Excellent High Temperature Oxidation Resistance Research Articles

Effect of laser cladding layers on microstructure and mechanical properties of NiCoCrAlY bond coat

The NiCoCrAlY alloy is usually used as a bonding layer material in thermal barrier coatings (TBCs) system. Owing its excellent high-temperature oxidation resistance and corrosion resistance, it plays a crucial role in maintaining the structural integrity and functionality of TBCs. Laser cladding technology is a flexible and efficient processing technique for the manufacturing of NiCoCrAlY bond coat, with the characteristics of precisely control the layers of the laser cladding and further modulate the quality of the bond coat. Therefore, the investigation on the influence of cladding layers on the microstructure and mechanical properties of the bond coat is of great significance for improving the service performance and lifetime of aircraft engines. To this end, the scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD) were used in this study to explore the influence of cladding layers on the microstructure and mechanical properties of the NiCrAlCoY bond coat. The results indicate that the microstructure of the cladded alloy can be divided into substrate, heat-affected zone, and cladded zone. The hardness of both the heat-affected zone and the cladded zone increases as the number of cladding layers increases, while the hardness of the substrate maintains constant. In addition, as the number of laser cladding layers increases, so does the residual stress of the cladded alloy.

Phase evolution and steam oxidation behavior of nickel-based composite coatings fabricated by laser cladding

In this study, a variety of synthesized carbides are used as reinforcing phases for Inconel 625 alloy to enhance the performance of surface coatings. The effects of scanning rate and laser power on the microstructure and mechanical properties of coatings were studied, and specific energy was introduced to comprehensively evaluate the mechanisms of their influence. The results show that the percentage of macroscopic defects in the nickel-based composite coatings decreases and the dilution rate increases gradually with increasing specific energy density and the mechanical properties of the composite coating deteriorate gradually. It is worth noting that when the specific energy density is 153.33 J/mm2, the comprehensive performance of the composite coating is the best, which is mainly due to the uniform distribution of the contained (Ti, W)C and Cr carbides. The coating exhibits excellent hardness, corrosion resistance, and high-temperature oxidation resistance. In addition, after determining the optimal process parameter combination, the high-temperature steam oxidation performance of the nickel-based composite coating is analyzed in depth. At 1200°C, the corrosion degree of the composite coating gradually increased with increasing oxidation time. Research shows that the ceramic phase structure of (Ti, W)C and Cr plays a significant inhibitory role in high-temperature environment, effectively decelerating the increase of corrosion rate. Interestingly, before the first 48 h of the oxidation process, the thickness of the oxidation layer on coating surface increases slowly. However, when the oxidation time reaches 96 h, large amounts of WO3 and NiCr2O4 phases are formed, leading to the generation of pores and cracks in the coating. This causes the oxide film to fall off, resulting in a sharp increase in the corrosion layer thickness. The research in this paper is of great significance for optimizing coating materials and preparation processes, as well as improving the long-term high-temperature corrosion resistance of coatings.

Study on the Oxidation Behavior of TiB2-CeO2-Modified (Nb,Mo,Cr,W)Si2 Coating on the Surface of Niobium Alloy.

A novel TiB2-CeO2-modified (Nb,Mo,Cr,W)Si2 coating was prepared on a Nb-5W-2Mo-1Zr alloy substrate using two-step slurry sintering and halide-activated pack cementation to address the limitations of a single NbSi2 coating in meeting the service requirements of niobium alloys at elevated temperatures. At 1700 °C, the static oxidation life of the coating exceeded 20 h, thus indicating excellent high-temperature oxidation resistance. This was due to the formation of a TiO2-SiO2-Cr2O3 composite oxide film on the coating surface, which, due to low oxygen permeability, effectively prevented the inward infiltration of oxygen. Additionally, the dense structure of the composite coating further enhanced this protective effect. The composite coating was able to withstand over 1600 thermal shock cycles from room temperature to 1700 °C, and its excellent thermal shock performance could be attributed to the formation of MoSi2, CrSi2, and WSi2 from elements such as Mo, Cr, and W, which were added during modification. In addition to adjusting the difference in thermal expansion coefficients between the layers of composite coatings to reduce the thermal stress generated by thermal shock cycles, the formation of silicide compounds also improved the overall fracture toughness of the coating and thereby improved its thermal shock resistance.

