Corrosion of heat exchanger alloys exposed to a non‐equilibrated CO‐based sulfidizing environment at 550°C

This paper presents results from a series of experiments involving the intermittent exposure of several steels to a CO‐based gas mixture containing 0.1% H₂S for periods of up to 1000 h. The flow of the gas was maintained at a rate sufficiently high to prevent equilibration of the mixture occurring at the reaction temperature of 550°C, thereby simulating conditions found in commercial coal gasification plants. Five materials were selected for this study, i.e. three commercial alloys containing 9, 12 and 20% Cr and two specially cast 12% Cr “model” alloys with additions of 1% and 2% Si. Several specimens of each alloy were exposed at the same time; some samples were also coated with a Cl‐containing ash mixture prior to exposure in order to study the influence of fly‐ash deposits upon degradation. In addition, after intermittently cooling the samples to room temperature, half were desiccated dry whilst the remainder were placed in moisture‐saturated air at 30°C in order to simulate conditions arising during periods of plant down‐time, i.e. down‐time corrosion (DTC). Corrosion kinetics are presented in terms of weight change and metal loss measurements indicating the significant improvement accompanying the addition of 1–2% Si to the basic 12% Cr composition. The contributions of ash deposits and periods of moist down‐time are also discussed.     

Corrosion behaviour of experimental alloys in simulated HTGR helium at temperatures in the 750–1050°C

A series of experimental alloys (basically Ni-20 Cr, with addition of one of more of the elements Al, Ti, Si, Nb and Y) has been examined after exposure to controlled purity helium for periods of 1000–6000 hours at 750–1050 °C. This environment was intended to simulate the primary coolant of a high temperature gas-cooled reactor. Alloys containing aluminum were found to be particularly susceptible to internal oxidation at the lower temperatures, but at 950 °C and above carburization became the dominant corrosion mechanism. The most corrosion resistant alloys were Ni-20Cr and Ni-20 Cr containing small amounts of silicon, titanium and niobium. The presence of 1% yttrium dramatically increased the incidence of carburization, even at temperatures as low at 850°C.     

Effect of Cr and Ni Contents on the Oxidation Behavior of Ferritic and Austenitic Model Alloys in Air with Water Vapor

Ferritic and austenitic model alloys with various contents of Cr and Ni ranging between 10–20% and 0–30%, respectively, were oxidized in air + 10% water vapor during 1 hr cyclic oxidation at 650°C and 800°C. Depending on the alloy composition and temperature, either a thin protective oxide scale was observed or accelerated attack occurred which sometimes included spallation. For austenitic model alloys, increasing either the Cr or Ni contents delayed the accelerated attack. For lower Cr and Ni contents at 800°C, accelerated attack, including spallation, occurred at short exposure times. No spallation was observed for the ferritic model alloys. However, accelerated attack can occur quickly with low Cr contents. Increasing the temperature delayed the breakaway observed on ferritic alloys whereas it reduced the protective-oxide-growth stage for austenitic alloys.     

Processing and mechanical properties of a molybdenum silicide with the composition Mo–12Si–8.5B (at.%)

Alloys with the nominal composition Mo–12Si–8.5B (at.%) were prepared by arc-melting or powder-metallurgical processing. Cast and annealed alloys consisted of approximately 38 vol.% α-Mo in a brittle matrix of 32 vol.% Mo₃Si and 30 vol.% Mo₅SiB₂. Their flexure strengths were approximately 500 MPa at room temperature, and 400–500 MPa at 1200°C in air. The fracture toughness values determined from the three-point fracture of chevron-notched specimens were about 10 MPa m^1/2 at room temperature and 20 MPa m^1/2 at 1200°C in air. The relatively high room temperature toughness is consistent with the deformation of the α-Mo particles observed on fracture surfaces. Three-point flexure tests at 1200°C in air and a tensile test at 1520°C in nitrogen indicated a small amount of high temperature plasticity. Extrusion experiments to modify the microstructure of cast alloys were unsuccessful due to extensive cracking. However, using powder-metallurgical (PM) techniques, microstructures consisting of Mo₃Si and Mo₅SiB₂ particles in a continuous α-Mo matrix were fabricated. The room temperature fracture toughnesss of the PM materials was on the order of 15 MPa m^1/2.     

Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr–25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr–25Ni–1Mn–0.5Si–Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 °C and 1,000 °C, in a sulphidising/chlorinating environment at 700 °C and in a nitriding atmosphere at 1,000 °C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 °C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.     

Das elektrochemische Verhalten von Kupferlegierungen in künstlichem Meerwasser bei 90 °C

Electrochemical behaviour of copper alloys in synthetic seawater at 90°C The electrochemical behaviour of materials for heat exchangers in desalination plants is studied in artificial seawater at a temperature of 90°C. The alloys are Cunifer 10, 30, 30Cr and 302, Hecorros 76–F34, CuNiA14 and 20, CuFe2 (Alloy 194) and NA-Copper. Potential-current density curves were taken by means of the potentiokinetic and the potentiostatic method. These curves indicate a clear differentiation of all alloys studied. The alloys CuNiA 14/20 show a larger range of passivity and appear to be more resistant to seawater of 90°C than Cunifer 10, 30 SoMs. The alloying elements Cr and Mn improve the tendency of passivation in the alloys CuNi30Cr and Cunifer 302. NA-Copper and CuFe2 (alloy 194) do not show any indication of passivation in seawater of 90°C.     

Das Aufkohlungsverhalten verschiedener Werkstoffe für Pyrolyserohre

Carburizing behaviour of various materials for tubular pyrolysis furnaces Chromium-nickel cast alloys with 25% Cr, 20–35% Ni, 0.4% C and silicon-contents ranging from 1.0 to 3.1% were pack-carburized for 200 h at 900–1150°C. Silicon reduces carburization most distinctly at 1050°C. At this temperature alloys with 1% Si heavily carburize, alloys with 2% Si very little carburize, and alloys with 3% Si do not carburize at all. It has been proved that alloys in carburizing atmospheres should be alloyed with 2.0 to 2.5% silicon. Aging tests up to 5000 h at 600–1000°C showed no significant aging temperature. Maximum aging seems to occur at 750°C and takes place during the first 500 h only. Up to a silicon-content of 2.5% there are no differences in the aging behaviour of the tested material. Stress rupture tests show that even with Si-contents of 3%, the underline of the scatter band is reached, and there are no concerns about silicon up to 2.5%.     

Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys

Two refractory high entropy alloys with compositions near Nb₂₅Mo₂₅Ta₂₅W₂₅ and V₂₀Nb₂₀Mo₂₀Ta₂₀W₂₀, were produced by vacuum arc-melting. Despite containing many constituents, both alloys had a single-phase body-centered cubic (BCC) structure that remained not only stable after exposure to 1400 °C, but also disordered, as confirmed by the absence of superlattice reflections in neutron diffraction data. Compressive flow properties and microstructure development of these alloys were determined from room temperature up to 1600 °C. Limited compressive plasticity and quasi-cleavage fracture at room temperature suggest that the ductile-to-brittle transition for these alloys occurs above room temperature. At 600 °C and above, both alloys showed extensive compressive plastic strain. The yield stress of both alloys dropped by 30–40% between room temperature and 600 °C, but was relatively insensitive to temperature above 600 °C, comparing favorably with conventional superalloys.     

The isothermal and cyclic high temperature oxidation behaviour of Ti–48Al–2Mn–2Nb compared with Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr

The isothermal and cyclic oxidation behaviour of Ti–48Al–2Mn–2Nb (at%) were studied at high temperatures in air in comparison with the intermetallic alloys Ti–48Al–2Cr–2Nb and Ti–48Al–2Cr. Tests were performed in air between 800 and 900°C. At 800°C Ti–48Al–2Mn–2Nb showed an excellent oxidation resistance under isothermal and cyclic conditions, comparable with Ti–48Al–2Cr–2Nb, and superior to Ti–48Al–2Cr. At 900°C the isothermal oxidation rate of Ti–48Al–2Mn–2Nb was similar as found for Ti–48Al–2Cr–2Nb, but much lower as that of Ti–48Al–2Cr. Upon cooling the oxide scale formed on Ti–48Al–2Mn–2Nb was prone to spallation. During the cyclic oxidation at 900°C, a steady state condition is reached for both niobium bearing materials, with a net linear mass loss rate, due to spallation and (re-)growth of the oxide scale. The linear mass loss rate for the Ti–48Al–2Mn–2Nb was higher than that of Ti–48Al–2Cr–2Nb, indicative of a higher susceptibility for spallation. During the initial stage of oxidation of all tested materials a complex multi-phased and multi-layered scale was formed consisting of α-Al₂O₃, TiO₂ (rutile), TiN and Ti₂AlN. After longer exposure times the outer scale was dominated by TiO₂. In case of the niobium containing materials no loss of protectivity of the oxide scale was found during the growth of the outer TiO₂ layer (under isothermal conditions). Two-stage oxidation experiments with isotope tracers were performed to study the oxidation mechanism in more detail.     

