EFFECT OF ELECTRODEPOSITED Cr2O3 OXIDE FILM ON THE OXIDATION OF Fe3Al

A uniform, dense and defect free Cr₂O₃ thin film, which is amorphous at ambient temperature, was applied on the surface of intermetallic Fe₃Al by electrodeposition reaction sintering, and the effect of this film on the oxidation of Fe₃Al at 900℃ in air was studied. The films and the oxide scales were analyzed by TEM, EDAX,SEM and XRD.It is proved that, by surface applied Cr₂O₃ thin film,a continuous, protective,fine grained α-Al₂O₃ scale was formed on Fe₃Al. As a result, the adherence of the scale and oxidation resistance of Fe₃Al were improved.     

Preparation and High Temperature Oxidation Properties of Micro-crystalline MGH754 ODS Alloy Coatings

A special coating technique, electro-spark deposition (ESD), was developed to produce micro-crystalline ODS MGH754 alloy coatings on a commercial 1Cr18Ni9Ti stainless steel and a cast Ni20Cr alloy substrates. The coatings have a very fine grain structure and metallurgical bonding with the substrates. The isothermal oxidation tests at 1000 ℃ in air showed that the micro-crystalline ODS alloy coatings had a much reduced oxidation rate and improved scale spallation resistance compared with the uncoated alloys. The selective oxidation of Cr was greatly promoted to form protective and continuous Cr₂O₃ scales on the alloy surface. Micro-crystallization and oxide dispersions have synergistic effects on the improvement of oxidation resistance. The beneficial effects were discussed based on the experimental results.     

Effect of thermal oxidation on the surface characteristics and corrosion behavior of a Ta-implanted Ti-50.6Ni shape memory alloy

A NiTi shape memory alloy (SMA) modified by Ta ion implantation was subjected to oxidation treatment in air at 723 and 873 K. Atomic force microscopy (AFM), Auger electron spectroscopy (AES), and grazing incidence X-ray diffraction (GIXRD) measurements were conducted to investigate the surface characteristics, including surface topography, elemental depth profiles, and surface phase structures. The surface roughness of the Ta-implanted NiTi increases after oxidation, and the higher the oxidation temperature is, the larger the value is. The surface of the Ta-implanted NiTi oxidized at 723 K is a nanolayer mainly composed of TiO₂/Ta₂O₅ and TiO with depressed Ni content. The Ta-implanted NiTi oxidized at 873 K is mainly covered by rutile TiO₂ in several micrometers of thickness. Potentiodynamic polarization tests indicated that the corrosion resistance of the Ta-implanted NiTi was improved after thermal oxidation at 723 K, but a negative impact was found for the Ta-implanted NiTi oxidized at 873 K.     

9Cr?5Si?3Al铁素体耐热钢的高温氧化行为

To improve the oxidation properties of ferritic heat-resistant steels, an Al-bearing 9Cr?5Si?3Al ferritic heat-resistant steel was designed. We then conducted cyclic oxidation tests to investigate the high-temperature oxidation behavior of 9Cr?5Si and 9Cr?5Si?3Al ferritic heat-resistant steels at 900 and 1000°C. The characteristics of the oxide layer were analyzed by X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. The results show that the oxidation kinetics curves of the two tested steels follow the parabolic law, with the parabolic rate constant k_p of 9Cr?5Si?3Al steel being much lower than that of 9Cr?5Si steel at both 900 and 1000°C. The oxide film on the surface of the 9Cr?5Si alloy exhibits Cr₂MnO₄ and Cr₂O₃ phases in the outer layer after oxidation at 900 and 1000°C. However, at oxidation temperatures of 900 and 1000°C, the oxide film of the 9Cr?5Si?3Al alloy consists only of Al₂O₃ and its oxide layer is thinner than that of the 9Cr?5Si alloy. These results indicate that the addition of Al to the 9Cr?5Si steel can improve its high-temperature oxidation resistance, which can be attributed to the formation of a continuous and compact Al₂O₃ film on the surface of the steel.     

