Oxide scale formation on Al containing Ni–Cr‐based high temperature alloys during application as flame tube material in recirculation oil burners

The flame tube is an important functional component of burners using the concept of the flame tube stabilised combustion. Under typical combustion conditions the material of the flame tube is exposed to high temperatures (≥900 °C) and to corrosion attack by the combustion gases. Furthermore as the burners are generally operated intermittently, the material suffers from extreme temperature and atmosphere changes. For flame tubes, a lifetime of approximately 8000 h is desired. Predominantly metallic high temperature materials are used. The scope of the present work was to test—under application conditions and for maximum material temperatures exceeding 900 °C—alternative high temperature alloys for use as tube material. The corrosion resistance of the austenitic Ni–Cr‐based alloys (601, 602 CA, 617 and 693) has been investigated in a burner rig at maximum material temperatures of 950 and 1000 °C and with exposure times from 50 to 3000 h. The chromium content of the alloys was between 20 and 30 wt% and that of aluminium between 1 and 3.4 wt%. Metallographic cross‐sections of samples of the alloys were analysed by electron microprobe yielding information about the microstructure and composition of the oxides in the surface zone and variations during exposure time. This study focuses on the observed specific effects of the alloying element aluminium on the development of the oxide scale and on the lifetime of the alloys. At the alloy surface after 500 h exposure time a chromium oxide scale had formed with aluminium oxides underneath predominantly along grain boundaries. For the alloys with the lower aluminium content, the aluminium oxides built up an open network but not a closed layer. For the alloy with the highest aluminium content (alloy 693) after 50 h two different characteristic microstructures at the surface were found. In one case, the grains at the surface were covered with chromium oxide on top and the remaining grain surface was completely enclosed by aluminium oxides. In the other case, the aluminium oxide formed a thin layer directly below the chromium oxide scale. After 500 h exposure time, a significantly thinner chromium oxide scale and massive internal chromium oxides were observed. Catastrophic corrosion, formation of internal oxides and aluminium nitrides started even after 500 h. It will be demonstrated that the early breakdown of alloy 693 is linked to the aluminium oxides which act as a barrier constricting the diffusion of chromium from the alloy matrix towards the surface. Under the conditions of extreme temperature changes given in the burner the aluminium oxide layer on its part did not provide corrosion protection.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

Ti-Cr合金的非等温氧化及着火预测(英文)

采用热重分析、XRD和SEM等方法研究Ti-Cr合金(0≤w(Cr)≤25%)从室温至1723K的非等温氧化行为及氧化膜的微观结构,探讨Cr元素对Ti-Cr合金抗氧化能力的影响机制。结果表明:当Cr含量小于某一临界值wC时,随着Cr含量的增加合金的抗氧化能力降低;当Cr含量高于wC时,随着Cr含量的增加合金的抗氧化能力提高;当温度高到1000K时,Ti-Cr合金的氧化仍符合抛物线规律,且主要发生钛的氧化;Ti-Cr合金氧化后基体中存在氧扩散层,氧化膜主要为金红石型TiO2,内层氧化膜出现富Cr现象,Cr氧化物的析出提高了Ti-Cr合金的抗氧化能力。金属和合金的着火是一个快速非等温氧化的过程,预测了Ti-Cr合金着火阶段的氧化机制。     

氧化时间及铬含量对Fe-Cr钢高温氧化行为的影响

为了测定不同氧化时间以及铬含量对高温条件下钢材表面氧化铁皮组织和厚度的影响,将Fe-5Cr钢与Fe-10Cr钢在1000 ℃空气条件下氧化60~180 min,采用增重法绘制其氧化动力学曲线,并利用光学显微镜、能谱仪和X射线衍射仪对氧化铁皮的断面形貌和物相进行研究。结果表明,两试验钢氧化初期为“气-固”反应,中后期为“气相-氧化铁皮-固相”反应。两试验钢氧化铁皮结构均由外氧化层、内氧化层和内氧化区域组成。当氧化时间为180 min时,Fe-10Cr钢检测到了内氧化物Cr₂O₃。空气中氧元素向内扩散与基体中铬元素发生反应生成内氧化物Cr₂O₃,再与氧化铁皮层中的FeO发生固相反应生成尖晶石结构产物FeCr₂O₄。随着氧化时间的增加,由于内氧化物Cr₂O₃不断受到内氧化层的包裹而转为外氧化,内外氧化的转变使得基体不断被腐蚀,氧化铁皮厚度不断增加。     

