Steam Oxidation Behavior of Advanced Steels and Ni-Based Alloys at 800 °C

This publication studies the steam oxidation behavior of advanced steels (309S, 310S and HR3C) and Ni-based alloys (Haynes^® 230^®, alloy 263, alloy 617 and Haynes^® 282^®) exposed at 800 °C for 2000 h under 1 bar pressure, in a pure water steam system. The results revealed that all exposed materials showed relatively low weight gain, with no spallation of the oxide scale within the 2000 h of exposure. XRD analysis showed that Ni-based alloys developed an oxide scale consisting of four main phases: Cr₂O₃ (alloy 617, Haynes^® 282^®, alloy 263 and Haynes^® 230^®), MnCr₂O₄ (alloy 617, Haynes^® 282^® and Haynes^® 230^®), NiCr₂O₄ (alloy 617) and TiO₂ (alloy 263, Haynes^® 282^®). In contrast, advanced steels showed the development of Cr₂O₃, MnCr₂O₄, Mn₇SiO₁₂, FeMn(SiO₄) and SiO₂ phases. The steel with the highest Cr content showed the formation of Fe₃O₄ and the thickest oxide scale.     

High-Temperature Oxidation Behavior of SIMP Steel at 800 °C

In this work, the high-temperature oxidation behavior of SIMP and commercial T91 steels was investigated in air at 800 °C for up to 1008 h. The oxides formed on the two steels were characterized and analyzed by XRD, SEM and EPMA. The results showed that the weight gain and oxide thickness of SIMP steel were rather smaller than those of T91 steel, that flake-like Cr₂O₃ with Mn_1.5Cr_1.5O₄ spinel particles formed on SIMP steel, while double-layer structure consisting of an outer hematite Fe₂O₃ layer and an inner Fe–Cr spinel layer formed on T91 steel, and that the location of the oxide layer spallation was at the inner Fe–Cr spinel after 1008 h, which led the ratio between the outer layer and the inner layer to decrease. The reason that SIMP steel exhibited better high-temperature oxidation resistance than that of T91 steel was analyzed due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel.     

Oxidation Behavior of Tribaloy T-800 Alloy at 800 and 1,000 °C

The oxidation behavior of Co-based Tribaloy T-800 alloy has been studied isothermally in air at 800 and 1,000 °C, respectively. The results showed that the oxidation mechanism was dependent on the exposure temperature. The oxidation of the alloy followed subparabolic oxidation kinetics at 800 °C. The oxide scale at this temperature exhibited a multi-layered structure including an outer layer of Co oxide, a layer composed of complex oxide and spinel, a nonuniform Mo-rich oxide layer, an intermediate mixed oxides layer and an internal attacked layer with different protrusions into Laves phase. During 1,000 °C exposure, it followed linear kinetics. The oxidation rendered a relatively uniform external Cr-rich oxide layer coupled with a thin layer of spinel on the top surface and voids at local scale/alloy interface and intergranular region together with internal Si oxide at 1,000 °C.     

Water Vapor Effects on the Oxidation Behavior of Fe–Cr and Ni–Cr Alloys in Atmospheres Relevant to Oxy-fuel Combustion

The oxidation behavior of a number of Fe–Cr- and Ni–Cr-based alloys was studied in atmospheres relevant to oxyfuel combustion at 650?°C. Oxidation was greatly enhanced in ferritic model alloys exposed in low p(O₂) CO₂?+?30%H₂O and Ar?+?30%H₂O gases. Rapidly growing iron oxides appear to be porous and gas permeable. Transition from non-protective to protective oxidation occurs on alloys with higher Cr contents between 13.5 and 22?wt% in H₂O. Excess oxygen, usually found in the actual oxyfuel combustion environments, disrupts the selective oxidation of Fe–Cr alloys by accelerating vaporization of early-formed Cr₂O₃ in combination with accelerated chromia growth induced by the H₂O. Rapid Cr consumption leads to the nucleation and rapid growth of iron oxides. On the contrary, Ni–Cr alloys are less affected by the presence of H₂O and excess O₂. The difference between Fe–Cr and Ni–Cr alloys is not clear but is postulated to involve less acceleration of chromia growth by water vapor for the latter group of alloys.     

