Fe Cr Spinel Research Articles

Interfacial microstructure and corrosion behavior of ODS FeCrAl alloy in oxygen-saturated lead-bismuth eutectic at 450 °C

The interfacial microstructure and corrosion behavior of oxide dispersion strengthening (ODS) FeCrAl alloy with 20 wt% Cr (e.g. 20Cr ODS FeCrAl alloy) exposed to static oxygen-saturated lead-bismuth eutectic (LBE) at 450 °C for 20 h, 40 h, 60 h, 80 h and 100 h as exposure time respectively have been investigated. The results show that multilayer corrosion films of 20Cr ODS FeCrAl alloy can exist at the corrosion interface. Due to formation of protective scales, the primary corrosion mechanism of 20Cr ODS FeCrAl alloy in LBE is oxidation rather than dissolution and penetration of liquid LBE. Specially, the spinel-shaped nano-sized dense external layer without any microcracks is obviously observed. Moreover, the dense and compact scale with micron-meters, i.e. Al-rich Fe-Cr spinel, generates at interface as the middle layer to effectively defend inward diffusion of oxygen and penetration of LBE. Meanwhile, a reinforced and continuous internal layer mixed with Cr2O3 scale and the ferrite inner oxidation zone (IOZ) existing in the form of the interlocking interface between the layers and the substrate can further promote the adhesion between corrosion layers and the substrate of 20Cr ODS FeCrAl.

In-situ study on the differential evolution of He bubbles in the multilayer oxide of Fe9Cr1.5W0.4Si F/M steel corroded in lead-bismuth eutectic

The combined effects of corrosion and irradiation on nuclear components have been an important but not yet fully revealed topic. Here, the irradiation behavior of the oxide scale formed on F/M steel after lead-bismuth corrosion was in-situ investigated during He+ irradiation. The results showed that the oxide scale included a Fe3O4 outer oxide layer, a nanograin Fe(FexCr2-x)O4 spinel inner oxide layer, and an internal oxide layer. He bubbles formed in Fe3O4, Fe-Cr spinel and F/M steel were polygon, irregular elongated pores and small spheres, respectively. These differences were attributed to variations in defect generation, migration, and corrosion-induced crystal defects in different oxides. Numerous corrosion-induced nanograin boundaries and vacancies in Fe-Cr spinel exhibited more effective absorption of irradiation-induced defects. Moreover, rhombic perfect dislocation loops were detected in Fe3O4 at the late stage of irradiation, their relative positional relationship with He bubbles indicated a potential interaction between bubbles and loops.

Corrosion kinetics and mechanisms of 15–15Ti steel in flowing liquid lead-bismuth eutectic at 500°C

The application of Liquid Lead-Bismuth Eutectic (LBE) as a coolant in Lead-cooled Fast Reactors (LFRs) presents significant challenges due to its corrosive nature, especially at elevated temperatures. This study investigates the corrosion kinetics and mechanisms of 15–15Ti stainless steel in flowing LBE at 500 °C with oxygen concentrations ranging from 1 to 3 × 10−6 wt% and a flow rate of 1 m s-1. The research highlights the formation and growth of the Fe-Cr spinel oxide layer, which follows a parabolic rate law indicative of a diffusion-controlled process, crucial for long-term material stability predictions. Despite the initial effectiveness of the Fe-Cr spinel layer in mitigating corrosion, its integrity is compromised over time due to spallation and dissolution, allowing penetration of LBE elements and selective dissolution of Fe, Ni, and Cr. The study reveals a complex interplay between direct dissolution of steel components and spallation of the oxide layer, providing critical insights into material loss mechanisms and the degradation of structural materials in LBE-cooled reactors.

Microstructural evolution of the inner oxide layer of a Si-modified austenitic stainless steel exposed to oxygen-saturated lead-bismuth eutectic (LBE): Toward the origin of LBE penetration

Microstructural evolution of inner oxide layer of a Si-modified austenitic stainless steel exposed to oxygen-saturated static lead-bismuth eutectic (LBE) at 550 °C up to 3000 h has been investigated. A dual-phase separated structure consisting of Fe-Cr spinel and (Fe, Cr)1-xNix phase is formed in inner oxide layer, attributed to partitioning of Ni and Cr associated with Fe-Cr spinel formation. SiO2 is preferentially formed preceding the Fe-Cr spinel at internal oxidation frontier, leading to dispersed SiO2 nanoparticles within Fe-Cr spinel. The mechanism of LBE penetration into inner oxide layer is further developed based on preferable dissolution process of (Fe, Cr)1-xNix phase.