Behavior of Oxide Scale on 9Cr–1Mo Steel Under Varying External Stresses

The effects of different external stresses on the oxidation behavior of9Cr–1Mo steel were investigated. Tensile specimens were subjected tostresses of 11, 20, 28, and 40 MPa and the oxidation behavior was studiedat a temperature of 973 K. The elongation of the specimen was determined byan extensometer. An acoustic-emission unit was employed to monitor theintegrity of the oxide scale. The oxide scale was found to undergo bucklingbefore spalling, in the case of unstressed specimens, and in the case ofthose specimens subjected to a stress up to 28 MPa. The specimen with 40 MPastress showed the development of cracks. The application of external stressup to 28 MPa (average strain rate for 28 MPa stress was1.2 x 10^-7 s^-1) had a beneficial effect with respectto the adherence of the oxide scale. The specimen with 40 MPa of stress(average strain rate of 5.2 x 10^-77 s^-1) showedsubstantial weight gain on oxidizing up to 140 hr. The unstressed specimenrevealed enhanced spallation when compared with the stressedones. Investigation by SEM revealed the spalling of the oxide scale when theduration of oxidation was higher than 70 hr at all stress levels employed inthe present investigation. Formation of cracks at 40 MPa exposed fresh areasto oxygen and caused accelerated oxidation. Analysis by energy-dispersiveX-ray spectrometry (EDS) revealed the segregation of chromium at the oxideridges. The segregation of silicon was also significant for the specimensubjected to a stress of 40 MPa. Analysis by XRD clearly revealed thepresence of oxides of chromium and iron (Cr₂O₃ andFe₂O₃) and spinel-type oxide (FeCr₂O₄).     

Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in H2O,Oxide-Growth Mechanisms,and Scale Adherence

Oxidation of 1-mm thick chromium samples with 7 to <1 wt. ppm hydrogenhas been studied at 900°C in a closed reaction chamber. The gases usedwere O₂, H₂O, and gas mixtures of O₂+H₂Oat a total pressure of about 20 mbar. By means of oxygen labeling intwo-stage oxidations, H₂ ¹⁶O followed by H₂ ¹⁸O,the position of oxide growth within the oxide scale was determined withsecondary-ion mass spectrometry (SIMS). The results are consistent withvisual observations and scanning electron microscopy (SEM), which showsflat oxides with good adherence when formed in H₂O. Whenreplacing O₂ with H₂O in the atmosphere, the oxidationrate increases by a factor of two with an increase of oxide growth at boththe inner and outer part of the oxide. An increased metal (cation) transportis observed when 5 wt. ppm hydrogen is present in the chromium metal priorto oxidation in both O₂ and H₂O. This is detrimentalfor the adherence of the oxide scale. The uptake of hydrogen in H₂Oexposure for 1600 min was measured to 1.5 wt. ppm. In exposures toO₂+H₂O mixtures, no H₂ is formed and no netwater is consumed as long as O₂ is present. A plausible reactionscheme based on an experiment with H₂ ¹⁸O is presentedto explain this observation.     

Influence of CeO2-coating on the high-temperature oxidation of chromium

The reactive-element effect on high-temperature oxidation of chromium metal was studied at oxygen pressures of 10⁵ Pa (1 atm) and 0.67 Pa and at temperatures of 800 and 1050°C under both isothermal and cyclic conditions. The reactive element, cerium, was applied as a cerium-oxide coating by sol-gel deposition. Growth and adherence of the chromia scale on the metallic substrate were investigated by kinetic weight-gain measurements, and the microstructure was characterized by scanning electron microscopy, transmission electron microscopy, EDX-analysis, X-ray diffraction, and Auger analysis. The study of the oxidation kinetics under isothermal conditions at 800°C clearly showed a reduction in the growth rate of the oxide film when cerium oxide was present. Improvement of the oxide-scale adhesion was also observed when ceria-coated chromium was subjected to thermal cycling. The applicability of a number of models which have been used to explain the reactive-element effect is discussed.     

