Fe Cr Spinel Research Articles

Effect of Chromium and Water Vapor of Low Carbon Steel on Oxidation Behavior at 1050 °C

The high-temperature oxidation behavior of Cr-containing steels are investigated by using continuous thermogravimetric analysis (TGA), in which the coupon samples are exposed at 1050 °C in dry and wet air for up to 60 min. The results showed that the oxidation kinetics of Cr-containing steels followed the parabolic law. The phase of oxide scale is examined by X-ray diffraction (XRD), and the surface and cross-sectional morphologies of the specimens after oxidation are studied in detail with scanning electron microscope (SEM) and the electron probe microanalysis (EPMA). Addition of chromium in steel can promote the formation of Fe–Cr spinel layer and Cr2O3 layer to reduce the oxidation rate at high temperatures, especially in dry air. In wet air, the rate of high-temperature oxidation is increased due to the reaction between H2O and FeO, Cr2O3 and Fe, which can generate volatile hydroxide of CrO2(OH)2 to transform the Fe–Cr spinel from dense layered structure to porous structure. In addition, the amount Cr lost in different steam content is calculated by the classical method of chromium vaporization rates. The oxidation mechanism is proposed based on ions diffusion, chemical reaction, and microstructure of oxide–substrate interface.

High Temperature Oxidation Behavior of SIMP Steel and T91 Steel at 800

Oxidation behavior of two ferrite/martensite(F/M) steels, namely a novel 9%-12% Cr modified F/M steel(SIMP steel) and a commercial T91 steel were comparatively studied in air at 800 oC. The oxide scales formed on the two steels were characterized by XRD, SEM and EPMA. The results show that the oxide scale formed on SIMP steel is single- layer composed of Cr_2O_3 and Mn_(1.5)Cr_(1.5)O_4 spinel particles,and Si was enriched at the interface between the chromia scale and matrix; while the oxide scale formed on T91 steel has a double layered structure with an outer hematite Fe_2O_3 layer and an inner Fe-Cr spinel layer. The SIMP steel has better high temperature oxidation resistance than T91 steel, which may be due to the higher content of Cr and Si beneficial to the formation of the compact oxide scale on the SIMP steel.

Stress corrosion behavior of T91 steel in static lead–bismuth eutectic at 480 °C

The corrosion behavior of stressed C-rings made of martensitic steel T91 was investigated through constant strain tests. The specimens with different initial hoop stresses (0 MPa, 150 MPa and 300 MPa) were exposed to static oxygen saturated lead–bismuth eutectic (LBE) at 480 °C for 500 h, 1000 h and 1500 h, respectively. The results showed that no crack was found on the outer surface of all the specimens after exposure; and the microscopic analysis showed that the specimens were covered with two oxide layers, which included a magnetite outer layer and a Fe–Cr spinel inner layer. The transformation of spinel into magnetite at the spinel/magnetite interface might be promoted by stress, which increased the difference between the thickness of the inner and outer layers. Moreover, the steel loss was estimated by the observed oxide layers; it increased rapidly when the stress was above 300 MPa, and was about 1.3 times of when the stress was absent.

Comparative oxidation behaviour of Fe-9Cr steel in CO2 and H2O at 550 °C: Detailed analysis of the inner oxide layer

Fe-9Cr steel was oxidized in pure water vapour and in CO2 at 550°C. In both environments the Fe–Cr spinel layer was composed of small equiaxed grains which stoichiometry evolved from Fe2.7Cr0.3O4 to Fe2CrO4 in H2O and from Fe2.6Cr0.4O4 to Fe1.9Cr1.1O4 in CO2. In both cases the mean stoichiometry was Fe2.3Cr0.7O4. Single oxidation mechanism was proposed: the “available space model”. Carbon for oxidation in CO2 and hydrogen for oxidation in H2O were detected at metal/oxide interface within the steel. These atoms could be responsible for nanometric cavities formation by trapping vacancies created by outward cationic diffusion.

