Fe Cr Spinel Research Articles

Thermal behaviour of hydrous nickel–magnesium silicates when heating up to 750°C

The thermal decomposition sequence of hydrous nickel–magnesium silicates from Colombia and Brazil was studied under air/Ar atmosphere from room temperature up to 750°C by differential scanning calorimetry–thermogravimetry followed by X-ray diffraction and scanning electron microscopy analyses. Differential scanning calorimetry curves of the samples obtained showed three endothermic peaks at 100, 250 and 600°C due to the release of free water, the dissociation of goethite and the release of crystalline water respectively. To determine the mineral species and microtexture, the ores were studied by scanning electron microscopy. Scanning electron microscopy–energy dispersive spectroscopy analyses showed that the ores are rich in Mg and Mg–Fe silicates, Cr spinel, Mn oxide, goethite and silica and exhibit complex alteration texture. X-ray diffraction analyses of Colombia-2 and Mirabela (Brazil) after the experiments showed that the dehydroxylation produces an amorphous intermediate phase, which is supposed to be due to the exsolution of silica. However, Colombia-1 sample, which was confirmed to contain antigorite mineral, was observed to undergo dehydration and recrystallisation simultaneously.

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe–Cr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe–Cr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite–spinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N2 gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.

Corrosion Resistance of Fe-Al-Alloy-Coated Ferritic/Martensitic Steel under Bending Stress in High-Temperature Lead-Bismuth Eutectic

The corrosion test was performed for ferritic/martensitic steel HCM12A with and without Fe-Al alloy coating in LBE at temperatures of 550 and 650°C under loading for an immersion time up to 500 h. After the corrosion test at 550°C for 500 h, both of the uncoated and Fe-Al-coated HCM12A showed a good corrosion resistance without the influence of the tensile stress on the LBE corrosion. On the other hand, after the corrosion test at 650°C, the Fe-Al coating layer on the specimen surface exhibited no LBE dissolution corrosion because of the formation of a stable oxide protection film on the coating layer surface, although the coating layer cracked. The LBE penetrated into the cracks and corroded the base metal and the precoating layer. The uncoated HCM12A exhibited the double oxide layers of FeO and Fe- Cr spinel. The FeO was damaged in the bent zone by the stress, and the Fe-Cr spinel layer was not destroyed by the influence of cracking and the tensile stress. The cracking and stress did not have a large influence on the overall oxidation corrosion rate.

Oxidation of Binary FeCr Alloys (Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr) in O2 and in O2 + H2O Environment at 600 °C

The oxidation behaviour of the binary alloys Fe–2.25Cr, Fe–10Cr, Fe–18Cr and Fe–25Cr (wt%) in dry and wet O2 at 600 °C is investigated by isothermal exposures of carefully polished samples for up to 168 h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe–2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O2, Fe–10Cr, Fe–18Cr and Fe–25Cr form a thin and protective (Fe,Cr)2O3 oxide similar to the chromia film formed on pure chromium. In wet O2, Fe–10Cr, Fe–18Cr and Fe–25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe–2.25Cr. Breakaway oxidation in wet O2 is triggered by the evaporation of CrO2(OH)2 from the protective (Fe,Cr)2O3 oxide.

Influence of alloy composition and temperature on corrosion behavior of ODS ferritic steels

The effects of alloying elements and temperature on corrosion behavior in supercritical water (SCW) were investigated for oxide dispersion strengthened (ODS) steels to design alloy compositions for corrosion-resistant ODS ferritic steels. Corrosion tests of the ODS steels were carried out at 400, 500, and 600°C under 25MPa with 8ppm of dissolved oxygen. The addition of 4wt.%Al is effective to improve the corrosion resistance of 16Cr-ODS steel, suggesting that an adequate combination between Cr and Al content ranges (14–16)Cr and (3.5–4.5)Al. The addition of Ce or Zr to W-added ODS steel is expected to increase its corrosion resistance. At higher temperatures, the addition of 4wt.%Al to 16Cr-ODS steel induces better corrosion resistance rather than the temperature of 400°C. It is considered that for 16Cr-ODS steels with Al, the continuous alumina mainly suppresses the inward oxygen diffusion and the Fe–Cr spinel adds to the suppression of oxygen diffusion.