Effect of Zn Additions on the Oxidation of Cu–2 at.% Al and Cu–4 at.% Al Alloys in 1 atm O2 at 800 °C

Four ternary Cu–Zn–Al alloys containing 5 or 10 at.% Zn and 2 or 4 at.% Al plus an alloy containing 2 at.% Al and 15 at.% Zn have been oxidized at 800 °C in 1 atm O₂, and their behavior has been compared with that of the corresponding binary Cu–Zn and Cu–Al alloys. For the alloy containing 4 at.% Al, which is already able to form external alumina scales, the addition of Zn is only effective in reducing the mass gain during the fast, initial-oxidation stage. Conversely, the addition of 15 at.% Zn to Cu–2Al is able to prevent the formation of external scales containing mixtures of the Cu and Al oxides, resulting in the formation of external alumina scales after an initial stage of faster rate, producing a limited third-element effect. Finally, the addition of Al to both Cu–5Zn and Cu–10Zn is able to prevent the internal oxidation of Zn, producing a kind of reversed third-element effect. Possible mechanisms for these effects are examined on the basis of general treatments concerning the scaling behavior of ternary alloys.     

Iron Oxidation at Low Temperature (260–500 °C) in Air and the Effect of Water Vapor

The oxidation of iron has been studied at low temperatures (between 260 and 500 °C) in dry air or air with 2 vol% H₂O, in the framework of research on dry corrosion of nuclear waste containers during long-term interim storage. Pure iron is regarded as a model material for low-alloyed steel. Oxidation tests were performed in a thermobalance (up to 250 h) or in a laboratory furnace (up to 1000 h). The oxide scales formed were characterized using SEM-EDX, TEM, XRD, SIMS and EBSD techniques. The parabolic rate constants deduced from microbalance experiments were found to be in good agreement with the few existing values of the literature. The presence of water vapor in air was found to strongly influence the transitory stages of the kinetics. The entire structure of the oxide scale was composed of an internal duplex magnetite scale made of columnar grains and an external hematite scale made of equiaxed grains. ¹⁸O tracer experiments performed at 400 °C allowed to propose a growth mechanism of the scale.     

Evolution of Oxide Film of T91 Steel in Water Vapor Atmosphere at 750 °C

In order to investigate the evolution of oxide film on T91 steel, oxidation tests were conducted in water vapor atmosphere at 750 °C. The phase compositions and microstructures of the oxide scales for early stage oxidation were investigated by using glancing angle XRD and SEM equipped with EDS. The results showed that during the initial oxidation stage Cr-rich oxide film formed and then it covered the sample surface rapidly. The initial Cr-rich oxide film was mainly composed of FeCr₂O₄, (Fe,Cr)₂O₃ and Fe₂O₃. This oxide film acted as a barrier against outward diffusion of iron and inward diffusion of oxygen. During the initial oxidation stage, chromium in the sample surface was consumed gradually, and then a large amount of iron ions penetrated the oxide film and diffused rapidly to the sample surface, resulting in forming an outer “non-protective” Fe₂O₃ layer.     