Structural features of copper ferrite-chromites

The structural features and the distribution of cations over crystallographic sites of copperchromite, copper ferrite, and mixed chromite-ferrites with the spinel structure obtained by thermal decomposition at 600°C and 900°C of mixed hydroxy compounds of copper, iron, and chromium with the composition Cu2+/(Fe3++Cr3+) = 1/2 and different Fe3+/Cr3+ ratios are studied using a set of physicochemical methods. It is shown that the spinel-structure phases formed exist in two modifications: cubic and tetragonal, depending on the Fe3+/Cr3 ratio. The crystallographic relationship between the cubic and tetragonally distorted phases of spinel is analyzed. The distribution of cations over crystallographic sites, the character and degree of tetragonal distortion of Cu–Fe–Cr spinel depend on the Fe3+/Cr3+ ratio: when Fe3+/Cr3+ > 1, the ratio of unit cell parameters c*/а* > 1; when Fe3+/Cr3+ < 1, the ratio c*/а* < 1; when Fe3+/Cr3+ = 1, the spinel is cubic irrespective of the temperature of thermal treatment. The oxygen coordination of copper ions in the spinel structure has a significant effect on the catalytic properties of the samples in the low-temperature water gas shift reaction.

High temperature oxidation behaviour of ferritic stainless steel SUS 430 in humid air

The high temperature oxidation behaviour of ferritic stainless steel SUS 430 was investigated over the temperature range from 1000 to 1150 °C in humid air containing 18% water vapour. Isothermal thermogravimetric analyses were performed to study the oxidation kinetics. The microstructure, composition and thickness of the oxide scale formed were investigated via optical microscopy (OM), X-ray diffraction and a scanning electron microscope equipped with an energy dispersive spectrometer. The results indicate that breakaway oxidation occurs at all temperatures and that its onset is accelerated by increasing temperature. The growth rate of the multilayer oxide scale follows a parabolic law with apparent activation energy of 240.69 kJ/mol, and the formation of FeO is decreased when the temperature is higher than 1120°C. The inner oxide scale, Fe-Cr spinel, grows mainly inward and internal oxidation is observed even in a short oxidation test at 1150°C for 105 s. The mechanism of high temperature oxidation of SUS 430 in humid air containing 18% water vapour is discussed.

High Temperature Oxidation Behavior of SIMP Steel

The oxidation behavior of two ferrite/martensite (F/M) steels, including a novel 9–12 % Cr modified F/M steel named SIMP steel and a commercial T91 steel, were studied in air at 700 °C for up to 1,000 h. The oxides formed on the two steels were characterized and analyzed by XRD, XPS, SEM and EPMA. The results showed that the oxide formed on SIMP steel was single-layer including flake-like Cr2O3 with Mn1.5Cr1.5O4 spinel particles, while the oxide on T91 steel exhibited a double layers structure consisting of an outer hematite Fe2O3 layer and an inner Fe–Cr spinel layer. The reason why the SIMP steel showed better high temperature oxidation resistance than T91 steel was analyzed to be due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel. Based on all the results, a kinetic model describing nucleation, growth and degradation of the oxide scale formed on surfaces of the two steels was proposed.

Effects of exposure temperature and time on corrosion behavior of a ferritic–martensitic steel P92 in aerated supercritical water

Oxidation behavior of a ferritic–martensitic steel P92 exposed to aerated supercritical water at 400–550°C and 25MPa was investigated by gravimetry, X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The weight gain approximately followed near-power oxidation kinetics. The oxidation rate was strongly dependent on temperature and followed Arrhenius behavior. The oxide film consisted of porous outer layer with hematite and magnetite and dense inner layer with Fe–Cr spinel. Exposure temperature and time showed significant influences on oxide surface morphologies, thickness and constituents. Higher temperature accelerated the formation of pores in oxide film. Related oxidation mechanism was also discussed.

