Initial Stages Of Oxidation Research Articles

An electrochemical investigation of oxygen adsorption on Pt single crystal electrodes in a non-aqueous Li+ electrolyte

Cyclic voltammetry has been used to probe the initial stages of oxygen reduction and oxidation in lithium-containing dimethyl sulfoxide at well-defined Pt single crystal electrodes in order to elucidate any catalytic effects ascribable to surface structure. In contrast to previous work involving sodium-oxygen, lithium-oxygen studies did not yield any significant differences for reaction on the three basal planes of platinum. Rather, all three planes generated a similar voltammetric response. However, by judicious use of various potential sweep limits, the formation of superoxide together with both a “conformal” or surface adlayer of lithium peroxide (Li2O2) together with a “microcrystallite” surface Li2O2 phase was resolved. Voltammetric peak intensity versus sweep rate measurements confirmed that superoxide electrooxidation was diffusion limited whereas electrooxidation of the two Li2O2 phases displayed behaviour typical of a surface-confined process. Under steady-state conditions for the formation of superoxide, it was found that for both the conformal and microcrystallite Li2O2 phases, electrooxidation followed zero-order kinetics, pointing to the importance of free surface sites in facilitating these reactions. A marked change in the rate of Li2O2 formation was found to coincide with a coverage of 0.25 monolayers, as measured by the charge density of the conformal Li2O2 electrooxidation peak. We postulate that electron tunnelling through both the conformal Li2O2 layer and microcrystallites deposited on this surface layer coincides with this coverage and accounts for such behaviour. This phenomenon of electron tunnelling through single conformal and mixed conformal/microcrystallite structures should prove vitally important in governing the overall electrooxidation rate.

Oxidation Resistance Failure of Dilute CuAl alloys at 800 °C

Oxidation is a major failure mode for the application of Cu in industry. To address this issue, alloying Cu with Al presents a potential solution due to the Al2O3-rich thin film formed during a pre-annealing process. The low oxidation resistance (OR) of dilute CuAl alloys that are oxidized at high temperatures is a critical issue due to their long-term exposure to an aerobic environment. Thus, the ORs of dilute CuAl alloys with 0.2–2.0 wt.% Al pre-annealed in a hydrogen atmosphere were examined at 800 °C and compared with the case at 400 °C. The OR of dilute CuAl alloys at 800 °C can be distinctly improved at the initial oxidation stage as well as 400 °C. However, at 800 °C, the OR gradually declines as the oxidation progresses, which considerably differs from the observation at 400 °C with the stable OR. The results show that the decline of the OR at 800 °C starts from the oxide button formation at the surface, and the growth and coarsening of these buttons cause nonuniform oxide scale formation. As the oxidation proceeds, the remnant Al2O3-rich thin film does not move with the Cu2O/CuAl interface, which is induced by the impeded outward diffusion of Cu atoms. The slowed diffusion of Cu atoms originates from the remnant Al2O3-rich thin film and the Al2O3 particles that are newly formed in the CuAl alloys due to the internal oxidation of Al.

Effect of boron on isothermal oxidation behavior of a nickel-base superalloy with high Al and Ti contents

The influence of boron on the oxidation resistance of conventionally cast U720Li alloy was investigated at 1130 °C and analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and electron probe microanalysis (EPMA). It has been found that the boron-undoped and -doped alloys all obey a super-parabolic rate law, but the oxidation rate is obviously increased by the boron addition. These alloys all show non-uniform oxidation between dendritic and interdendritic areas due to the serious microsegregation, while the oxide scale morphology of the high-boron alloy is quite different from that of the boron-undoped alloy. For boron-doped alloys, in the initial stage of oxidation the outward diffusion of boron from borides formed in the interdendritic region results in the formation of discontinuous and non-uniform B-rich oxide layers beneath and inside the Cr2O3 layer, while as the oxidation proceeds further boron can also transfer from the B-rich oxide to outer layers which eventually leads to disappearance of the B-rich oxide layers. The formation of B-rich oxide layers destroys the continuity of Cr2O3 layer and decomposition of B-rich oxide layers makes Cr2O3 layer more porous, which significantly reduce the protective property of the Cr2O3 layer and in turn decrease the oxidation resistance. Moreover, the eutectic (γ + γ′) highly enriched in Ti, Al is prone to be oxidized and form a thicker scale than the other regions, and hence the significant increase of its precipitation with boron addition also contributes to increasing the oxidation rate.

