High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 2 ₁ ⁴ Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of -Fe₂O₃. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.     

Inherent oxidation protection of Fe-5Cr-15Ni-2Si-4.5Mo

The oxidation mechanism of Fe-5Cr-15Ni-2Si-4.5Mo alloy was investigated in order to determine the role of Si and Mo in providing oxidation resistance. It was determined that the oxidation protection in the temperature range 750–950°C resulted from formation of a continuous oxide sublayer of SiO ₂ (or possibly Fe ₂ SiO ₄).Molybdenum formed an intermetallic Fe ₂ Mo _1–x Si _x that eventually diffused out into the grain boundaries and formed a protective barrier to the oxidation process. The mechanism behind the improved oxidation is the formation of a SiO ₂ layer at the metal-oxide interface that retards the outward diffusion of Fe. It was also established that the oxidation mechanism was controlled by an activation energy equal to that of Fe ³⁺ ions diffusing through SiO ₂.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

EFFECTS OF ACTIVE ELEMENTS Y AND Ce ON HIGH TEMPERATURE OXIDATION OF Fe-25Cr-40Ni ALLOY

The effects of small amount(≥0.05%)of active elements Y and Ce in Fe-25Cr-40Ni al-loy have been investigated on the kinetics of early stage of high temperature oxidationas well as composition and microsructure of oxide film by ion backscattering and slowpositron beam.The results show that Y and Ce can reduce evidently the high tem-perature oxidation rate of FeCrNi alloy at early stage and can effectively enhance theCr_2O_3 formation and retard the oxide formation of Fe and Ni in the surface layer thusthe microstructure of the oxide film is improved.Y and Ce enter the oxide film andconcentrate within several tens nanometres from surface of the outer layer.The mech-anism of oxidation resistance for Ce is different from Y.Ce can decrease the densityof vacancy defects in oxide layer,so the cations are restrained from diffusion outwardthrough vacancies.While Y can control the outward diffusion of cations through va-cancies at the early statge of oxidation(shorter than 12 min),aften that restrained thecations from diffusion outward along grain boundaries up to 60 min oxidation.     

High temperature corrosion of nanoceria coated 9Cr-1Mo ferritic steel in air and steam

In this article, air and steam oxidation resistance of bare and nanoceria coated 9Cr-1Mo ferritic steel at 650 °C for 500 h is reported. The kinetics of the oxidation process and the changes in the morphology and the chemistry for both the specimens were analyzed by backscattered electron and EPMA mapping. The nanoceria coated 9Cr-1Mo ferritic steel showed better oxidation resistance than uncoated steel under similar conditions. The thickness of the oxide scales formed on the coated samples was significantly reduced as compared to the uncoated substrates. The effectiveness of such a thin and simple nanocoating to improve the oxidation resistance of 9Cr-1Mo ferritic steel compared to the pristine is attributed to the change in the migration of cations from outward to inward diffusion of oxygen as the major mechanism.     

Oxidation mechanism of an Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy at 470 °C (Part II)

This paper is the second part of a global study on the oxidation process of an Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth mechanism of a duplex oxide scale. The oxide layer has a duplex structure composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer grows by iron diffusion until Pb-Bi/oxide interface whereas the Fe-Cr spinel layer grows, at the metal/oxide interface, inside the space kept “available” by the iron vacancies accumulation due to iron outwards diffusion for magnetite formation. This growth mechanism is close to the “available space model”. However, this model is completed by an auto-regulation process based on oxygen supply.     