Achieving excellent mechanical and antioxidant properties in MoAlB ceramics through spark plasma sintering-high-frequency (SPS-HF) sintering

A fully dense MoAlB ceramic was synthesized utilizing MoAlB powder through SPS-HF technology. The Vickers hardness, fracture toughness and bending strength of the MoAlB ceramic reached 14.08 GPa, 6.70 MPa m1/2 and 407 MPa, respectively. The electrical conductivity and thermal conductivity of the MoAlB ceramic at room temperature were measured to be 3.20 × 106 S m−1 and 23.50 W m−1 K−1, respectively. The ceramic exhibited excellent high-temperature oxidation resistance at 800–1200 °C for up to 64 h. Even after 64 h of oxidation at 800 °C and 1000 °C, the thickness of the oxide layer was only 2.09 µm and 2.20 µm, respectively. The oxidation behavior was controlled by a diffusion process. The rapid formation of a continuous and dense Al2O3 oxide layer effectively inhibited the inward diffusion of oxygen and the oxide layer is well bonded to the matrix.

Research status on the effect of energy density on the forming microstructure and properties of nickel-based superalloys for laser additive manufacturing

Abstract Nickel-based superalloys have excellent high-temperature mechanical properties, corrosion resistance and oxidation resistance, and strong machinability. It is widely used in aerospace, submarine and shipbuilding, petrochemical, electronic industry and other industries. However, there are still challenges in the popularization and application of nickel-based superalloys for alloy components with complex structures and extremely harsh working conditions. In this paper, the research status of the influence of energy density on the microstructure and properties of laser additive fabrication of nickel-based superalloys at home and abroad is reviewed. The influence of energy density on the microstructure evolution behavior and mechanical properties improvement effect of laser additive manufacturing nickel-based superalloys is summarized. The mechanism of energy density was discussed from the perspectives of microstructure evolution and macroscopic performance change. Based on the individual effects and synergistic effects of each process parameter, the influence of laser energy density on dendrite growth, phase precipitation characteristics, element distribution and porosity defect control effect of nickel-based superalloy was expounded, as well as the influence mechanism on microhardness, wear resistance and residual stress. Finally, the energy density optimization and development prospect of laser additive fabrication of nickel-based superalloys are prospected.

Exploring Nb[sbnd]Si based alloys with excellent fracture toughness and oxidation resistance

The main objective of this paper is to give a comprehensive discussion of the fracture toughness and oxidation resistance of Nb-16Si-20Ti-3Al-5Cr-xSn-yZr (x=0.5, 1.5; y=2, 4, 6) alloys, based on experimental evidence, phase formation and diffusion theory. Results demonstrate that the microstructure of Sn0.5Zr2 and Sn1.5Zr2 alloys consist of Nbss and α-Nb5Si3, and the other four alloys are composed of Nbss, α-Nb5Si3 and γ-Nb5Si3. At the same time, the area fraction of γ-Nb5Si3 increases with the increase of Zr content. The coarsening of silicides increases with the addition of Sn. The KQ value of the Sn0.5Zr4 alloy reaches the highest with the mean of 12.64 MPa·m1/2. After oxidation at 800 °C for 24 h, Sn0.5Zr4 alloy alloy did not undergo pest oxidation. Sn0.5Zr4 alloy forms a continuous and dense TiO2 oxide layer oxidized at 1200 °C, which makes it have the best high temperature oxidation resistance. Sn0.5Zr4 alloy has excellent fracture toughness, medium temperature and high temperature oxidation resistance.

Simultaneous improving high temperature oxidation resistance and room temperature toughness of Nb[sbnd]Si based alloy via appropriate Ta content addition strategy

NbSi based alloys are expected to become the application materials for high pressure turbine blades of aircraft engines. This paper investigates the impact of Ta addition on room temperature fracture toughness and isothermal oxidation behavior at 1250 °C of four NbSi based alloys with compositions of Nb-16Si-20Ti-8Zr-4Hf-1.5Al-xTa (x = 0, 1, 2, and 4 at.%) fabricated through vacuum non-consumable arc-melting. The results show that the room temperature fracture toughness and 1250 °C oxidation resistance of the alloys increase first and then decrease with the addition of Ta element. The room temperature fracture toughness of 1Ta alloy is the best, reaching 17.1 MPa·m1/2. Adding a moderate amount of Ta (1 at.%) significantly enhances the alloy's oxidation resistance, but an excessive Ta content diminishes this enhancement. This research has obtained a Nb-16Si-20Ti-8Zr-4Hf-1.5Al-1Ta alloy with high room temperature fracture toughness and excellent high temperature oxidation resistance simultaneously, which provide an ideal path for designing NbSi alloys with superior comprehensive properties.