Mechanical and oxidation behavior of textured Ti2AlC and Ti3AlC2 MAX phase materials

Highly textured Ti2AlC and Ti3AlC2 ceramics were successfully fabricated by a two-step fabrication process, and the Lotgering orientation factors for {00l} planes of textured Ti2AlC and Ti3AlC2 were calculated as 0.82 and 0.71, respectively. The effect of texturing was evaluated in terms of elastic modulus and hardness by macro- and micro-indentation. Moreover, the oxidation behavior of the MAX phases was investigated at 1300 °C in air, revealing that the oxidation was markedly anisotropic, where the textured side surface exhibited much better oxidation resistance, resulting from the rapid diffusion of Al element within its basal planes to form a protective Al2O3 scale on it. Furthermore, Ti2AlC had larger difference regarding oxidation behavior between the top and side surface than Ti3AlC2, correlated to its higher Al ratio, leading to higher texturing degree and more diffusion pathways to the outer surface to produce an Al2O3 layer already at the initial oxidation stage.

A comparative study on the micro-surface characteristics at black shale initial oxidation stage

The pyrite oxidation is crucial to the overall black shale oxidation process. A. ferrooxidans was documented an effective oxidation ability on pure pyrite, but its role in black shale oxidation is unclear. In this study, a comparative study of acid solution and A. ferrooxidans on the micro-surface characteristics at the initial stage (7 days) was conducted on black shale slices, a comprehensive approach combining the micro-morphologies, micro-structures, micro-environmental pH and micro-surface elemental content were investigated by using polarizing microscopies, SEM, fluorescent staining and EDX line scan analysis. The pyrite oxidation rate was employed to the index for black shale oxidation degree, and analyzed by XRD, aqueous pH, oxidation-reduction potential (ORP), ferrous and ferric ions concentrations measurement. The results show that the micro-surface characteristics are different in acid solution and A. ferrooxidans groups, which significantly impact the pyrite oxidation rate. A. ferrooxidans promote the jarosite formation and elemental C accumulation on the rocks micro-surface, which is assumed to inhibit further reactions. Two reaction phases named “pyrite oxidized phase” and “jarosite formation phase” are proposed to occur in the initial stage of A. ferrooxidans oxidizing black shale. These findings provide experimental data to evaluate the micro-surface reactions during black shale oxidation process.

A systematic study of the oxidation behavior of Cr coatings on Zry4 substrates in high temperature steam environment

Cr coatings have been deposited on the outer surface of zircaloy-4 tube with well control of thickness variations along the tube axis. A series of samples have been oxidized in high temperature steam environment at 1200 °C for different durations. Cr2O3 layers are formed on the surface of the Cr coatings after high temperature oxidation, however, we have found that the thickness of the Cr2O3 layers increases first, and then decreases. The reason is because the Cr2O3 is reduced by the zirconium substrate when the thickness of Cr2O3 reaches its maximum. More detailed study revealed that the entire process can be divided into three stages: initial oxidation stage (I); reduction stage (II); and the oxidation of the sub-coating zirconium at a lower rate than the uncoated Zry4 (III). In addition, precipitation of secondary phases both within the coatings and the zirconium substrate has been studied in detail which plays an important role during the oxidation process. Therefore, the oxidization behavior of the Cr-Zr system is more complex than generally assumed. This study has provided sufficient experimental evidences for the redox reaction between the Cr coating and Zry4 substrate. Diffusion channels of oxygen ions during the reduction reaction have also been studied systematically.

