Inward Diffusion Of Oxygen Ions Research Articles

Influence of water vapor partial pressure on self-healing and oxidation of SiC-dispersed yttrium silicate composites

The self-healing and oxidation behavior of SiC-dispersed Y2Si2O7-Y2SiO5 composites were elucidated under various water vapor partial pressures PH2O. Samples were exposed in a furnace for self-healing experiments at 1100 to 1300°C and for high-temperature oxidation at 1200 to 1400°C, both for 1 to 24h, under PH2O ranging from 2×104 to 5×104Pa, while keeping PO2constant at 2×104Pa. Water vapor promoted the self-healing effectiveness of the composites. A mechanism contributing to the closure of surface cracks was consistent with volume expansion derived from the oxidative conversion of SiC into SiO2 and the outward diffusion of yttrium cations. The formation of the internally oxidized zone signifies oxidation behavior, resulting from the inward diffusion of oxygen ions. Growth of the internally oxidized zone obeys the parabolic pattern. The influence of PH2O on the high-temperature oxidation of SiC/Y2Si2O7-Y2SiO5 is discussed in this study.

Effect of molybdenum addition on oxidation behavior and secondary protection mechanism of FeCrAl coatings

FeCrAl alloy coatings are widely used for their excellent oxidation resistance at high temperatures, but they can suffer from rapid material degradation due to breakaway oxidation of the coating. Composition manipulation is an essential way to prompt the oxidation resistance of the coatings. In this study, we prepared Fe-13Cr-7Al (at.%) alloy coatings with varying Mo contents (Mo = 0, 1.5, 3, 6 at.%) by magnetron sputtering and investigated their oxidation behaviors at 550 °C. The surface and cross-sectional morphologies of the coatings before and after oxidation were examined using electron microscopy. The results showed that the crystalline grain and particle size of the coatings decreased significantly with the increase of Mo content, leading to a notable increase of grain boundaries and further enhancing element diffusion during the oxidation tests. As a result, a protective and stable oxide scale rapidly grew on the surface of FeCrAlMo coatings during oxidation. Moreover, FeCrAlMo coatings demonstrated improved secondary protection mechanisms with increasing Mo contents. This was attributed to the formation of an internal oxidation layer of a spinel barrier (M3O4, M = Fe, Cr, Al, Mo) beneath the outer oxidation layer. The spinel barrier impeded inward diffusion of oxygen ions and outward diffusion of metal cations, resulting in enhanced oxidation resistance of the spinel-type oxide. The thermodynamic calculations were used to evaluate the oxidation mechanisms of the coatings. These results provide a deeper understanding of the effect of Mo addition on the oxidation behavior and underlying protection mechanisms of FeCrAl coatings, and can be beneficial for designing and developing materials for use in oxidation environments.

Tailoring microstructure and improving oxidation resistance of an additively manufactured high Nb containing TiAl alloy via heat treatment

In this paper, the effects of heat treatment on the microstructure and oxidation behavior of Ti-45Al-8Nb alloy prepared by selective electron beam melting were investigated. The results show that the microstructure of alloy changes from near-γ structure to duplex structure after heat treatment, but both present the heterostructure characteristics. The oxidation tests reveal that heat treatment can effectively improve the oxidation resistance, which mainly results from the optimization of phase constitution, the introduction of more twin boundaries and the formation of continuous AlNb2 barrier layer. These factors will inhibit the inward diffusion of oxygen ions into the substrate.

High-temperature oxidation behaviors and mechanisms in newly designed L12-strengthened high-entropy alloys

The oxidation behaviors of a newly-designed L12-strengthened multi-component Ni-Co-Fe-Cr-Al-Ti-Nb system were studied at 800 ℃, where the synergistic alloying effects of Ti, Nb, and Cr on the oxidation resistance were uncovered. The findings show that the doping of Ti facilitates the outward diffusion of metal cations and inward diffusion of oxygen ions through the Cr2O3 and Al2O3 scales, deteriorating the oxidation resistance. By substituting the Ti with Nb, a continuous and dense Al2O3 scale was potentially promoted benefiting from the “unlocked” Al atoms from the L12 particles. The increased additions of Cr and Nb contribute to the improved oxidation resistance.

