was observed after surface modification. XRD and SEM–EDS investigations showed that the protective Cr₂O₃ layer formed the main part of the scale. Measurements of Cr vaporization rate in the air–H₂O gas mixture revealed that both surface modifications of the steel significantly suppressed the formation of volatile chromium compounds to a large degree. The yttrium-implanted steels oxidized both in air and the Ar–H₂–H₂O mixture were characterized by the lowest area specific resistance and thereby did not exceed the acceptable ASR level (0.1 Ω cm²) for interconnect materials in the temperature range of 973–1,073 K, unlike pure steel and the steel coated with Y₂O₃.     

Compatibility of CVD SiC and SiCf/SiC Composites with High Temperature Helium Simulating Very High Temperature Gas-Cooled Reactor Coolant Chemistry

The chemical compatibility aspects of CVD β-SiC and SiC_f/SiC composites with a VHTR specific helium coolant were examined. The specimens were exposed to helium gas containing 20 Pa H₂, 5 Pa CO, 2 Pa CH₄, and 0.02–0.1 Pa H₂O, which is an expected VHTR coolant chemistry. Oxidation tests were carried out at 900 and 950 °C for up to 250 h. β-SiC and SiC_f/SiC composites had an excellent compatibility with the expected VHTR helium coolant environment. The oxidation of β-SiC as a matrix material of the SiC_f/SiC composite reacted in a passive oxidation regime owing to the presence of water vapor. A condensed version of the oxide SiO₂ formed at an early stage of oxidation and the growth of this oxide layer was very limited as the oxidation time increased up to 250 h. The recession of the pyrolytic carbon interphase of SiC_f/SiC composite could not be observed in the test range.     

A Black Phosphate Conversion Coating on Steel Surface Using Antimony(III)-Tartrate as an Additive

A novel black phosphate conversion coating was formed on steel surface through a Zn-Mn phosphating bath containing mainly ZnO, H₃PO₄, Mn(H₂PO₄)₂, and Ca(NO₃)₂, where antimony(III)-tartrate was used as the blackening agent of phosphatization. The surface morphology and composition of the coating were characterized by scanning electron microscopy, energy dispersion spectroscopy, and x-ray photoelectron spectroscopy. Corrosion resistance of the coating was studied by potentiodynamic polarization curves and electrochemical impedance spectroscopy. The pH value of the solution had significant influence on the formation and corrosion resistance of the coating. The experimental results indicated that the Sb plays a vital role in the blackening of phosphate conversion coating. The optimal concentration of antimony(III)-tartrate in the phosphating bath used in this experiment was 1.0 g L^?1, as higher values reduced the corrosion resistance of the coating. In addition, by saponification and oil seals, the corrosion duration of the black phosphate coating in a copper sulfate spot test can be as long as 20 min.     

Effect of Pyrolysis Temperature on Properties of Porous Si3N4-BN Composites Fabricated Via PIP Route

The Si₃N₄-BN composites have been prepared via die pressing and the precursor infiltration and pyrolysis route using borazine as the precursor. The Si₃N₄-BN composites are composed of h-BN, α-Si₃N₄, and β-Si₃N₄ produced at a pyrolysis temperature from 1200 to 1750 °C with only 0.17-3.9 wt.% phase transition of Si₃N₄. The effect of pyrolysis temperature on properties of the composites has been investigated. The density and mechanical properties of the composites, at both room temperature and 1000 °C, increase along with the elevating of the pyrolysis temperature. The density of the composites achieves 2.33 g/cm³ at 1750 °C with the porosity of 14.1%. The flexural strength, elastic modulus, and fracture toughness at room temperature of the Si₃N₄-BN composites pyrolyzed at 1750 °C are 219.1 MPa, 75.5 GPa, and 2.62 MPa m^1/2, respectively. A desirable flexural strength of 184.9 MPa with a residual ratio of 84.4% has been obtained when the composites are exposed at 1000 °C in the air. Micrographs of SEM and TEM illustrate the bonding structure of the pyrolysis BN and Si₃N₄ grains.     