High-temperature oxidation behavior of CuAlNiCrFe high-entropy alloy bond coats deposited using high-speed laser cladding process

A promising new bond coat using CuAlNiCrFe high-entropy alloy was proposed in this paper. The difference in pre-oxidation conditions between the traditional thermal-sprayed MCrAlY bond coat and CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding was investigated. The result confirmed that the CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding can complete the pre-oxidation faster and form a continuous α-Al2O3 thermally grown oxide (TGO), which shortens the initial oxidation stage, thereby avoiding the formation of other oxides and spinel structures. In the subsequent isothermal oxidation process, the block-like structure of the high-speed laser cladding layer and the sluggish diffusion effect of the high-entropy alloy work together to ensure the slow and continuous supply of aluminum element to TGO layer and obtain a low growth rate. Also, the diffusion between the bonding layer and the substrate is controlled at a low level, and the new CuAlNiCrFe bond coat exhibits excellent oxidation and diffusion resistance. With the consumption of aluminum, the phase structure of the high-entropy alloy bond coat changed from BCC to FCC, but it still maintained a stable simple solid solution structure.

2-Mercaptobenzimidazole films formed at ultra-low pressure on copper: adsorption, thermal stability and corrosion inhibition performance

2-mercaptobenzimidazole (2-MBI) is considered as an effective corrosion inhibitor for copper. In this study, the adsorption of 2-MBI on pristine and pre-oxidized Cu(111) surfaces was investigated by sublimation at ultra low pressure, using Auger Electron Spectroscopy, Scanning Tunneling Microscopy and X-ray Photoelectron Spectroscopy in order to understand its corrosion inhibition properties. 2-MBI adsorbs with S and N atoms bonded to Cu. On pristine Cu(111) surface, a self-assembled monolayer is formed at about 5 L, with the adsorption of atomic S resulting from molecule decomposition to form a (7×7)R19.1° structure, and that of the molecule forming a (8 × 8) structure. 2-MBI is lying flat in the adsorbed multilayer. Oxidation of copper prior to exposure results in compact and homogeneous molecular films, with dissociation and substitution of 2D oxide by 2-MBI, but much more slower than that for 2-mercaptobenzothiazole (2-MBT). A multilayer of 2-MBI can block the initial stages of oxidation of copper under low oxygen pressure at room temperature, and the molecular layer is stable until 500°C. The comparison with 2-MBT suggests that the latter is a better corrosion inhibitor for copper at room temperature.

XPS Studies of the Initial Oxidation of Polycrystalline Rh Surface

Increased interest in the oxidation process of polycrystalline rhodium, observed in recent years, is the result of its application in exhaust catalytic converters. However, most studies have involved sample surfaces with low Miller indices. In our research, we investigated polycrystalline rhodium foil containing crystallographically different, highly stepped, µm-sized crystallites. These crystallites were exposed to identical oxidizing conditions. To determine crystallographic orientation, the electron backscattering diffraction (EBSD) method was used. To investigate the initial stages of oxidation on the individual crystallites of Rh, X-ray photoelectron spectroscopy (XPS) studies were performed. The results obtained for the individual crystallites were compared and analyzed using chemical state quantification of XPS data and multivariate statistical analysis (MVA).

The evolution of microstructure and electrical performance in doped Mn-Co and Cu-Mn oxide layers with the extended oxidation time

Mn-Co and Cu-Mn based alloy coatings are deposited on 430 SS by high energy micro-arc alloying process, and then alloy coatings are oxidized at 750 °C in air to form the spinel oxides. Some composite oxides, such as Co3O4, Mn2O3 and (Mn,Co)3O4, are formed at the initial stage of oxidation. The single-phase spinel (pure MnCo2O4) without impurity phases is only found on the surface of Co-33Mn-17Cu oxide layer with the extended oxidation time, its area-specific resistance values at 500 °C to 800 °C are low and close to that of Mn-35Cu oxide layer which mainly composed of Cu1.2Mn1.8O4 spinel.

Oxidation Behavior of Ta-W Alloy

The oxidation behaviors of tantalum-tungsten alloy with 10-20% W was investigated between temperature range of 700 to 900 °C exposed in air. The kinetics of Ta-W alloy was determined by TG-DTA, the characteristics of oxides were analyzed by SEM, EDS and XRD. The oxidation tests revealed that the alloys obeyed parabolic kinetic in the initial stage, then translated in linear law. The addition of W has a good effect on the oxidation resistance of Ta-W alloys at experimental temperature. Solid solution of Ta2O5form in case of oxidation product of Ta-10W, Ta-15W alloys, while the complex oxide Ta22W4O67form after Ta-20W alloy oxidized. The formation of solid solution and complex oxide impeded the volatilization. The compact oxide film protects the penetration of oxygen in the initial oxidation stage. The large compressive stresses and mismatch of the coefficient of thermal expansion between oxide scale and matrix alloys make the oxides layer be broken, which cause kinetic of oxidization obeying linear law.