Microstructure and oxidation behavior of CoCrxCuFeMnNi high-entropy alloys fabricated by vacuum hot-pressing sintering

Herein, CoCrxCuFeMnNi (x = 0, 0.5, 1.0, 1.5, and 2.0, in molar ratio) high-entropy alloys (HEAs) are fabricated by vacuum hot-pressing sintering (VHPS). The effect of Cr content on the microstructure and oxidation behavior are studied. When x ≤ 1.5 mol, the phases of the four alloys were all composed of FCC2 major phase and FCC1 secondary phase, while Cr2.0 alloy consisted of a small amount of FCC1 phase and ρ phase in addition to FCC2 main phase. The elemental segregation increased with the increase of Cr content. Cr2.0 alloy exhibited the lowest oxidation rate constants in the oxidation stage and the slow oxidation stage, which were 2.29 × 10−11 and 3.46 × 10−12 g2 cm−4 s−1, respectively, showing the best oxidation resistance. The oxidation products of CoCrxCuFeMnNi HEA system were mainly Mn4O3, Mn3O2, Cr2O3 and (M,Cr)3O4-type spinel oxides. The oxidation mechanism is mainly selective oxidation, that is, the outward diffusion of metal cations and the inward diffusion of oxygen ions. The oxidation resistance of the Cr-rich FCC1 and ρ phases is better than that of the copper-rich FCC2 phase.

Study of High-Temperature Corrosion Characteristics Fe-2.25Cr-1.6W Steel for Eco-friendly Power Plants

To evaluate the corrosion mechanism of Fe-2.25Cr-1.6W(wt.%) steels were corroded between 600 and 800 ℃ for up to 200 h in Na2SO4. Corrosion rates increased progressively as the corrosion time increased. The formed outer scale grew into the air by the salt during corrosion, which facilitated outward diffusion of Fe2+ ions. The inner mixed layer grew by the inward diffusion of oxygen ions. The Fe-2.25Cr-1.6W(wt.%) steel corroded little fast, forming thick scales that consisted primarily of an outer and middle Fe-O layer and an inner (Fe, Cr, O)-mixed layer. At the interface of the outer and inner scales, voids developed and cracking occurred. Fe-2.25Cr-1.6W(wt.%) steel had a small amount of Cr, which cannot decreased the scaling rate and the adhesion of scale.

In situ nanoparticle-induced anti-oxidation of FeCr alloys

Effective oxidation control of metal materials is a long-standing challenge. Here we show a new strategy for oxidation control induced by nanoparticles (NPs). The results demonstrate that the in situ formed NPs can promote the formation and growth of a continuous and dense protective oxide scale while enhancing its adhesion with the matrix. Furthermore, the assembly of NPs on the grain boundaries (GBs) and phase boundaries (PBs) can be effective in stifling the inward diffusion of oxygen ions and the outward diffusion of cations. The effectiveness of this approach is verified in FeCr alloys in high temperature conditions. Our approach can break through the inherent limitations of conventional alloying treatment, e.g. the detrimental element interaction, the high costs and limited availability of noble metals and rare earth elements, which may pave a new avenue for the remarkable improvement in the oxidation resistance of metal materials.

In situ nanoparticle-induced anti-oxidation mechanisms: Application to FeCrB alloys

In this work, in situ nanoparticles (TiB2 and TiC) can be demonstrated to induce significant improvement in the oxidation resistance of Fe-12 wt.%Cr-3.5 wt.%B alloys. To unravel the roles of nanoparticles in the anti-oxidation of Fe-12 wt.%Cr-3.5 wt.%B alloys, samples with various nanoparticle contents were synthesized to investigate their oxidation resistance. Results show that nanoparticles facilitate the formation of a continuous and dense Cr2O3 protective film and enhance its adhesion with the matrix. Furthermore, the nanoparticle assembly on the surface of borides can inhibit the inward diffusion of oxygen ions and the outward diffusion of cations, thereby effectively controlling the growth of oxide scales.