粉末冶金TiAl基合金显微组织及力学性能的研究

采用粉末冶金方法制备多种成分的ＴｉＡｌ基合金,并研究其显微组织及室温、高温力学性能,结果表明,采用粉末冶金方法能制备成分均匀、显微组织细小的Ｔｉ－Ａｌ－Ｃｒ－Ｎｂ系列合金。添加合金元素对粉末冶金ＴｉＡｌ基合金的显微组织具有显著影响。粉末冶金ＴｉＡｌ基合金的力学性能与其显微组织有密切的关系,显微组织越细小,其室温强度及延性越高,但在高温下,其屈服强度随晶粒尺寸增加而增加。所制备出的Ｔｉ－４７Ａｌ－３     

Anticorrosion properties of textured substrates made of copper-based binary alloys

The tendency of some copper-based binary alloys to oxidize has been studied by the thermogravimetry method. It has been established that, apart from their perfect cubic texture and the high strength properties, substrates made of the Cu–Ni, Cu–0.4% Cr, and Cu–1.6% Fe alloys also possess better anticorrosion properties at the temperature of the deposition of the epitaxial layers (700°C) than the substrates of pure copper. In the alloys containing disperse particles of a second phase, the oxidation resistance decreases in the inverse proportion to the particle size. The Cu–0.6% V ribbon, in which the vanadium particles reach a size of several microns, was even less resistant to oxidation than the copper substrate; therefore, it cannot be used for the deposition of functional layers at elevated temperatures.     

Metal dusting of high temperature alloys

Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H₂-H₂O atmosphere in exposures at 650°C, 600°C, 500°, and 450°C. The materials were pretreated by annealing in H₂ at 1000°C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Cr₂O₃ scale. The resistance to metal dusting is only dependent on the ability to form a protective Cr₂O₃ scale, thus the high Cr ferritic steels proved to be very resistant, the ferritic steels with 12–13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC 66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M₃C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate.     

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract

The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO₂) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr₂O₃ film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.     

High temperature oxidation behavior of chromium‐rich alloys containing high carbides fractions. Part I: Nickel‐base alloys

Six wear‐resistant alloys based on nickel, containing 30 wt.% Cr and from 2.5 to 5.0 wt.% C, were elaborated by foundry and subjected to oxidation by air at 1000, 1100, and 1200 °C, for evaluating the oxidation behavior of hard bulk alloys. Their microstructures are rich in chromium carbides, eutectic, or pro‐eutectic, and they also contain graphite for the highest carbon contents of interest. All the studied alloys obviously display a chromia‐forming behavior, despite the initial low chromium content in the matrix. During oxidation, carbides disappear over an increasing distance from the oxidation front, with the consequence of the enrichment of the neighbor matrix in chromium. The minimal chromium content on the surface after oxidation decreases when the alloy is richer in carbon and increases with the oxidation temperature. The carbide‐free zone tends to be deeper when the oxidation temperature increases, and also when the alloy's carbon content increases, in contrast with low‐C Ni‐30Cr alloys previously studied. The disappearance of carbides means a carbon loss as gaseous oxidized species. This probably disturbs the oxide scales growing on the external surface and may influence the oxidation behavior of the alloys. When present, graphite does not deteriorate dramatically the oxidation resistance of the alloys. The hardness of the alloys are lowered by the exposures to high temperature and by the presence of graphite.     