Metal dusting and coking of alloy 803

Investigation was made by SEM examination on metal dusting and coking behaviours of alloy 803 in a flowing gas mixture of H₂-CO-H₂O. It was found that an oxide scale arisen on the sample surface at the beginning of exposure. Metal dusting started when graphite deposition occurred earlier at the local defects in the oxide scale than the defects were repaired by enough supply of chromium from the interior of alloy matrix. Coke consisted of graphite filaments and metallic particles produced by disintegrating of alloy matrix, and grew up from the defects in the oxide scale with pit left in the sample surface. Increasing chromium content, doping a small amount of silicon and reducing grain size to create fast diffusion paths for chromium and silicon to alloy surface, all promote the formation of a dense oxide scale and favor early self-repairing of the defects in the oxide scale before occurrence of graphite deposition. The resistance of an alloy to metal dusting can be improved generally by means of these methods.     

Corrosion Behavior of 9Cr-1Mo Steel in Sulfur Dioxide Environment

Corrosion behavior of annealed 9Cr-1Mo steel was studied in SO₂ environment at 1173 K, at flow rates from 8.33 × 10^?7 to 33.33 × 10^?7 m³/s, and parabolic rate law was followed. The rate constants were found to be independent of flow rate, within the range of flow rate investigated. Corrosion at temperatures from 973 to 1173 K, at a constant flow rate of 16.66 × 10^?7 m³/s, at 1 atmospheric pressure, for 6 h also exhibited parabolic law, however, the rate constants were observed to increase significantly with rise in temperature. The outer layer of the scale formed at 973 K was essentially of iron oxide, with small amount of chromium oxide whereas the inner layer was predominantly of chromium sulphide and chromium oxide. The scale formed at 1173 K was multilayered, in contrast to double layered formed at 973 K and 1073 K. The outer thick layer of the scale formed at 1173 K, consisted of iron oxide followed by thin substrate of chromium sulphide, iron sulphide/iron oxide, and chromium sulphide/chromium oxide toward the substrate. A model is proposed for the process of corrosion of 9Cr-1Mo steel in SO₂ environment, based on the present investigation.     

Oxidation behaviour of Ni–Cr based alloy containing Si during high temperature application in an oil burner

Abstract

In certain modern oil burners, the combustion reaction is started in a flame tube. In the combustion atmosphere, the tube material is exposed to high temperatures and temperature changes. Nickel–chromium alloys are used to meet the requirement of high oxidation resistance. The paper presents the results on the oxidation behaviour of the silicon containing alloy 603 exposed to a low NO_x burner at temperatures of 950 and 1000°C up to 2000 h. Beneath a chromia scale silica precipitates formed at the beginning of exposure, which grew laterally establishing a nearly continuous interlayer. The interlayer disintegrated during the continued exposure. A thinner chromia scale was observed for alloy 603 compared with the scales observed for aluminium containing nickel–chromium alloys. This was attributed to a pronounced scale spallation. After 1000 h at 1000°C catastrophic oxidation of alloy 603 occurred involving Mo oxide and internal chromium oxide.     

The oxidation of chromium- and nickel-implanted nickel at high temperatures

The isothermal oxidation behaviour in oxygen of nickel, implanted with nickel and chromium ions, has been studied in the range 950–1150°C using kinetic and electronoptical techniques. Implantation with nickel ions has a significant and lasting effect on the oxidation kinetics and on the morphology and grain configuration of the NiO scale. The oxide grains are generally smaller, more facetted and more mismatched for the implanted surfaces and there is more extensive stress generation during oxide growth. After an initially slower rate, the oxide on the implanted surfaces develops at a faster rate than that on the unimplanted surfaces, particularly at the higher temperatures. Implantation with chromium ions inhibits the initial development of the NiO scale. However, subsequently, the oxidation rate is more rapid for the implanted surfaces and increases progressively with increasing chromium ion dose. The results can be accounted for in terms of doping of the NiO and the much decreased grain size of the oxide. It is concluded that short-circuit grain boundary diffusion processes, as well as bulk lattice diffusion of Ni²⁺ ions, are important in the oxidation of nickel at high temperatures.     