Preparation of BN@SiO2 nanofiber ceramic aerogel with ultralight, wave-transparent and heat-insulating for microwave sintering furnace

Thermal insulation devices play an extremely important role in enhancing microwave heating efficiency. However, existing wave-transparent insulation materials often exhibit high heat storage, low microwave transmittance at high temperatures, and poor oxidation and thermal shock resistance, hindering high-temperature applications for microwave sintering. To address this issue, we developed an ultralight and wave-transparent BN@SiO2 aerogel, which was fabricated through the in situ growth of SiO2 on the surfaces of BN fibers. The resulting composite aerogel demonstrated excellent high-temperature oxidation resistance and an impressively low density of 0.07 g/cm3. Furthermore, the dielectric constant of the BN@SiO2 aerogel ranged from 2.58 to 2.80, and its dielectric loss tangent was as low as 5.13 × 10−3–7.98 × 10−3 within the 2–10 GHz frequency range. The successful synthesis of BN@SiO2 aerogel provides new progress and research ideas for the development of wave transparent and thermal insulation materials.

Enhancing the high-temperature oxidation performance of AlCrTiSiN coating by in-situ generation of multi-layer antioxidant composite structures

A novel AlCrTiSiN high-entropy coating with superior thermal stability and high-temperature oxidation resistance, has been produced. The coating is characterized by unique fcc single-phase solid solution structure with uniform composition, severe lattice distortion and low fcc-AlN solubility, which results in the superior thermal stability. The excellent high-temperature oxidation resistance of coating is attributed to the multi-layer composite structures generated by in-situ reaction of element selective diffusion during the oxidation process. The inner dense SiO2-typed amorphous sub-layer and the intermediate (Al,Cr)2O3-typed sub-layer effectively block the O inward diffusion, and thus enhance the high-temperature oxidation resistance of the coating.

Fabrication of mechanically alloyed high entropy alloys (20-Fe-20Cr-20Ni-20Mn-20Ti) modified carbon paste electrode sensor for the cyclic voltammetric determination of methyl orange

We have successfully fabricated high entropy alloys (HEA) of composition 20-Fe-20Cr-20Ni-20Mn-20Ti by ball milling method for 20 h after optimizing the milling parameters. Generally, the HEAs are very popular for their multi-elemental cocktail effect and exhibit excellent high temperature oxidation resistance, high strength, electromagnetic, wear and corrosion resistance properties. But, HEAs also exhibit significant electrocatalytic properties due to their unique morphology, composition, and structure. Therefore, we have used planetary ball milled HEA powders for the electrochemical determination of methyl orange (MO). MO is recalcitrant and very difficult to degrade and can affect the humans as well as the environment. Therefore, we must determine the presence of MO. We have investigated the effect of scan rate, modifier and analyte concentration, and the pH variation on the anodic peak current (Ipa) of MO. The calculated active syrface area of bare carbon paste electrode (BCPE) and HEA-modified carbon paste electrode (HEA-MCPE) were found to be 0.041 cm2 and 0.137 cm2 respectively. We have obtained the maximum oxidation peak current for the 8 mg HEA-MCPE at a pH of 7.2 during the electrochemical oxidation of MO. The increased Ipa in basic medium is due to structural transformation of MO from quinoid form (in acidic medium) to azo structure (in basic medium). We have also calculated the number of electrons and the protons involved in the electrochemical reaction. The calculated values of detection limit (DL) and quantification limit (QL) of 8 mg HEA-MCPE was found to be 0.114 × 10−8 M and 0.38 × 10−8 M respectively.