Oxidation Behavior of Graded NiCrAlYRe Coatings at 900, 1000 and 1100 °C

A graded NiCrAlYRe coating was prepared by combining arc ion plating (AIP) with chemical vapor deposition (CVD) aluminizing. Quasi-isothermal oxidation tests of the graded NiCrAlYRe coating and the conventional NiCrAlYRe coating were performed in air at 900, 1000 and 1100 °C for up to 1000, 1000 and 200 h, respectively. The results showed that the graded NiCrAlYRe coating exhibited better long time oxidation resistance than the conventional NiCrAlRe coating. This favorable oxidation behavior was attributed to the rapid formation of a protective α-Al₂O₃ scale and a sufficient Al reservoir. The structures and morphologies of oxide scales varied under different oxidation conditions. θ-Al₂O₃ was observed on both coatings during oxidation at 900 °C, however, the graded coating showed more favorable conditions for θ-Al₂O₃ to grow than the conventional coating. For the graded coating, phase transformation from θ-Al₂O₃ to α-Al₂O₃ resulted in a sharp decrease in the parabolic rate constant k_p between 900 and 1,000 °C.     

Steam Oxidation Resistance of Advanced Steels and Ni-Based Alloys at 700 °C for 1000 h

The aim of the work was to investigate corrosion resistance of highly alloyed steels and Ni-based alloys in a steam atmosphere for 1000 h at 700 °C. In these steam oxidation experiments, two solid solution strengthened alloys; Haynes^® 230^®, 617 alloy, two gamma-prime (γ′) strengthened alloys; 263 and Haynes^® 282^® and three Cr+Ni- rich stainless steels: 309S, 310S and HR3C austenitic steels were exposed. The study showed that the materials exposed commonly developed thin oxide scales; in Ni-based alloys, these consisted of mainly MnCr₂O₄ spinels and Cr₂O₃, with the exception of 617 alloy where NiCr₂O₄ spinels and Cr₂O₃ were found. In Fe-based alloys, Cr₂O₃, MnCr₂O₄ spinels, Fe,Mn(SiO)₄, and finally Fe₃O₄ developed. No evaporation of chromia has been found within 1000 h test period. Furthermore, the development of TiO₂ was not observed into a large extent in Haynes^® 282^® and 263 alloy, in contrast to the study performed at 800 °C under the same steam environment conditions.     

High Temperature Oxidation Behavior of High Speed Steel for Roll in Water Vapor

The oxidation behavior of high speed steel (HSS) was researched by high temperature thermo balance at 500 to 800℃ in water vapor. The morphology was observed by scanning electron microscope, the microstructure of oxide scale was analyzed by energy dispersive spectrometer and X-ray diffraction spectrum. The results indicate that the mass gain of HSS increases with oxidation temperature rising, the effect of oxidation temperature on the morphology is obvious, water vapor temperature only affects mass gain and affects hardly morphology of oxide scale at the same oxidation temperature. The relevant oxidation mechanisms are also discussed.     

Pre-oxidation Effect on Oxidation Behavior of F91 in Carbon Dioxide at 550 °C

Upon exposure to CO₂ at 550 °C, F91 tends to form rapidly growing scales consisting of an outer Fe oxide and an inner Fe–Cr spinel oxide. A comparative study has been carried out between the pre-oxidized and non-pre-oxidized F91, to determine the influence of pre-oxidation upon the oxidation behavior of F91 in CO₂. Formation of a rapidly growing scale and carburization could be inhibited by a pre-oxidation treatment in air prior to oxidation in CO₂. Although during exposure to CO₂, a fast growing scale still would form locally, pre-oxidation changed its structure. Effects of pre-oxidation time on the oxidation resistance in CO₂ are investigated.     

Oxidation of AM60B Mg Alloys Containing Dispersed SiC Particles in Air at Temperatures Between 400 and 550 °C

AM60B magnesium alloys, with and without dispersed SiC particles, were oxidized between 400 and 550 °C in air. The scales generated consisted primarily of MgO and a small amount of Mg₃N₂ formed by the outward diffusion of cations (Mg, Al, Mn) and the inward diffusion of anions (N, O). The SiC particles were stable in the AM60B alloy during oxidation and increased its oxidation resistance to a certain extent. However, given the predominance of the non-protective MgO as the main oxide, the SiC_p/AM60B composites were inevitably destroyed as oxidation progressed.     