Burn-resistant behavior and mechanism of Ti14 alloy

The direct-current simulation burning method was used to investigate the burn-resistant behavior of Ti14 titanium alloy. The results show that Ti14 alloy exhibits a better burn resistance than TC4 alloy (Ti–6Al–4V). Cu is observed to preferentially migrate to the surface of Ti14 alloy during the burning reaction, and the burned product contains Cu, Cu₂O, and TiO₂. An oxide layer mainly comprising loose TiO₂ is observed beneath the burned product. Meanwhile, Ti₂Cu precipitates at grain boundaries near the interface of the oxide layer, preventing the contact between O₂ and Ti and forming a rapid diffusion layer near the matrix interface. Consequently, a multiple-layer structure with a Cu-enriched layer (burned product)/Cu-lean layer (oxide layer)/Cu-enriched layer (rapid diffusion layer) configuration is formed in the burn heat-affected zone of Ti14 alloy; this multiple-layer structure is beneficial for preventing O₂ diffusion. Furthermore, although Al can migrate to form Al₂O₃ on the surface of TC4 alloy, the burn-resistant ability of TC4 is unimproved because the Al₂O₃ is discontinuous and not present in sufficient quantity.     

Oxidation Behavior of Micro-Crystalline Coatings of 310S Stainless Steel

Micro-crystalline coatings of 310S stainless steels were produced by unbalanced magnetron sputter deposition. Isothermal oxidation behavior of the coated and uncoated specimens has been studied using a thermogravimetric analysis (TGA) station. The oxidation time was 50h and the temperature was 1 000℃. The oxidation rates of the coated specimens was found to be much lower than the uncoated specimens after 50 h of oxidation. The oxidation kinetic curves of the coated specimens consisted of three stages: approximately parabolic at the first stage, speeding up at the second stage, and slow down at the third stage. The increase of the oxidation rate at the second stage nasattributed to the fast diffuison of Fe though the fine grained Cr₂O₃ layer formed on the micro-crystalline coatings.The top view and cross-section microstructures of the oxides formed on the coated and the uncoated specimens were studied with SEM and EDS. It was observed that the nucleation of oxide on the coated specimens was much enhanced at the initial oxidation stage. This was explained as the result of reduction in the critical free energy change and increase in the supply of chromium ions.     

一种高铪镍基高温合金在900、1000和1100℃条件下的氧化行为研究

To investigate the oxidation behavior of a nickel-based superalloy with high hafnium content (1.34wt%), this study performed isothermal oxidation tests at 900, 1000, and 1100°C for up to 200 h. X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy were applied to study the oxidation behavior. The weight gain of the high Hf nickel-based superalloy exhibited a parabola-like curve, and no spallation of the oxide scale was observed during the oxidation tests. The alloy presented excellent oxidation resistance, and no HfO₂ was observed in the oxide scale at 900°C. With the increase of the oxidation temperature to 1000°C, HfO₂ particles formed in the spinel phases of the scale, and “peg-like” HfO₂ was observed within and beneath the inner layer of Al₂O₃ after 200 h. As the oxidation temperature rose to 1100°C, “peg-like” HfO₂ was observed at the early stage of the oxidation test (within 25 h). The formation mechanism of HfO₂ and its impact on oxidation resistance were investigated based on the analysis of the oxidation test results at different temperatures.     

