Microstructure and Volatile Species Determination of Ferritic/Martensitic FB2 Steel in Contact With Ar + 40 %H2O at High Temperatures

Volatile species were identified by means of mass spectrometry during the initial stages of oxidation of ferritic/martensitic steel FB2 (Fe–9Cr–1.5Mo–1.1Mn–0.05Si–0.1C–0.1Ni–1Co–0.2V–0.05Nb–0.008B) at 650 °C and 800 °C in a steam atmosphere of Ar + 40 %H₂O for 180 and 150 h, respectively. The greater amount of Cr-containing volatile species detected, (i.e.) CrO₃(g), CrO₂(OH)(g) and CrO₂(OH)₂(g), from the oxidized alloy at 800 °C compared to 650 °C showed that loss and breakdown of chromia (Cr₂O₃) are accelerated by increasing the temperature. For instance, the CrO₂(OH)₂(g), which is responsible for the breakdown of chromia scales, was observed at 650 °C for 150 h of oxidation, while the same species was detected at 800 °C for 75 h. It was found that the hematite (Fe₂O₃) that formed in the initial stage of oxidation was constantly evaporating during the test. Therefore, the sample gradually lost protection against oxidation, with chromia evaporating followed by volatilization of products that formed, such as the magnetite (Fe₃O₄) and wüstite (Fe_1?x O). Eventually, oxidation breakaway of the sample was reached. Simultaneously, the oxidation kinetics of the samples was determined. The morphology/composition and structure of oxidized samples were also studied using the scanning electron microscopy–electron back-scatter diffraction/energy dispersive X-ray spectroscopy and X-ray diffraction techniques.     

Kinetic Features of Iron Oxidation in Liquid Lead Saturated with Oxygen Below and Above the Chaudron Point (570 °C)

The oxidation kinetics of Armco–Fe in stagnant lead melts saturated with oxygen at 550 and 650 °C was investigated and the peculiarities of structure, phase and elemental composition of scales were determined. At 550 °C oxidation follows a parabolic law up to 1,500 h and then (1,500–2,000 h) oxidation accelerated. At 650 °C during 100 h the scale grew rapidly according to a quadratic time dependence. Then (100–500 h) the oxidation rate decreased sharply. From 500–1,000 h oxidation accelerated. The scales formed at 550 and 650 °C consisted of Fe₃O₄ and FeO/Fe₃O₄, respectively. The scales had a duplex structure. The outer-oxide layer grew from the initial solid metal–liquid metal interface towards the melt while the inner layer grew towards the iron substrate. The defectiveness of scale altered with time. The possible reaction mechanisms in the Fe–Pb[O] system are discussed.     

Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000?��C

The 4509 alloy (Fe?C18Cr?CNb?CTi) was oxidised in dry and wet air in the 800?C1000 °C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 °C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 °C, under wet air 7.5 vol.% H₂O. It is proposed that the oxidant H⁺/OH^? species react at the internal interface with iron in the chromium-depleted alloy zone. Wüstite reacts with Cr₂O₃ to form FeCr₂O₄. Outward iron diffusion leads to Fe₃O₄ and Fe₂O₃ formation. The chromia scale was consumed by reaction with wüstite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.     

Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coating in H<Subscript>2</Subscript>–H<Subscript>2</Subscript>O Environment

Isothermal oxidation behavior of an HVAF-sprayed NiCoCrAlY coating on AISI 304L was studied in an Ar–10 %H₂–20 %H₂O environment at 600 °C. Techniques such as BIB/SEM, EDS, and XRD were used to comprehensively characterize the coating and the coating/substrate interface to investigate the oxidation mechanisms. Results were also compared with those obtained from an uncoated AISI 304L substrate. The alumina-forming NiCoCrAlY coating was found to exhibit superior oxidation behavior due to the formation of a slow-growing and protective Al₂O₃ scale, while the chromia-forming bare 304L substrate lost its protective capability due to the formation of a duplex [Fe₃O₄ on (Fe,Cr)₃O₄ spinel oxide] corrosion product layer.     

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.     

Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe₂O₃ and (Fe,Co,Cr)₃O₄ spinel with a small amount of Cr. However, (Fe,Co,Cr)₃O₄ spinel and Fe₂O₃ were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co₃O₄ and (Fe,Co)₃O₄ with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)₃O₄ spinel layer and an inner Cr₂O₃ layer was developed on the sputtered coating.     

