Cr-rich Oxide Layer Research Articles

Investigation of Oxide Layer Development of X6CrNiNb18-10 Stainless Steel Exposed to High-Temperature Water.

The oxide layer development of X6CrNiNb18-10 (AISI 347) during exposure to high-temperature water has been investigated. Stainless steels are known to form a dual oxide layer in corrosive environments. The secondary Fe-rich oxide layer has no significant protective effect. In contrast, the primary Cr-rich oxide layer is known to reach a stabilized state, protecting the base metal from further oxidation. This study's purpose was to determine the development of oxide layer dimensions over exposure time using SEM, TEM and EDX line scans. While a parabolic development of Cr in the protective primary layer and Fe in the secondary layer was observed, the dimensions of the Ni layer remained constant. Ni required the presence of a pronounced Fe-rich secondary layer before being able to reside on the outer secondary layer. With increasing immersion time, the Ni element fraction surpassed the Cr element fraction in the secondary layer. Oxide growth on the secondary layer could be observed. After 480 h, nearly the entire surface was covered by the outer oxide layer. In the metal matrix, no depletion of Cr or Ni could be observed over time; however, an increased presence of Cr and Ni in the primary layer was found at the expense of Fe content. The Nb-stabilized stainless steel was subject to the formation of Niobium pentoxide (Nb2O5), with the quantity and magnitude of element fraction increasing over exposure time.

Effect of high-energy shot peening process on the oxidation behaviour of AISI 321 stainless steel at 900 °C

ABSTRACT The present work comparatively investigates the oxidation behaviour of AISI 321 austenitic stainless steel at 900 °C in Ar-21vol.%O2 in two conditions of solution annealed (SAed) and after a high-energy shot peening process (HESPed). Kinetic observations indicated that during the initial 25 h of oxidation, the HESPed sample exhibits higher oxidation kinetics. However, as oxidation progresses, the kinetics of the SAed sample surpasses in a way that after 150 h, the weight gain for SAed is 0.93 mg/cm2, and for HESPed, it is 0.71 mg/cm2. GI-XRD results after 150 h of oxidation revealed similar oxide phases on both samples, including (Cr,Fe)2O3, Mn(Fe,Cr)2O4, and Fe2O3. An important point is that the oxide formed on the surface of the SAed sample, unlike HESPed, undergoes severe spallation and cracking after 150 h, leading to sub-surface oxidation and the formation of Ni-rich oxide phases such as Ni(Fe,Cr)2O4. The enhancement observed in the oxidation performance of the HESPed sample can be ascribed to the augmented density of high-diffusivity paths, a consequence of the high-energy shot peening process, and the prompt development of uniform inner Si-rich and intermediate Cr-rich oxide layers.

High-temperature oxidation behavior of Cu-Fe-based immiscible alloy produced by laser powder bed fusion

The oxidation behavior of Cu-Fe-based immiscible alloy produced by laser powder bed fusion at 500 °C ∼ 700 °C were analyzed. The results show that the Cu-, Fe-, and Cr-rich oxide layers with the sandwich-like structure can be formed after the oxidation of different temperatures. In particular, after the oxidation for 10 h and 100 h at 500 °C and 600 °C, the island-like CuO were formed on the surface of alloy. However, CuO layer formed at 700 °C was continuous and severely wrinkled so that the internal stress and the strength loss led to fracture of the oxide scales.

Effect of Pt Addition on the Oxidation and Corrosion Resistance of Al0.25CoCrFeNi High-Entropy Alloy

The effect of minor platinum alloying on the microstructure, phase composition, oxidation and corrosion resistance of Al0.25CoCrFeNi high-entropy alloy (HEA) has been studied. It was observed that Pt does not segregate as a separate phase, but it is incorporated into the fcc solid solution. High-temperature oxidation of the as-cast Al0.25CoCrFeNi and Al0.25CoCrFeNiPt0.1 high-entropy alloys was carried out in a muffle furnace under isothermal conditions at 900 °C for 0–50 h (weighing was carried out every 10 h) in air atmosphere. The specific weight gain decreased from 0.58 mg/cm2 for Al0.25CoCrFeNi to 0.31 mg/cm2 for the platinum-doped Al0.25CoCrFeNiPt0.1 sample. It was determined that Pt triggers the formation of an interlayer Al-rich oxide phase between the outer Cr-rich oxide layer and the interior of the alloy, significantly reducing the oxidation rate of Al0.25CoCrFeNiPt0.1 HEA. Electrochemical tests in 0.5 M H2SO4 solution demonstrated passive behavior with anodic control of the process.