Oxidation Behavior of Sputtered DD98M Nanocrystalline Coating at 1000 °C

Increasing the efficiency of coal fired steam power plants is an important contribution towards clean coal power. In fact, new ferritic steels are expected to withstand 325 bar and 650 °C. Moreover, in order to facilitate CO₂ capture oxygen can be used instead of air for combustion (oxycombustion) so that no NO_X emissions are produced. Boiler components, such as superheater tubes, are exposed to both steam and fireside corrosion and at higher temperatures, ferritic steels corrode at very fast rates under both atmospheres. A solution can be found in the use of protective coatings, a number of which, applied by techniques capable of depositing said coatings both on the inner and outer surfaces of tubes, are being studied within nationally and European funded projects. In particular, two new Ni and Cr modified aluminide coatings deposited on P92 by non-line-of-sight hybrid processes have been produced and the preliminary results of on-going laboratory testing, both under oxycombustion model atmospheres as well as under pure steam at 650 °C are promising, in particular those exhibited by the Cr enriched aluminide coating. Moreover, results obtained in a pilot oxycombustion boiler operated by CIUDEN in Leon, Spain are also shown.     

Air Oxidation of FeCoNi-Base Equi-Molar Alloys at 800–1000°C

The oxidation behavior of FeCoNi, FeCoNiCr, and FeCoNiCrCu equi-molar alloys was studied over the temperature range 800–1000 °C in dry air. The ternary and quaternary alloys were single-phase, while the quinary alloy was two-phase. In general, the oxidation kinetics of the ternary and quinary alloys followed the two-stage parabolic rate law, with rate constants generally increasing with temperature. Conversely, three-stage parabolic kinetics were observed for the quaternary alloy at T 900°C. The additions of Cr and Cu enhanced the oxidation resistance to a certain extent. The scales formed on all the alloys were triplex and strongly dependent on the alloy composition. In particular, on the ternary alloy, they consist of an outer-layer of CoO, an intermediate layer of Fe₃O₄, and an inner-layer of CoNiO₂ and Fe₃O₄. Internal oxidation with formation of FeO precipitates was also observed for this alloy, which had a thickness increasing with temperature. The scales formed on the quaternary alloy consisted of an outer layer of Fe₃O₄ and CoCr₂O₄, an intermediate layer of FeCr₂O₄ and NiCr₂O₄, and an inner layer of Cr₂O₃. Finally, the scales formed on the quinary alloy are all heterophasic, consisting of an outer layer of CuO, an intermediate-layer of CuO and Fe₃O₄, and an inner-layer of Fe₃O₄, FeCr₂O₄, and CuCrO₂. The formation of Cr₂O₃ on the quaternary alloy and possibly that of CuCrO₂ on the quinary alloy was responsible for the reduction of the oxidation rates as compared to the ternary alloy.     

Transition Between Different Oxidation Modes of Binary Fe–Si Alloys at 600–800 °C in Pure O2

The oxidation behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si has been studied at 600–800 °C under 1 atm O₂. Fe–5Si and Fe–9Si followed multi-stage parabolic kinetics, while Fe–13Si showed more complex kinetics at all temperatures. The increase in Si content resulted in a transition from the internal oxidation of Si beneath an external scale of iron oxides to the exclusive external formation of silica. The critical contents required for the transitions between the various possible oxidation modes of the binary Fe–Si alloys were calculated and compared with the experimental results. A thermodynamic mechanism for the formation of Fe-rich oxide nodules observed in the oxidation of Fe–5Si and Fe–9Si has been proposed.     

Oxidation Comparison of Alumina-Forming and Chromia-Forming Commercial Alloys at 1100 and 1200 °C

21 Commercial alumina-forming and chromia-forming high-temperature alloys were tested at 1100 and 1200 °C for up to 1000 h. Exposure was performed in air using 100-h cycles on ~2-mm-thick test coupons placed in highly sintered alumina crucibles. Visual appearance, gross mass gain and amount of spallation were monitored after every 100 h. Investigation of topography and cross sections was performed in LOM and SEM with EDS at breakaway oxidation or at 1000 h depending on what occurred first. In order to investigate the earlier part of the oxidation process in more detail, separate samples from all materials were exposed under the same conditions for a single 100-h cycle and were then investigated in the same manor.     