Effect of Pre-Oxidation at 800?��C on the Pitting Corrosion Resistance of the AISI 316L Stainless Steel

In situ X-ray diffraction was used to identify the oxides formed on the AISI 316L (containing 2% Mo) stainless steel during isothermal oxidation at 800 °C, in air. Good oxidation behavior was observed on this steel when considering kinetics, structural characteristics and scale adherence. It was demonstrated that molybdenum plays a protective role in that it hinders the outward iron diffusion and leads to the lower growth rate and the better scale adherence. The oxide scale was then composed of Cr₂O₃ with a small amount of Mn_1,5Cr_1,5O₄ at the external interface. Pre-oxidation of the AISI 316L also improved its aqueous corrosion resistance. No pitting corrosion occured during the corrosion test. Aqueous corrosion testing also showed that the oxide scale formed at 800 °C is crack-free and still adherent after cooling to room temperature.     

Cyclic oxidation of AISI 316L stainless steel – influence of water vapour between 800 and 1000°C

At 800 and 900°C, 10 vol.-%H₂O in air has little effect on the AISI 316L stainless steel oxidation under isothermal and cyclic conditions. The oxide scale is composed of Cr₂O₃ with a small amount of Mn_1·5Cr_1·5O₄ at the external interface. Results show that water molecules or protons can modify the diffusion process in the scale and lower the oxidation rate. At 1000°C, a deleterious effect of water vapour on the scale structure is observed. In situ X-ray diffraction was used to analyse the oxide formation on AISI 316L specimens during isothermal oxidation at 1000°C in moist air. Results show that the breakaway oxidation is due to the iron oxidation starting after 31 h oxidation. This leads to an external Fe₂O₃ scale growth and an internal multilayered FeCr₂O₄ scale formation. In wet air, thermal cycling conditions lead to continuous weight losses at 1000°C, whereas the scale remains adherent in dry air.     

Effects of Grit Blasting and Annealing on the High-Temperature Oxidation Behavior of Austenitic and Ferritic Fe-Cr Alloys

Grit blasting (corundum) of an austenitic AISI 304 stainless steel (18Cr-8Ni) and of a low-alloy SA213 T22 ferritic steel (2.25Cr-1Mo) followed by annealing in argon resulted in enhanced outward diffusion of Cr, Mn, and Fe. Whereas 3 bar of blasting pressure allowed to grow more Cr₂O₃ and Mn _x Cr_3?x O₄ spinel-rich scales, higher pressures gave rise to Fe₂O₃-enriched layers and were therefore disregarded. The effect of annealing pre-oxidation treatment on the isothermal oxidation resistance was subsequently evaluated for 48 h for both steels and the results were compared with their polished counterparts. The change of oxidation kinetics of the pre-oxidized 18Cr-8Ni samples at 850 °C was ascribed to the growth of a duplex Cr₂O₃/Mn _x Cr_3?x O₄ scale that remained adherent to the substrate. Such a positive effect was less marked when considering the oxidation kinetics of the 2.25Cr-1Mo steel but a more compact and thinner Fe _x Cr_3?x O₄ subscale grew at 650 °C compared to that of the polished samples. It appeared that the beneficial effect is very sensitive to the experimental blasting conditions. The input of Raman micro-spectroscopy was shown to be of ground importance in the precise identification of multiple oxide phases grown under the different conditions investigated in this study.     

The Effect of Cr on the Lifetime of Al-Rich Amorphous Oxide Layer Formed on Fe–Cr–Al Alloys at 650 °C

The oxidation behavior of Fe–6Al with different Cr contents (0–24 at.%) at 650 °C in air was investigated so as to clarify the role of Cr in the oxidation resistance of the Al-rich amorphous oxide layer. The time to breakdown of the Al-rich amorphous layer was found to increase with increasing alloy Cr content. This corresponds to the time to increase the rate of oxidation by formation of Fe₃O₄. Such a beneficial effect of Cr to maintain the protective Al-rich amorphous layer for a longer oxidation time is attributed to the enhancement of outward diffusion flux of Al by positive “cross-term effect” of Cr in the Al- and Cr-depleted zone.     