Characteristics of oxide scale formed on ferritic stainless steels in simulated reheating atmosphere

The aim of this study is to optimise the stainless steel oxidation behaviours during hot rolling. The high temperature oxidation behaviours of ferritic stainless steels B443NT and B445J1M were studied over the temperature range from 1000 to 1150°C in a humid atmosphere containing 18% water vapour, as measured by a thermogravimetric analyser (TGA). The results indicate that breakaway oxidation occurs at 1090°C for the B443NT steel, which is 60°C lower than that for the B445J1M steel. The occurrence of iron oxide nodules on the steels marks the onset of breakaway oxidation; however, the breakaway oxidation phenomenon of B445J1M is different from that of B443NT due to a compact and continuous Mn–Cr spinel which is formed on the surface of B445J1M. The oxide nodules with regenerated Cr2O3 scale underneath the Fe–Cr spinel display better adhesion without showing pores at the metal–scale interface.

铁素体-马氏体P92钢在600℃/25MPa超临界水中的氧化特性

The oxidation behavior of ferritic-martensitic P92 steel was studied in supercritical wa- ters containing different dissolved oxygen of 0, 100 and 300 μg/L at 600 ℃ under 25 MPa. The re- sults show that a typical double-layered oxide scale is formed on all the samples, which consists an outer layer Fe-rich magnetite and an inner layer Cr-rich Fe-Cr spinel. Cracks on the surface of ox- ide scales are found at the initial oxidation stage, which is different from the experimental phenom- enon in supercritical water at 550 ℃. A distinct gap between inner oxide layer and substrate alloy is observed in some regions of the sample which is exposed to the supercritical water containing 300 μg/L dissolved oxygen. The mechanism concerning the influence of dissolved oxygen on oxi- dation behavior and the gap formation is further discussed.

Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H2O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn–Cr spinel particles on the top of an inner and continuous Fe–Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.

HCM12A Cr-rich oxide layer investigation using 3D atom probe

Three-dimensional atom probe techniques were applied to the investigation on the oxide scale in 12Cr ferritic/martensitic steel, HCM12A. A duplex oxide scale was formed in lead bismuth eutectic at 450–500°C, during 5500h. Samples were located 500–700nm away from the boundary between magnetite and Fe–Cr spinel layers, while the total oxide layer thickness is 18μm. It detected super enrichment of Cr with a size of ten nm roughly, as well as depletion of Fe and enrichment of O at the same site. Surrounding the Cr super enrichment area, enrichment of Si was newly noticed due to the scanned profile of detected atom counts. It is also confirmed that Pb and Bi concentration in the observed spinel region is almost null or less than 0.01atomic%, which is possible detecting lowest limit.

Steam-side scale morphologies associated with scale exfoliation from ferritic steel T22

Observations of steam-formed scales on type T22 ferritic steel reheater tubes have provided some insights into the scale morphologies associated with exfoliation. Measurements indicated that as the total thickness of the adherent scales increased, the ratio of the thicknesses of the main inner (Fe–Cr spinel) and outer (magnetite) layers increased continuously to a maximum of approximately 2.5. The often-assumed thickness ratio of 1.0 did not persist after a laminated structure of pairs of spinel and magnetite layers was developed in the main inner layer. The development of such multi-laminations appeared to be associated with a decrease in the rate of growth of the main outer, magnetite layer. In this particular set of samples, exfoliation occurred in scales of total thickness greater than approximately 270 μm, and involved separation of a single pair of oxide layers of similar thickness that consisted of the original outer magnetite layer and part of the original spinel layer. The scale lost in subsequent exfoliation events at essentially the same site consisted of a further pair of layers comprising the new magnetite layer re-grown after the initial event, and a further part of the original spinel layer. Exfoliation events did not appear to involve the whole of the spinel layer existing at that time, only the equiaxed region immediately subjacent to the magnetite and a portion of the laminated structure.

Oxidation of Oxide Dispersion Strengthened Steels: II. Kinetics and Mechanism

The oxidation resistance of 18%Cr-oxide dispersion strengthened (ODS) ferritic steels with and without 5%Al has been investigated in air at 700900 °C for time period up to 540 h. The oxidation rate of ODS steels is significantly dependent on the oxidation time and temperature. Compared to Al-containing ODS steel, the finer grains of Al-free ODS steel are due to the formation of smaller coherent oxide particles which suppress the steel's grain growth. The grain refinement of ODS steels is expected to allow rapid segregation of Cr or Al to the steel surface, so that the continuous Fe-Cr spinel or alumina layer is formed quickly in comparison to the alloys without oxide particles dispersion. Therefore, the excellent oxidation resistance of ODS steels is owing to the formation of continuous, protective oxide layers which correlate with oxide nanoparticles and grain refinement.