The Oxidation Behavior of Ni–15Cr–5Al–xSi (x = 0, 1, 3, 5 wt%) Alloys in Air at 1100 °C

Oxidation of Metals - In order to explore effect of silicon on the oxidation resistance of Ni-based superalloys, the cyclic oxidation behavior of Ni–15Cr–5Al–xSi...     

Improved Oxidation Resistance of a New Aluminum-Containing Austenitic Stainless Steel at 800 °C in Air

The oxidation resistance of austenitic stainless steels modified with various aluminum contents was investigated. The weight gain per unit area is in parabolic relation to oxidation time, and the oxidation rate significantly decreases with increased aluminum content. Outer layer oxides of austenitic stainless steel transform from Cr₂O₃ to a composite oxide layer comprising Cr and Al, and more dense Al-containing oxides formed with increasing the added Al contents. Since the diffusion of element Al is enhanced and the diffusion of element Cr is inhibited, the oxides enriched in Al dramatically contribute to the improved oxidation resistance of austenitic stainless steels at high temperature. The possible oxidation mechanisms are also proposed based on microstructural observations.     

Corrosion of Binary Fe–Si Alloys in Reducing Oxidizing and Sulfidizing–Oxidizing Atmospheres at 700 °C

The corrosion behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si was studied at 700 °C in an H₂–CO₂ gas mixture providing 10^?20 atm O₂ as well as in an H₂–H₂S–CO₂ gas mixture providing the same oxygen pressure coupled to an S₂ pressure of 10^?8 atm. All the alloys followed complex kinetics which were mostly linear for Fe–5Si, but showed one or two parabolic stages for the other two alloys. Simple oxidation produced essentially two-layered scales in which Si was confined to the alloy consumption zone in the form of silicon oxide and iron-silicon double oxide. Corrosion in the oxidizing–sulfidizing gas mixture produced scales composed of a thick external zone of pure FeS followed by an internal region containing complex mixtures of FeS with Si and Fe oxides. Internal oxidation of silicon was only observed for the oxidation of Fe–5Si in both environments. The extent of corrosion decreased in both gas mixtures with an increase in the Si content of the alloys. Finally, the addition of sulfur produced a significant increase of the overall mass gains for each alloy.     

Effect of Water Vapor on the Oxidation Behavior of Fe–1.5Si in Air at 1073 and 1273 K

The oxidation behavior of Fe–1.5Si was investigated at 1073 and 1273 K in air, air–H₂O, Ar–H₂O, O₂–H₂O, and O₂ atmospheres. The extent of corrosion in atmospheres containing H₂O increased rapidly after an incubation period of slow oxidation, the incubation period becoming shorter in the order, O₂–H₂O, air–H₂O, and Ar–H₂O. With increasing H₂O contents in air–H₂O, the incubation time decreased. During the incubation period, oxidation was slow, because of the formation of an inner Si-rich oxide layer and a Pt marker was located between the external Fe₂O₃ (Fe₃O₄ included) and an inner Si-rich oxide layer. During the rapid oxidation, the inner FeO+Fe₂SiO₄ layer thickened and a Pt maker was at the interface between an external Fe-oxide and an inner FeO+Fe₂SiO₄ layer. Observations of scale cross sections indicated that voids made channels along the boundaries of columnar FeO crystals, suggesting transport of water molecules. The Si-rich oxide layer changed into an FeO+Fe₂SiO₄ mixture due to penetration of water molecules. A combined process of perforating dissociation and transport of water molecules is suggested to be the cause of the rapid growth of the inner FeO+Fe₂SiO₄ layer.