Microstructural investigation of the oxide formed on TP 347H FG during long‐term steam oxidation

AbstractThe long‐term oxidation behaviour of TP347H FG in ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal‐fired power plant and characterising the oxide layer with reflective light and electron microscopy. Double layered oxide scales formed during steam oxidation. TEM investigations reveal that the inner oxide layer consists of particles of metallic Ni/Fe and FeCr spinel in the interior of the former alloy grains and a compact layer of FeCr spinel and Cr2O3 along the former alloy grain boundaries. The morphology suggests that the inner layer grows by internal oxidation of the interior of the alloy grains. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposure times up to 30 000 h. Faster Cr diffusion within the fine‐grained alloy at higher temperatures is held responsible for this observation. This hypothesis is supported by kinetic data. The oxide thickness at low and high temperatures after 58 000 h exposure was higher than expected.

A stability of oxide layers formed in LBE on HCM12A to external loading

Stability of oxide layers associated under applied external load was investigated for oxide layers formed on the inner surface of a steel pipe in flowing LBE with respect to fracture behavior, fracture mechanism and nano-hardness. Ring specimens obtained from the pipe were compressed by a compression test machine, and a fracture of the inner oxide layers was observed. Cracks crossed the oxide layers occurred from the interface between the oxide layers and the base metal to the surface of oxide layers and delamination of oxide layers occurred at the Fe–Cr spinel layer in flatted part of compressed ring specimen. In the compressed part of the ring specimens, all oxide layers were spalled off from the base metal. Engineering strains in the base metal near oxide layers were determined from the deformation of micro-indentation sizes. A tensile strain component to the circumferential direction was responsible for the cracking. It appears that the tensile strain component in the radial direction was responsible for the delamination of the oxide layers. Nano-hardness and Young’s modulus of each oxide layers were measured by a nano-indentation method for the evaluation of the fracture behavior. Young’s modulus of Fe–Cr spinel layer is lower than that of Fe 3O 4 layer by 10% with the same hardness in both layers, which yields excess strain to the spinel layer rather than to the Fe 3O 4 layer and consequently causes cracking.

Effects of temperature on the oxide film properties of 304 stainless steel in high temperature lithium borate buffer solution

The paper mainly investigated the effects of temperature on the oxide film properties of 304SS in lithium borate buffer solution by electrochemical measurements and XPS analysis. As temperature increased, the protective property of the film degraded and structure varied from a single layer to a double-layer. Whatever the temperature, the oxide film exhibited an n-type and p-type semiconductor in the potential range above and below the flat band potential, respectively. The electronic properties were assigned to a Fe–Cr spinel inner layer and a defective Fe–Cr oxide outer layer. The related growth mechanisms of the oxide film were also discussed.

Development of the inner oxide zone upon steam oxidation of an austenitic stainless steel

The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated in the temperature range 500–700°C for a fixed oxidation time of 336 h. The samples were characterised using reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500°C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700°C. The major part of the scale grew into the former alloy grains, whereas Fe—Cr spinel grew along the former alloy grain boundaries. TEM and EELS investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe—Cr spinel embedded in a metallic Fe—Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. Growth of the internal oxidation zone at high humidity (46%) is not significantly affected by the type of carrier gas used.

Oxidation of X20 in Water Vapour: The Effect of Temperature and Oxygen Partial Pressure

The oxidation behaviour of X20 in various mixtures of water, oxygen, and hydrogen was investigated at temperatures between 500 °C and 700 °C (time: 336 h). The samples were characterised using reflected light microscopy and scanning electron microscopy equipped with energy dispersive spectroscopy. Double-layered oxides developed during oxidation under all conditions. The morphology of the oxide layers was strongly influenced by temperature, whereas the influence of the oxidising environment appeared to be less pronounced, as long as it contained water vapour. The inner layer consisted of converted M23C6 embedded in Fe–Cr spinel after oxidation at 500 and 600 °C, while alternating layers of Cr-rich and Cr-poor oxide were observed after oxidation at 700 °C. An internal oxidation zone developed during oxidation at 500 and 600 °C, with its depth influenced by the oxidising environments. The results are discussed based on the various hypotheses of the accelerating effect of water vapour that have been put forth in the literature.

Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

SUMMARY This paper presents the mineralogy and magnetic properties of two varieties of chromitite, sampled in the same exposure of a serpentinite massif regarded to be a fragment of the Sudetic ophiolite. The varieties differ mineralogically and magnetically. One of them, labelled TaA and altered to a low degree according to scanning electron microscope (SEM) and microprobe results, comprises an unaltered Al–Cr spinel core, some secondary chromite and less abundant Cr-magnetite. It has a high magnetic susceptibility and natural remanence (NRM), and its dominating magnetic phase is ferrichromite with T b/T c of about 530 ◦ C occurring in grains in the single domain (SD) + pseudo-single domains (PSDs) and superparamagnetic (SP) states. The second variety, labelled TaB, is highly altered and comprises, apart from Al spinel, two kinds of secondary chromites that do not differ much from the primary Al spinel and no Cr-magnetites. This chromitite variety does not reveal one well defined T b/T c, but a series ranging between 200 and 450 ◦ C, their grain sizes correspond to the SD + SP domain states. The same pattern was observed in chromitite TaA after annealing in air at a temperature of 700 ◦ C. Dunites associated with the chromitites contain, apart from magnetite, chromite grains. The majority of specimens reveal magnetic characteristics similar to the TaA-like variety, but in some specimens TaB-like characteristics were also observed. A standard palaeomagnetic study performed earlier on the dunites showed that their NRM has three components—Lower Devonian carried by magnetite, Permian and Tertiary-Recent. The results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the TaA and TaB chromitites.

High Temperature Oxidation of Fe-9Cr-1Mo Steel in Liquid Metal

The oxidation mechanism of the T91 martensitic steel in oxygen-saturated Pb-Bi eutectic at 470°C has been investigated to develop a long term predictive model of the steel oxidation kinetic. This work is performed in the frame of life duration studies carried out for the MEGAPIE spallation module demonstrator dedicated to the feasibility demonstration of an hybrid reactor. Our scientific approach has been based on an experimental characterization of the oxide scales and of the T91 steel oxidation kinetics. From these experimental results, an oxidation mechanism has been elaborated and then simulated. The oxide scale formed at the T91 surface has a duplex structure, constituted of an external magnetite scale and an internal Fe-Cr spinel scale. A scale growth mechanism has been proposed: the magnetite scale growth seems to be limited by the iron lattice diffusion inside the duplex oxide scale. At the same time, a self-regulation mechanism seems to govern the Fe-Cr spinel scale growth. This mechanism consists of a non-limiting oxygen diffusion step, which is carried out, across the oxide scale, inside liquid lead nano-channels and a limiting iron oxide lattice diffusion step. Considering the proposed oxidation mechanism, a simulation of the growth of the two oxides scales has been carried out and compared to the experimental oxidation kinetics. The excellent agreement between the experimental results and the simulations supports to accept the proposed mechanism, leading to prediction of kinetics for long oxidation durations.

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.

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

This paper is the first 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 oxygen transport mode across the oxide scale. The oxide layer has a duplex structure composed of an internal Fe–Cr spinel layer and an external magnetite layer. Oxygen 18 tracer experiments are performed: they show that the magnetite layer grows at the Pb–Bi/ oxide interface whereas the Fe–Cr spinel layer grows at the metal/oxide interface. Oxygen seems to diffuse across the oxide scale dissolved inside nanometric lead penetrations called nano-channels. Specific experiments are performed to characterize the nano-channels.

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.

