Nickel Oxide Scales Research Articles

Nickel oxide scale microstructure and accelerated growth in combustion flue gas: Effect of ash particles

Model Ni-25Cr and Ni-25Cr-2Mn-1Si (wt%) alloys coated with an industrial fly ash were exposed to a wet CO2 gas at 750 °C. It was found that the outer thermally-grown NiO scale encapsulated ash particles at an accelerated rate. TEM combined with TKD analysis revealed that nano structured NiO grains on the ash surface contributed to the observed NiO encapsulating effect.

Oxidation behavior of porous Ni- 4 and 8 wt %Cr alloys in air at 1273K

Oxidation behavior of porous Ni-4 and 8 wt% Cr pellets prepared by conventional powder metallurgy was investigated. The pellets were oxidized in air at 1273K for 100 h. The oxidation kinetics were measured by using gravimetric method. The oxide layer morphology and, the surfaces topography and the cross-section microstructure of the oxidized pellets were investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and X-ray energy dispersive analysis. The oxidation rate was initially rapid and then gradually decreased until realizing a constant rate after oxidation time of about 40 h for the 4 wt %Cr pellets with linear rate constant of about 8x10-8 (g/cm2)/sec. For the 8 wt %Cr pellets, the linear rate constant was about 20x10-8 (g/cm2).sec which was achieved after 72 h. Porous nickel oxide scales were formed on the 4 and 8 wt %Cr substrates. Finally, the growth of the oxide scale was explained on the basis of inward migration of molecular oxygen through the porous oxide scale; accordingly, the newly oxide atomic layers were added at the scale/metal interface.

Investigations for the Validation of the Defect Based Scale Failure Diagrams—Part I: Nickel Oxide

Mechanical stresses imposed on protective oxide scales can lead to cracking and failure of the scale and consequently to a loss of the protective properties. Therefore, an assessment of the mechanical stability limits is of great interest. In this work, a new concept using defect based oxide scale stability diagrams to assess the mechanical stability limits is discussed. In contrast to mechanical failure diagrams proposed earlier, the presented model is based on physical defect size instead of scale thickness. Nickel oxide scales on high purity (99.99 %) nickel were thermally grown in dry and humidified synthetic air to provide a model oxide system. SEM investigations were carried out to examine the physical defect structure in the oxide scales and mechanical 4-point bend testing was used to measure the critical strain for through scale cracking. The data served for establishing a data base for the validation of the defect based scale failure model.

The mechanisms of gross slip fretting wear on nickel oxide/Ti6Al4V mated surfaces

The objective of this work was to determine the mechanisms of tribofilm formation during the gross slip fretting wear of Ti6Al4V and a thick nickel oxide film. This was accomplished by conducting bench level fretting wear experiments on Ti6Al4V surfaces mated with ∼75 μm thick nickel oxide scale. The nickel oxide scale was grown by oxidizing commercially pure nickel at 1000 °C. The fretting wear experiments were conducted at room temperature, 150, 300, and 450 °C, with a stroke length of 250 μm, at oscillation speeds of 2 and 30 Hz, and an approximate Hertzian contact stress of 650 MPa. It has been shown that at elevated temperatures, lubricious nanocrystalline tribofilms were formed in the fretting wear contact via a tribo-sintering mechanism. These tribofilms formed as transfer layers on the mated Ti6Al4V interface, and reduced the friction and wear on the mated surface at elevated temperatures.

Mill Scale for Synthesis of Fe–Ni and Fe–Ni–Co Alloys through Gaseous Reduction: Reaction Kinetics and Mechanism

Nickel and Nickel cobalt ferrite powders were prepared through the ceramic route by calcination of a stoichiometric mixture of nickel oxide, cobalt oxide and mill scale as source for iron oxide. The produced ferrites powders were isothermally reduced in pure hydrogen at 800–1100°C. Based on thermogravimetric analysis, the reduction behavior of the synthesized ferrite and the kinetics reaction mechanism were studied. The initial ferrite powder and the various reduction products were characterized by XRD, SEM and reflected light microscope to reveal the effect of hydrogen reduction on composition, microstructure and reaction kinetics of synthesized ferro-alloys. The activation energy values were calculated from Arrhenius equation. The approved mathematical formulations for the gas solid reaction were applied to confirm the estimated rate controlling reaction mechanism. Complete reduction of home made ferrite powder was achieved with synthesize of nanocrystalline Fe–Ni and Fe–Ni–Co alloys.

