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.     

The oxidation of cupro-nickel alloys—II. The kinetics of diffusion in porous inner layers

The oxidation of cupro-nickel alloys at temperatures > 160°C produces a duplex oxide structure. In Part I, diffusion of copper across the inner nickel oxide layer was shown to be rate controlling. This conclusion is tested in this paper by examining the oxidation rate curves of a series of nine metals and alloys extending from pure copper to pure nickel. The isochronal curves are found to pass through a maximum at 75%Cu which is explained by surface diffusion of the copper through a porous nickel oxide structure extending through the metal consumption zone. The surface diffusion coefficient derived from this model is in the range 0.8–2.0 × 10^?14 m² s^?1 in accord with results of other workers.     

A fundamental study of the kinetics of zinc oxidation in the temperature range 320–415°C in atmospheres of pure oxygen and oxygen doped with gaseous impurities

The oxidation of pure zinc in the temperature range 320–415°C was investigated gravimetrically. Oxidation in pure oxygen was found to obey parabolic rate equations in the temperature range 375–415°C and logarithmic time-dependence in the range 320–370°C. The oxidation rate at temperatures above 370°C was generally proportional to the logarithm of oxygen pressure, although this was not obeyed at all temperatures and pressures in this range. A reaction mechanism involving control of oxidation at the oxide/oxygen interface has been proposed with two rate-determining surface reactions of oxygen. One of these leads to a logarithmic rate equation and the other to parabolic kinetics.The results obtained from zinc oxidation in atmospheres of oxygen mixed with small concentrations of water, hydrogen, carbon dioxide, iodine and ammonia have also been explained with reference to this mechanism.     

The role of interface structure in oxidation reactions

The dynamics of the scale–metal interface exhibits an asymmetry in behavior in oxidation and other scaling reactions depending on whether the mechanism is one of cation or anion-diffusion. For the cation case, the growth step is at the free surface and line defects at the metal–scale interface serve to annihilate vacancies in the scaling mechanism. For the anion case, the growth step is at the scale–metal interface. Disconnections and dislocations provide the growth sites for the reaction and both their topological and elastic properties influence the mechanism. Scaling reactions to form nickel oxide and molybdenum disulfide are considered as examples. Conditions favoring scale fracture are discussed. The results are related to the reactive element effect and to the significance of the Pilling–Bedworth ratio.     

Kinetics and mechanisms of reactive solid state dewetting in the system Ag–Ni–O

Thick silver films vapour deposited under ultra high vacuum onto high purity nickel foils, dewet the substrate when annealed at high temperature in air. No hole is observed in the silver film after an annealing at the same temperature in a pure argon atmosphere. SEM observations of the film surface and of cross-sections reveal that dewetting occurs when a nickel oxide layer is formed at the silver–nickel interface as a result of oxygen diffusion through the silver film [Phil. Mag. Lett. 80(2000)33]. The kinetics of the phenomenon has been studied for films 1.8 microns thick annealed at 1123 K. Four dewetting mechanisms have been identified and are described. In the case of short heat treatments (1 h at 1123 K in air) the main dewetting mechanism is observed to be the condensation at the silver–nickel oxide interface of vacancies into voids which grow towards the free surface of the silver film.     

Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

1　INTRODUCTIONTheoxidationofunalloyedcopperandnickelhasalreadybeenstudiedindetail.Whilethehigh tem peratureoxidationofCu Nialloys ,alsostudiedanumberoftimestodate[13] ,isanexampleofarela tivelysimpleclassofscalingofbinaryalloysbyasin gleoxidant,becausethemetalsformacontinuousse riesofsolidsolutions ,whiletheiroxides ,CuO ,Cu2 OandNiO ,exhibitsmallmutualsolubilitiesandshowsignificantdifferencesinthethermodynamicstabilityandparabolicgrowthrates .Thus ,copper richalloysformexternalscales…     

Effect of CeO<Subscript>2</Subscript> Coating on the Isothermal Oxidation Behaviour of Ni-Based Alloy 230