Ni segregation in the oxide film of 15–15Ti austenitic steel at high-temperature CO2

A double-layer oxide film was formed on 15–15Ti exposed to high-temperature CO2, with outer Fe-rich layer and Cr/Ni-rich inner layer comprising Ni-poor and Ni-rich zones. The Ni-poor zone contained Fe-Cr spinel oxides and numerous nanopores, whereas the Ni-rich zone consisted of Fe-Cr spinel oxides and Ni-rich phases with austenite structure. The Ni segregation showed a specific evolution with exposure time and temperature: Ni-poor zones grew up and evolved into a laminar structure as the oxide film thickened. The formation and evolution mechanism of Ni-poor zones and the effect of Ni segregation on the oxidation were discussed in present work.

Effect of deformation twinning on the early oxidation behaviour of 316L steel exposed to liquid lead-bismuth eutectic

Twins were introduced into 316L austenitic stainless steel by shot peening treatments. The twinned 316L was then exposed along with pristine 316L to oxygen-saturated liquid lead-bismuth eutectic at 550 °C for 120 h. We observed that the inner oxide layer on the pristine 316L consisted of Fe–Cr spinel and reticulated nickel-rich phases, whereas that on the twinned 316L consisted of Fe–Cr spinel and Cr2O3 without the nickel-rich phases. The factor contributing to the different microstructure and composition of the oxide layer on pristine and twinned 316L is that the twin boundaries provide additional channels for Cr diffusion into the oxide front, allowing the formation of Cr2O3 and high Cr dense Fe–Cr spinel. As a result, the Ni rich phases do not have sufficient space to exist and are pushed back into the Cr poor steel matrix.

Influence of Si addition on the corrosion behavior of 9Cr ferritic/martensitic steel in static liquid lead-bismuth eutectic

Abstract This study investigated the corrosion behavior of 9Cr ferritic/martensitic (F/M) steel with varying silicon contents when exposed to static lead-bismuth eutectic at temperatures between 450 °C and 550 °C. Except for samples corroded at 450 °C for 500 h, which developed a bilayer consisting of magnetite layer and Fe-Cr spinel layer, the oxide scale at 450 °C and 500 °C consists of exterior magnetite layer, Fe-Cr spinel layer, and disorganized internal oxide zone. Most of samples that corroded at 550 °C underwent dissolution corrosion. With the addition of Si, 9Cr F/M steel demonstrated enhanced corrosion resistance to LBE at low temperatures and effectively delaying the onset of dissolution corrosion at 550 °C.

Corrosion kinetics of T91 steel under low oxygen conditions (10−7 wt% O) in lead-bismuth eutectic at 500 °C

Among the advanced reactor designs of the fourth generation, the Lead-cooled Fast Reactor (LFR) stands out as one of six distinct types. The corrosion caused by the liquid Lead-Bismuth Eutectic (LBE) on structural materials is recognized as a significant obstacle hindering the development of LFR. The corrosion kinetics of T91 steel in static LBE with 10−7 wt% oxygen at 500 ºC is analyzed in this study. The formed oxide scale consists of an outer (Fe, Cr)3O4 layer with a Fe-Cr spinel structure and an inner internal oxidation zone (IOZ). The thickness evolution of the oxide layers follows a parabolic rule, with rate constants determined for different layers. The dissolution rate of metals into the LBE decreases over time, indicating diminishing the concentration gradient of Fe across the oxide layer-LBE interface. The observed deviations from expected norms may be attributed to dynamic flow conditions and non-isothermal loop experiments, which accelerate Fe diffusion into LBE.

Revealing the effect of O2, SO2 and H2O in supercritical CO2 on the corrosion behavior of 316 L steel at elevated temperature

Closed supercritical CO2 (sCO2) power systems require steels that can resist corrosion in high temperature CO2 containing different impurities at ppm levels. However, potential impurity effects on Fe-16Cr steels are not well understood. Herein we evaluated the corrosion behavior of 316 L steel in sCO2 containing 100 ppm O2, SO2 or H2O at 600 °C and 20 MPa. All investigated impurities enhanced Fe-oxide nodule formation, leading to higher carburization in underlying substrate. Presence of O2 caused transition of chromia scale into Fe-Cr spinel, because Cr supply from underlying substrate could not meet the consumption. Spallation occurred when 316 L was exposed with SO2 impurity, attributed to the formation of sulfides within and beneath the oxide scales. H2O impurity showed relatively low damage to protective chromia layer with localized aggravation. Corrosion mechanisms related to impurities were unraveled based on experimental results and thermodynamic analysis.