Assessment of the Likelihood of Underground Coal Oxidation and Self-Ignition: A Review

Standard and alternative methods of assessing the likelihood of coal oxidation and self-ignition in the bed are analyzed. These methods are based on the change in the petrographic, physicomechanical, chemical, and thermal properties of the coal on oxidation. Most of the methods are unsuitable for identification of the initial stage of coal oxidation. The onset of self-heating may be better determined on the basis of the surface properties of the coal: in particular, the hydrophobic–hydrophilic balance and the sorptional properties of internal surfaces. The extraction, transportation, and analysis of the coal sample from the bed must prevent contact with the air.

Microstructure-dependent oxidation behavior of Ni-Al single-crystal alloys

The effect of the γ′+γ two-phase structure on the oxidation behaviors of Ni-Al single-crystal alloys at 650 °C was investigated by scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray diffraction and Auger electron spectroscopy. In the initial oxidation stage, the oxidation behavior is primarily determined by the growth pattern of oxides in the γ channel. The outward convex NiO was formed in unprotected wide γ channels. And Ni-Al spinel oxide provides a great number of short-circuit paths, accelerating the inward diffusion of oxygen and outward diffusion of Ni. In the late stage of oxidation, the elongated internal oxide in the large γ′ phase contributes to the diffusion of oxygen along the oxide/metal interface. Consequently, the Ni-Al single-crystal alloy with wide γ channels and large γ′ precipitates exhibited poor oxidation performance.

In Situ Observation of Reaction Fronts During the Initial Stages of Iron Surface Oxidation at 1150 °C

This study focuses on the initial stages of iron surface oxidation, studied by confocal scanning laser microscopy. Pure iron cubes were prepared and polished for in situ observation. Samples were oxidized for 2–60 s by dry air at 1150 °C. A dynamic oxidation process was observed, with two reaction fronts that moved from the sample edge to the center at around 20–30- and around 40–50-s oxidation, respectively. Scanning electron microscopy (SEM) was used to characterize the iron oxide surface morphology. Pyramidal features appeared first on the surface, and then the surface became flatter with time. X-ray diffraction was used to characterize the phases of the outermost oxide layer. Overall, the content of wustite decreased and hematite content increased with oxidation time. SEM with a backscattered electron detector was used to measure the oxide layer thicknesses, and it was found that the oxide thickness increased parabolically with time. The different oxide layers within the scale were identified by an optical microscope with a polarized light source. Even after 2-s oxidation, an obvious magnetite layer was found on the wustite layer, and after 10-s oxidation, an obvious hematite layer was observed on the magnetite layer. A thicker hematite layer was observed at the sample edge. The reasons for the morphology changes are discussed.

Improved High-Temperature Oxidation Properties for Mn-Containing Beta-Gamma TiAl with W Addition

TiAl-based alloys containing relatively high Mn (e.g., Ti–42Al–5Mn (in at.%)) possess excellent hot workability and cost-effective advantages, but may also have poor oxidation resistance at high temperature. Hence, additional attention needs to be paid on the oxidation resistance of these alloys. In this study, a Mn-containing γ-TiAl-based alloy with the composition of Ti–42Al–5Mn has been chosen, and the oxidation behavior of this alloy with different W additions under thermal cycling conditions at 800 °C was investigated. The morphology and the composition distribution of the oxide scale were analyzed by means of scanning electron microscopy equipped with energy-dispersive spectroscopy, X-ray diffractometry and X-ray photoelectron spectroscopy. Auger electron spectroscopy depth profiling was used to investigate the effect of W on the initial stage of oxidation. It shows that the addition of 1 (at.%) W can significantly reduce the oxidation mass gain, and the oxidation reaction rate constant is decreased by an order of magnitude. The adherence of oxide scale is greatly enhanced with no spallation observed in Ti–42Al–5Mn–(0.8, 1)W after 100-h cyclic oxidation. Tungsten promotes the selective oxidation of Al and formation of a denser oxide scale which prevents the inward diffusion of oxygen; in this way, high-temperature oxidation resistance of the alloy is improved.

