Cr2O3 Oxide Film Research Articles

High-temperature oxidation behavior of GH4169 and Inconel617 nickel-based superalloys in SOFC environment

Field exposure for 5000h combined with weight gain measurement and morphology/composition analysis of oxide film were conducted to examine high temperature oxidation behavior of GH4169 and In617 nickel-based super alloys in SOFC system environment. The In617 alloy has been found to demonstrate excellent oxidation resistance in the SOFC environment because of its high Ni content. An integral and dense NiO oxide film can be developed in the outer layer with a continuous Cr2O3 oxide film formed in inner layer. It indicates that NiO played a crucial role in the formation of an integral and dense oxide film. In contrast, GH4169 alloy failed to develop a continuous and dense oxide film probably due to high Fe content, in which case Fe-dopped chromium oxide (Fe,Cr)2O3 was formed. As a result, the oxide film was loose and porous, leading to relatively poor oxidation resistance.

Oxidation behavior of 9Cr-RAFM steel at high temperature

The effect of yttrium on the mechanism of oxidation film formation of the 9Cr reduced activation ferritic/martensitic (9Cr-RAFM) steel was investigated. Four alloys yttrium contents ranging from 0 to 0.065% were melted in a vacuum induction furnace and subjected to continuous isothermal oxidation experiments. The alloys were oxidized in air at 600, 700, 800 and 900 °C for 100 h. Oxidation kinetic curves were obtained and analyzed to study the effect of yttrium on the oxidation resistance of the alloys at high temperatures. The oxide films formed on the alloys were observed and analyzed by X-ray diffractometry, scanning electron microscopy and electron probe X-ray microanalysis. The results indicated that the oxidation weight gain of the samples varied with the yttrium content, with the highest gain observed in the yttrium-free alloy sample, and the least in those containing 0.065% yttrium. Adding yttrium significantly enhanced the high temperature oxidation resistance of 9Cr-RAFM alloy by promoting the formation of a selective Cr2O3 oxide film. The film improved the adhesion of the oxide layer to the base metal, reduced the oxidation rate, and thereby enhanced the high-temperature oxidation resistance of the alloy.

B and Ce composite microalloying for improving high-temperature oxidation resistance of 254SMO super-austenite stainless steel

Aiming at serious oxidation problem of 254SMO super-austenitic stainless steel during hot working, the influence of B and Ce composite microalloying on its oxidation behavior was comparatively investigated at 1050 and 1100 °C. The results demonstrated that the combination of B and Ce can significantly alter the composition of the oxide film in 254SMO. Particularly, B and Ce composite microalloying can effectively promote the diffusion of Cr to the surface, and form a dense Cr2O3 oxide film at a faster rate in the initial stage, which is more conducive to inhibiting the Mo volatilization and thus improving the oxidation resistance of 254SMO steels. Additionally, compared to the 0.005 wt% B (50B) and 0.005 wt% B together with 0.002 wt% Ce (50B + 20Ce) samples, the addition of 0.005 wt% B together with 0.005 wt% Ce (50B + 50Ce) had a more significant effect on improving high-temperature oxidation resistance of 254SMO. This research provides a valuable scholarly reference for improving the oxidation resistance of super-austenite stainless steels.

Failure analysis of the end surface of the burner nozzle made of UMCo50 material in long-term high temperature oxygen-enriched sulfur-chlorine environment

As the core device of coal water slurry gasification technology, the failure of the burner nozzle will bring serious technical and safety problems. The purpose of this paper is to determine the mechanism of burner failure based on the characteristics of corrosion products and cracks. The results showed that the Cr2O3 oxide film on the surface of UMCo50 was damaged under the effect of mechanical erosion, high temperature and stress, and the corrosive elements S and Cl in the combustion chamber made it further cracked and peeled off. Therefore, the matrix was corroded and cracks occurred. During the crack propagation process, more M23C6 carbides were precipitated at the grain boundary, it caused Cr-depleted at the grain boundary, which reduced the corrosion resistance of the matrix near the grain boundary. The exfoliation of carbides distributed along the grain boundary also accelerated crack propagation. Through XPS and XRD analysis, it was found that the main corrosion products on the crack surface of the failed parts were: Cr2O3, CoCr2O4, CoO, Fe2O3, FeS. According to the causes of failure analysis, the corresponding improvement measures are put forward.