Die Möglichkeiten einer Wasserstoffversprödung von Spannstahl bei Einwirkung von Kohlensäure auf sulfidhaltigen Zementstein

The possibility of hydrogen embrittlement of reinforcing steel during carbon dioxide attack ou sulfide containing concrete Cement mortar (furnace cement, alumina-silicate cements) has been carbonated and the H₂S being generated has been quantitatively determined. The gas volumes measured are by orders of magnitude below the values theoretically expected. During the carbonate formation in mortar tubes advancing from the external surface only a small proportion of the H₂S penetrates into the interior of test specimens, e. g. 5·10^?6 H₂S (with reference to the mortar weight) in the case of a high furnace mortar containing 1,19% S, while a H₂S concentration by three orders higher was found in a tube of transformed alumina-silicate cement stone. This goes to show that the mechanism of reinforcing steel embrittlement as described in literature for alumina-silicate cements cannot be correct.     

The Corrosion Behavior of Carbon Steel in Sulfide Aqueous Media at 30°C

In this paper, we studied the effect of sulfide ions on the corrosion behavior of carbon steel to simulate the geological disposal of high-level radioactive waste. In geological storage conditions, sulfidogenic environment was sustained by sulfate-reducing bacteria. Corrosion tests were conducted in systems in a controlled atmosphere of 5% H₂/N₂. Batch experiments were conducted at 30°C for 1 month with steel coupons immersed in Na₂S solutions. The structural characterization of the corrosion products was investigated by scanning electron microscope/energy dispersive x-ray spectroscopy, confocal micro-Raman spectrometry, and x-ray diffraction. In the absence of sulfide ion, a magnetite (Fe₃O₄) corrosion product layer was formed on steel surface while in the presence of sulfide ions we observed the formation of a poorly crystallized irons sulfide at low-sulfide concentration (1 mg/L) and a solid adherent pyrrhotite layer at higher sulfide concentration (5-15 mg/L). The strong drop in steel corrosion rate with sulfide concentration was revealed and related to the formation of well-crystallized pyrrhotite.     

Removal of hydrocarbon films from the surface of metal materials in air glow discharge

Selective oxidation of amorphous hydrocarbon (a-C:H) films deposited on tungsten, molybdenum, and stainless steel by chemically active oxygen (with a source of glow discharge in air) has been studied. Film oxidation was carried out both directly in the discharge and in the area of afterglow. It has been shown that plasmolysis products of a-C:H films in air are CO, CO₂, H₂O, and H₂. The rate of film oxidation depended on the position in relation to plasma and decreased in the hollow cathode-positive column-afterglow series. The coefficients of carbon erosion in afterglow increased from 10⁻³ to 10⁻² at. C/at. O at a temperature increase from room temperature to 130°C.     

Structural,Microstructure, Mechanical and Electrical Properties of Porous Zr4+-Cordierite Ceramic Composites

Zirconium-cordierite ceramic composites have been synthesized by a co-precipitation method using MgCl₂·6H₂O, NaAlO₂, Na₂SiO₃·5H₂O, and ZrOCl₂·8H₂O as starting materials. XRD, FT-IR, and SEM techniques were employed to study the effect of zirconium on the crystal structure and microstructure of the samples. XRD results revealed that spinel MgAl₂O₄ and t-ZrO₂ phases were predominant in the samples with low Zr⁴⁺ content (10 wt.%), whereas zircon ZrSiO₄ was predominant with high Zr⁴⁺ content (≥15 wt.%). The densification behavior was improved from 30.4 to 44.6% of the theoretical density (2.6 g/cm³) at 15 wt.% of Zr⁴⁺. However, microhardness of the sintered samples was enhanced from 7.1 to 7.5 GPa with increasing the Zr⁴⁺ dose from 0 to 25 wt.%. On the other hand, the gradual increase in Zr⁴⁺ content from 0 to 25 wt.% led to suppression in the electrical resistivity (ρ) from 16.6 to 2.8 × 10⁹ Ω/cm, respectively. In addition, the dielectric permittivity (ε) of the pure cordierite was decreased with Zr⁴⁺ ion addition. The maximum dielectric permittivity (ε) at low frequencies (10 MHz) was 18.7 at 10 wt.% Zr⁴⁺ content, whereas at high frequencies (1 GHz) it was 38.8 at 15 wt.% Zr⁴⁺ content.     