Ceria coating was deposited on alloy 230 using an electro-deposition process in a cerium nitrate electrolyte. Reactive element effects were investigated by comparing the oxidation behaviour of the samples with and without the ceria coating. The prepared ceria coating reduced the oxidation rate and improved the adherence of the oxide layer. A thicker oxide layer with large spallation areas formed on the uncoated sample, while a thin and protective oxide layer was found on the coated sample after oxidation for 1000 h at 900 °C. The oxidation mechanisms of alloy 230 with and without ceria coating were discussed. Furthermore, as the ceria coating changed the grain shapes of chromium oxides from columnar to an equiaxed structure, discussion also advanced proposals regarding the mechanisms of formation of these different oxides. The equiaxed grains enhanced the adhesion of the oxide to the alloy surface.     

A comparison of the effects of small additions of various second elements on the oxidation of nickel at 1200°C

The oxidation behaviour of Ni-4.2% Mo, Ni-4.0% W, Ni-2.5% Al, Ni-4.2% V and Ni-5.0% Cr (all wt. %) at 1200 °C in flowing oxygen at 1 atm. pressure has been studied using various techniques. In particular, the solubility of the second element in NiO, its distribution across the NiO scale and the effects of these on the oxidation rates and scale morphologies have been examined. The oxidation rates of all the alloys are greater than that of nickel, although for Ni-4.2% Mo, where incorporation of internal oxide into the scale does not occur and molybdenum does not dope the oxide, the small increase in weight gain during oxidation Compared with that for nickel is due to internal oxide formation only. As the internal oxide particles pileup at the alloy/oxide interface, they exert a blocking effect to outward diffusion of Ni²⁺ ions, especially in the later stages of oxidation. Ni-4.0% W behaves similarly, although a few internal oxide particles are incorporated into the scale and a small amount of doping of the oxide ensures that it thickens at a slightly faster rate than the scale on nickel and for Ni-4.2% Mo. The oxidation rates of the other alloys are significantly faster than that of nickel and increase in the order Ni-2.5% Al, Ni-4.2% V, Ni-5.0% Cr. These increased rates are largely caused by increases in the total cation vacancy concentrations in the scales, although internal oxide formation can make a significant contribution to the oxidation kinetics. The influence on the oxidation behaviour of a number of factors, namely doping of the scale, internal oxidation, dissociation of NiO and transport of gaseous oxygen within the scale, blocking effects in the oxide and at the alloy/oxide interface, and grain growth of the oxide, are considered in detail.     

Thin Film oxidation of metals at the Curie temperature

Anomalies in the Arrhenius plot of the thin film (10²–10⁵ Å) oxidation of nickel and iron at the Curie temperature are discussed. Amongst the various theories, a mechanism which takes into account the importance of both the surface-state charge at the metal/oxide interface and the space charge int he oxide during the logarithmic oxidation of metals, is considered suitable to explain such anomalies.     

Determination of Oxygen Diffusion Along Nickel/Zirconia Phase Boundaries by Internal Oxidation

Internal oxidation tests with nickel alloys that contained up to 8 at.% zirconium were carried out. All alloys were two-phase consisting of γ-Ni and the intermetallic phase Ni₅Zr. Their behavior under low oxygen partial pressures in the range of 800–1,000?°C could not be described by the Wagnerian analysis. Oxygen diffusivity along the interface nickel/monoclinic zirconia plays an important role for the rate of internal oxidation. The early stages of internal oxidation show the in situ mode where diffusion of the less noble element zirconium cannot diffuse in the matrix and is oxidized instantly. Later in the process the mode shifts from in situ towards the diffusive mode as zirconium has the possibility to diffuse. This change could also be observed as the size of the oxide particles varied with ongoing oxidation. A method for the determination of the oxygen diffusivity in nickel/monoclinic zirconia phase boundaries is presented.     

Formation and Healing of Voids at the Metal–Oxide Interface in NiAl Alloys

The present investigation concerns the formation and healing of voids at the metal–oxide interface in β-NiAl alloys. It is shown that transient cavities form at the metal–oxide interface during the initial stage of isothermal oxidation at 1,050 °C. These voids disappear after a few hours of oxidation, due to their filling with oxide. It is proposed that the filling of the transient voids occurs through cracks in the outer oxide scale. This allows for an oxygen transport into the cavities where a new oxide is created and grows outwards from the metal surface. Pt-containing β-NiAl alloys that have better resistance to oxide spallation show suppressed formation of cavities and/or a faster filling of voids. This is explained by the enhanced diffusion of Al in these materials.     