Multimodal in Situ synchrotron Analysis Reveals the Effect of the Oxygen Evolution Reaction on Corrosion of a Ni-Cr-Mo Alloy

Nanometer-thin and spontaneously formed oxide films, or passive films, govern the corrosion resistance of many advanced alloys [1]. Our current understanding of the structure and chemistry of the passive film is primarily based on ex situ surface analysis using X-ray Photoelectron Spectroscopy (XPS) [2] and Scanning Tunneling Microscopy (STM) [3]. XPS has historically been limited to Ultra High Vacuum (UHV) conditions, so the results do not represent the true passive film/electrolyte interface. To observe corrosion initiation and progression and obtain a fundamental understanding of corrosion mechanisms, there is a need for techniques that can combine in situ capabilities with detailed chemical and structural information from the surface region. We have previously used Ambient Pressure (AP) XPS to study passive film growth [4] [5] and a combination of X-ray Reflectivity (XRR), Grazing Incidence X-ray Diffraction (GI-XRD), X-ray Fluorescence (XRF) to study the passivity breakdown of duplex stainless steel [6].Ni-based alloys are known for their excellent mechanical properties and corrosion resistance and are used in many demanding industrial environments. Ni is commonly alloyed with Cr and Mo, where Cr is known to improve the corrosion resistance due to the formation of a Cr2O3 oxide film on the surface, while the role of Mo is debated. Commonly used electrochemical techniques for studying passivity breakdown of other alloys may not be applicable to Ni-Cr-Mo alloys because the measured electrochemical current is not only due to corrosion reactions [7]. Ni and Mo are good catalysts for the Oxygen Evolution Reaction (OER) [8, 9], which adds another dimension of complexity to the material system since OER is known to be coupled with dissolution and degradation in other metallic systems [10]. A fundamental holistic understanding is missing regarding how the chemistry and structure of the passive film on Ni-Cr-Mo alloys evolve in realistic aqueous conditions and how that correlates with the onset of dissolution, which determines the breakdown of passivity.Here we present a comprehensive study combining several synchrotron-based techniques to study the surface region of a Ni-Cr-Mo alloy in NaCl solutions in situ during electrochemical polarization, as shown in Figure 1 a). XRR and AP-XPS were used to investigate the thickness and chemistry of the passive film. GI-XRD was used to determine the change in the metal lattice underneath the passive film. XRF was used to quantify the dissolution of alloying elements. X-ray Absorption Near Edge Structure (XANES) was used to study the chemical state of the dissolved species in the electrolyte and the chemical state of corrosion products formed on the surface. The combination of these techniques allowed us to study the corrosion process and detect the passivity breakdown in situ and correlate it to the onset of OER.Growth of the passive film and enrichment of Mo6+ oxide was observed in the passive range below 800 mV vs. Ag/AgCl, followed by a drastic increase in the electrochemical current coupled with the formation of a thick film of MoO3, and Cr(OH)3, as seen from the AP-XPS data in Figure 1 b). The current increase at potentials above 800 mV vs. Ag/AgCl coincided with the dissolution of Ni2+, Cr3+, and Mo6+, as seen in Figure 1 c). Quantitative analysis revealed that a substantial part of the measured current was due to oxygen evolution, as shown in Figure 1 d). The experimental techniques and the unique information they provide will be discussed, as well as the role of OER on the passivity breakdown of the Ni-Cr-Mo alloy. D. D. Macdonald, Pure and Applied Chemistry, 71 (6), 951-978 (1999). H. Strehblow and P. Marcus, CORROSION TECHNOLOGY-NEW YORK AND BASEL-, 22 1 (2006). V. Maurice and P. Marcus, Progress in Materials Science, 95 132-171 (2018). A. Larsson, K. Simonov, J. Eidhagen, A. Grespi, X. Yue, H. Tang, A. Delblanc, M. Scardamaglia, A. Shavorskiy, J. Pan, and E. Lundgren, Appl Surf Sci, 611 155714 (2023). M. Långberg, C. Örnek, J. Evertsson, G. S. Harlow, W. Linpé, L. Rullik, F. Carlà, R. Felici, E. Bettini, U. Kivisäkk, E. Lundgren, and J. Pan, npj Materials Degradation, 3 (1), 22 (2019). E. Bettini, C. Leygraf, and J. Pan, Int J Electrochem Sc, 8 11791-11804 (2013). H. Liao, X. Zhang, S. Niu, P. Tan, K. Chen, Y. Liu, G. Wang, M. Liu, and J. Pan, Applied Catalysis B: Environmental, 307 121150 (2022). A. Lončar, D. Escalera-López, S. Cherevko, and N. Hodnik, Angewandte Chemie International Edition, 61 (14), e202114437 (2022). Figure 1