Thermal Plasma Spraying Applied on Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs), attractive for diverse applications in a broad range from small portable and auxiliary power units, up to central power systems, are conventionally produced by sintering methods. However, plasma spraying promises some advantages particularly for cells with metal support. In the present paper, research activities conducted in recent years at DLR as well as latest developments on plasma sprayed functional layers for SOFC as cathodes, electrolytes, and anodes are reported. Power densities of more than 800 mW/cm² were achieved for plasma sprayed single cells of 12.56 cm² size, and 300 mW/cm², respectively, with a 250 W stack made of 10 cells. These values were attained at 0.7 V and 800 °C, with H₂:N₂ = 1:1 as fuel gas and air as oxidizing gas. Furthermore, continuous operation of more than 5000 h was attained with a plasma sprayed metal-supported SOFC stack which could also withstand more than 30 redox and thermal cycles.     

Carburization/Oxidation Behavior of Alloy Haynes-214 in Methane–Hydrogen Gas Mixtures

The excellent resistance of Alloy Haynes-214 to carburization at elevated temperatures is attributed to the formation of a protective surface layer of Al₂O₃, resulting from the reaction of trace oxygen impurity in the gas phase with the Al-enriched surface. Lower carburization resistance below 1000 °C is manifested by increased carbon pick-up and degradation of mechanical properties. Exposures in CH₄/H₂ gas mixtures containing trace levels of oxygen below 1000 °C reduce the supply of outwardly diffusing aluminum from the bulk of the alloy to the metal surface, thus enhancing inward-oxygen diffusion and the subsequent formation of fine lamellar dispersion of internal Al₂O₃ within the sub-surface zone. In the meantime, inward-carbon diffusion forms internal carbides, thus leading to increased weight gain, relative loss of protection, and possible degradation of mechanical properties. Effective mitigation of carburization is limited only to exposure at extreme temperatures (1100 °C) which enhances the outward diffusion of aluminum towards the gas/metal interface as well as the nucleation of Al₂O₃ at the surface, thus causing an early transition from internal to external oxidation, which promotes the formation of an external protective layer of Al₂O₃ which effectively blocks carbon ingress in the alloy.     

Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C‐5, P‐11 and P‐22 steel samples were tested in two different environments: air and CO₂ + 2H₂O + 7.52N₂, a gas composition which simulates the combustion products of natural gas, at 450 and 500 °C. P‐22 steel had the best oxidation resistance at both temperatures in air. In CO₂ + 2H₂O + 7.52N₂ environment, the oxidations of all the steels were accelerated and C‐5 exhibited better oxidation resistance than P‐22 and P‐11. Analyses of oxidation products by optical microscopy, SEM‐EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P‐22 in air was explained with reference to the formation of a protective Cr‐containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO₂ + 2H₂O + 7.52N₂ environment were attributed to the depletion of protective Cr‐containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe? Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P‐22 (2.25%) even at 450 and 500 °C.     

Microwave Power Absorption in Materials for Ferrous Metallurgy

The characteristics of microwave power absorption in materials for ferrous metallurgy, including iron oxides (Fe₂O₃, Fe₃O₄ and Fe₀.₉₂₅O) and bitumite, were explored by evaluating their dielectric loss (Q _E) and/or magnetic loss (Q _H) distributions in the 0.05-m-thick slabs of the corresponding materials exposed to 1.2-kW and 2.45-GHz microwave radiation at temperatures below 1100°C. It is revealed that the dielectric loss contributes primarily to the power absorption in Fe₂O₃, Fe_0.925O and the bitumite at all of the examined temperatures. Their Q _E values at room temperature and slab surface are 9.1311 × 10³ W m^?3, 23.7025 × 10³ W m^?3, and 49.5999 × 10³ W m^?3, respectively, showing that the materials have the following heating rate initially under microwave irradiation: bitumite > Fe_0.925O > Fe₂O₃. Compared with the other materials, Fe₃O₄ has much stronger power absorption, primarily originated from its magnetic loss (e.g., Q _H = 1.0615 × 10⁶ W m^?3, Q _H/Q _E = 2.4185 at 24°C and slab surface), below its Curie point, above which the magnetic susceptibility approaches to zero, thereby causing a very small Q _H value at even the surface (Q _H = 1.0416 × 10⁵ W m^?3 at 880°C). It is also demonstrated that inhomogeneous power distributions occur in all the slabs and become more pronounced with increasing temperature mainly due to rapid increase in permittivity. Characterizing power absorption in the oxides and the coal is expected to offer a strategic guide for improving use of microwave energy in ferrous metallurgy.