Quantitative Prediction of Voids Formation in a Growing Nickel Oxide Scale at 1373 K

Voids which form in oxide scales strongly influence the mechanical properties of the scale and the adherence between the metal substrate and the scale. The purpose of this study is to demonstrate a method for quantitative estimation of voids formation in NiO scale, and compare the microstructure of the scale formed on nickel at 1373 K with the estimation. Calculations of ion fluxes and their divergence in NiO scales can predict the annihilation of the oxides which mostly occurs in the vicinity of the metal/oxide interface and the volume fraction of the voids. These predictions are in good agreement with the observed morphology of NiO scale obtained in high temperature oxidation of nickel at 1373 K. [doi:10.2320/matertrans.MER2007122]

Voids Formation in a Growing Nickel Oxide Scale and Cobaltous Oxide Scale at 1373 K

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The Nuclear Microprobe in Materials Science: Application to Ceramics

The aim of this paper is to present the analytical capabilities of the nuclear microprobe for the characterization of ceramics. The nuclear microprobe uses the interactions of a focused ion beam of MeV light ions ( 1H, 2H, 3He, 4He, ..) with a solid target to determine local elemental concentrations at the μm 3 scale. The main analytical techniques are based on the spectrometry of the induced X and gamma-rays, and of the particles scattered or produced by nuclear reactions. Examples are shown such as concentration profiling of additives and dopants in alumina, oxygen-18 diffusion profiles and interfacial enrichment in nickel oxide scales grown under external mechanical loading, local oxygen stoichiometry and carbon contents in superconductor oxides, boron and carbon determination for local characterization in multiphase quaternary borocarbides.

Anin situ investigation of the tensile failure of oxide scales

A small four-point-bend jig has been used in a scanning electron microscope (SEM) to monitor the tensile fracture processes in iron and nickel oxide scales in situ. The scales were 3–40 μm thick, the strain rate was 4×10−5 sec−1 and acoustic emission (AE) was used to correlate signals with specific cracking events. The technique provided detailed information of the failure processes, and several micrographs were taken as the crack pattern developed during testing. Failure started with short random through-scale cracks. These cracks soon formed a regular pattern. Spallation only occurred at much higher strains and resulted in very energetic AE signals. These signals were used together with the SEM observations to determine the strain to cause spallation. Measurements of the crack spacing as a function of strain showed that plastic stress relaxation by interfacial slip and/or substrate yielding processes affected cracking. Thus, the oxide remained attached to the substrate longer than would be expected from purely elastic behavior. An analysis based on the observed crack spacing at the onset of spallation gave ratios of 0.8–1.9 and ∼0.9 between tensile and interfacial strength for iron and nickel oxides scales, respectively.

Experimental data on oxide fracture

Protective oxide scales on high temperature materials often fail in service leading to increased oxidation rates. Thus, a knowledge of oxide failure behaviour and failure strains is required. This paper reviews the measured failure strains of chromium oxide and aluminium oxide scales at room and oxide growth temperatures under applied strain. Data on iron and nickel oxide scales are also included for modelling purposes. Failure strains in tension show a power-law dependence on the measured void size within the layer, as expected from fracture mechanics models. Residual tensile or compressive growth and cooling strains prior to loading can increase or decrease the measured failure strains depending on the loading mode, i.e. tension or compression. At growth temperature, lateral oxide growth and creep may act to increase the failure strains, leading in some cases to measured failure strains in excess of 2%. However, the effective failure strains in tension, after taking account of the residual strains, oxid...