Black Ti–Zr-based oxygen defective oxide film with visible light absorption prepared via atmospheric oxidation

A black-TiZrO4 oxide film with light absorption in the visible and near-infrared regions was successfully formed on a surface using the atmospheric oxidation of a Ti–Zr alloy. The absorption of light in a wide range of wavelengths in the black-TiZrO4 oxide film was caused by a decrease in the energy gap owing to the oxygen deficiency, as indicated from the X-ray photoelectron spectroscopy analysis, and from a decrease in the refined lattice parameters, based on X-ray diffraction results. Moreover, no cracks or voids were observed within the TiZrO4 oxide film or at the interface between the oxide film and metallic Ti–Zr matrix, resulting in an ideal film. The rapid atmospheric oxidation in Ti–Zr alloy is attributed to both the promotion of the inward diffusion of oxygen ions by the substitution of Zr for Ti, and the faster diffusion of oxygen ions in a Ti–Zr-based oxide film.

Oxidation behaviour of thin Ni/Al2O3 nanocomposite coatings electrodeposited on steel substrate

This paper presents investigations of the oxidation behaviour of nanocomposite Ni/Al2O3 coatings electrodeposited on steel substrates. The high-temperature processes of the coating oxidation were studied using the thermo-gravimetric analysis, scanning electron microscopy and XRD phase analysis. Simultaneous DSC-TGA oxidation experiments were conducted with continuous heating within the temperature range 200 °C–1200 °C and isothermally at 750 °C and 900 °C in oxygen flux. A proposed oxidation mechanism of Ni matrix nanocomposite coatings that includes diffusion of the substrate element was developed. The oxidation of the Ni/Al2O3 coating occurred via complex routes such as the outward diffusion of Ni and Fe ions and the inward diffusion of oxygen ions. The iron atoms diffused outward from the steel substrate through the Ni/Al2O3 coating to the surface and formed iron and nickel oxides layers. This thermally grown scale on the Ni/Al2O3 coating deposited on the steel substrate consisted of a Fe2O3 layer at the surface, a (Fe,Ni)3O4 spinel sublayer, and an inner NiO layer. Temperature-dependent isothermal kinetics at the steady-state stage of the oxidation process could be described by the linear growth law at 750 °C, while at 900 °C the quasi-linear growth rate is slightly decreased by the quadratic function of time.

Electrochemical study on the corrosion behaviors of 316 SS in HITEC molten salt at different temperatures

Corrosion of 316 SS in HITEC molten salt was investigated at 450 °C, 600 °C, and 680 °C by electrochemical methods including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. Corrosion products were investigated by XRD and SEM equipped with EDS. Corrosion of 316 SS in HITEC molten salt exhibited active corrosion characteristics at these temperatures. 316 SS exhibited resistance to corrosion in HITEC molten salt at 450 °C, suffered severe corrosion at 680 °C, and demonstrated discontinuously increasing corrosion rate from 600 °C to 680 °C. A thin layer of Cr2O3 and Fe-based oxides formed at 450 °C. At 600 °C and 680 °C three distinct oxide layers could be distinguished with a loose outer Fe-based oxide layer. After corrosion at 600 °C and 680 °C for 200 h, Na2O and K2O were detected in the HITEC molten salts, which caused the decomposition of the protective Cr2O3 layer and enhanced corrosion of 316 SS. EIS results show that the corrosion processes of 316 SS in molten salt at 450 °C and 600 °C are controlled by outward diffusion of metal ions and oxygen vacancies, while corrosion mechanism at 680 °C change to was inward diffusion of oxygen ions. The change in corrosion mechanism became more prominent when corrosion time was prolonged to 197 h.

