Influence of Preoxidation on High-Temperature Corrosion of a FeCrAl Alloy Under Conditions Relevant to Biomass Firing

Preoxidation of a commercial FeCrAl alloy (Kanthal APM) was evaluated as a surface modification approach to reduce alkali chloride-induced corrosion during biomass firing in power plants. Samples of the alloy preoxidized at 900 °C in O₂ or O₂ + 10 vol% H₂O, and at 1100 °C in O₂, were coated with KCl and exposed at 560 °C to a gas mixture comprising of 12 vol% CO₂, 6 vol% O₂, 3 vol% H₂O, 400 ppmv HCl and 60 ppmv SO₂. The oxide formed at 1100 °C showed no reactivity with the corrosive species. By contrast, all samples preoxidized at 900 °C suffered severe attack, resulting in formation of Fe-, Cr- and Al-containing corrosion products in a heterogeneous morphology, similar to non-preoxidized samples. The observed differences with respect to the degree of corrosion attack on the preoxidized samples are discussed in terms of the composition and thickness of the different types of Al₂O₃ layers obtained by the preoxidation treatment.     

Effect of Pressure on Metal Dusting Initiation on Alloy 800H and Alloy 600 in CO-rich Syngas

The effect of pressure on metal dusting initiation was studied by exposing conventional alloys 600 and 800H in CO-rich syngas atmosphere (H₂, CO, CO₂, CH₄, H₂O) at ambient and 18 bar total system pressure and 620 °C for 250 h. It was verified that, at constant temperature, increasing the total system pressure increases both oxygen partial pressure (pO₂) and carbon activity (a _C), simultaneously. Both samples exposed at ambient pressure showed very thin oxide scale formation and no sign of metal dusting. By contrast, samples exposed in the high-pressure experiment showed severe mass loss by metal dusting attack. Iron- and chromium-rich oxides and carbides were found as corrosion products. The distinct pressure-dependent behavior was discussed by considering both thermodynamic and kinetic aspects with respect to the protective oxide formation and pit initiation.     

Facile regeneration vitreous microfibrous entrapped supported ZnO sorbent with high contacting efficiency for bulk H2S removal from reformate streams in fuel cell applications

In this study, a microfibrous carrier consisting of 3 vol.% of 8 μm (diameter) glass fibers is used to entrap 45 vol.% of 150 to 250 μm (diameter) SiO₂ support particulates. ZnO is then nanodispersed onto the support by impregnation at the loading of 17 wt.%. At equivalent bed volumes, ZnO/SiO₂-entrapped materials provide twofold longer breakthrough times for H₂S (with a 67% reduction in sorbent loading) compared with packed beds of 1 to 2 mm commercial extrudates. Five-log reductions in H₂S concentration with up to 75% ZnO utilization at breakthrough are achieved. H₂S concentrations from 60 to 2000 parts per million in volume (ppmv) can be reduced to as little as 0.6 ppmv at 400°C in 30% H₂O at a face velocity of 1.7 cm/s for layers as thin as 1.0 mm. At 500 to 600°C, ZnO/SiO₂-entrapped materials provide much higher regenerability in air than do 1 to 2 mm commercial extrudates. The use of glass fibers permits greater than 50 regeneration cycles. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

不同脉冲频率条件下2219-T87高强铝合金焊缝成形行为

采用复合超高频脉冲方波变极性钨极氩弧焊接技术完成了2219-T87高强铝合金平板堆焊试验,分析了脉冲电流频率对焊缝成形特征的影响及其规律.结果表明,脉冲电流频率对电弧特性及熔池流动行为有较大影响,造成焊缝熔宽、熔深及熔透率出现显著变化.脉冲频率f_H<60 kHz时,焊缝熔宽、熔深随脉冲频率的增加而增大,熔透率在f_H<35 kHz时基本保持不变,在f_H>35 kHz时出现显著提升,较常规变极性氩弧焊(VP-GTAW)至少提高了34%;当脉冲频率达到60 kHz时,焊缝熔透率达到最大,较VP-GTAW提高了约60%;脉冲频率f_H>65 kHz时,熔宽、熔深及熔透率呈现回落趋势.     