Microstructure development of plasma sprayed dual-phase high entropy ceramic coating derived from spray dried and induction plasma spheroidized powder

Dual phase high entropy ceramics (DPHECs) exhibit excellent high temperature mechanical properties and oxidation resistance. Nevertheless, they cannot be applied in the structural components on large scale due to their intrinsic brittleness. Hence, DPHEC coating is a necessary development direction for industrial applications. In this study, we first fabricated micron-order DPHEC powder with high quality by precursor synthesis, spray drying (SD), and induction plasma spheroidization (IPS). The DPHEC coating was prepared by supersonic atmospheric plasma spraying (SAPS). The microstructure, phase compositions, and mechanical properties of the DPHEC powder and coating are investigated in detail. Results show that the SD&IPS powder and SAPS coating are all composed of HEB and HEC phases, showing slight composition segregation and typical global eutectic microstructure. The nanohardness and Young's modulus are 22.17 ± 2.76 GPa and 309.79 ± 35.71 GPa, respectively. The fracture toughness was calculated to be 2.23 ± 0.17 MPa m1/2 from the energy analysis method. The mechanical properties are enhanced by the solid solution and fine grain strengthening effects and are weakened by the intrinsic defects of plasma sprayed coating. This work provides a new method and a potential material for thermal spraying ultra-high temperature ceramic coatings.

Fire-resistant and thermal insulation performance of SiC nanofiber batt fabricated from cotton fiber batt

The cotton fiber batt is one of the most commonly used thermal insulator in our lives at room temperature because of its flexible and porous structure. The SiC nanofibers have excellent mechanical, physical, and chemical properties such as high mechanical strength and modulus, low density, high-temperature resistance, which is a promising material for high temperature structural materials or thermal insulator. In this work, a flexible SiC nanofiber batt was fabricated from the cotton fiber batt by sol-gel carbothermal reduction reaction. The fluffy carbonized porous cotton batt was used as a template for the fabrication of sponge-liked SiC nanofiber batt composed of single crystalline β-SiC fibers. It exhibits good compressive flexibility, ultra-high porosity (a specific surface area was 30.90 m²/g), excellent high-temperature stability (ranging from −196 °C in liquid nitrogen to 1300 °C under the flame of butane blowtorch) and low thermal conductivity (0.02 W/(m·K) at room temperature). The SiC nanofiber batt, which has low density, excellent high-temperature resistance and oxidation resistance, is a promising material for high-temperature structural materials or thermal insulators under extreme conditions. The facile fabrication method of SiC nanofiber batt from common cotton batt provides a novel technique for economic industrial production of high temperature thermal insulation material.

Effect of non-metallic silicon content on the microstructure and corrosion behaviour of AlCoCrFeNi high-entropy alloys

AlCoCrFeNi high-entropy alloys (HEAs) have shown excellent mechanical properties and high-temperature oxidation resistance but poor corrosion resistance owing to the high contents of the aluminium passivation film. In this study, Si was added to these alloys with the aim of improving the composition and stability of the passivation films. The microstructure and corrosion behaviour of AlCoCrFeNiSix (molar ratio x = 0, 0.1, 0.2, 0.3, and 0.4) HEAs were investigated. Dendritic and spinodal decomposition regions were observed in all five alloys. In the dendritic region, the dendritic structures transitioned from a petal shape to a dendritic shape with increasing Si content. Phase separation occurred in the spinodal decomposition region, which predominantly consisted of ordered body-centred cubic (B2) and disordered body-centred cubic (A2) phases. The size of the A2 phase decreased, and the amount of the A2 phase increased with increasing Si content. An optimised Si content could enhance the stability of Al and Cr passivation films, whereas excessive Si imposed an adverse effect by forming a Cr passivation film. The corrosion resistance of AlCoCrFeNiSi0.2 HEAs was optimum with a passive current density of 3.2 μA/cm2, which was 0.2 times that of the obtained AlCoCrFeNi HEAs without Si. The type of corrosion changed from local corrosion of AlCoCrFeNiSix (x ≤ 0.3) HEAs to general corrosion of AlCoCrFeNiSi0.4 HEAs. The corrosion mechanisms of the AlCoCrFeNi HEAs with various Si contents were elucidated.