Oxidation of Minor Elements from an Iron–Nickel–Chromium–Cobalt–Phosphorus Alloy in 17.3% CO2–H2 Gas Mixtures at 700–1000 °C

Fe–Ni–Cr–Co–P alloys were exposed to 17.3% CO₂–H₂ gas mixtures to investigate the oxidation of minor elements in metallic alloys in the early solar system. Reaction temperatures varied between 700 and 1000 °C. Gas-phase equilibrium was attained at 800, 900, and 1000 °C, yielding H₂–H₂O–CO–CO₂ gas mixtures. Experiments at 700 and 750 °C did not achieve gas-phase equilibrium and were performed in H₂–CO₂ gas mixtures. Reaction timescales varied from 1 to 742 h. The experimental samples were characterized using optical microscopy, electron microprobe analysis, wavelength-dispersive-spectroscopy X-ray elemental mapping, and X-ray diffraction. In all experiments Cr experiences internal oxidation to produce inclusions of chromite (FeCr₂O₄) and eskolaite (Cr₂O₃) and surface layers of Cr-bearing magnetite [(Fe,Cr)₃O₄]. At 900 and 1000 °C, P is lost from the alloy via diffusion and sublimation from the metal surface. Analysis of P zoning profiles in the remnant metal cores allows for the determination of the P diffusion coefficient in the bulk metal, which is constant, and the internally oxidized layer, which is shown to vary linearly with distance from the metal surface. At 800 and 900 °C, P oxidizes to form a surface layer of graftonite [Fe₃(PO₄)₂] while at 700 and 750 °C P forms inclusions of the phosphide-mineral schreibersite [(Fe,Ni)₃P].     

Evaluation of High Temperature Corrosion in Simulated Waste Incinerator Environments

In this study, high temperature reactions of Fe–Cr alloys at 500 and 600 °C were investigated using an atmosphere of N₂–O₂ 8 vol% with 220 vppm HCl, 360 vppm H₂O and 200 vppm SO₂; moreover the following aggressive salts were placed in the inlet: KCl and ZnCl₂. The salts were placed in the inlet to promote corrosion and increase the chemical reaction. These salts were applied to the alloys via discontinuous exposures. The corrosion products were characterized using thermo-gravimetric analysis, scanning electron microscopy and X-ray diffraction.The species identified in the corrosion products were: Cr₂O₃, Cr₂O (Fe_0.6Cr_0.4)₂O₃, K₂CrO₄, (Cr, Fe)₂O₃, Fe–Cr, KCl, ZnCl₂, FeOOH, σ-FeCrMo and Fe₂O₃. The presence of Mo, Al and Si was not significant and there was no evidence of chemical reaction of these elements. The most active elements were the Fe and Cr in the metal base. The Cr presence was beneficial against corrosion; this element decelerated the corrosion process due to the formation of protective oxide scales over the surfaces exposed at 500 °C and even more notable at 600 °C; as it was observed in the thermo-gravimetric analysis increasing mass loss. The steel with the best performance was alloy Fe9Cr3AlSi3Mo, due to the effect of the protective oxides inclusive in presence of the aggressive salts.     

Oxidation Behavior of High-Speed Steel Used for Hot Rolls

Non-isothermal oxidation kinetics of the high-speed steel (HSS) were studied by thermal gravimetric analysis. The surface and cross-sectional morphology of the HSS oxide film formed at different temperatures and durations were observed by scanning electron microscopy, and the corresponding chemical composition was analyzed by using energy dispersive spectrometer. The composition and structure of the oxide film were also investigated by X-ray diffraction. The results showed that the oxide scale of the HSS is mainly composed of Fe₂O₃, Fe₃O₄ and FeCr₂O₄. Temperature is the main factor on the quality of the oxide film. Below 600 °C, the oxidation rate of the steel is slow and the thickness of the oxide film is below 5 µm. However, the oxidation rate sharply increased as the temperature reaches 600 °C. Cr, Mo and V tend to concentrate at the scale/steel interface and form FeCr₂O₄ and other oxides.     

High-Temperature Oxidation Behavior of CrMoV,F91 and Mar-M247 Superalloys Exposed to Laboratory Air at 550 °C

The oxidation behavior of three commercial superalloys, CrMoV, F91 and Mar-M247, was studied at 550 °C in laboratory air for 1000 h. Mar-M247 superalloy showed the best oxidation resistance, which is attributed to the formation of a scale rich in Cr₂O₃ and Al₂O₃, followed by F91 and CrMoV. A thick duplex oxide formed on CrMoV alloy and spallation was observed. The results for CrMoV alloy showed that calculated Fe diffusion in magnetite was 200 times faster than literature values for Fe diffusion in Fe₃O₄, which is attributed to grain-boundary diffusion and the effect of impurity on diffusion. F91 initially formed a protective chromium-rich oxide layer followed by formation nodules, leading breakaway oxidation. The oxide nodules consisted of a duplex structure with different morphologies and oxide phases from duplex oxide scale in CrMoV.     