Investigation of Surface Oxide Layer Structure to Improve Durability of Stainless Steels Under Humidified Hydrogen

We study degradation mechanism of stainless steels under SOFC operating conditions to improve durability of SOFC stack. To reduce material cost, a low-cost stainless steel is preferred because metal interconnect occupies considerable amount in the stack. From commercially available stainless steels, four types of alloy composition were chosen as model materials, which are widely used in the other applications. The alloy test pieces were exposed to hydrogen with 10%-50% H2O at 600oC-700oC, and mass gain after the test was measured. The mass gain increased with time, indicating that steam oxidation proceeded. The oxidation rate was smaller for JIS SUS430J1L than the other three types of the alloy for all the test conditions. The proficient durability came mainly from the thin Cr-rich oxide layer covering the alloy uniformly with little defect or segregation. The additives in the alloy such as Nb also may have an effect on suppressing elemental diffusion through the oxide layer. Because morphology of the surface oxide layer initially formed on the surface should affect the process of steam oxidation during SOFC operation, we investigated influence of pre-heat treatment condition (700oC-900oC) of the alloy. The surface oxide layer formed by the pre-heat treatment at 900oC in air for 2 h had an effect of reducing steam oxidation rate. Microstructure of the surface oxide layer before and after the durability test was analyzed by STEM. From the difference in the microstructure of the surface oxide layer, predominant factor to improve durability will be discussed. This paper is based on results obtained from a project, (JPNP20003), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Key Processing Factors in Hydrothermal Liquefaction and Their Impacts on Corrosion of Reactor Alloys

Despite intensive efforts to develop hydrothermal liquefaction for the conversion of wet biomass and biowaste feedstocks into valuable bio-oils, severe corrosion of conversion reactor alloys and other core components, induced by the pressurized hot water medium, catalysts, and inorganic and organic corrodants generated during the conversion process, has significantly hindered the industrial deployment of this attractive technology. In this paper, a general review of major operating parameters, including biomass feedstock types, temperature, pressure, and catalysts, was conducted to advance the understanding of their roles in conversion efficiency and the yield and properties of produced oils. Additionally, the corrosion performance of a representative constructional alloy (Alloy 33) was investigated in both non-catalytic and catalytic HTL environments at temperatures of 310 °C and 365 °C, respectively. The alloy experienced general oxidation in the non-catalytic HTL environment but suffered accelerated corrosion (up to 4.2 µm/year) with the addition of 0.5 M K2CO3 catalyst. The corrosion rate of the alloy noticeably increased with temperature and the presence of inorganic corrodants (S2− and Cl−) released from biowastes. SEM/XRD characterization showed that a thin and compact Cr-rich oxide layer grew on the alloy in the non-catalytic HTL environment, while the surface scale became a double-layer structure, composed of outer porous Fe/Cr/Ni oxides and inner Cr-rich oxide, with the introduction of the K2CO3 catalyst. From the corrosion perspective, the alloy is a suitable candidate for construction in the next phase of pilot-scale validation assessment.

Effect of laser shock peening and surface mechanical attrition treatment on the oxidation resistance of a 20Cr-25Ni-Nb stainless steel

In this study, the effects of laser shock peening (LSP) and surface mechanical attrition treatment (SMAT) were comparatively investigated on the oxidation behaviors of 20Cr-25Ni-Nb stainless steel in a high-temperature (650 °C) CO2 environment up to 1000 h. The results show that the oxidation resistance of stainless steel can be improved by LSP but deteriorated by SMAT. LSP promoted the formation of a thin and uniform Cr-rich oxide layer on the 20Cr-25Ni-Nb stainless steel instead of a thick duplex oxide layer with Fe-rich outer layer and Cr-rich inner layer which appeared on the untreated sample. For the SMAT-treated sample, besides the oxide scale, a thick internal oxidation zone (IOZ) and oxides spallation were also observed. The formation of a Cr-rich oxide layer was facilitated by the enhancement of the Cr diffusion within the LSP-induced hardened layer. The unsatisfying oxidation performance of the SMAT-treated sample is due to the surface micro-cracks and significant grain refinement which synergistically contributed to the visible carburization and oxidation in a high-temperature CO2 environment.