Study on the arc erosion resistance and mechanism of Cr- doped W[sbnd]Cu composites

WCu composites have been widely used in electrical contacts and arc resistance electrodes. To improve the arc erosion resistance, the WCu composites doping Cr was fabricated by high energy ball milling and SPS process. The arc erosion characteristics of W-Cu-Cr composites under 1000, 5000 and 10,000 electric contact operations were studied, and the mechanism of arc erosion resistance of WCu composites doped with Cr was revealed. The results indicated that the average arc energy of W-Cu-1Cr composites decreased by 35.7%, 53.7%, and 63.8% respectively after 1000, 5000, and 10,000 electrical contact tests compared with WCu composites undoped Cr phase. After 10,000 operations, the average welding force of W-Cu-1Cr composites is 24.3% lower than that WCu composites undoped Cr phase, and had lower arc erosion loss (0.57 mg). In addition, the strong oxygen absorption of Cr phase results in the formation of an oxide film on the surface of WCu composites. The arc root moves smoothly and continuously along the Cr2O3 oxide film at the grain boundary, thereby reducing arc energy and improving the arc erosion resistance of WCu composites.

Corrosion Behavior of Alumina-Forming Austenitic Steel in Supercritical Carbon Dioxide Conditions: Effects of Nb Content and Temperature.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr2O3 oxide film and an inner Al2O3 oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ'-Ni3Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe2(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.

Oxidation Property of a Fourth-Generation Powder Metallurgy FGH4108 Nickel-Based Superalloy

Isothermal oxidation kinetics of a fourth-generation powder metallurgy FGH4108 nickel-based superalloy is investigated at 800 °C to 1100 °C by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). At 800 °C and 900 °C, the oxidation kinetic curves of the FGH4108 superalloy follow parabolic law. At 1000 °C, the oxidation kinetic curve follows cubic law. At 1100 °C, the oxidation kinetic curve has two distinct parts: the first part follows a parabolic law, and the second one obeys a linear law. Cross-sectional morphologies and elemental distributions show that the oxide film consists of two parts at 800 °C: the outer layer is a continuous dense protective Cr2O3 oxide film, and the inner layer is a discontinuous Al2O3 oxide layer. At 900–1100 °C, the oxides consist of three layers: the outermost is the oxides of Cr2O3 and TiO2, the middle is a continuous oxide of Al2O3, and the innermost is dotted oxides of TiO2. The thickness of the inner TiO2 oxide layer increases with the increase of oxidation temperature. On this basis, the oxidation behavior of the FGH4108 superalloy at high temperatures is confirmed to be controlled by the diffusion of Cr, Al, Ti, and O. From the aspect of oxidation resistance, the long-term service temperature of the FGH4108 superalloy should not exceed 1000 °C.