Mechanism of electrolytic pickling of stainless steels in a neutral sodium sulphate solution

Oxide scale that mainly consists of spinel‐type metal oxides is formed on stainless steels when they are heated up during the manufacturing cycle. The removal of the oxide scale and chromium depleted subscale by pickling is one of the most important processes during the production. Electrolytic pickling in neutral sodium sulphate is widely used and provides relatively fast removal of the oxide scale. Despite its vast use, the essence of the electrochemical mechanism of electrolytic pickling is not clear. Stainless steel EN 1.4301 and EN 1.4404 samples were annealed in a laboratory furnace to produce an oxide scale similar to process conditions. The oxide scale was protective at potentials equal to the passive region but dissolved at anodic potentials above 1050 mV vs. SCE. The dissolution was found to proceed by electrochemical reactions of the scale. Three different stages were discernible in the dissolution process. At the beginning chromium and manganese of the outer layer were preferentially dissolved. When the chromium content in the outer layer decreased, the scale was enriched of iron and dissolution was hindered causing the electrode potential to increase. At the third stage the potential first decreased and then steady state was obtained. The oxide thickness was greatly reduced and the chromium content was lowered below the base metal level. At the steady state the remaining oxide was rich in iron and silicon. Silicon was not dissolved by electrochemical reactions and remained at the surface after prolonged polarisation.     

Corrosion behaviour of Nickel-chromium alloys in molten carbonate

The corrosion behaviour of nickel-chromium alloys with chromium contents ranging from 10 to 25% in molten carbonate under reducing gas atmospheres was investigated with electrochemical methods (cyclic voltammetry). The oxide scale was investigated, both after quenching from stationary conditions at fixed potentials and shortly after interrupting a cyclic voltammogram. The composition and the properties of the corrosion products, present as oxide layers, are potential-dependent. At potentials in the range of -1500 to -900 mV the layer consists of lithium chromite (LiCrO₂). For 18- and 25%-chromium alloys a continuous layer is formed. The lithium chromite formed on the 10%-chromium alloy at potentials in the same range does not form a continous oxide layer. At potentials in the range of -700 to +100 mV a continuous oxide layer is formed on all alloys. The corrosion product is a cubic solid solution of nickel oxide (NiO) and lithium chromite. In cyclic voltammetry experiments a hexagonal solid solution of lithium chromite and nickel oxide is formed at -700 mV. The hexagonal solid solution is an intermediate corrosion product between lithium chromite, which is stable at more cathodic potentials and a cubic solid solution of nickel oxide and lithium chromite which is stable at more anodic potentials. On the 10% chromium alloy with a non-continuous oxide scale, a cubic solid solution is formed at -500 mV. At about -300 mV the chromate formation and dissolution starts. For the 18- and 2.5%-chromium alloys, on which a continuous lithium chromite-scale is formed at the open circuit potential, a hexagonal solid solution of lithium chromite and nickel oxide is formed during the anodic scan at -700 mV. When the chromate formation and dissolution starts at about -300 mV, the chromium content of the scale decreases, the nickel content increases and the hexagonal solid solution becomes a cubic solid solution. The cathodic scan is very similar to the cathodic scan of pure nickel. During the cathodic scan the oxide scale mainly consists of a nickel-rich cubic solid solution of nickel oxide and lithium chromite because chromium has dissolved as chromate ions during the preceding anodic scan. The reactions are essentially the same as those on pure nickel [1, 2, 3].     

Intergranular oxidation and internal void formation in Ni-40% Cr alloys

Internal void formation and intergranular oxidation behaviour have been studied during the oxidation of two Ni-40Cr alloys in 1 atm oxygen at 1000° to 1200°C. The development of an external Cr₂O₂ scale causes vacancies to be generated in the alloy at the alloy-scale interface as chromium diffuses into the scale, and others to be generated in the alloy due to the different diffusion rates of chromium towards the interface and of nickel back into the bulk alloy. At 1200°C, internal void formation results from condensation of such vacancies at inclusions in the grains and at the grain boundaries. The intergranular oxidation observed at 1000°C, 1100°C and to a lesser extent. 1200°C results from preferential condensation of vacancies to form voids in the alloy grain boundaries. Significant depletion of chromium in the alloy adjacent to the scale facilitates the supply of oxygen from the scale and its penetration into the alloy grain boundaries to form intergranular oxide. Such intergranular oxide develops deep into the alloy following diffusion of this oxygen through a porous network in the oxide, which arises because of the vacancy condensation, and oxidation of chromium at the tip of the intergranular penetration.     