Study on the corrosion behavior of laser surface remelted and laser cladding of ferritic/martensitic steels after exposure to lead-bismuth eutectic at 700 °C

This study explores two vital structural materials, T91 (Fe-9Cr) and SIMP (Fe-11Cr) steels, in the context of lead-cooled fast reactors and accelerator-driven sub-critical systems (ADS). Lead-bismuth eutectic (LBE) functions as a key coolant and spallation target material due to its impressive thermal conductivity, neutron yield, and chemical properties. Unfortunately, materials in contact with LBE are prone to severe corrosion at elevated temperatures (T>500 °C), compromising their integrity. To bolster corrosion resistance, we utilized laser remelting and laser cladding to apply FeCrAl/TiN coatings on the steel surfaces. Our study scrutinizes the corrosion behavior of steel in LBE saturated with oxygen at 700 °C and investigates the underlying causes. Following 240 h of exposure to corrosion, T91 and SIMP steels subjected to laser remelting displayed substantial oxide scale formation. Lead-bismuth atoms infiltrated the outer oxide layer (Fe3O4), diminishing adhesion between the inner and outer oxide layers, leading to the detachment of the outer oxide layer. The inner oxide layers, composed of Fe-Cr spinel, were approximately 123 μm thick for T91 steel and 77 μm for SIMP steel, underscoring SIMP steel's superior corrosion resistance. For T91 steel treated with laser cladding FeCrAl/TiN coating, a characteristic duplex oxide layer with a total thickness of around 83 μm was formed, with noticeable deposition of Pb-Bi atoms at the interface between the outer and inner oxide layers. Conversely, only a protective alumina layer safeguarded SIMP steel from LBE corrosion. This outcome emphasizes the efficacy of laser cladding FeCrAl/TiN coating in providing superior protection for SIMP steel over T91 steel. Our research significantly contributes to the development of anti-corrosion coatings for high-temperature LBE environments.

Enhanced corrosion resistance of 15–15Ti austenitic steel in liquid lead-bismuth eutectic at 550 ℃ by shot peening processing

The effect of shot peening on microstructure and corrosion of 15–15Ti austenitic steel in liquid LBE at 550 ℃ was investigated. The nanocrystallization and preferential recrystallization were observed, improving the resistance to corrosion and intergranular cracking in LBE. The intensive grain boundaries and dislocations on steel surface improved the compactness and adhesion of the formed oxide scale, efficiently preventing LBE penetration into steel. The Cr-rich oxide formed in boundaries of the recrystallized grains is Fe-Cr spinel with porous structure, it significantly prohibited the counter diffusion of elements, suppressing the growth of outer magnetite layer and inner transgranular oxide.

Study of the High‐Temperature Oxidation Behavior of SUS430 Ferritic Stainless Steel Under Different Content of Water Vapor Atmosphere

The oxidation behavior of SUS430 ferritic stainless steel is studied during isothermal oxidation at 1100 °C for 2 h under a mixture of 10, 30, and 50% water vapor and air, and oxidation kinetics curves are drawn. The surface morphology and phase composition of oxide scale are analyzed using X‐Ray diffractometer, scanning electron microscope, and energy dispersive spectrometer. The oxidation kinetic curves under three different humidities are S‐shaped and include three stages: induction period, acceleration period, and deceleration period. The controlling steps under 10% water vapor and air are nucleation and growth, while under 30% and 50% water vapor and air are both phase boundary control reactions. The induction period becomes shorter and the breakaway oxidation occurs earlier with increasing water vapor content. The structure of oxide scale produced is layered, with the outer layer being iron oxide and the inner layer being mainly the Fe–Cr spinel phase. The volatilization of Cr and the growth stress of the Cr2O3 layer cause the formation of nodules, and a large number of cracks are generated in oxide scale. The cracks provide diffusion paths, accelerating the diffusion of Fe ions and electrons, as well as the erosion of the substrate.

