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.     

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.     

Effect of reactive element oxide inclusions on the growth kinetics of protective oxide scales

A model is presented for describing the growth kinetics of a protective oxide scale containing reactive element (RE) oxide inclusions (pegs). The formation of RE oxide inclusions due to dissolution and diffusion of the RE from intermetallic precipitates along grain or phase boundaries in the alloy is considered. The average oxide scale growth kinetics depend on the RE content, the parabolic rate constant of the protective oxide scale, the alloy grain/phase size and the size of the RE containing precipitates. The specimen thickness determines the amount of RE available for oxidation. If the RE in the alloy has been consumed completely, then the RE oxide inclusions attain a maximum size. After this point, a decrease in the average oxidation kinetics occurs. Very good agreement between experiments and calculations was obtained for the oxidation of a free-standing NiCoCrAlY coating at 1373 K.     

The growth and structure of thin oxide films on ceria-sol-coated nickel

The influence of 14-nm thick ceria ceramic coatings deposited by the sol-gel technique on the early-stage oxidation of polycrystalline nickel at 973 K was studied by analytical electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction. The size of the ceria particles in the coating was modified prior to oxidation by vacuum annealing. It was found that ceria particle size is a crucial factor affecting the oxidation kinetics, oxide microstructure, and distribution of cerium within the oxide film. Coarse ceria particles applied to the nickel surface were ineffective in the inhibition of oxidation and were spread throughout the whole oxide. Coatings with small ceria particles markedly improved the oxidation resistance. After oxidation such particles were present in the surface region of nickel oxide, acting as the sources of cerium ions segregated at the nickeloxide grain boundaries. The stereological analysis of oxide microstructure as well as microchemical examination supported the predominant role of grain-boundary segregation of cerium ions decreasing the oxidation rate. The results are discussed in terms of reactive-element effect on the development of microstructure of nickel oxide film during initial stages of oxidation.     

Comments on the use of the two-stage-oxidation method and surface-analytical techniques in studying growth processes of oxide scales

Application of the two-stage-oxidation methods and surface-analytical techniques, SIMS and SNMS, for determining transport processes occurring in thin oxide scales is discussed briefly. The most fundamental rules relevant to the interpretation of results are formulated, and the practical implications are inferred. The effects related to the morphological and microstructural features are considered.on leave from the Academy of Mining and Metallurgy, Krakow, Poland.     

镍基带上取向生长纳米二氧化铈颗粒及薄膜

采用低成本的无机铈盐与有机溶剂进行螯合,制备出稳定的前驱溶液,采用旋涂的方法,在金属镍基带上沉积了二氧化铈前驱薄膜。通过差热-热重分析和X射线衍射确定了前驱薄膜的晶化起始温度。通过控制涂膜溶液浓度,可以得到均匀分布的氧化铈颗粒和致密薄膜。XRD分析表明薄膜外延了镍基底的(200)取向。成功探索了在金属基底上低成本取向生长纳米二氧化铈颗粒和薄膜的可行性。     

Morphology of oxide scales formed on titanium

High-temperature oxidation of several pure metals and their alloys gives rise to multilayered corrosion scales. This curious morphology has not been the subject of a specific investigation, and its interpretation remains widely open. This paper presents results on titanium foil oxidized in pure oxygen leading to this phenomenon. It analyzes the significant features of the stratification as a function of the main parameters of the reaction kinetics such as temperature, oxygen pressure, oxidation time, and initial thickness of the metallic foil. All the results, including those previously reported, show the importance and the consistency of the studied phenomenon as well as the incapability of providing an unequivocal intepretation at the present time. The analysis of the main results, such as the appearance of a macroscopic order, the existence of boundary limits, and the nonequilibrium state of the structure, shows that a consistent set of data exists to give a meaningful interpretation of multilayered corrosion scales in terms of a nonlinear, far-from-equilibrium organization.     

The growth of oxide platelets on nickel in pure oxygen. II. Surface analyses and growth mechanism

The structural properties of NiO platelets emerging from a primary oxide layer by oxidation of pretreated nickels in pure oxygen between 650 and 800° C have been investigated in relation with the initial metallic layers and the primary oxide. Surface composition and segregation of impurities were also studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy. Textural properties and structural orientation of both the primary oxide layer and the platelets were analyzed by X-ray diffraction and transmission electron microscopy. Platelets grew along {111} planes, leading to elliptical or semicircular bicrystals. The driving force for the present type of growth originates from the stresses accumulated in the thin, primary oxide layer. Impurities do not directly interact with this growth, but enable a specific grain structure to be developed thereby favoring platelet growth.     

Spalling of protective oxide scales

The bulk mole fraction of an element A in a binary alloy AB required to form (1) a coherent, continuous layer of oxide AO_v, (2)a second layer of AO _v should the first layer spall from the surface, and (3) further layers of the protective oxide if repeated spalling is likely, is calculated as a function of the ratio of the parabolic rate constant for oxidation to the alloy interdiffusion coefficient. The implications of spalling of the protective oxide layers are also discussed.     