Revealing the role of SO2 in the high-temperature corrosion diffusion of two superalloys in CO2 through molecular dynamics and thermal stability

The corrosion behavior of HR230 and 740 H alloys in CO2 and CO2 with 0.5% SO2 at 1000 ℃ was studied by molecular dynamics, thermodynamics and experiments. The weight gain of both alloys in CO2 environment was increased with the addition of 0.5% SO2, in which the weight gain of 740 H alloy was higher than that of HR230 alloy. Some CrS sulfides were observed in Cr2O3 oxide film of both alloys, while TiO2 oxides on 740 H alloy could reduce the infiltration of S element. Thermodynamic calculations of the corrosion reactions showed that serious internal vulcanization and internal oxidation would occur in both alloys when the content of SO2 exceeded 3%.

High temperature oxidation enhanced by laser remelted of CoCrFeNiAlx (x = 0.1, 0.5 and 1) high-entropy alloys

In order to further improve the oxidation resistance of CoCrFeNiAlx (x = 0.1, 0.5, and 1) high entropy alloys (HEAs), the surface of CoCrFeNiAlx prepared by vacuum arc melting is controlled by laser surface remelting (LSM) and the high-temperature oxidation properties of CoCrFeNiAlx before and after LSM is systematically studied. The results revealed that, with the increase of Al content, the parabolic velocity constant kp value gradually decreased after LSM treatment. Moreover, CoCrFeNiAl0.1 formed a continuous Cr2O3 oxide layer, CoCrFeNiAl0.5 formed an outer layer of Cr2O3 oxide film and an inner layer of Al2O3 precipitation, and CoCrFeNiAl1 formed a uniform and dense Al2O3 oxide layer on the entire area. After 100 h of oxidation, the oxide film thickness of three alloys is around 5 µm, 4 µm, and 3 µm, respectively. The improvement in oxidation resistance after LSM can be ascribed to the formation of a dense re-melted layer, uniform distribution of the elements, refinement of the re-melted layer.

Achieving superior hot corrosion resistance by PVD/HVOF duplex design

In this study, an optimized duplex coating, consisting of a thick Cr3C2-NiCr interlayer and a top CrN film, were designed and deposited by HVOF and PVD. And then hot corrosion behaviors were systematically investigated, and corresponding corrosion mechanisms are discussed in detailed. Results showed that the thickness of the oxide scale of duplex coating is approximately one-tenth that of the Cr3C2-NiCr coating. Due to the formation of Cr2O3 oxide film, the CrN/Cr3C2-NiCr duplex coating exhibits superior hot corrosion resistance. However, chlorine-induced corrosion in uncovered holes is the main cause of local failure of duplex coating.

Effect of CeO2 on High‐Temperature Oxidation Performance of Electron Beam Cladding NiCoCrAlY Coating on Ni‐Based Alloy

In order to enhance the high‐temperature oxidation resistance properties of Inconel 617 alloy, NiCoCrAlY and NiCoCrAlY CeO2 composite powder coatings metallurgically bonded to substrate were prepared on the surface of Inconel 617 alloy by electron beam cladding. The effect of rare earth oxide CeO2 on the oxidation resistance of NiCoCrAlY coating was investigated. The isothermal oxidation behavior of the substrate and NiCoCrAlY cladding layer with different CeO2 contents (1%, 2%, 3%, and 4%) and without CeO2 oxidized at 1050°C for 20 h, 40 h, 60 h, and 100 h was analyzed. The microstructure and phase composition of the coating after electron beam treatment were tested. The results show that the self‐repair of Al2O3 and Cr2O3 oxide film can be improved under a high‐temperature oxidation environment with the addition of CeO2, and the oxidation resistance of NiCoCrAlY coating can be effectively strengthened by adding CeO2. The improvement effect is most obvious when the content of CeO2 is 2%.