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.     

The Mechanism of Phase Transformation in Thermally-Grown FeO Scale Formed on Pure-Fe in Air

The isothermal phase transformation behavior of thermally grown oxide scale of FeO, which was formed on Fe at 700 °C in air for 16 min, was investigated at 320, 450, 500, 520, and 560 °C in air. The phase transformation of FeO was found to consist of four transformation modes: (1) growth of outer Fe₃O₄ layer; (2) precipitation of Fe₃O₄; (3) formation of magnetite seam; and (4) eutectoid decomposition of FeO. The transformation was always completed by the eutectoid decomposition at all temperatures in the present study; however, the proportion of transformation mode (1) and (2) strongly depended on temperature. At higher temperatures growth of the outer Fe₃O₄ layer initially predominates, but the precipitation of Fe₃O₄ controls the initial transformation at lower temperature before the eutectoid reaction. The eutectoid reaction was found to be initiated by Fe nucleation from Fe-saturated FeO. Fe saturation in FeO was due to growth and/or precipitation of Fe₃O₄ and formation of the magnetite seam layer, which acts as a diffusion barrier for Fe inward diffusion into Fe substrate. It was proposed that these transformation modes, growth and/or precipitation of Fe₃O₄ and magnetite seam formation, are necessary to begin the eutectoid reaction, i.e., completion of FeO scale transformation.     

Characterization of Oxide Scales Formed on Ferritic Stainless Steel 441 at 1,100 °C under water vapor

This study focuses on the characterization of the oxide scales formed after different exposure times in the range of 2.5–20 min. A commercially available ferritic steel grade AISI 441 was exposed to wet argon at 1100 °C with 5, 9 and 13% H₂O. Raman microspectroscopy, XRD, EDS and XPS were used to fully characterize the oxide scale. For all samples exposed for over 4 min, the scale was constituted of three layers in this order: a thin top layer of spinel phases (Fe,Cr,Mn)₃O₄ with local outgrowths; a second and main layer of Cr₂O₃ + (Mn,Cr)₃O₄; and finally a bottom layer of SiO₂. The uncommon presence of Fe in the top layer was also observed.     

High Temperature Oxidation of the Austenitic (35Fe27Cr31Ni) Alloy Sanicro 28 in O2 + H2O Environment

The present study investigates the high temperature oxidation of alloy Sanicro 28 (35Fe27Cr31Ni) in 5% O₂ and in 5% O₂ + 40% H₂O. Polished steel coupons were isothermally exposed in a tube furnace at 600, 700 and 800 °C for up to 168 h. The samples were investigated by gravimetry, grazing angle X-ray diffraction (XRD), Auger electron spectroscopy (AES), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy/energy dispersive X-rays (STEM/EDX). The results show that the material forms a protective scale in both environments. The scale is duplex. The inner part of the scale consists of corundum type chromium-rich (Cr _x Fe_1?x )₂O₃, and the outer layer consists of spinel type oxide. Chromia is lost from the protective oxide by vaporization of CrO₂(OH)₂ in O₂ + H₂O environment. The capacity of Sanicro 28 to suffer chromia vaporization without forming a rapidly growing iron-rich oxide is attributed to its high Cr/Fe ratio. The spinel formed at the oxide/gas interface could in addition be beneficial for oxidation behavior in wet oxygen because it may slow down chromia evaporation.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

Influence of dissolved oxygen concentration and exposure temperature on the steam oxidation behaviour of HR3C

ABSTRACT

Oxidation behaviour of an austenitic steel HR3C at 620～650°C for 1000?h in steam with two different levels of dissolved oxygen (DO), 20?ppb and 5?ppm, was investigated by gravimetry, X-ray diffraction and scanning electron microscopy. Results showed that mainly a single-layered (Cr, Mn)₂O₃ scale was developed. The oxide growth followed near-power kinetics and was accelerated with DO concentration and exposure temperature. At the higher DO concentration, the formation of (Cr, Mn)₂O₃ with a much smaller size was favoured. Besides, the oxide scale became undulated and interfacial voids formed. Higher exposure temperature resulted in the growth of the (Cr, Mn)₂O₃ crystals and the change of their morphology from acicular to granular. Fe₃O₄ nodules and Ni-Cr spinel occurred at 650°C/5?ppm DO. Related oxidation mechanisms were discussed.