Investigation of Surface Oxide Layer Structure to Improve Durability of Stainless Steels Under Humidified Hydrogen

Durability of low cost stainless steels, selected from the materials widely used in other applications, was investigated under humidified hydrogen of SOFC anode condition. JIS SUS430J1L exhibited higher resistance for water oxidation. The feature came mainly from the thin Cr-rich oxide layer covering the alloy uniformly with little defects or segregations. Under humidified H2 at 700oC, Mn migrated outward and the morphology changed. Durability of the alloy seems to depend on the inner side of the surface oxide layer. Although the Cr-rich oxide layer blocked outward diffusion of metal ions, the layer could not block inward diffusion of oxygen ion completely. Improvement of the durability is possible by controlling the thickness of the surface oxide layer by the pre-heat-treatment.

Effect of Al content on 1200 °C steam oxidation behavior of Cr-based coatings on TZM alloy

In this paper, the 1200 °C high temperature steam oxidation behavior of Cr-based coatings with different Al content (0, 15 at%, 30 at%, 50 at%) deposited on titanium-zirconium-molybdenum (TZM) alloy has been investigated. The oxidation resistance of TZM alloy is significantly increased by the Cr-based coatings. The Cr70Al30 coating exhibits the best performance due the protectiveness of a triple oxide layer formed on coating surface, including outer Al2O3 and Cr2O3 mixed layer, intermediate Cr-rich oxide (Cr2O3) layer and inner Al-rich oxide (Al2O3) layer. The effect of Al content on the oxidation resistance of Cr-based coatings is discussed in detail.

Irradiation accelerated corrosion of alumina-forming austenitic steels in supercritical CO2: The oxide scale formed within an individual grain or affected by grain boundary

The irradiation accelerated corrosion within monolayer grains of a new AFA steel exposed to sCO2 are studied. The presence of irradiation induced dislocation loops accelerate element diffusion and reduces corrosion resistance by ∼44%. Many oxide scale features are firstly found, including a two-phase inner layer consisting of Ni-rich austenite and Cr-rich spinel, and Al2O3/SiO2 oxide precipitates. Irradiation leads to an additional Cr-rich oxide layer at the bottom of inner layer and a narrower Al2O3 band in the internal oxidation layer. Irradiation also induces Cr depletion at grain boundaries, thereby impeding the formation of a protective Cr-rich oxide scale.

Synergistic Roles of Nb and Mo in the Formation of Oxides on Fe-20Cr-25Ni-Nb Stainless Steels in High-Temperature CO2

Fe-20Cr-25Ni-Nb steel is an important material for developing highly compact and efficient nuclear power systems by using the supercritical CO2 Brayton cycle. The in-core materials should possess excellent oxidation resistance in a high-temperature CO2 environment. In this work, a new 20Cr-25Ni-Nb steel with a minor Mo addition was developed, and its oxidation behavior was investigated in a pure CO2 environment at 650 °C under 3.5 MPa. The experimental results show that the oxidation kinetics of the steels followed the parabolic law with the test time, and the oxidation process was controlled by diffusion. The 20Cr-25Ni-Nb steel showed better oxidation resistance after Mo addition, which was attributed to the synergistic effects of Nb and Mo during the oxide scale formation process. In a high-temperature environment, the volatilization of Mo promoted the outward diffusion of Nb, resulting in the formation of an Nb-rich layer at the oxide/metal interface, which slowed the outward diffusion of Fe for the formation of the outermost Fe-oxide layer. Although the volatile nature of Mo at high temperatures promoted the outward diffusion of Nb, the addition of Mo had no significant influence on the overall structure of the oxide scales, which consisted of an outer Cr-rich oxide layer and an inner spinel oxide layer.

Evolution of oxide film of a new martensitic steel 9Cr3W3Co in supercritical water

The oxide evolution of 9Cr3W3Co martensitic steel in the supercritical water environment at 650 °C and 30 MPa was studied. The results indicated that a double-layer oxide film, composed of a columnar Fe3O4 outer layer and a fine grain FeCr2O4/Fe3O4 spinel mixed inner one, that thicknesses followed a sub-parabolic law, was formed. The FeCr2O4 first grew along various boundaries, then grew into lath along subgrain boundary, dislocation line or by bulk diffusion. After a continuous Cr-rich oxide layer was formed at the inner oxide layer/matrix interface, the inner layer was gradually completely oxidized, which significantly slowed down the oxide rate.

Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al2O3 scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr2O3 layer, beneath which Al was internally oxidized. However, a continuous protective Al2O3 scale was developed below a thin Cr2O3 layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr2O3 layer and an amorphous Al2O3 layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr2O3 layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr2O3 and amorphous Al2O3 layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al2O3 layer until its transition to a protective crystalline Al2O3 scale.

The Nanostructure of the Oxide Formed on Fe-10Cr-4Al Exposed in Liquid Pb.

An Fe–10Cr–4Al alloy containing reactive elements developed for application in high-temperature liquid lead environments was analyzed after exposure in 600 and 750°C lead with dissolved oxygen for 1,000–2,000 h. Atom probe tomography, transmission electron microscopy, and X-ray scattering were all used to study the protective oxide formed on the surface. Exposure at 750°C resulted in a 2-μm thick oxide, whereas the 600°C exposure resulted in a 100-nm thick oxide. Both oxides were layered, with an Fe–Al spinel on top, and an alumina layer toward the metal. In the 600°C exposed material, there was a Cr-rich oxide layer between the spinel and the alumina. Metallic lead particles were found in the inner and middle parts of the oxide, related to pores. The combination of the experimental techniques, focusing on atom probe tomography, and the interpretations that can be done, are discussed in detail.

Unveiling the mechanism of yttrium significantly improving high-temperature oxidation resistance of super-austenitic stainless steel S32654

Aiming at serious catastrophic oxidation problem of super-austenitic stainless steel S32654, the influence of different rare earth elements on its oxidation behavior was comparatively investigated at 1200 °C. The mechanism of Y significantly improving high-temperature oxidation resistance of S32654 was unveiled. The results demonstrated that Y played much better beneficial roles than Ce and La in the initial formation of oxide layer: (1) Y promoted Cr segregation to steel surface to combine with O; (2) its preferential oxidation provided nucleation cores for Cr2O3. Both roles jointly promoted the selective oxidation of Cr and then the formation of protective Cr-rich oxide layer. This provided good prerequisites for inhibiting the formation and volatilization of MoO3. Additionally, Y cation segregation to oxide grain boundaries further promoted the selective oxidation of Cr and Si to form more protective oxide layer. These beneficial roles of Y essentially eliminated the synergistic effects of MoO3 volatilization and lamellar Cr2N precipitation on catastrophic oxidation. Accordingly, the oxidation resistance of Y-bearing S32654 was improved by 22%–45%.

Microstructure, mechanical properties and corrosion behavior of AlxCrMoNbZr multiprincipal element alloy coatings for accident-tolerant fuel claddings: Effect of Al content

AlxCrMoNbZr (x = 0, 0.2, 0.4, 1.0, 2.7) multiprincipal element alloy (MPEA) coatings were prepared on Zircaloy-2 (Zr-2) substrates by radio frequency magnetron sputtering. The effects of Al content on the microstructure, mechanical properties and corrosion resistance of the coatings were systematically investigated. The amount of BCC nanocrystals in the as-deposited coatings was found to increase with the addition of Al, leading to an increase in hardness and elastic modulus. Al is found to be a promoting factor for weight gain and oxide layer thickening of the coatings during an autoclave test in static pure water at 360 °C and 18.7 MPa for 45 days, which is related to the vacancies introduced by rapid outward diffusion of Al and slow inward diffusion of O. However, these effects can be ignored when the Al content is low. All the coatings provide superior protection for the Zr-2 substrate due to the inner protective Cr-rich oxide layer adjacent to the coatings. These preliminary screening results provide a criterion and indication for the composition design of Al-containing MPEA coatings applied to accident-tolerant fuel (ATF) claddings.