Oxidation of differently prepared Al-Mg alloy powders in oxygen

Powders of both commercial atomized spherical Al-Mg alloy and mechanically alloyed Al-Mg were oxidized in oxygen using thermo-gravimetry (TG). For both powders, the Al/Mg mass ratio was equal to 1. Fully and partially reacted powders were recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy particles for both atomized and mechanically alloyed powders. Results were interpreted accounting for the measured particle size distribution for the spherical powder and distributing the TG-measured weight gain among the individual particle size bins. The reaction interfaces were always located at the internal surface of the oxide shell as determined by matching the oxidation dynamics for particles with the same sizes but belonging to powders with different particle size distributions. Thus, the reaction is always rate limited by inward diffusion of oxygen ions through the growing oxide shell. Two oxidation stages were identified for both materials. Both Al and Mg oxidize during both observed oxidation stages. The second oxidation stage is caused by formation of the spinel phase, most likely occurring at a threshold temperature. In the present measurements, the step in the oxidation rate, or switch between the oxidation stages, occurs when the oxide shell grows above a certain thickness of approximately 1.5 μm. The apparent activation energy during the first oxidation stage energy changes during the first oxidation stage suggesting that more than one reaction occur in parallel, e.g., causing formation of MgO and amorphous alumina. For the second oxidation step, controlled by diffusion of oxygen through spinel layer, the activation energy remains nearly constant around 185 kJ/mol.

Oxidation behaviour of Mg–2.1Gd–1.1Y–0.82Zn–0.11Zr alloy at high temperatures

The oxidation behaviour of Mg–2.1Gd–1.1Y–0.82Zn alloy at high temperatures was studied by thermogravimetric analysis. Results showed that the weight gain of samples mainly occurred in the temperature-rise stage. This result was ascribed to the absence of protective scales and the rapid transport of oxygen inward through the macro-defects of discontinuous oxide scales. After the formation of protective (Gdx, Y2−x)O3 film, dense scales retarded the inward diffusion of oxygen ions (O2−) and improved the oxidation resistance of the alloy at high temperatures. Moreover, when alloy coupons were set in a box furnace at 1003K, the molten alloy did not burn until 120min, indicating high safety.

Reaction Interface for Heterogeneous Oxidation of Aluminum Powders

Heterogeneous oxidation of aluminum is rate limited by diffusion through a growing aluminum oxide layer. If inward diffusion of oxygen ions is faster than outward diffusion of aluminum, the reaction will occur at the inner interface of the oxide. Conversely, the reaction will occur at the outer oxide surface if outward diffusion of aluminum is faster. The location of the heterogeneous reaction is identified processing results of thermogravimetric measurements for two oxidizing spherical aluminum powders with different but overlapping particle size distribution. For each experiment, the measured weight gain is distributed among particles of different sizes assuming that the rate of oxidation is proportional to the reactive interface area. Different models are considered to determine the interface area. For a ductile oxide shell, when there is no void between oxide and aluminum, two cases with the reaction occurring at both inner and outer surfaces of the shell were evaluated. In addition, a case with the reaction at the outer surface of a rigid oxide shell is considered, for which a void inside the particle forms when the aluminum core is shrinking. Oxidation weight gains for the same size particles present in different aluminum powders are expected to be identical to each other when the calculated reactive interface area reflects the true oxidation mechanism. It is concluded that the reaction at the outer surface of a rigid oxide shell describes the experiments most accurately. Thus, the outward diffusion of aluminum ions controls the rate of heterogeneous oxidation of aluminum in a wide range of temperatures of approximately 400–1500 °C. The conclusion is further supported by the electron microscopy of particles quenched at different temperatures, showing oxide surface features consistent with the identified reaction mechanism.

Microstructural analysis of Ti‐added Cr2AlC compounds after air‐oxidation

Bulk Cr2AlC compounds alloyed with 5% Ti were synthesized via a powder metallurgical route and oxidized between 700 and 1300 °C for up to 30 days in air. They were oxidized according to the equation: Ti‐added Cr2AlC+O2→Al2O3+TiO2+Cr7C3+(CO or CO2). Most of the scale consisted primarily of the thin, uniform α‐Al2O3 layer that contained a small amount of Cr ions, which grew primarily by the inward diffusion of oxygen ions. Nodules formed locally at the Ti‐rich region of the compound. The outer part of the nodule that consisted of TiO2 grew by the outward diffusion of Ti4+ ions, while the inner part of the nodule that consisted of (Ti, Al, Cr)‐mixed oxides grew by the inward diffusion of oxygen ions. Copyright © 2012 John Wiley & Sons, Ltd.