Effects of laser surface remelting on damage resistance of oxide scales on T91 exposed to flowing oxygen-saturated lead-bismuth eutectic

Forming suitable oxide scales on the surface of T91 steel by controlling the oxygen concentration has been considered as an effective method to restrain the liquid metal corrosion of T91 steel exposed to lead-bismuth eutectic (LBE). Notably, the successful precondition of this method is that the oxide scales on T91 steel can not be damaged easily, especially in flowing LBE environment. In the present study, laser surface remelting (LSR) treatment is employed to enhance the damage resistance of oxide scales on T91 steel under the flowing (∼3 m/s) oxygen-saturated LBE conditions at 500 ℃. The experimental results reveal that although the T91 steel specimens subjected to LSR treatment still process a double-layered oxide scales comprising an outer magnetite layer and an inner Fe-Cr spinel layer, their damage resistance ability is remarkably stronger compared the as-received T91 steel specimens in flowing LBE environment. Further observations indicate that such phenomenon should be ascribed to fewer microstructural defects, such as LBE-filled holes within the oxide scales, resulting from the refined microstructure and high density of dislocations activated in LSR specimens. Accordingly, the possible reasons behind these phenomena were proposed, and a corresponding damage model of oxide scales in flowing LBE condition was established.

Sr-doped LaFeO3 thin coatings for protection of ferritic stainless steel interconnects in solid oxide cells: A study on Cr-barrier and electrical properties

A high-temperature conversion process in molten carbonate baths was applied to produce thin Sr-doped LaFeO3 perovskite coatings on the surface of a K41 steel for interconnect applications in Solid Oxide Cells. Two conversion coatings with identical structure and phase composition, but slightly different thickness were produced in nitrate-accelerated baths containing the nitrate additive at two concentration levels. The as-prepared coatings consisted of a dual-layered perovskite-spinel structure with a top sub-micrometric Sr-doped LaFeO3 layer grown onto a micron-range thick inner Fe-Cr spinel oxide layer. Both coatings showed high protection against oxidation and chromia scale formation at 700°C. Further, excellent Area Specific Resistance (ASR) behavior was found for both coatings at 700°C, although an optimal balance of ASR and Cr-diffusion barrier properties was achieved with the coating produced in the lower nitrate content bath (N-02 coating). The causes for the enhanced barrier properties of the N-02 coating were ascribed not to a different coating thickness, but rather to the evolution of the pristine dual-layer into a more impervious three-layer structure during oxidation aging at 700°C. The evolved structure was formed by decomposition of the pristine sub-coating Fe-Cr spinel into two new spinel sublayers: an outer ferrite and an inner chromite layer, whereas the top thin-film perovskite layer remained essentially unaltered. Partial change in the Fe oxidation state and other possible causes involved in the observed decomposition of the pristine Fe-Cr spinel layer during the thermal aging are analyzed and discussed. Measurements of ASR activation energy indicated that the contact resistance behavior of the thin perovskite coatings was dominated by the internal spinel layer conductivity.

Synergy of stress and corrosion in a 12Cr ferritic/martensitic steel exposed to static liquid lead-bismuth eutectic at 550 °C

In this work, uniaxial creep tests under 300, 350 and 380 MPa were performed to study the synergy of stress and corrosion in a 12Cr ferritic/martensitic (F/M) steel exposed to static liquid lead-bismuth eutectic (LBE) at 550 °C. The results showed that LBE had a strong degradation effect especially under low-oxygen condition, leading to drastic acceleration of creep deformation as compared to that in air. Although oxide scales, such as magnetite, Fe-Cr spinel and chromium oxide, could slow down crack initiation and propagation, this effect is load and oxygen concentration dependent. The overall creep behavior is governed by multiple competing mechanisms.

Steam oxidation behavior of T92 steel tube for high temperature reheater after long-term service in ultra-supercritical boilers

The steamside oxidation behavior of T92 steel tube for high temperature reheater after long-term service in ultra-supercritical boilers was investigated. The structure evolution process, phase composition and element distribution of the oxide film were studied. The oxide film consisted of outermost layer with hematite, subouter layer with magnetite and inner layer with Fe-Cr spinel. The structure of the transition zone of the oxide film and related oxidation mechanism was discussed in detail. A steam oxidation kinetic model of the T92 reheater based on the evolution of the inner layer thickness of the oxide film was established.