Corrosion behavior of Fe–14Cr–2W and Fe–9Cr–2W ODS steels in stagnant liquid Pb with different oxygen concentration at 550 and 650°C

Corrosion behavior of ferritic (Fe–14Cr–2W+Y2O3) and ferritic–martensitic (Fe–9Cr–2W+Y2O3) oxide dispersion strengthened (ODS) steels in the static isothermal liquid Pb was investigated at 550 and 650°C depending on the oxygen concentration CO in the melt for duration up to 1000h. It was determined that the interaction mode of steels changes from the dissolution in the pure Pb (CO⩽10−14wt%O) to the formation of protective oxide layers on the surface of steels in the oxygen-added Pb (CO∼10−6wt%O) and to formation of multiphase non-protective scales in the Pb saturated by oxygen (CO∼10−3wt%O). In general, the observed corrosion behavior of ODS steels coincides with that of traditional chromium steels. However, specific structure of ODS steels causes some variations in corrosion process. In the pure Pb (CO⩽10−14wt%O) the fine-grained structure promotes inter-granular corrosion attack and penetration of lead into steel matrix along grain boundaries. Increase in Cr content in the steel promotes corrosion attack. In the oxygen-added Pb (CO∼10−6wt%O) the fine-grained structure, vice versa, ensures formation of oxide layers with higher Cr content due to fast diffusion of Cr into growing oxide along grain boundaries. The protective properties of oxide layers are improved with temperature rise (550→650°C) and chromium content in steel. In the oxygen-saturated Pb (CO∼10−3wt%O) the ODS steels undergo severe oxidation accompanied by the formation of non-protective multiphase scale which consist of mixture of different oxide phases: plumboferrite, magnetite, Fe–Cr spinel and free Pb. The oxidation kinetics intensifies drastically with temperature and decelerates with increasing chromium content in the steel. Based on the experimental data the scheme of interaction of components in the "steel - liquid Pb” system depending on temperature and oxygen content is proposed.

Oxidation of Fe–10Cr in O2 and in O2+H2O environment at 600°C: A microstructural investigation

Oxidation of Fe-10Cr in dry and wet O-2 was studied at 600 degrees C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O-2 gives a Cr-rich protective (Fe1-xCrx)(2)O-3 scale. The same protective oxide initially forms in O-2 + H2O environment, but after an incubation period scale breakdown is triggered by CrO2(OH)(2) evaporation that depletes the substrate in Cr and converts (Fe1-xCrx)(2)O-3 to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.

Changes in an Australian laterite ore in the process of heat treatment

This paper examines an Australian garnieritic-type ore and changes in phase composition and morphology caused by heating in argon at 400–1000°C using XRF, XRD, DTA/TG, SEM/EDS and BET analyses. The mineral phases detected by XRD in the original ore include chlorite, talc, hematite and quartz. Traces of iron silicate, Fe–Cr spinel and monoxide phase (predominantly manganese oxide) were observed by EDS. Nickel was detected in chlorite, talc, iron silicate and monoxide phase. Heat treatment at 400–500°C did not change XRD patterns. At 600°C, dehydroxylation of the brucitic phase of chlorite occurred. Chlorite was converted into olivine (forsterite) and enstatite at 600–800°C. Upon heating to 900–1000°C, talc was also converted into olivine and enstatite. Ni-bearing phases after heat treatment at 800–850°C were forsterite, enstatite, talc, iron silicate and monoxide.