Oxidation-sulfidation behavior of iron-chromium-nickel alloys

Oxidation-sulfidation studies of Fe-Cr-8Ni alloys with 4, 12, and 22 wt. % Cr were conducted at 750 and 875°C in multicomponent gas mixtures that contained CO, CO₂, CH₄, H₂, and H₂S. The reaction processes resulted in parabolic kinetics. A chromium concentration in the range 0–12 wt. % in the alloy had a negligible effect on the parabolic rate constant; however, the rate constant for the alloy with 22 wt. % Cr was significantly lower. For a given sulfur partial pressure, the oxygen partial pressures required for the formation of a continuous oxide layer in an Fe-22Cr-8Ni alloy were 10² to 10³ times those calculated for Cr-Cr₂O₃ equilibrium at temperatures of 875 and 750° C, respectively.Work supported by the U.S. Energy Research and Development Administration.On visiting appointment from the Academy of Mining and Metallurgy, Cracow, Poland through a fellowship of the International Atomic Energy Agency.     

Mechanical properties and the structure of porous titanium obtained by sintering compacted titanium sponge

Mechanical properties and characteristics of the porous material obtained by sintering of compacted granules of titanium sponge of grade TG-OP-01 have been investigated with the purpose to develop a technology of production of biocompatible orthoimplants. The content of pores θ was from 20 to 60%. As θ increased from 20 to 50%, the Brinell hardness H _B, maximum bending strength σ_bend, and impact toughness K _c decreased, following the empirical dependences H _Bθ = const, σ_bend × θ² = const, and K _c × θ^1.5 = const, respectively. At θ > 50%, there was observed a stronger drop in these values. The experiments on the absorption of water by the pores and an analysis of the filtration properties of the material made it possible to establish that the basis of the system of pores connected with the surface is composed by the gaps between the pressed granules 0.1–0.2 cm sintered during annealing rather than by the small (～ 100 μm) pores in the granules.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

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.     

The influence of sulfur pressure on Sulfidation behaviour of NiCoCrAl(Y) alloys at high temperature

Sulfidation of alloy having nominal composition Ni-23Co-19Cr-12Al (wt%) with and without the addition of 0.6% yttrium was studied at temperatures 1073–1273 K in sulfur vapor at atmospheric pressure and in H₂/H₂S gas mixtures at sulfur pressure of 10^?3 and 10^?1.5 Pa. Sulfidation runs were followed thermogravimetrically. Phase and chemical composition of sulfide scales and scale morphologies were determined by means of XRD, EDX, EPM and SEM analyses. After certain initial period sulfidation of both materials followed approximately a parabolic rate law. The estimated sulfidation rates for each alloy increased with sulfur pressure and temperature. The sulfide scales on both materials showed complex microstructures and compositions, depending on sulfidation conditions, with several sulfide and sulfospinel phases present, such as (Ni,Co)S, (Ni,Co)₃S₄, (Ni,Co)Cr₂S₄, (Cr,Ni,Co)Al₂S₄ or (Cr,Ni,Co)S and (Cr,Ni,Co)₃S₄. There was no evidence of yttrium segregation either to the grain boundary regions in the scale or to the alloy/scale interface. Yttrium dissolved in the sulfide phases and accelerated the sulfidation process. This behaviour was ascribed to the doping effect.     

Microstructural Investigation of Protective and Non-Protective Oxides on 11% Chromium Steel

FIB, SEM and STEM/EDX were used to investigate X20 stainless-steel samples exposed to dry O₂, or O₂ containing 40% H₂O, with a flow velocity of 0.5 cm/s or 5 cm/s, for 168 hr or 336 hr at 600°C. Thin protective Cr-rich (Cr,Fe)₂O₃ was maintained on the samples exposed to dry O₂, even after 336 hr, and on the sample exposed to O₂/H₂O mixture with the low-flow velocity (0.5 cm/s) for 168 hr. The oxide scale formed in the latter environment contained less Cr, due to Cr loss through CrO₂(OH)₂ evaporation. Breakaway oxidation occurred on the samples exposed in high-gas-flow velocity for shorter time (168 hr) or in low-gas-flow velocity (0.5 cm/s) for longer time (336 hr). The breakaway scales featured a two-layered structure: an outward-growing oxide “island” consisting of almost pure hematite (α-Fe₂O₃), and an inward-growing oxide “crater” consisting of (Cr,Fe)₃O₄. The transition from a thin protective (Cr,Fe)₂O₃ scale to a non-protective thick scale on this martensitic/ferritic steel originated locally and was followed by rapid oxide growth, resulting in a thick scale that covered the whole sample surface.     

Development of Nitrogen-Hydrocarbon Atmospheric Carburizing and Process Control Methods

Atmospheric pressure carburizing and neutral carbon potential annealing in nitrogen containing small additions of hydrocarbon gases can offer cost and steel surface quality alternatives to the comparable, endothermic atmosphere, or vacuum operations. An experimental program was conducted for refining real-time process control methods in carburizing of AISI 8620 steel under N₂-CH₄, N₂-C₃H₈ blends containing <5 vol.% of hydrocarbon gas at 900 and 930 °C. Multiple types of gas analyzers were used to monitor residual concentrations of H₂, CO, CO₂, H₂O, O₂, CH₄, C₃H₈, and other hydrocarbons inside furnace. A modified shim stock technique was additionally evaluated for correlation with gas analysis and diffusional modeling using measured carbon mass flux values (g/cm²/s). Results of this evaluation work are presented.     