The re-oxidation of pre-formed nickel oxide scales

The effect of a pre-existing thermal nickel oxide scale on the subsequent re-oxidation of nickel at a different temperature has been investigated. The scale microstructure, oxidation kinetics and through-scale transport of molecular oxygen during re-oxidation have been determined. Grain growth during re-oxidation is consistent with a simple parabolic rate law. The activation energy for grain growth is approximately 4 eV, suggesting control of grain growth by an anion vacancy diffusion mechanism. Reoxidation kinetics are described by a parabolic rate law. The re-oxidation rate is reduced, relative to isothermal oxidation at the same temperature, when the initial oxidation is at a higher temperature than the re-oxidation. The through-scale transport of oxygen during re-oxidation is determined by the preoxidation temperature and is independent of both re-oxidation temperature and time. It is proposed that the amount of oxide formed at the scale-metal interface is determined by the structure of the interface formed during the initial oxidation. Subsequent annealing at higher temperatures does not substantially alter the interface structure although the scale microstructure is changed.

The tensile failure of nickel oxide scales at ambient and at growth temperature

The tensile failure of nickel oxide (NiO) scales formed on nickel was investigated using a four-point bend test technique at room temperature and at the oxide growth temperature (900°C). Strain rates used were ∼10 −4 and ∼10 −5 s −1. Acoustic emission (AE) was employed to monitor failure and interpret the mechanism of failure from the first fracture events through crack propagation to final spallation. The results are compared with similar measurements previously made on iron oxide scales at room temperature and at the growth temperature (550°C). Plasticity of the NiO scale at its growth temperature resulted in higher critical failure strains than at room temperature. The different failure strains for oxides of iron and nickel can be explained by differences in residual stress and plasticity. The overall crack patterns were similar for both oxides. K IC values of 0.41 and ∼1.61 MN m − 3 2 were found for NiO at room temperature and at 900°C, respectively, and the residual stress in the oxide at room temperature was ∼ 175 MPa. The fracture surface energy values were 0.8 and 2.5 J m −2 for nickel and iron oxides, respectively.

Determination of growth stresses in nickel oxide using X-ray diffraction

Residual stress measurements using X-ray diffractometry have been carried out on nickel oxide scales formed at 1000°C on pure nickel and nickel containing 0·5 wt-% alloying additions of yttrium, hafnium, cerium, or zirconium. The residual stresses present in nickel oxide scales formed on the alloys containing the additions are significantly higher than those in the nickel oxide scale formed on pure nickel. It is proposed that the principal cause of this increase in stress is associated with oxide grain refinement due to the presence of the added elements in the parent metal. The eventual consequence of these high stress levels is the formation of a weak porous inner oxide that is susceptible to cracking and dramatic stress relief on cooling, leading ultimately to scale decohesion and spalling.MST/1472

Influence of Grain Boundary Specialness Character on Grain Texture and Boundary Texture in Nickel Oxide Scales

Influence of Grain Boundary Specialness Character on Grain Texture and Boundary Texture in Nickel Oxide Scales

Oxygen transport in growing nickel oxide scales at 600?800�C

The effect of oxidation conditions on the penetration of oxygen into growing nickel oxide scales has been examined using oxygen isotope tracers and SIMS analysis. Depth profiles and cross-section images of oxide scales show that the proportion of oxygen penetration is increased as the temperature is reduced. At 600°C, the amount of oxygen tracer transported to the scale-metal interface is considerable, approaching that observed for a conventional “duplex” microstructure. The formation of healing oxide along numerous fissures through the oxide is directly observed in SIMS images. These fissures seem to coincide with grain boundaries in the preexisting oxide. As the oxidation temperature is increased, the oxygen transport becomes less uniform, some regions of the scale showing little or no transport. Reduction in the oxygen pressure reduces the amount of oxygen penetrating the scale.