High-Temperature Cyclic Oxidation of 800H Superalloy at 750°C–950°C in Air

Abstract High-temperature oxidation behavior of iron-based superalloy (800H) was investigated at 750°C, 850°C, and 950°C in air for 96 h. Weight gain was used to determine the kinetics of oxidation. The surface of the alloy was characterized by using scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy after oxidation process. The results showed that mass gain increased rapidly in the initial stage of oxidation at high temperature. The alloy obeyed parabolic behavior at 750°C, 850°C, and 950°C. The oxide layer mainly contains oxides of chromium oxide, nickel oxide, iron oxide, and spinel chromium nickel oxide. The thickness of the oxide scales increased with time and temperature.

Efficient Global Structure Optimization with a Machine-Learned Surrogate Model

We propose a scheme for global optimization with first-principles energy expressions of atomistic structure. While unfolding its search, the method actively learns a surrogate model of the potential energy landscape on which it performs a number of local relaxations (exploitation) and further structural searches (exploration). Assuming Gaussian processes, deploying two separate kernel widths to better capture rough features of the energy landscape while retaining a good resolution of local minima, an acquisition function is used to decide on which of the resulting structures is the more promising and should be treated at the first-principles level. The method is demonstrated to outperform by 2 orders of magnitude a well established first-principles based evolutionary algorithm in finding surface reconstructions. Finally, global optimization with first-principles energy expressions is utilized to identify initial stages of the edge oxidation and oxygen intercalation of graphene sheets on the Ir(111) surface.

First-principles study of the initial stage of aluminum oxidation

The adsorption and migration of O atoms on Al (1 1 1) surface, and the infiltration of O atom to Al crystal was calculated by the first-principles method, to analyze the initial stage of aluminum oxidation. The results show that: the stable adsorption of O atom on the Al (1 1 1) surface is the Fcc position, and O atoms migrate on surface to form atomic islands, but the migration needs to pass through the Hcp position, reducing effectively the activation energy; until reaching some coverage, the adsorbed O atoms enter into Al crystal by the “abstract effect”.

Reversible oxidation–reduction of epitaxial iron oxide films on Pt(111): Magnetite–hematite interconversion

We studied magnetite-hematite-magnetite transformations in an ultrathin epitaxial film on Pt(111) using surface sensitive and bulk sensitive methods. At initial oxidation stages at elevated temperature (810 K), a 5-nm thick magnetite Fe3O4(111) film became non-stoichiometric toward maghemite and then showed the first signs of hematite phase formation under an oxygen exposure of 3 × 103 L. Finally, under 2 × 104 L, the film fully transformed to hematite α-Fe2O3(0001), maintaining a high single-crystal quality. A comparison of the conversion electron Mössbauer spectra and low energy electron diffraction pattern showed that at intermediate oxidation stages, hematite dominated at the surface, whereas a spinel phase was still observed in the deeper layers. The magnetite-hematite conversion was fully reversed by annealing under ultra-high vacuum at temperatures exceeding 600 K, and despite a change in morphology, the magnetite film preserved the original crystal structure and orientation.

Effect of High Temperature Oxidation on Infrared Irradiation of Stainless Steel 304

The effect of 304 stainless steel oxidation at 800℃ for 0.5, 2, 10, 25, 100 and 200 h, respectively, on its weight gain, surface microphotograph, composition, and infrared emissivity were investigated in the paper. The results show that, with increasing the oxidation time, the weight gain rate decreased gradually. At the initial stage of surface oxidation, Fe oxide was mainly formed. The surface composition changed from Fe oxide to large flake Cr Mn oxide with the increasing of oxidation time. Polished and unoxidized stainless steel 304 showed low infrared emissivity, the infrared emissivity increased rapidly with the extension of oxidation time.