Studies on thermally grown oxide as an interface between plasma-sprayed coatings and a nickel-based superalloy substrate

A thermally grown oxide layer formed by hot corrosion was investigated as an interface between plasma-sprayed coatings and a nickel-based superalloy substrate. The hot corrosion mechanism of NiCr-Cr₂O₃ and Al₂O₃-40wt% TiO₂ (A40T) plasma coated Inconel 617 was evaluated. The experiments were carried out at 1000℃ using a combination of Na₂SO₄, NaCl, and ₂O₅ salts to simulate the conditions of a gas turbine in a marine environment. The hot corrosion results revealed the spallation and dissolution of oxides upon prolonged exposure. Optical images and scanning electron micrographs of the exposed samples revealed the formation of oxide scale and provided details of its morphology in NiCr-Cr₂O₃ coated samples. Microstructure characterization of A40T coatings demonstrated a thermally grown oxide (TGO) layer at 1000℃. Increasing the thickness of the TGO layer decreased the corrosion resistance. The elemental analysis and image mapping revealed the migration of active elements from the substrate and coatings toward the corrosive environment.     

Effect of niobium alloying level on the oxidation behavior of titanium aluminides at 850°C

This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α₂ + γ(Ti₃Al + TiAl) and orthorhombic Ti₂NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti₂NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.     

Effect of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on the oxidation induration mechanism of Hongge vanadium titanomagnetite pellets

As part of a research project to develop a novel clean smelting process for the comprehensive utilization of Hongge vanadium titanomagnetite (HVTM), in this study, the effect of Cr₂O₃ addition on the oxidation induration mechanism of HVTM pellets (HVTMPs) was investigated in detail. The results showed that the compressive strength of the HVTMPs was greatly weakened by the Cr₂O₃ addition, mainly because of a substantial increase in the porosity of the HVTMPs. The Cr₂O₃ addition marginally affected the phase composition but greatly affected the microstructural changes of the HVTMPs. Increased amounts of Cr₂O₃ resulted in a decrease in the uniform distribution of the hematite grains and in an increase in the Fe-Cr solid solutions (Fe_1.2Cr_0.8O₃ and Fe_0.7Cr_1.3O₃) embedded in the hematite grains. Moreover, the compact hematite was destroyed by forming a dispersed structure and the hematite recrystallization was hindered during the oxidation induration, which adversely affected the compressive strength. On the basis of these results, a schematic was formulated to describe the oxidation induration mechanism with different amounts of added Cr₂O₃. This study provides theoretical and technical foundations for the effective production of HVTMPs and a reference for chromium-bearing minerals.     

Micro-analysis of high-temperature oxidation-resistance of a new kind of heat-resistant grid plate in grate-kiln

To further improve the oxidation-resistance of materials and reduce the cost of grid plates in grate-kiln, a new kind of heat-resistant grid plate was developed. The microstructure of this grid plate with a life more than 18 months was studied by XRD, SEM and EDS techniques. The results show that high hardness, high intensity and good impact property make the new kind of heat-resistant grid plate and its oxide film have a higher resistance to deformation and abrasion at 900-1000℃ Besides, small grain size is beneficial to form a complete protective oxide film. The oxide film composed of SiO₂ layer, Cr₂O₃ layer and Fe₂O₃ layer is rather thin and bonds closely with the backing. The forming of the chemical stable nickel-rich layer increases the density of Cr₂O₃ layer.     

Rare earth effects on high temperature oxidation of pure nickel at 1000 ℃

Isothermal and cyclic oxidation behaviors of pure and yttrium-implanted nickel were studied at 1000 ℃ in air.SEM and TEM were used to examine the oxide scales formed on nickel substrate.It was found that Y-implantation greatly improved the anti-oxidation ability of nickel both in isothermal and cyclic oxidizing experiments.Acoustic emission(AE)technique was used to study the size and number distribution of defects at the oxide/metal interface.Laser Raman microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium.The main reason for the improvement in anti-oxidation and adhesion of oxide scale was that Yimplantation greatly reduced the grain size of NiO and lowered the compressive stress within the scale.In the meantime,Y-implantation inhibited ion diffusion rate in the oxide scale and reduced the size and number of interfacial defects,hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate.     