Kinetics and microstructure of TiN coatings by CVD

Titanium nitride (TiN) coatings obtained by chemical vapor deposition with TiCl₄, H₂, and N₂ as gas sources at the atmospheric pressure have been examined in this study. The results showed that the deposition rate increased and reached a saturated value with increasing TiCl₄ partial pressure at different process parameters including deposition temperatures and partial pressures of H₂ and N₂. The reaction orders of TiCl₄, H₂, and N₂ were extracted by the least-square regression approximation of the experimental data. For TiCl₄ on N₂, their reaction order is influenced by the partial pressure of the other. A degraded growth rate at a high temperature of 1200°C was mainly related to the endothermic CVD TiN reaction and the hot-wall reactor. Microstructural examination by scanning electron microscope revealed the surface morphology changed with process parameters. Compositional analysis of the CVD TiN showed non-stoichiometric at low temperatures and 1200°C, but stoichiometric at 1000°C. The TiN stoichiometry could be affected by the partial pressures of N₂ and H₂.     

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.     

Improvement of the hydrogen-storage kinetics of Mg by reactive mechanical grinding with 10 wt.% Fe2O3 and 5 wt.% Ni

The addition of Fe₂O₃ to Mg is believed to be able to increase the hydriding rate of Mg, and the addition of Ni is thought to be able to increase the hydriding and dehydriding rates of Mg. A sample Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) was prepared by mechanical grinding under H₂ (reactive mechanical grinding). The as-milled sample absorbed 4.61 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min. Its activation was accomplished after two hydriding-dehydriding cycles. The activated sample absorbed 4.59 wt.% of hydrogen at 593 K under 12 bar H₂ for 60 min, and desorbed 3.83 wt.% hydrogen at 593 K under 1.0 bar H₂ for 60 min. The activated Mg-(10wt.%Fe₂O₃, 5 wt.%Ni) had a slightly higher hydriding rate at the beginning of the hydriding reaction but a much higher dehydriding rate compared with the activated Mg-10 wt.%Fe₂O₃. prepared via spray conversion. After hydriding-dehydriding cycling, Fe₂O₃ was reduced, Mg₂Ni was formed by the reaction of Mg with Ni, and a small fraction of Mg was oxidized.     

Modelling and analysis of pulsed Nd:YAG laser machining characteristics during micro-drilling of zirconia (ZrO2)

Pulsed Nd: YAG laser beam has great ability for micro-machining of ceramic materials because of high laser beam intensity at low mean beam power, good focusing characteristics due to very small pulse duration, small kerf widths and narrow heat effected zones. In the paper, experimental investigations into CNC pulsed Nd:YAG laser micro-drilling of zirconium oxide (ZrO₂) have been carried out. Influence of laser machining parameters on the HAZ thickness and phenomena of tapering of the machined micro-holes has been experimentally investigated. Response Surface Methodology-based optimal parametric analysis has been performed to determine the optimal setting of process parameters such as pulse frequency and pulse width, lamp current, assist air pressure for achieving minimum HAZ thickness and taper of the micro-hole machined by pulsed Nd:YAG laser. Minimum HAZ thickness has been obtained as 0.0675 mm when the lamp current, pulse frequency, assisted air pressure and pulse width are set at optimal parametric setting i.e. 17 amp, 2.0 kHz, 2.0 kg/cm² and 2% of the duty cycle, respectively. Minimum taper has been achieved as 0.0319 at optimal parametric setting i.e. the lamp current of 17 amp, pulse frequency of 2.0 kHz, assisted air pressure of 0.6 kg/cm² and pulse width of 2% of the duty cycle. Analysis has also been carried out for multi-optimisation of both the responses i.e. HAZ thickness and taper during pulsed Nd:YAG laser micro-drilling on ZrO₂.     