Formation and Oxidation Behavior of a Diffusion-Barrier-Coating System on a Ni–Mo base Alloy at 1,373 K in Air

A Ni-aluminide/Re-base diffusion-barrier-coating system and, for comparison, a sole Ni-aluminide coating were formed on the Ni–Mo based alloy Hastelloy-X, and the oxidation behavior of the coated alloys was investigated in air at 1,100 °C for up to 400 h. After 100 h of oxidation the diffusion-barrier-coating system consisted of a duplex structure comprised of an inner σ-(Re, Mo, Cr, Ni) and an outer β-NiAl layers and covered by a protective Al₂O₃ scale. By contrast, the Ni-aluminide coating had degraded, forming a reaction diffusion zone, γ-Ni(Cr, Mo, Fe, Al), with voids, and the external scale exfoliated extensively. After 400 h of oxidation of the diffusion-barrier-coating system, an intermediate reaction-diffusion zone was present. The Mo of the alloy substrate was enriched in the inner σ-layer, changing it from a Re-base alloy to a Mo-base alloy. The Mo-base alloy appeared to enhance both inward-Al diffusion and outward diffusion of alloying elements such as Mo, Fe, and Cr to form the intermediate reaction-diffusion zone. The outer layer consisted mainly of β-NiAl with channels of γ-Ni(Cr, Mo, Fe, Al). It was concluded that the Re-base alloy such as a σ-phase in the Ni–Cr–Re system acts as an effective diffusion-barrier layer, but that this is not the case for the Mo-base alloy layer.     

Oxidation behavior of Mn and Mo alloyed Fe-16Ni-(5-8)Cr-3.2Si-1.0Al

Oxidation tests were conducted on a master alloy, Fe-16Ni-(5–8)Cr-3Si-lA1, to which (0–4) wt/o pct Mn and/or Mo were added. Tests were conducted at temperatures ranging from 1073–1273 K for times up to 1000 hr. Additions of Mn resulted in formation of a dual oxide structure and decreased oxidation protection. Addition of Mo significantly improved oxidation protection by formation of an intermetallic Fe(Mo)Si precipitate that eventually formed a protective SiO₂ oxide sublayer. Increasing the Cr concentration in an alloy containing both Mn and Mo resulted in a slight increase in weight gain. To first order, the apparent oxidation activation energy for all the alloys was nearly constant, 121 kJ/mol, suggesting that the same mechanism controlled the oxidation for all compositions. The oxidation protection was related to the alloy components and concentration.     

Oxidation resistance of aluminum-coated Fe-20Cr alloys containing rare earths or yttrium

Aluminum-coated Fe-20Cr-(rare earth or yttrium) alloy foils were developed with oxidation resistance equivalent or superior to Fe-20Cr-5Al-(rare earth or yttrium) alloy foils. The coated foils were made by dipping Fe-20Cr sheet into a salt-covered aluminum bath and then rolling the sheet to foil. Oxidation resistance of the coated foil was enhanced by adding rare earths or yttrium to the Fe-20Cr substrate alloys to insure oxide adherence. Test results indicate that only sufficient addition to tie up sulfur as a stable sulfide is needed in the Fe-20Cr alloy. Aluminum-coated foils show lower oxide growth rates than similar Fe-Cr-Al alloys, most likely the result of fewer impurities (particularly Fe) is the coated foils' growing oxide scale.     

A study of the Cr-depleted surface layers formed on four Cr-Ni steels during oxidation in steam at 600° C and 800° C

Three commercial steels with 18% Cr-11% Ni, 19% Cr-25% Ni, and 19% Cr-35% Ni and a laboratory-made 20% Cr-35% Ni steel have been oxidized in steam at 600°C and 800°C. Two types of oxides are distinguished: a thin layer rich in Cr₂O₃ and patches of thick oxide of Fe, Ni, Cr spinels. The Cr concentration profile in the steels under the first type has been determined by microprobe analysis of ground and pickled or annealed specimens oxidized for various times. The variation of the Cr concentration in the alloy at the alloy-oxide interface with surface treatment, time, and temperature is discussed. It is concluded that one reason for the more extensive nodular growth on pickled or annealed specimens is their greater Cr depletion after short time oxidation. The interdiffusion coefficients in the Cr-depleted zones have been determined on ground and annealed specimens oxidized at 800°C. The acceleration of the diffusion rate due to grinding is clearly demonstrated. A fair agreement with diffusion data from diffusion couples is found.     