两种Co-Nb合金在60O℃～800℃1大气压纯氧中的氧化

研究了含Ｎｂ１５和３０ｗｔ％的Ｃｏ－Ｎｂ二元合金在ｌａｔｉｎ纯氧中６００～８００℃的氧化特性。它们的氧化动力学近似地遵循抛物线规律,而其瞬时氧化速率常数随时间而减低、且以６００℃氧化者尤甚。两合金的氧化速率均高于纯Ｃｏ,但其速率增量颇低。在所有的实验条件下,两合金都发生了外氧化与内氧化,外氧化膜的外侧为连续的纯氧化钴带,其下为两个二元Ｃｏ－Ｎｂ氧化物（ＣＯＮｂ２Ｏ６和ＣＯ４Ｎｂ２Ｏ９）与基金属氧化物的混合。内氧化带为氧化钴、氧化铌（Ｎｂ２Ｏ５或／和ＮｂＯ２）的混合,而在该带的最外侧还有源于富Ｎｂ合金相的二元氧化物。在合金－氧化膜界面处都没有观察到贫铌带。从合金的和所生成氧化膜的显微组织特征,尤其是从铌在钴中溶解度低的角度,对合金的氧化行为进行了讨论。     

Reactive molecular dynamics study of the oxidation behavior of iron-based alloy in supercritical water

A theoretical study of the oxidation behavior of iron-based alloy in the supercritical water (SCW) has been carried out based on ReaxFF force-field molecular dynamics simulation. An atomic model has been proposed to simulate the initial chemisorption reactions and atoms diffusion behavior across the oxide layer. Simulation results imply that Cr addition has an important effect on the oxidation behavior of iron-based alloy. In the initial stage of oxidation, H₂O prefers to adsorb on the Cr atom, and some species in the form of Cr(OH)₄ are observed on the FeCr alloy surface. Once an initial oxide layer is formed, further oxidation is controlled by the migration of vacancy. The O vacancies are formed at the oxide/FeCr alloy interface and migrate toward the steam, whereas Fe vacancies are formed at the oxide/steam interface and migrate toward the FeCr alloy. Attributed to the stronger binding energy of O–Cr bond than O–Fe bond, the Cr diffusivity in the oxide is less than Fe atoms. Thus, double oxide layers, including the inner Fe–Cr–O layer and outer Fe–O layer, are formed on the FeCr alloy, which is in good agreement with previous experimental observation.     

Early Stages of High Temperature Cyclic Oxidation of an Electrodeposited Ceria Coating on Nickel Superalloys Under Water-drop Tests

Ceria coatings having a well-defined cracks network were electrodeposited onto Ni-based René N5 superalloys. After a subsequent annealing under argon for promoting the formation of a stable alpha alumina scale at the ceria layer/substrate interface, the samples were isothermally oxidized at 1,100 °C for 50 h to thicken the oxide scale. They were then subjected to cyclic oxidation at 1,100 °C for 1-h cycles. 10 μL water drops were put down onto the surface of the samples during the cold stages to study the influence of water onto the mechanical integrity of the pre-oxidized system. The multi-cracked ceria microstructure exhibited a widening of the cracks as well as refinement of the oxide grains. However, no spallation was observed and the composition remained stable for both ceria and alumina. It was demonstrated that the crystallization of ceria into the CeO₂ fluorite structure conferred to the coating chemical inertness towards water.     