Effect of TiAlN diffusion barrier at MoSi2-SiO2 composite coating/Mo alloy interface

In order to reduce the interdiffusion at interface of the silicide coating/Mo alloy and improve the service life of the coatings. MoSi2-SiO2 composite coatings with TiAlN diffusion barrier were prepared on Mo alloy by magnetron sputtering and followed by hot press sintering in the present study. Oxidation and interdiffusion behaviors of the samples were investigated at 1200 °C in static air. The results show that MoSi2-SiO2 composite coatings have excellent high temperature oxidation resistance, and the inward diffusion of Si is greatly inhibited with the addition of TiAlN diffusion barrier. After oxidation at 1200 °C for 100 h, the mass change of the MoSi2-SiO2 composite coatings is only -0.059 mg/cm2 to -0.022 mg/cm2, the thickness of the interdiffusion zone is just only 3.4 μm that is one order of magnitude smaller than TiAlN-free coating. The TiAlN diffusion barrier can effectively inhibit the interdiffusion of elements between the MoSi2-SiO2 composite coatings and the Mo alloy substrates.

Microstructure and properties of CoCrFeNiAlxMo2-x high-entropy alloy coating by laser cladding

CoCrFeNiAlxMo2-x (x = 0, 0.5, 1, 1.5, and 2) high-entropy alloy coatings were applied to Q235 steel, and this study investigated the effect of x variations on microstructure evolution and properties. It was fount that as x increased, the crystal structure of the coatings transitioned from a face-centred cubic (FCC) + body-centred cubic (BCC) + σ + B2 phase to a BCC + σ + B2 phase, and eventually it become a BCC phase. Concurrently, the microhardness, wear resistance, corrosion resistance, and high-temperature oxidation resistance of the coatings initially increased and then decreased with the rise in x. The optimal values were achieved at x = 1. The enhancement in microhardness and wear resistance of high-entropy alloy (HEA) coatings was attributed to the change of phase type, dispersion strengthening and solid solution strengthening. Adding of the Al element generated a passivation film on the coatings’ surface, which mitigated galvanic corrosion. Furthermore, an appropriate amount of Mo increased the thickness and density of the passivation film, thereby improving its corrosion resistance. During early-stage high-temperature oxidation, the Al underwent element undergoes rapid oxidation, while Mo promoted the formation of Al2O3, leading to an excellent high-temperature oxidation resistance. Additionally, mathematical model equations were used to fit the results, adding insights into the effects of Al and Mo content changes (x value) on the wear resistance, corrosion resistance, and high-temperature oxidation resistance of the coatings.

Gamma prime formation in nickel-based superalloy IN738LC manufactured by laser powder bed fusion

Complex components for high-temperature gas turbine applications require materials that offer a combination of excellent high-temperature strength and oxidation resistance. Nickel-based superalloys with high gamma prime (γ´) volume fractions are particularly suited for these applications, especially combined with additive manufacturing for intricate geometries. Despite the complex thermal history that these materials experience during laser powder bed fusion (LPBF) processing, γ´formation is suppressed when manufacturing IN738LC, which has a medium equilibrium γ´content of about 40–50 vol%. This study follows γ´formation in LPBF IN738LC during subsequent annealing treatments at temperatures ranging from 745 °C to 865 °C, creating an experimentally determined TTT (temperature-time-transformation) diagram. This diagram is largely based on scanning electron microscopy (SEM) imaging supported by Vickers hardness measurements and scanning transmission electron microscopy (STEM) bright field imaging. Atom probe tomography (APT) of the as-built material indicated nm-sized regions depleted in Cr and enriched in Ni, Al, and Ti, but show no characteristic superlattice patterns in TEM diffraction. APT and TEM diffraction analysis of material annealed at 850 °C for 3 min confirmed the presence of the γ´phase but indicated that γ´had formed through spinodal decomposition instead of precipitation.

Comparative study on mechanical properties and high temperature oxidation behaviour of Hastelloy X and Inconel 718 fabricated by laser directed energy deposition

The high temperature oxidation resistance is crucial to assure the reliability of nickel-based superalloy parts serving at elevated temperature. Hastelloy X is a solid solution strengthening superalloy featured by decent mechanical properties and excellent high temperature oxidation resistance, having the potential to replace the commonly used nickel-based superalloy Inconel 718 in appropriate high-end manufacturing fields. In this work, a comparative study on mechanical properties and high temperature oxidation behaviour of Hastelloy X and Inconel 718 fabricated by laser directed energy deposition was carried out. Under the same processing parameters, the as-built Hastelloy X consisted of columnar and cellular γ phase with dispersed carbides, but Inconel 718 contained columnar γ phase with dispersed carbides and γ′ phase. At ambient temperature, Hastelloy X showed better ductility but inferior strength and hardness. Additionally, the isothermal oxidation tests at 1000 and 1100 °C demonstrated that the oxidation kinetics curves of both as-built superalloys followed the parabolic law. Hastelloy X owned a better high temperature oxidation resistance, which could be attributed to the uniformly distributed dense Cr2O3 and the relatively slight exfoliation of oxides in the oxide scales.