The reactive element effect on the growth rate of nickel oxide scales at high temperature

The restriction of oxide-scale growth by an incorporated reactive element has been investigated for the oxidation at 900°C of CeO2-coated and Ce-alloyed Ni. Analytical electron microscopy of scales in transverse section revealed that significant inhibition of diffusion along a network of grain-boundary pathways in NiO was associated with segregation of Ce at a high concentration. The development of this form of Ce distribution depended critically on the provision of sufficiently small and closely spaced CeO2 source particles within the scales.

Development of grain structure in nickel oxide scale

Outward diffusion of metal cations through the oxide scale controls the oxidation of a number of transition metals including Ni, Cr, and Fe. Grain boundary diffusion of cations is more predominant than that in the oxide lattice at temperatures in the range 500–1000°C. Thus, the grain structure of the scale is an important factor at these temperatures. The Ni/NiO system is focused on since much is known concerning this system and it can be used as a model for technologically important protective scales, such as Cr2O3. Grain growth in bulk ceramics is outlined and the possible differences from that in scales are highlighted. Factors controlling the initial grain size in oxide scales are discussed and observations of grain structure and grain growth are surveyed. Recent parallel and transverse TEM results on NiO scale and quantitative image analysis results for traced grain boundary networks are used to develop the discussion. Surface morphology observations are briefly reviewed and research on the microstructure of scales grown on transition metals other than Ni is summarised.MST/949

Development of grain structure in nickel oxide scale

Outward diffusion of metal cations through the oxide scale controls the oxidation of a number of transition metals including Ni, Cr, and Fe. Grain boundary diffusion of cations is more predominant than that in the oxide lattice at temperatures in the range 500–1000°C. Thus, the grain structure of the scale is an important factor at these temperatures. The Ni/NiO system is focused on since much is known concerning this system and it can be used as a model for technologically important protective scales, such as Cr2O3. Grain growth in bulk ceramics is outlined and the possible differences from that in scales are highlighted. Factors controlling the initial grain size in oxide scales are discussed and observations of grain structure and grain growth are surveyed. Recent parallel and transverse TEM results on NiO scale and quantitative image analysis results for traced grain boundary networks are used to develop the discussion. Surface morphology observations are briefly reviewed and research on the microstructure of scales grown on transition metals other than Ni is summarised.MST/949

The Adherence of Nickel Oxide on Nickel During High-Temperature Oxidation

AbstractA combined experimental/analytical study is being conducted to investigate the fundamental nature of adherence of oxide scales formed on metals during high-temperature oxidation. Isothermal oxidation experiments were conducted on high-purity nickel (Ni 270) specimens in air at temperatures of up to 1200°C. None of the experiments resulted in physical separation of nickel oxide scales from the nickel substrate, even though substantial amounts of porosity were found in scales close to the scale/matal interfaces. The results of numerical and analytical calculations of stresses in the NiO/Ni system suggest that metal and oxide plasticities play a role in the experimental observations.

Evolution of grain structure in nickel oxide scales

In systems such as the oxidation of nickel, in which grain-boundary diffusion in the oxide can control the rate of oxidation, understanding of the factors governing the grain structure is of importance. High-purity mechanically polished polycrystalline nickel was oxidized at 700°C, 800°C, and 1000°C for times up to 20 hr in 1 atm O2. The scale microstructures were examined by parallel and transverse cross section transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Texture coefficients were found by X-ray diffraction (XRD). Each grain in the transverse section grain boundary networks was systematically analyzed for width parallel to the Ni-NiO interface and perpendicular length, for boundary radius of curvature and for number of sides. The variation of these parameters with depth in the scale was examined. In particular, grains were increasingly columnar (i.e., with ratio of grain length to width >1) at higher temperatures and longer times. Columnar grain boundaries tended to be fairly static; the columnar grain width was less than the rate controlling grain size predicted from the oxidation rate. The mean boundary curvature per grain provided a guide to the tendency for grain growth, except in the region of the Ni-NiO interface, where the boundaries were thought to be pinned.