The attack of Co-Cr alloys by Ar-SO2 atmospheres

Cobalt alloys containing up to 25% chromium have been exposed to Ar-10% SO₂ atmospheres at temperatures between 600 and 1000° C. The results show that, although an increase in chromium content leads to a reduction in the reaction rate, even to negligible rates in the cases of the higher chromium contents, all of the alloys are eventually subjected to rapid attack at more or less longer times, depending on the chromium content. The mechanism of the reaction appears to involve the formation of a more or less protective oxide layer which is eventually penetrated by sulfur. The sulfur forms chromium sulfides at the metal-scale interface, removing the chromium from solution and causing an expansion that cracks the protective scale, allowing both the ingress of gas and the formation of rapidly growing cobalt compounds. The process occurs rapidly with Co-5% Cr alloys, whereas, only the initial sulfur penetration is observed with Co-25% Cr alloys during the time scale of the investigation. The penetration of sulfur is thought to occur as a molecular gas species permeating through the scale down physical defects.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

An investigation of the growth-mechanism of Cr2O3 on pure chromium in 1 atm oxygen at 950° C

The growth mechanism of Cr₂O₃ scale formed when pure chromium was oxidized at 950°C in oxygen at 1 atm pressure has been investigated. Isotope tracer techniques were used to determine the growth sites of the oxide. The scale was buckled extensively, with the convex side always toward the gas, never the reverse. The following growth-mechanism is proposed. Initially growth occurs entirely by cation diffusion, with new oxide being formed at the oxide-gas interface. Then, at a time that is not the same for all parts of the scale, the growth mechanism changes to one in which new oxide is formed within the outer part of the scale.     

An Investigation of the Growth Mechanism of Scale Formed on Chromium Containing Low Amounts of Sulfur,Chlorine, and Phosphorus

The growth mechanism of the flat and adherent scale formed at 900°C in 0.1 atm oxygen on chromium, which contained 1 ppm (wt) sulfur, 0.08 ppm (wt) chlorine, and 0.2 ppm (wt) phosphorus, has been found to be very different in adjacent parts of the scale, consistent with the results of previous tracer studies of the growth mechanism of chromia formed on chromium. In some places, it grew primarily by chromium transport, while in others it grew primarily by oxygen transport and in still others it grew by a mixture of both. New oxide formed within the outer part of the scale and, in some cases, throughout the scale. A tentative hypothesis is proposed to explain why growth mechanisms were different in different parts of the scale on the same specimen. The scale formed at 950°C on Fe–20%Cr–0.11%Si, which contained 15 ppm sulfur, grew predominantly by cation transport, with only a small amount of oxygen transport. In this case also, new oxide formed within the outer part of the scale.     

The influence of chromium on the Oxidation of β-NiAl at 1000°C

It is known that chromium influences the growth of an alumina oxide layer on β-NiAl at high temperatures. It has been claimed that oxidized chromium will act as nuclei for the θ → α-alumina phase transformation. This study presents RBS-data on the presence of chromium in the oxide scale which tends to substantiate this claim. Also XRD results will affirm the above mentioned claim.     

The air oxidation of two-phase Cu-Cr alloys at 700–900°C

The oxidation in air of three two phase Cu-Cr alloys with nominal Cr contents of 25, 50, and 75 wt. % was studied at 700–900°C. The alloys corroded nearly parabolically, except at 900°C, when the corrosion rates decreased with time more rapidly than predicted by the parabolic rate law. The corrosion rate decreased for higher Cr contents in the alloy under constant temperature and generally increased with temperature for the same alloy composition. The scales were complex and consisted in most cases of an outermost copper oxide layer free from chromium and an inner layer composed of a matrix of copper oxide or of the double oxide Cu₂Cr₂O₄, often containing particles of chromium metal surrounded by chromia and then by the double oxide. Metallic copper was also frequently mixed with chromia. Cr-rich regions tended to form continuous chromia layers at the base of the scale, especially at the highest temperature. No chromium depletion was observed in the alloy.