High-temperature oxidation of nano-multilayered TiAlCrSiN thin films in air

Nano-multilayered TiAlCrSiN films consisting of alternating, crystalline TiCrN and AlSiN nanolayers were deposited by cathodic arc plasma deposition. Their oxidation characteristics were studied between 600 and 1000 °C for up to 70 h in air. The formed oxides consisted primarily of Cr 2O 3, α-Al 2O 3, SiO 2, and rutile-TiO 2. The TiAlCrSiN films oxidized slower than the TiN films and faster than the CrN or CrAlSiN films, with an apparent activation energy of 36.4 kJ/mol. During their oxidation, an outermost TiO 2 layer was formed by outward diffusion of Ti ions, and the outer Al 2O 3 layer was formed by outward diffusion of Al ions. Simultaneously, an inner (Al 2O 3, Cr 2O 3)-mixed layer and an innermost TiO 2 layer were formed by the inward diffusion of oxygen ions. SiO 2 was present mainly in the lower part of the oxide layer due to its immobility.

High-Temperature Oxidation of TiN-Ti5Si3 Composites Prepared by Polymer Pyrolysis

Composites consisting of a TiN matrix containing about 32 vol.% Ti5Si3 were synthesized via polymer pyrolysis, and oxidized in the temperature range of 700 to 900°C for up to 120 h in air. Oxidation occurred mainly in the TiN matrix, because Ti5Si3 was resistant to oxidation. The oxidation of the TiN matrix was preceded by the outward diffusion of Ti ions to form an outer TiO2 layer, and the inward diffusion of oxygen ions to form an inner TiO2 layer, together with nitrogen evolution from the TiN matrix.

Oxidation of Ni films electroplated on steel

The high-temperature oxidation of Ni films electroplated on a steel substrate was studied at 700 and 800°C in air. During oxidation, outward diffusion of Ni and Fe ions and inward diffusion of oxygen ions occurred. NiO formed initially, and as the oxidation progressed, Fe2O3 and NiFe2O4 also formed. The oxidation resistance of Ni films was inferior to that of bulk Ni, which was attributed to the involvement of Fe during oxidation.

High Temperature Oxidation of Thermomechanically Treated Ti-45.4%Al-4.8%Nb Alloys

The thermomechanically treated <TEX>$Ti-45.4\%Al-4.8\%Nb(at\%)$</TEX> alloy was oxidized between 800 and <TEX>$1000^{\circ}C$</TEX> in air, and the oxidation characteristics were studied. The dissolution of Nb in the oxide scale was observed from the TEM study. The Pt marker test revealed that the oxidation process was controlled by the outward diffusion of Ti ions and the inward diffusion of oxygen ions. During oxidation, the evaporation of Nb-oxides was found to occur to a small amount. Niobium tended to pile-up at the lower part of the oxide scale, which consisted primarily of an outer <TEX>$TiO_2$</TEX> layer, and an intermediate <TEX>$Al_{2}O_{3}-rich$</TEX> layer, and an inner mixed layer of (<TEX>$TiO_{2}+Al_{2}O_{3}$</TEX>).

Thermal Oxidation of Tungsten‐Based Sputtered Coatings

The effect of the addition of nickel, titanium, and nitrogen on the air oxidation behavior of W‐based sputtered coatings in the temperature range 600 to 800°C was studied. In some cases these additions significantly improved the oxidation resistance of the tungsten coatings. As reported for bulk tungsten, all the coatings studied were oxidized by layers following a parabolic law. Besides and phases detected in all the oxidized coatings, and were also detected for W‐Ti and W‐Ni films, respectively. was present as an inner protective compact layer covered by the porous oxide. The best oxidation resistance was found for W‐Ti and W‐N‐Ni coatings which also presented the highest activation energies ( and 218 kJ mol−1 respectively, as opposed to for the other coatings). These lower oxidation weight gains were attributed to the greater difficulty of the inward diffusion of oxygen ions for W‐Ti films, owing to the formation of fine particles of , and the formation of the external, more protective layer of for W‐N‐Ni coatings.