Structural and magnetic microinhomogeneities in accessory spinel of the system Fe2+(Cr2–xFex3+)O4 from the Kytlym massif (Urals platinum-bearing belt)

The microstructure and magnetic properties of accessory Fe–Cr-spinels from the Kytlym massif of the Urals platinum-bearing belt were studied. Atypical Fe–Cr-spinels in the form of magnetic microareas in grains of primary nonmagnetic Fe–Cr-spinel have been revealed for the first time in the bed dunites of the Kytlym multiphase concentrically zoned massif, North Urals. These spinels are responsible for the magnetic properties of the dunites. It has been established that the microareas are separations in solid solution Fe2+(Cr2–xFex3+)O4, which are enriched in Fe3+ and are probably an intermediate product of the transformation of primary accessory Fe-Cr-spinel during the formation of the dunite massif. These are magnetic microphases with particular chemical composition, cation distribution, and corresponding reversed crystal lattice, which determine the main magnetic properties of the microarea: the magnitude and direction of magnetization vector and Curie temperature. The formation of this earlier unknown type of magnetic Fe–Cr-spinel is probably conjugate with the formation and concentration of PGE mineralization in the bed dunites of the Kytlym platinum-bearing massif. The presence of such magnetization carriers in rocks and ores must be taken into account in geophysical research at the Urals chromite and platinum–chromite deposits.

Spinel-peridotites of the Frido Unit ophiolites (Southern Apennine-Italy): evidence for oceanic evolution

The ophiolitic rocks of the Frido Unit include serpentinites derived from a lherzolitic to cpx-rich harzburgitic mantle, as suggested by microstructural and petrographical features. The serpentinites are englobed in tectonic slices where they are associated with mafic dykes and medium to high-grade metamorphic rocks such as amphibolites, gneiss, granofels as well as gabbros and basalts with pillow structures. The matrix of tectonic slices is mainly composed of calchschists and phyllites. The serpentinites of the Frido Unit show pseudomorphic and vein textures. Primary mantle minerals are represented by olivine, orthopyroxene, clinopyroxene and spinel and are identifiable either for the occurrence of mineral relics or pseudomorphs maintaining the mineral shape. Pseudomorphic minerals are serpentine, magnetite, chlorite, and tremolite. Olivine is replaced by serpentine forming a mesh texture; orthopyroxene is mostly altered to bastite; in some cases it shows exsolution lamellae of clinopyroxene and kink bands; fresh orthopyroxene is preserved as exsolution inside clinopyroxene porphyroclasts. Clinopyroxene is armoured by tremolite rim. Spinel shows a holly-leaf habit and is often armoured by a corona of Cr-chlorite. The core of the analyzed spinel has a Cr-Al spinel composition corresponding to chromite (Al2O3=29-31 wt %; Cr2O3= 28-37 wt%), whereas the rim has a Fe-Cr spinel composition corresponding to ferrit-chromite (Al2O3= 1-2% wt; Cr2O3=28-30 wt %). The Cr-Al spinel/ferritchromite ratio may be various in different spinel porphyroclasts. Magnetite replaces spinel or occurs within the mesh textured serpentine. The metamorphic assemblages in the Frido Unit serpentinites allow to infer qualitatively the physical conditions operating during serpentinization. The metamorphic mineral assemblages are typical of the greenschist-amphibolite transition and greenschist facies conditions, as suggested by the occurrence of tremolite replacing clinopyroxene and of Cr-chlorite and ferrit-chromite after Al-rich chromite. These minerals were produced by interactions between hydrothermal fluids and mantle peridotites. Serpentinites are cut by veins filled with mineralogical assemblages typical of the prehnite-pumpellyite facies likely related to later orogenic Apennine evolution. HP/LT metamorphic conditions (blueschist facies) have been documented in mafic dykes enclosed in serpentinites, but similar conditions are not recorded in serpentinites. This suggests that P-T conditions during the orogenic event were not able to produce a HP/LT mineral assemblage in serpentinites.

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature I: Mechanism of Void-Induced Duplex Oxide Formation

Under CO2 exposure at an intermediate temperature, typically 550 °C, 9Cr–1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe–Cr rich spinel oxide layer. It is proposed that the inner Fe–Cr spinel layer grows according to a mechanism involving void formation at the oxide/metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO2, which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.