Effect of Cr2O3 addition on oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets with simulated coke oven gas

The oxidation induration and reduction swelling behavior of chromium-bearing vanadium titanomagnetite pellets (CVTP) with Cr₂O₃ addition were studied, and the reduction swelling index (RSI) and compressive strength (CS) of the reduced CVTP with simulated coke oven gas (COG) injection were investigated. The results showed that the CS of the CVTP decreases and the porosity of the CVTP increases with increasing amount of Cr₂O₃ added. The Cr₂O₃ mainly exists in the form of (Cr, Fe)₂O₃ solid solution in the CVTP and as Fe-Cr in the reduced CVTP. The CS of the reduced CVTP increases and the RSI of the reduced CVTP decreases with increasing amount of Cr₂O₃ added. The limited aggregation and diffusion of metallic iron contribute to the formation of dense lamellar crystals, which leads to the slight decrease for reduction swelling behavior of reduced CVTP. This work provides a theoretical and technical basis for the utilization of CVTP and other Cr-bearing ores such as chromite with COG recycling technology.     

PREPARATION OF COMPLEX ZrO2-Y2O3 SOL

Complex ZrO₂-Y₂O₃ sol were prepared from precursors of inorganic salts and metal alkoxides. The processes for complex ZrO₂-Y₂O₃ sol preparation and purification, the sol-gel transformation and the characterization of the sol from the two kinds of precursors were discussed and compared. It was found that the colloidal particles were amorphous in nature, and extremely fine (<10nm) in the both cases. ZrO₂-Y₂O₃ coatings on GH220 superalloy samples MCrAlX (M=Fe, Co, Ni, X=Y, Ta,Hf…) pre-coated) were prepared using sols from the two different processes and tested at 1000 ℃ for 100 h in laboratory air. Results showed that only the coatings from the metal alkoxides derived sol offered excellent oxidation resistance.     

Hot-corrosion behavior of Cr2O3-CNT-coated ASTM-SA213-T22 steel in a molten salt environment at 700℃

The present work investigates the hot-corrosion behavior of carbon nanotube (CNT)-reinforced chromium oxide coatings on boiler steel in a molten salt (Na₂SO₄-60wt%V₂O₅) environment at 700℃ under cyclic conditions. The coatings were deposited via the high-velocity oxygen fuel process. The uncoated and coated steel samples were subjected to hot corrosion in a silicon tube furnace at 700℃ for 50 cycles. The kinetics of the corrosion behavior was analyzed through mass-gain measurements after each cycle. The corrosion products were analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray analysis techniques. The results revealed that uncoated steel suffered spallation of scale because of the formation of nonprotective Fe₂O₃ scale. The coated steel samples exhibited lower mass gains with better adhesiveness of oxide scale with the steel alloy until the end of exposure. The CNT-reinforced coatings were concluded to provide better corrosion resistance in the hot-corrosion environment because of the uniform dispersion of CNTs in the coating matrix and the formation of protective chromium oxides in the scale.     

Photocatalytic activity of ferric oxide/titanium dioxide nanocomposite films on stainless steel fabricated by anodization and ion implantation

A simple surface treatment was used to develop photocatalytic activity for stainless steel. AISI 304 stainless steel specimens after anodization were implanted by Ti ions at an extracting voltage of 50 kV with an implantation dose of 3 × 1015 atoms·cm?2 and then annealed in air at 450℃ for 2 h. The morphology was observed by scanning electron microscopy. The microstructure was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The photocatalytic degradation of methylene blue solution was carried out under ultraviolet light. The corrosion resistance of the stainless steel was evaluated in NaCl solution (3.5 wt%) by electrochemical polarization curves. It is found that the Ti ions depth profile resembles a Gaussian distribution in the implanted layer. The nanostructured Fe₂O₃/TiO₂ composite film exhibits a remarkable enhancement in photocatalytic activity referenced to the mechanically polished specimen and anodized specimen. Meanwhile, the annealed Ti-implanted specimen remains good corrosion resistance.     