Einfluß von Chloriden auf die Oxidation des 2¼ Cr-1 Mo-Stahls

Effects of chlorides on the oxidation of the 2¼ Cr-1 Mo steel Effects of NaCl deposited on oxide scales were investigated for the oxidation of the 2¼Cr-1 Mo steel in the temperature range 450–650 °C. The presence of NaCl causes accelerated oxidation under formation of porous and cracked Fe₂O₃ layers and of solid FeCl₂ at the metal/oxide interface. By reaction of the chloride with the oxide scale Cl₂ is formed, which penetrates into the scale and forms FeCl₂ at the inner phase boundary, this is evaporating continuously and is oxidized to iron oxide upon diffusing to the surface, whereby the chlorine is set free again. An active oxidation results which is catalysed by Cl₂. The rate of the accelerated oxidation is determined mainly by the evaporation and outward diffusion of gaseous FeCl₂ and therefore by temperature by temperature and thickness of the oxide scale. Other parameters such as the flow velocity, the area covered with NaCl or the chlorine pressure, which is present at the outer oxide scale have less effect on the overall rate.     

Oxidation mechanism of a Fe-9Cr-1Mo steel by liquid Pb-Bi eutectic alloy (Part III)

This paper is the third part of a global study on the oxidation process of a Fe-9Cr-1Mo martensitic steel (T91) in static liquid Pb-Bi. It focuses on the growth kinetics simulation of a duplex oxide scale. The oxide layer is composed of an internal Fe-Cr spinel layer and an external magnetite layer. The magnetite layer growth is controlled by iron diffusion inside the Fe-Cr spinel and the magnetite lattice. An analytical simulation, matching with the experimental kinetics, is performed. The Fe-Cr spinel layer growth kinetics is also simulated. The protective scale is determined by simulation and by an original experiment using GD-OES.     

The Influence of Temperature on Oxide-Scale Formation during Erosion--Corrosion

Four steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, 353 MA) and one Ni-based superalloy (Inconel 625) were exposed in an erosion--corrosion test rig at the temperatures 20, 350, 550 and 700°C for 1 week. The atmosphere was air and the particle velocity 1.2 m/s. The composition and thickness of the developed surface layers were determined by Auger electron spectroscopy and X-ray photoelectron spectroscopy. The ferritic and Ni-based alloys investigated show a minimum in wastage rate around 350°C due to the development of a particle strengthened/toughened composite layer on the surface. The greatest wastage rates were measured at 700°C. Rapid diffusion paths created in the oxide from the particle bombardment results in the growth of oxide nodules at the oxide/metal-interface causing protruding oxide flakes which are chipped away. At this temperature the ferritic steel Fe9Cr1Mo is degraded to a larger extent than the austenitic steels.     

Oxidation behavior of TiAl coated with a fine-grain Co-30Cr-4Al film

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-m thickness has been studied at 1100–1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K. Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded. This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature II: Mechanism of Carburization

In parallel to the formation of a duplex oxide scale, 9Cr–1Mo steel carburizes strongly under CO₂ at 550?°C and this carburization accelerates with time. It is observed that an increase of the total CO₂ pressure in the environment from 1 to 250 bars induces a higher carbon deposition in the inner Fe–Cr rich spinel oxide layer. In order to explain this phenomenon, modelling of the carburization process was carried out. A mechanism involving gas diffusion of CO₂ and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.     

The initiation of selective oxidation of a ferritic stainless steel at low temperatures and oxygen pressures

Oxidation of a ferritic stainless steel of type Fe-18Cr-2Mo has been performed in the temperature range 285–495°C and oxygen partial pressure range 10^?9-10^?8 torr. The chemical composition of thin oxide layers formed has been analysed by means of Auger Electron Spectroscopy and interpreted in terms of available chromium, iron and oxygen at the solid/gas interface. The selective oxidation of chromium is considered by different probabilities for the oxidation of available chromium and iron respectively. At oxide thicknesses below 100 Å the supplies of chromium and iron are ruled by diffusion in the steel matrix. The results have been used to predict the chemical compositions of two subsequently growing oxide layers provided the thickness of the first oxide layer is below 50 Å.     