Cyclic Oxidation of Heat Resisting Steels

Standard cast, heat resisting steels containing 25–29 w/o (weight percent) chromium and 30–36 w/o nickel together with cast alloys containing 45 or 60 w/o nickel plus low levels of aluminium were subjected to cyclic oxidation in air at 1000 and 1150°C. The standard materials suffered rapid weight loss which was somewhat mitigated by the presence of cerium. The 45 w/o nickel alloys were much more resistant and the 60 w/o nickel alloys showed superior resistance to cyclic oxidation. This improvement was due to alumina formation at or near the alloy surface. In the absence of aluminium, alloys underwent subsurface chromium carbide oxidation at a rate independent of alloy chromium content. This effect is shown to be a consequence of rapid oxygen diffusion along internal phase boundaries.     

Temperature and gas composition dependence of internal oxidation kinetics of an Fe?10%Cr alloy in water vapour containing environments

Recently it was proposed, that the hampered formation of external protective chromia scales on FeCr‐alloys in water vapour containing, low‐pO₂ gases is correlated with enhanced internal oxidation of chromium. In the present study the internal oxidation kinetics of Fe? 10Cr (in mass%) during isothermal oxidation in Ar? H₂? H₂O mixtures at temperatures in the range 800–1050 °C has been investigated. It was found that the tendency for Cr to become internally oxidized decreased with decreasing temperature. At the higher test temperatures the internal oxide precipitates consisted of Fe/Cr‐spinel. With decreasing temperature the precipitates near the oxidation front gradually exhibited increasing amounts of chromia. At 900 °C the oxidation morphology in the Ar? H₂ base gas mixture changed from exclusive internal oxidation of Cr at a water vapour content of 2% towards a combined internal Cr oxidation and external Fe‐oxide formation at higher water vapour partial pressures.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

Cyclic oxidation of high temperature coatings on new γ′-strengthened cobalt-based alloys

An emerging class of cobalt-based γ′-strengthened alloys promises higher temperature capabilities compared to current Ni-base superalloys commonly used in aerospace and power generation applications. As with Ni-base alloys, high temperature coatings that enhance environmental resistance are desirable. Single crystal samples of Co–9.2Al–9.0W and Co–7.8Al–7.8W–4.5Cr–2.0Ta (at.%) were coated with vapour phase nickel aluminide and MCrAlY. Samples were subjected to cyclic oxidation at 1100 °C with 300–450 1-h cycles. Compared to NiAl-based coatings, the MCrAlY coatings exhibited superior adherence and an interdiffusion zone free of detrimental intermetallic phases. Evolution of microstructure during cycling is discussed with reference to the available thermodynamic data.     

High temperature corrosion of metallic materials in simulated waste incineration environments at 300–600 °C

The durability of the materials employed in waste incineration plants is affected by different factors that together result in severe degradation of the materials. The effects of alloying elements, as well as deposit composition and temperature range are investigated in this work. Different alloys were exposed in laboratory experiments to a specifically simulated waste incineration atmosphere N₂ – 8 vol.% O₂ – 2000 vppm HCl – 15 vol.% H₂O – 200 vppm SO₂ at 300–600 °C. The samples are covered with different salt mixtures especially chlorides and sulphates. Experiments with PbCl₂‐KCl‐ZnCl₂ at 450 °C, which is molten, showed catastrophic corrosion, but this was not observed in the case with KCl, which is solid. Mixtures of five different sulphates did not cause significant mass losses at 600 °C, but sulphate‐chlorided mixtures caused bigger mass losses. The presence of Al and Si in the alloys decreased mass change effects, the presence of Mo in alloys tested showed a negative effect.     

Influence of Titanium on the High Temperature Oxidation and Chromia Volatilization of Ternary Ni–Cr–C Alloys

Two ternary alloys, Ni–25Cr–0.25C and Ni–25Cr–0.50C (wt%) and three versions containing also titanium (1 and 2 wt%) were cast and submitted to oxidation in dry synthetic air at 1200 °C to observe the effects of titanium on the behaviors of the nickel-based alloys belonging to this category. The mass gains are wholly parabolic in all cases and the values of the parabolic and chromia volatilization constants are typical of a chromia-forming behavior. The mass gains of the Ti-containing alloys are faster than for the Ni–Cr–C alloys, and these kinetic differences are consistent with the differences in chromia thickness and in Cr-impoverishment of the subsurface. In addition, the presence of Ti led to the development of thin a TiO₂ outer scale isolating chromia from hot air, but without benefit for the protection of Cr₂O₃ against volatilization. The obtained results also suggest that Ti may perturb the Cr diffusion in volume but also delay the oxide spallation during cooling.