Experimental Study of the Evolution of Creep-Resistant Steel’s High-Temperature Oxidation Behavior

This study shows that in an atmosphere containing water vapor, the oxide layer on the surface of the 9CrNB steel MarBN (Martensitic 9Cr steel strengthened by Boron and MX Nitrides) was formed by an outer layer of hematite Fe2O3 and Cr2O3 and an inner two-phase layer of Fe3O4 and Fe3O4 + (Fe, Cr)2O4, which was confirmed by XRD analysis. Part of the layer consisted of nodules and pores that were formed during the increase in oxides when the present H2O(g) acted on the steel surface. The diffusion mechanism at temperatures of 600 and 650 °C and at longer oxidation times supported the “healing process” with a growing layer of Fe oxides and the presence of Cr and minor alloying elements. The effects of alloying elements were quantified using a concentration profile of the oxide layer based on quantitative SEM analysis, as well as an explanation of the mechanism influencing the structure and chemical composition of the oxide layer and the steel-matrix–oxide interface. In addition to Cr, for which the content reached the requirement of exceeding 7.0 wt. % in the inner oxide layer, W, Co, Mn, and Si were also found in increased concentrations, whether in the form of the present Fe-Cr spinel oxide or as part of a continuously distributed layer of Mn2O3 and SiO2 oxides at the steel-matrix–oxide interface. After long-term high-temperature oxidation, coarser carbides of the M23C6 type (M = Fe,W) significantly depleted in Cr were formed at the oxide-layer/matrix interface. In the zone under the oxide layer, very fine particles of MC (M = V, Nb, and to a lesser extent also Cr in the particle lattice of the given phase) were observed, with a higher number of particles per unit area compared to the state before oxidation. This fact was a consequence of Cr diffusion to the steel surface through the subsurface zone.

Effect of Au-ion irradiation on the morphology, microstructure and lead-bismuth eutectic corrosion behavior of refractory TiNbZrMoV high-entropy alloy coating

The effect of Au-ion irradiation on the morphology, microstructure and lead-bismuth eutectic (LBE) corrosion behavior of refractory TiNbZrMoV high-entropy alloy coating was investigated. After irradiation, the surfaces of coatings were smoothened, and crystalline phase was formed in the coatings. After LBE corrosion at 650 °C, the irradiated samples presented a triple layer (interface Fe-Cr spinel, coating and surface Fe-oxide layer) consistent with the pristine sample, and the coating still maintained the low-consumption characteristic. Irradiation accelerated LBE corrosion, which could be attributed to the formation of grain boundary and defects that provided more diffusion channels for atoms.

Accelerated steam corrosion behavior of Fe-ion irradiated RAFM steel: The role of displacement damage

In this study we investigated the accelerated corrosion behavior of Fe-ion irradiated RAFM steel (SIMP) in high-temperature steam. The thickness, elementary composition, surface morphology, crystal structure and microstructure of the formed oxide films were studied. Consistent with the unirradiated sample, the oxide films of the irradiated samples consisted of an outer layer of Fe3O4, a porous inner layer of Fe-Cr spinel, and an internal oxide zone composed of (Cr, Si)2O3. The thickness of the oxide films increased with the increasing displacement damage. Compared with austenitic steels, the increase of corrosion rate of SIMP steel by displacement damage is rather moderate.

Fretting behavior of oxide scale formed on ferritic/martensitic steel after pre-exposure to lead-bismuth eutectic

The microstructure and fretting behavior of oxide scales formed on ferritic/martensitic steel (T91) after pre-exposure to lead-bismuth eutectic (LBE) at 500 ℃ have been investigated by jointly utilizing multiple characterization methods. The results reveal that the oxide scales are mainly composed of a loose magnetite layer and a relatively dense Fe-Cr spinel layer with some LBE-filled holes located between them. Compared to as-received material, the oxide scales themselves possess weaker fretting resistance. In particular, the oxide scales close to the wear track prefer to desquamate layer by layer in the form of brittle failure under combined shear and normal stress during fretting. Such phenomenon may be related to the reduction of oxide scale stability caused by the microcracks located on the spinel/substrate interface and the LBE-filled holes. Accordingly, a possible damage model of oxide scales during fretting was proposed and discussed.