Highly controllable additive manufacturing of heterostructured nickel-based composites

Owing to hetero-deformation induced (HDI) strengthening and HDI work hardening, heterostructured materials with both “hard” and “soft” features have been proven to achieve strength–ductility synergy. Laser-directed energy deposition (LDED) has shown tremendous potential in the fabrication of heterostructured materials, but faces challenges in accurately placing the required structures or materials at specific times and locations. This study developed a novel Ti2AlC (MAX phase)-modified Inconel 718 composite material (MAX/Inconel 718) with multiscale precipitation (γ’, carbides, Laves phase) characteristics during solidification, highly sensitive to changes in cooling rates, and exhibiting excellent controllability of strength. A method called multibeam diameter laser-directed energy deposition (MBD-LDED), which allows the dynamic adjustment of the beam diameter during the building process to alter the cooling rate during solidification, is proposed. This enabled the placement of MAX/Inconel 718 with different strengths at suitable positions within the part. Different combinations of beam diameters can form periodic distributions and spatial interlocking structures with alternating “soft” and “hard” features perpendicular and parallel to the building direction. Compared to commercial Inconel 718, MAX/Inconel 718 demonstrated excellent manufacturability, strength, and high-temperature oxidation resistance. This study provides new insights into the design and performance optimisation of heterostructures using homogeneous materials and offers guidance for the integrated manufacturing of heterostructured components in the context of comprehensive material–structure–performance design.

Stability of the ternary τ11-Al4Fe1.7Si intermetallic phase from experiment and Ab initio calculations

The Al-Fe-Si ternary system provides an opportunity for the design of new lightweight low cost alloys that have high melting temperature, superior strength, and excellent high-temperature corrosion and oxidation resistance. This system is exceptionally complex and exhibits up to 21 reported binary and ternary compounds. The intermetallic compound τ11-Al4Fe1.7Si is of particular interest due to its low theoretical density, promising mechanical properties, and low cost constituent elements. Knowledge of the exact composition range and thermal stability of this phase is crucial for further development. In this study, Al-Fe-Si alloys were fabricated and equilibrated at 800 and 950 °C. The composition range of τ11-Al4Fe1.7Si was analyzed using scanning electron microscopy with energy dispersive spectroscopy and X-ray diffraction to confirm the presence of the phase. Density functional theory calculations show that entropic contributions drive and stabilize the τ11-Al4Fe1.7Si phase. Based on experimental measurements, the composition range of τ11-Al4Fe1.7Si phase was determined to be Al-(24.3–25.5)Fe-(8.2–10.8)Si at% and Al-(24.6–25.2)Fe-(9.6–11.0)Si at% at 800 °C and 950 °C, respectively.

Tribological, oxidation and corrosion properties of ceramic coating on AISI H13 steel by rare earth-Cr composite boronizing

In this paper, a rare earth-Cr composite boronizing method to prepare boride ceramic coatings for AISI H13 steel was proposed. The effect of rare earth elements and Cr on the microstructure, hardness, wear resistance, high temperature oxidation resistance, and corrosion of boride ceramic layers was researched. The characteristics of the boride layers were investigated by means of X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe microanalysis (EPMA), wear tests, high-temperature oxidation tests, and corrosion immersion tests, etc. The results showed that the boride layer comprised of Fe2B, FeB, CrB and Cr2B phases. The addition of rare earth CeO2 and Cr nanopowder promoted the growth of the boride layer, and its surface hardness increased by 3.5 times compared with that of tempered AISI H13 steel. In addition, the addition of CeO2 and Cr significantly reduced the wear rate of the boride layer, with the surface wear mechanism being abrasive oxidative wear. Furthermore, the oxidation rate of the boride layer was 71 % lower than that of AISI H13 steel, demonstrating excellent high-temperature oxidation resistance; and the boride layer showed good acid corrosion resistance in corrosion immersion tests.