采用HVOF涂层提高异种合金825和AISI 321 CO2激光焊件在900°C腐蚀盐环境中的热腐蚀性能

This study investigated the hot corrosion performance of a dissimilar weldment of Ni-based superalloy and stainless steel joined by CO₂-laser welding and improved by high-velocity oxy-fuel (HVOF) coating in a Na₂SO₄?60wt%V₂O₅ environment at 900°C. A dissimilar butt joint of AISI 321 and alloy 825 was fabricated by CO₂-laser welding with low heat input after obtaining the optimum welding parameters by bead-on-plate trials. The metallurgical and mechanical properties of the laser weldment were evaluated. The tensile test results indicated the occurrence of fracture in the base metal AISI 321 side. The HVOF process was employed to coat Ni?20wt%Cr on the weldment. To evaluate the surface morphology of the corrosion products formed on the uncoated and Ni?20wt%Cr-coated weldments, scanning electron microscopy (SEM) analysis was performed. Energy-dispersive spectroscopy (EDS) was used to determine the different elements present on the surface scales. The existence of oxide phases on the weldments was determined by X-ray diffraction (XRD). The cross sections of the weldments were characterized by SEM with EDS line mapping analysis. The results indicated that the Ni?20wt%Cr-coated weldment exhibited superior hot corrosion resistance due to the development of Cr₂O₃ and NiCr₂O₄ protective oxide scales.     

Roles of Metallic Oxides in the Elimination Reactions of NO

The catalytic effect of several metallic oxides(Fe₂O₃, Cr₃O₃ and Ni₂O₃) on elimination of NO has been studied in a flow type reactor, and the roles of metallic oxide in the elimination reactions of NO has also been discussed. The results show that in the system of NO-CO-CO₂-N₂, the catalytic reaction of NO/CO on the metallic oxide plays a leading role in NO elimination especially for Fe₂O₃ and Cr₂O₃ samples. However, Ni₂O₃ and its pre-reduced samples show extremely remarkable catalytic effect on direct decomposition of NO.     

Effect of interaction of refractories with Ni-based superalloy on inclusions during vacuum induction melting

This study documents laboratory-scale observation of the interactions between the Ni-based superalloy FGH4096 and refractories. Three different crucibles were tested—MgO, Al₂O₃, and MgO–spinel. We studied the variations in the compositions of the inclusions and the alloy–crucible interface with the reaction time using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy and X-ray diffraction. The results showed that the MgO and MgO–spinel crucibles form MgO-containing inclusions (Al–Mg oxides and Al–Mg–Ti oxides), whereas the inclusions formed when using the Al₂O₃ crucible are Al₂O₃ and Al–Ti oxides. We observed a new MgAl₂O₄ phase at the inner wall of the MgO crucible, with the alloy melted in the MgO crucible exhibiting fewer inclusions. No new phase occurred at the inner wall of the Al₂O₃ crucible. We discuss the mechanism of interaction between the refractories and the Ni-based superalloy. Physical erosion was found to predominate in the Al₂O₃ crucible, whereas dissolution and chemical reactions dominated in the MgO crucible. No reaction was observed between three crucibles and the Ti of the melt although the Ti content (3.8wt%) was higher than that of Al (2.1wt%).     

AISI 321不锈钢钢钢包精炼过程中钙含量对夹杂物的影响

The effect of three heat processes with different calcium contents on the evolution of inclusions during the ladle furnace refining process of AISI 321 stainless steel was investigated. The size, morphology, and composition of the inclusions were analyzed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. After the addition of aluminum and titanium, the primary oxide in the AISI 321 stainless steel was an Al₂O₃–MgO–TiO_x complex oxide, in which the mass ratio of Al₂O₃/MgO was highly consistent with spinel (MgO·Al₂O₃). After calcium treatment, the calcium content in the oxide increased significantly. Thermodynamic calculations show that when the Ti content was 0.2wt%, the Al and Ca contents were less than 0.10wt% and 0.0005wt%, respectively, which was beneficial for the formation of liquid inclusions in molten steel. Moreover, the modification mechanism of calcium on TiN-wrapped oxides in combination with temperature changes was discussed.