新型304不锈钢软钎焊用钎剂的研制

研究制备了以有机金属盐甲基磺酸亚锡为主要活性成分的新型304不锈钢软钎焊用H36钎剂,并配合Sn-Cu-Ni钎料比较了其与传统H3PO4-C2H5OH钎剂、ZnCl2-HCl钎剂和ZnCl2-NH4Cl钎剂在304不锈钢基板上的润湿铺展性能.结果表明,新研制的H36钎剂的铺展性能明显优于其它钎剂,其最大铺展面积达到109.56 mm2,相比H3PO4-C2H5OH钎剂和ZnCl2-NH4Cl钎剂分别提高了249.4％和239.3％；添加乳化剂6500能进一步提高钎剂的润湿铺展性能,最大铺展面积可达到157.49 mm2.对优化后的H36-2钎剂匹配Sn-Cu-Ni钎料钎焊304不锈钢钎焊接头力学性能的结果表明,钎焊接头的抗拉和抗剪强度分别达到22.72 MPa和33.93 MPa,优于现有文献报道的数据.     

Dual-environment effects on the oxidation of metallic interconnects

Metallic interconnects in solid oxide fuel cells are exposed to a dual environment: fuel on one side (i.e., H₂ gas) and oxidizer on the other side (i.e., air). It has been observed that the oxidation behavior of thin stainless steel sheet in air is changed by the presence of H₂ on the other side of the sheet. The resulting dual-environment scales are flaky and more friable than the single-environment scales. The H₂ disrupts the scale on the air side. A model to explain some of the effects of a dual environment is presented where hydrogen diffusing through the stainless steel sheet reacts with oxygen diffusing through the scale to form water vapor, which has sufficient vapor pressure to mechanically disrupt the scale. Experiments on preoxidized 316L stainless steel tubing exposed to air-air, H₂-air, and H₂-Ar environments are reported in support of the model. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Effect of environmental and metallurgical factors on hydrogen induced cracking of HSLA steels

Hydrogen induced cracking (HIC) resistance of two high strength low alloy (HSLA) steel plates equivalent to API X70 grade was evaluated in various test solutions with different H₂S partial pressures and pH values. Results showed that H₂S partial pressure is the key parameter affecting HIC resistance. Hydrogen permeation rate was affected by both H₂S partial pressure and pH of test solutions, whereas the apparent hydrogen diffusivity was determined mainly by pH value in case of H₂S partial pressure less than 0.1 atm. HIC in the steels primarily nucleated at inclusions and/or clusters containing the Al and Ca oxides. HIC resistance was determined by diffusible hydrogen amount with different microstructures.     

Influence of Water Vapor on the Cyclic-Oxidation Behavior of a Low-Pressure Plasma-Sprayed NiCrAlY Coating

The oxidation of a low-pressure plasma-sprayed (LPPS) NiCrAlY coating on a nickel-base superalloy was studied at 1050 °C in flows of O₂, and mixture of O₂ and 5% H₂O under atmospheric pressure. Water vapor has an obvious effect on the cyclic oxidation of the NiCrAlY coating. There is more decrease in weight gain when exposure to O₂ is replaced by exposure to O₂ + 5% H₂O. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in pure oxygen is composed mainly of Cr₂O₃, and a thin Al₂O₃-rich layer is formed at the interface between the Cr₂O₃-rich layer and the coating. The oxide formed on the LPPS NiCrAlY coating after cyclic oxidation in a mixture of O₂ + H₂O is composed of NiCr₂O₄, NiO and Cr₂O₃. The effect of water vapor on the oxidation of the NiCrAlY coating may be attributed to an increase in Ni and Cr cation transport, stress-corrosion cracking of Al₂O₃ and moisture-enhanced volatility of the Cr₂O₃ scale.     

Influence of the Oxygen Partial Pressure on the Oxidation of Inconel 617 Alloy at High Temperature

Ni-based superalloy Inconel 617 (IN617) is one of the main candidate structural materials for high temperature components (heat exchanger) of the gas-cooled fast reactor (GFR), a possible candidate for generation IV nuclear reactor. The material in operating conditions will be exposed to impure He at a temperature of around 850 °C. The impurities are expected to be oxidizing (such as O₂, H₂O) but since no feedback experience is available for this type of reactor, the level of impurities is completely unknown. Hence, an attempt has been made to understand the influence of oxygen partial pressure on oxide composition and on the oxidation mechanisms of IN617 at 850 °C. To achieve this, oxidation tests were performed at 3 different range of partial pressure: 10^?5, 0.2 and 200 mbar. Tests were performed from 1 h to 28 days and the obtained oxide layers were characterized using MEB, EDX, XPS, XRD and GD-OES. The oxide layers were mainly composed of chromia containing TiO₂ and thickening with time. Aluminium oxide formed internally. Other oxides were detected in the scale, such as NiO, CoO, MoO₃ and MnO₂, except for the lowest oxygen partial pressure experiments, where a selective oxidation took place. The scale-growth mechanism was cationic for low and medium oxygen partial pressure conditions. A growth following a transient oxidation mechanism was observed for high oxygen partial pressure.