Characterisation of passive films on 300 series stainless steels

The formation and breakdown of the passive films on stainless steels are mainly controlled by ionic and electronic transport processes. Both these processes are in part controlled by the electronic properties of the oxide film. Consequently, it is vital to gain a detailed perception of the electronic properties of the passive films together with structural and compositional information for a comprehensive understanding of mechanisms behind passivity and localised corrosion. As a step towards this goal the passive films formed on two main austenitic stainless steels AISI 316L and AISI 304L in borate solution were characterised by in situ Raman spectroscopy and photocurrent spectroscopy coupled with electrochemical measurements. This revealed the formation of an Fe-Cr spinel as the dominant constituent in the passive films with more Cr enrichment in the oxide film on 316L than that of 304L. Bandgap readings and semiconductivities of the two stainless steels suggested that three different applied potential regions existed; 800mV(SCE) to 300mV(SCE), 200mV(SCE) to −300mV(SCE) and below −500mV(SCE).

Electrochemical & optical characterisation of passive films on stainless steels

The formation and breakdown of the passive film are mainly controlled by ionic and electronic transport processes; processes that are in turn controlled by the electronic properties of the film. Consequently a comprehensive understanding of mechanisms behind passivity and localised corrosion require a detailed perception of the electronic properties of the passive films together with compositional and structural information. As a step towards this goal the passive film on austenitic stainless steel, AISI 316L, formed in borate solution was characterised by in situ Raman spectroscopy and photocurrent spectroscopy coupled with electrochemical measurements. The composition, structure and semiconductivity of the passive films depended on the potential; Fe rich n-type oxide and a Cr rich p-type oxide dominated at more positive potentials and more negative potentials respectively whilst n-type dual layered film formed at intermediate potentials. Analyses of the bandgap determined for these oxides suggested their structures to be Fe2O3 and a Fe-Cr spinel. This hypothesis was supported by the results of in situ Raman spectroscopy.

Oxidation Mechanism of Steels in Liquid–Lead Alloys

The oxidation mechanism of steels in liquid–lead alloys (lead or lead–bismuth) was studied. Parametric dependencies of oxidation, including oxygen-concentration effects, oxidation-rate constant and corrosion-rate effects, are analyzed. An oxidation model is developed based on the assumptions that the chemical reactions are at equilibrium locally, and scale removal is due to mass-transfer corrosion. The model shows that outward-iron diffusion in the solid phase (oxide layer) controls the oxide growth and mass-transfer rate in the flowing-boundary layer determines the corrosion-product transport in the liquid phase (liquid–lead alloy). The oxide thickness depends on both the parabolic oxide-growth-rate constant and the mass-transfer-corrosion rate. For long-term operation, the outer layer of a duplex-oxide layer can be completely removed by flowing lead alloys and it is expected that a pure-chromium-oxide layer forms underneath the Fe–Cr spinel if iron is heavily depleted. The oxide thickness and steel weight change are very different from those of the pure parabolic law and they are classified into distinct and universal categories. The model is validated partially by application to interpreting the measured oxide behavior of several steels in a lead-bismuth eutectic-test loop.

The kinetics of oxidation of diffusion coatings in superheated steam

The kinetics of oxidation of Cr-rich, Si–Cr-rich and Al-Cr-rich diffusion layers have been studied in superheated steam within the temperature range 650 and 750°C. The diffusion layers were generated in test-pieces of alloy type P-92 using the pack cementation approach. The thermogravimetric analytical technique was used to determine the kinetic in superheated steam over a period of 100 h. The results were benchmarked against the oxidation kinetic of the bare P92 alloy at 650°C. The results of these short-term experiments indicated a significant improvement in oxidation resistance where the diffusion layers did not experience any apparent attack. The oxidation kinetics of a test-piece of alloy type S304H were also determined at 750°C. The latter type of samples experienced oxidation with the formation of a 1 μm thick Fe–Cr spinel type oxide with discrete patches of break-away oxidation. By comparison the diffusion layers showed no apparent attack. The diffusion layer chemistry remained stable even at 750°C for all coated composition. The characteristics of the diffusion layers will be presented.

Oxidation of Stainless Steel in H<sub>2</sub>O/O<sub>2</sub> Environments – Role of Chromium Evaporation

Oxidation of Stainless Steel in H<sub>2</sub>O/O<sub>2</sub> Environments – Role of Chromium Evaporation