Effect of medium temperature precipitation phase and Mn element diffusion mechanism on high temperature oxidation process of repair and remanufacture CoCrFeMnNi high-entropy alloy cladding

In order to study the effect of medium temperature precipitation phase and Mn element diffusion mechanism on the high temperature oxidation resistance of CoCrFeMnNi high-entropy alloy cladding layer, the high-entropy alloy cladding layers were prepared on Q235 substrate by micro-plasma arc process, which were annealed at different temperatures in atmosphere, and then subjected to high temperature oxidation at 900 °C for 100 h. Although the results showed that the microstructure of CoCrFeMnNi high-entropy alloy cladding layer was dendritic structure, the content of Fe, Co and Cr in dendrites was higher, while the content of Ni and Mn among dendrites was higher rather than the generally uniform elements compositions. As the annealing temperature increased, the crystal size in the layer grew slightly. The cladding layer of unannealed and annealed at 900 °C and 1160 °C for 4 h was a single FCC phase structure. However, the annealing cladding layer at 500 °C showed a small amount of Cr-rich BCC phase at the grain boundary, and the 700 °C annealing cladding layer appeared Cr–Fe-rich δ phase at the grain boundary. The alloy cladding layer annealed at different temperatures was subjected to high temperature oxidation experiments at 900 °C for 100 h. The oxidation kinetics curve was parabolic, the diffusion of Mn element controlled the entire oxidation process. The Mn element diffused faster than the other metal cations in the Cr2O3 oxide film and the grain boundary of the matrix, forming an intimate oxide film on the surface and a pore layer depleted in Cr and Mn at an interface portion between the substrate and the oxide film. With the increase of annealing temperature, the grain boundary effect and the interface of Cr and Mn were more favorable for the outward diffusion of Mn element, which formed an intimate oxide film, leading to the gradual severity of oxidation.

Hydrogen Permeation, and Mechanical and Tribological Behavior, of CrNx Coatings Deposited at Various Bias Voltages on IN718 by Direct Current Reactive Sputtering

In the current work, the microstructure, hydrogen permeability, and properties of chromium nitride (CrNx) thin films deposited on the Inconel 718 superalloy using direct current reactive sputtering are investigated. The influence of the substrate bias voltage on the crystal structure, mechanical, and tribological properties before and after hydrogen exposure was studied. It was found that increasing the substrate bias voltage leads to densification of the coating. X-ray diffraction (XRD) results reveal a change from mixed fcc-CrN + hcp-Cr2N to the approximately stoichiometric hcp-Cr2N phase with increasing substrate bias confirmed by wavelength-dispersive X-ray spectroscopy (WDS). The texture coefficients of (113), (110), and (111) planes vary significantly with increasing substrate bias voltage. The hydrogen permeability was measured by gas-phase hydrogenation. The CrN coating deposited at 60 V with mixed c-CrN and (113) textured hcp-Cr2N phases exhibits the lowest hydrogen absorption at 873 K. It is suggested that the crystal orientation is only one parameter influencing the permeation resistance of the CrNx coating together with the film structure, the presence of mixing phases, and the packing density of the structure. After hydrogenation, the hardness increased for all coatings, which could be related to the formation of a Cr2O3 oxide film on the surface, as well as the defect formation after hydrogen loading. Tribological tests reveal that hydrogenation leads to a decrease of the friction coefficient by up to 40%. The lowest value of 0.25 ± 0.02 was reached for the CrNx coating deposited at 60 V after hydrogenation.

The Influence of Temperature on the Oxidation Mechanism in Air of HR3C and Aluminum-Containing 22Cr–25Ni Austenitic Stainless Steels

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr2O3 healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr2O3 oxide film on HR3C was unstable and partly converted into a less protective MnCr2O4 with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr2O3 to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al2O3 and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.