Oxidation behavior of 2.25Cr-1Mo steel with prior tempering at different temperatures

The oxidation behavior of a normalized 2.25Cr-1Mo steel tempered previously for 10 hr at different temperatures between 873 and 1023 K has been studied up to a maximum duration of 1000 hr in air at 773–973 K. The oxidation resistance of the steel was found to decrease significantly with the temperature of tempering. Tempering of this steel is reported to cause microstructural changes involving precipitation of Cr as carbides and a decrease in the effective (free) Cr contents, that could influence the oxidation resistance of the Cr-containing alloys. Relative compositions across the thickness of the oxide scales, as analyzed by SEM/EDX and SIMS, suggest that a less Cr-rich (and less protective) and thicker scale on the steel formed because previous tempering caused extensive depletion of free Cr.     

活性元素Y和Ce对Fe—25Cr—40Ni合金高温氧化的影响

用离子背散射和慢正电子束研究了活性元素Y和Ce对Fe-25Cr—40Ni合金在高温初期氧化动力学、氧化膜表层成份和微观缺陷结构的影响,实验结果表明了微量活性元素(≥0.05%)在高温氧化初期显著减少了合金的氧化速率,有效地促进了Cr_2O_3的生长,抑制了Fe和Ni氧化物的形成,改善了氧化膜的微观结构,活性元素结合进入氧化膜并在外层膜中(约几十nm)富集,活性元素Ce抗氧化机理不同于Y,Ce使合金氧化膜的空位缺陷显著降低,主要控制了阳离子沿晶格空位向外扩散,而含Y合金由氧化初期主要控制阳离子沿晶格扩散转变为主要控制阳离子沿晶界向外扩散。     

Influence of Cryomilling on the Morphology and Composition of the Oxide Scales Formed on HVOF CoNiCrAlY Coatings

Commercially available, gas-atomized CoNiCrAlY powder was cryomilled to produce powder with nanocrystalline grains. The cryomilled powder and conventional gas-atomized powder were thermally sprayed using the HVOF process to prepare two coatings with fine-grain (～15 nm) and coarse-grain (～1 μm) microstructure, respectively. The two coatings were isothermally oxidized in air at 1000° C for up to 330 hr. The morphology and composition of the oxide scales formed on the two coatings were compared with each other. The results indicate that, while a fine-grain microstructure can promote the formation of a pure alumina layer on the coating by increasing the Al diffusion rate toward the surface, it can also accelerate the Al depletion by increasing the Al diffusion rate toward the substrate, which results in the formation of non-alumina oxides after long-term oxidation. The mechanisms governing the oxide formation are discussed in terms of atomic diffusion and thermodynamic stability.     

Oxidation kinetics,breakaway oxidation,and inversion phenomenon in 9Cr-1Mo steels

The oxidation behavior of 9Cr-1Mo ferrritic steels has been studied in air, oxygen, and steam at 1 atm pressure at various temperatures. Long-term experiments in air were carried out from 500–800°C by measuring the weight gains by interrupting the experiment at regular intervals of time. Short-term experiments in oxygen from 500–950°C and in air at 900 and 950°C were carried out by continuous recording of weight gain versus time in a continuousrecording thermogravimetric balance. Short-term experiments in steam were carried out using a special atmosphere furnace attached to the thermogravimetric balance. In air/oxygen, the weight gains at 700°C were lower than those at 600°C, while in steam, the weight gains at 800°C were lower than those at 700°C. This inversion phenomenon was observed for all the three steels viz. 9Cr-1Mo (high Si), 9Cr-1Mo (low Si), and 9Cr-1Mo-Nb steel. Examination of the oxide scales was carried out using SEM/EDAX, AES/ESCA, and X-ray diffraction techniques, and a mechanism is proposed for the occurrence of the inversion phenomenon.