Mechanical properties of oxide scales

At high temperatures most alloys rely on the protective effect of oxide scales formed by the reaction between oxygen from the environment and components of the alloy. The protective effect of these scales may, however, be impaired if stresses lead to cracking or spalling. Therefore, the mechanical properties of the scales play a vital part in protection, in particular under service conditions of the materials where the presence of stresses cannot be excluded. The paper provides a survey of the existing models that describe the mechanical properties of scales (the emphasis being on mechanical scale failure), the measuring techniques and the data. It will be demonstrated that, in the case of tensile conditions, the situation is relatively well understood and that a fair amount of data exists. With regard to compressive conditions scale failure consists of a combination of different failure steps, which can each be described by a quantitative model, but there is still clearly a need for experimental verification. While the measuring techniques for the properties under tensile stresses are fairly well developed, those for the behavior under compressive stresses, in particular those characterizing scale adhesion, are still under discussion and relatively few experimentally determined data are available in this case. Nevertheless, existing knowledge can be put to good use in assessing scale behavior under the effect of stresses, based on the models and data given in the paper.     

纯铜表面纳米化对镍扩散的影响

利用表面机械研磨处理(SMAT)在纯铜表面制备出纳米结构表层,并对其表面进行了电镀镍处理,采用扫描电子显微镜分析了镍原子在纳米晶铜中的扩散行为.结果表明:表面纳米晶层内存在有大量非平衡态缺陷和晶界,尤其是三叉晶界数量增加,降低了镍原子扩散的激活能,提高了其扩散系数,从而加快了镍原子的扩散.     

The penetration by sulfur of NiO scales growing on nickel

The transport of sulfur through growing scales may occur by chemical (solution and diffusion) or physical (gas molecule permeation) mechanisms. Both possibilities are examined theoretically for the case of NiO growing on nickel. Experiments are designed and carried out to establish which mechanism plays the major role in sulfur transport. The results indicate that the physical mechanism is likely to be predominant.     

Etching of oxide scales on chromium ferritic steels

Abstract

The oxide scale formed on chromium ferritic steels (21/4–9% Cr) by reaction with water/steam in the temperature range 200–570°c consists of an inner Fe/Cr spinel layer and an outer layer of magnetite. These layers are often indistinguishablefrom one another when the scale is sectioned and viewed under an optical microscope. Possible etching techniques, able to reveal the two layers (often without affecting the metal surface), have been investigated. Successful techniques are listed and the mechanisms involved are discussed in relation to current knowledge of oxide dissolution and passivation of metal surfaces.     

Quantification of growth kinetics and adherence of oxide scales formed on Ni-based superalloys at high temperature

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales.     

Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.     

The use of 18O as a tracer to study the growth mechanisms of oxide scales

A nuclear technique for the determination of ¹⁸O concentration as a function of depth is described. Examples are given of the application of this technique to studies of growth mechanisms of oxide scales.     

Carbon transport through oxide scales on Fe-Cr alloys

The penetration of carbon through Cr₂O₃ layers was studied for a series of different Fe-Cr alloys using a radioactive tracer method. Preoxidized samples were exposed at 900°C for 700 hr in a H₂ -H₂O -CO-CO₂ atmosphere tagged with¹⁴C; carbon penetration profiles were then determined, and the lateral distribution of carbon was observed by autoradiography. Even minute amounts of carbon (0.05 ppm) within the scale and in the alloy could be detected. The carbon uptake into different Fe-Cr alloys decreased with increasing Cr content to a minimum for the alloys with 12.5–20% Cr, indicating low porosity and good adherence of the Cr₂O₃ layers. Poor scale adherence was observed for Fe-10% Cr but could be improved by Ce additions. Porosity increased with contents >20% Cr of the alloys. Pore formation could be induced by impurities, e.g., SiC particles distributed on the surface.     

Tracer isotope distribution in growing oxide scales

Two methods are presented for the analysis of oxygen tracer isotope double oxidation experiments. Mass balance criteria are presented for inferring oxide growth mechanisms from the oxygen isotope profile. For the case of inward growing scales, a diffusion model is presented which describes the tracer distribution as a function of lattice and grain boundary diffusivities, grain size and the parabolic growth rate.List of symbols used J _y ¹⁸ Flux of tracer down the grain boundary - J _y ¹⁶ Flux of O-16 down the grain boundary - J _y ^TOT Total flux of oxygen down the grain boundary - J _x ¹⁸ Flux of tracer out of grain boundary - J _x ¹⁶ Flux of O-16 into grain boundary - D _b Grain boundary diffusion coefficient - D Lattice diffusion coefficient - ₀ Oxygen chemical potential change across the oxide scale - Y Total oxide thickness - Y _old Thickness of oxide formed during O-16 oxidation - Y _new Thickness of oxide formed during O-18 oxidation - Y _new ⁱⁿ Thickness of oxide formed during O-18 oxidation due to inward anion diffusion - Y _new ^out Thickness of oxide formed during O-18 oxidation due to outward cation diffusion - K _p Parabolic rate constant - t O-18 oxidation time - O-16 oxidation time - _y Tracer concentration in grain boundary at depthy - C Tracer concentration in the grain - C _i Initial tracer concentration in grain - C _g Average tracer concentration in oxide grain - ¯C Average tracer concentration at depthy - C _max Maximum tracer concentration in the oxide scale - C _s Tracer concentration in gas during O-18 oxidation - Grain boundary width - r Grain size     

The growth of oxide platelets on nickel in pure oxygen. I. Morphology and oxidation kinetics

A very specific oxide morphology consisting in rounded oxide platelets a few microns in diameter has been obtained on pure nickel. The main experimental factors in the achievement of such a controlled morphology have been thoroughly investigated. Platelets are produced in the temperature range from 600 to 850°C and for oxidation times varying from a hundred hours to a very few minutes. Platelets could not be grown on very pure nickel. Scanning electron microscopy has been used extensively, and the corresponding observations are reported in the first part of this study together with thermogravimetric results.     

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective —Al₂O₃ scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.