Microstructure and thermal fatigue resistance of Co-based alloy coating on H13 steel by laser cladding

Thermal fatigue failure is easy to occur on H13 steel surface under working condition, the Co-based coating was fabricated by Nd:YAG laser on H13 steel. Microstructure, distribution of elements and composition of phases were investigated by means of optical microscope(OM), scanning electron microscope(SEM), energy dispersive spectrometer(EDS) and X-ray diffraction(XRD).The effect of thermal fatigue on microhardness of Co-based coating and quenching-tempering H13 steel was tested by microhardness tester. The results show that: From bottom to the surface of Co-based coating, planar crystal, cellular crystal, dendrite crystal and equiaxed crystal were observed; Co-based alloy coating is mainly composed of -Co and M23C6, M2O3 and M3O4(M=Fe, Co, Cr)are produced on the surface of coating after thermal cycling; Microhardness of Co-based coating which could reach 706HV0.2, decreased gradually. After 1000 thermal cycles, the microhardness decreases of Co-based alloy coating and H13 steel are 24.4% and 37.7%, respectively; The surface of the Co-based coating has no thermal cracks, while that of H13 steel has a large number of thermal cracks; Cr2O3 oxide film is formed on Co-based alloy coating, which makes thermal fatigue resistance of coating better than that of H13 steel.

Microstructure and high temperature oxidation resistance of nickel based alloy GH4169 irradiated by high current pulsed electron beam

The surface of nickel-based superalloy GH4169 was irradiated by high-current pulsed electron beam (HCPEB). Oxidation test of the irradiated specimens was conducted at 900°C in air for 80h. The microstructure evolution and surface morphology of the irradiated surface were analyzed by means of X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM). XRD result showed that the width of peaks had a broadened trend, indicating grain refinement was obtained in the rapid-quenched. Microstructure observations revealed that a compact remelted layer was formed on the sample surface after HCPEB irradiation, inside which were mainly nanocrystallines with their size smaller than 100nm. Besides, high-density dislocations and deformation twins were also formed in remelted layer. The observations after oxidation test showed that the oxide film on 20-pulsed sample surface was more continuous and compact than that on the initial sample surface, and the protection mechanism of 20-pulsed sample surface changed from the combined action of outer NiCr2O4 spinel layer and inner Cr2O3 oxide film layer into outer Cr2O3 oxide film and inner Al2O3 oxide film. The results indicated that the irradiated sample had much higher oxidation resistance than the initial one, among which 20-pulsed sample own the best oxidation resistance.

Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.

Oxidation Behavior and Electrical Conductivity between Ln0.8Sr0.2MnO3 Cathode and Crofer 22 APU with Spinel Coatings

The modified method of electrostatic spray pyrolysis was designed to apply protective coatings based on Mn-Co spinel to ferritic stainless steels (Crofer22APU and 08X18T1). The comparative thermogravimetric (TG) studies of ferrite stainless steels with and without protective coatings were carried out. The electrochemical characteristics of protective coatings exposed to long current loading were studied. The formation of the Mn-Cr spinel observed and studied at the contact between the ferrite stainless steel and the Mn-Co spinel. The coatings of Mn-Co spinel were shown preventing formation of continuous Cr2O3 oxide film on the stainless steel surface.

Influence of Lanthanum Ion-Implantation on Adhesive Property of Oxide Film Formed on Co-40Cr Alloy

The isothermal and cyclic oxidizing kinetics of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1 000 °C in air by thermal gravimetric analysis (TGA). Scanning electron microscopy (SEM) was used to examine the Cr2 O3 oxide film’s morphology after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on the formation of Cr2 O3 film. Acoustic emission (AE) method was used in situ to monitor the cracking and spalling of oxide films during oxidizing and subsequent air-cooling stages. Laser Raman spectrum was used to examine the stress changes within oxide films. A theoretical model was proposed relating to the film fracture process and was used to analyze the AE spectrum both on time domain and AE-event number domain. It was found that lanthanum implantation remarkably reduced the isothermal oxidizing rate of Co-40Cr and improved the anti-cracking and anti-spalling properties of Cr2 O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduced the grain size and internal stress of Cr2 O3 oxide, increased the high temperature plasticity of oxide film, and remarkably reduced the number and size of Cr2 O3/Co-40Cr interfacial defects.