Effect of surface finishing on the oxidation characteristics of a Fe-21Cr-32Ni alloy in supercritical carbon dioxide

The effect of surface finishing on the oxidation behavior of a Fe-21Cr-32Ni alloy in supercritical carbon dioxide (650 ºC for 1000 h) was studied. After exposure, the cross-section of the specimen showed a typical inner-outer oxide layers structure with subsurface carburization. Reduced oxidation and carburization rates were identified for the ground surface, where a much thinner Cr-rich oxide scale and a shallower carburization depth were observed. Grinding introduced an ultra-fine grained layer and a highly deformed layer on the metal surface, where Cr diffusion rate was significantly enhanced. The high Cr diffusion led to the transition from internal to external oxidation by favoring the formation of a thin/protective Cr-rich oxide layer. This continuous Cr-rich oxide layer not only protected the substrate matrix from further oxidation, but also reduced the permeability of carbon into the oxides, thus improving the oxidation and carburization resistance of the ground specimen.

Effect of Deformation Structure of AISI 316L in Low-Temperature Vacuum Carburizing

The effect of plastic deformation applied to AISI 316L in low-temperature vacuum carburizing without surface activation was investigated. To create a difference in the deformation states of each specimen, solution and stress-relieving heat treatment were performed using plastically deformed AISI 316L, and the deformation structure and the carburized layer were observed with EBSD and OM. The change in lattice parameter was confirmed with XRD, and the natural oxide layers were analyzed through TEM and XPS. In this study, the carburized layer on the deformed AISI 316L was the thinnest and the dissolved carbon content of the layer was the lowest. The thickness and composition of the natural oxide layer on the surface were changed due to the deformed structure. The natural oxide layer on the deformed AISI 316L was the thickest, and the layer was formed with a bi-layer structure consisting of an upper Cr-rich layer and a lower Fe-rich layer. The thick and Cr-rich oxide layer was difficult to decompose due to the requirement for lower oxygen partial pressure. In conclusion, the oxide layer is the most influential factor, and its thickness and composition may determine carburizing efficiency in low-temperature vacuum carburizing without surface activation.

Pitting Corrosion Resistance and Electrochemical Properties of Equimolar CoCrFeMnNi and Non-Equimolar Alcocrfeni High-entropy Alloys

Equimolar CoCrFeMnNi high entropy alloy (HEA) exhibits excellent mechanical properties, but the corrosion resistance is relatively poor because of high Mn content. Thus, this study focused on Al addition and non-equimolar AlCoCrFeNi HEAs were fabricated. To investigate the effect of alloy composition on corrosion resistance, non-equimolar HEAs, Al8Co19Cr23Fe32Ni18 and Al25Co14Cr10Fe19Ni32, were fabricated by arc-melting. Their pitting corrosion resistance was studied by polarization in 0.1 M NaCl. Moreover, Al-containing HEAs have great potential to develop high corrosion resistant alloys by anodizing. Thus, the properties and the pitting corrosion resistance of passive oxide film formed by polarization from -0.55 to 0.5 VSCE in H2SO4 were also investigated.On the basis of the XRD results and surface images, Al8Co19Cr23Fe32Ni18 alloy was single phase alloy. On the other hand, equimolar AlCoCrFeNi and Al25Co14Cr10Fe19Ni32 alloys have multi-phase structures of Al and Ni rich phase and Cr and Fe rich phase. The pitting potential of Al8Co19Cr23Fe32Ni18 alloy was higher than that of equimolar CoCrFeMnNi alloy. After H2SO4 treatment, both alloys have apparent increase in their pitting potential. In the case of multi-phase alloy, Al25Co14Cr10Fe19Ni32 alloy has a lower pitting potential than that of equimolar AlCoCrFeNi alloy. After H2SO4 treatment, the Al and Ni rich phase was dissolved, and a Cr rich oxide layer was formed. This was likely to contribute the improvement of pitting corrosion of both AlCoCrFeNi and Al25Co14Cr10Fe19Ni32 alloys.

The formation mechanism and effect of amorphous SiO2 on the corrosion behaviour of Fe-Cr-Si ODS alloy in LBE at 550 °C

The corrosion behaviour of the newly developed Fe-Cr-Si oxide-dispersion-strengthened alloys in liquid lead–bismuth eutectic at 550 °C was microstructurally studied. The corrosion resistance was enhanced with increasing Si content, attributing to the preferred formation of amorphous SiO2 during the inward diffusion of oxygen. SiO2 promoted the formation of Cr-rich oxides that were uniformly distributed in the inner-oxide layer and matrix, thereby reducing the Fe and O diffusion coefficients and ultimately delaying the formation of the outer magnetite layer and the growth of the inner Cr-rich oxide layer while reducing interfacial defects.