Effects of 0.1 wt.% Manganese, Aluminum, and Silicon on Oxidation and Copper-rich Liquid Phase Formation in an Iron–0.3 wt.% Copper–0.15 Wt.% Nickel Alloy

This study investigated the effect of easily oxidizable impurities on the oxidation behavior of iron containing small amounts of copper and nickel. The motivation for this work stems from a cracking phenomenon in low carbon steels known as hot shortness. This type of cracking is caused by formation of a copper-rich liquid layer and is reduced in the presence of easily oxidizable impurities. This work studied iron alloys with 0.3 wt.% copper, 0.15 wt.% nickel, and 0.1 wt.% (manganese, aluminum, or silicon) oxidized in air at 1,150 °C. Parabolic oxidation rates were not affected by manganese or aluminum but were decreased with silicon additions. Manganese and aluminum additions led to internal MnO and hercynite formation. These slightly increased the amount of material entrapped into the oxide. Silicon additions led to a nearly continuous fayalite layer near the oxide/metal interface that decreased the oxidation rate and therefore the amount of copper-rich liquid.     

Investigation of Adhesion and Fracture Toughness of Thermally Grown Oxide Scales by Interface Indentation Test

HIGH TEMPERATURE ALLOYS rely on theformation of a dense layer against oxidation.Howeverthe crack,spallation and detachment of oxide layeunder growth stress and thermal stress fromtemperature difference cause the failure of theprotective layer.Therefore,high temperature materialsare required forming a good oxide scale with highstrength and high interface bonding strength.Anappropriate measuring method and evaluation fooxide/metal interfacial adhesion is of great importanceto understand …     

The corrosion of nickel in 1 atm of pure SO2 at 600–1000°C and the mechanism of formation of the two-layered scales

The reaction of pure nickel with 1 atm SO₂ at 600–1000°C follows an irregular type of kinetics, tending to become parabolic at the highest temperature. The reaction rate tends to decrease with an increase of temperature while the scale has the same type of overall structure at all temperatures. The scales consist of an inner region of sulfide free from oxide and containing metal particles and an outer region composed of an oxide matrix containing dispersed particles of sulfide, generally extending up to the outer scale surface. The main mechanism of reation is considered to be the simultaneous formation of a mixture of oxide plus sulfide at the scale-gas interface by direct reaction of the metal with molecules of SO₂. The formation of an inner layer of sulfide not containing oxide is attributed to an inward migration of sulfur through the outer duplex layer, most likely inside the network of sulfide particles.     

The High Temperature Oxidation Behavior of Mg–Gd–Y–Zr Alloy

The oxidation behavior of pure Mg and Mg–Gd-Y-Zr alloy was studied in O₂ at 300 °C with and without the presence of water vapor. The kinetics curves revealed improved oxidation resistance of Mg–Gd–Y–Zr alloy in O₂, compared with pure Mg. However, when water vapor co-existed with oxygen, the oxidation rate of Mg–Gd–Y–Zr alloy was accelerated; whereas, the oxidation rate of pure Mg was restrained. Detailed XPS analysis of pure Mg oxidized with water vapor revealed that the reduced oxidation rate could be strongly linked with the outer Mg(OH)₂ film. On the contrary, for Mg–Gd–Y–Zr alloy, an incomplete Mg(OH)₂ film was present in the outer region of oxide layer, which can provide a ready pathway for water vapor transport to the inner part of the oxide film and which has little oxidation resistance to water vapor.     

Some interactions in the erosion-oxidation of alloys

The modes of interaction of erosion and high-temperature oxidation, occurring simultaneously on an alloy surface, have been studied using Ni-30Cr, MA754, Ni-20Al, and Co-22Cr-11Al-0.2Y alloys to examine the influence of chromia and alumina scale formation on the erosion of nickel and cobalt base alloys. The results have shown that, in the presence of a rapidly flowing oxidizing gas stream, the evaporation of volatile metal oxides becomes important at lower temperatures where normally it can be ignored. Otherwise, the erosion and oxidation of alloys parallels the behavior of pure metals but also introduces additional factors derived from lengthening of the period of transient oxidation and modification of concentration profiles in the alloy adjacent to the alloy/scale interface. Higher erosion intensities extend the transient oxidation behavior which adversely affects the formation of protective scales. As with pure metals, the presence of erosion and oxidation together always produced increased rates of degradation.     

Factors affecting the high-temperature oxidation behavior of some dilute nickel- and cobalt-base alloys

Oxidation of nickel- and cobalt-base alloys, containing small additions of a higher valent second metal, in oxygen or air at high temperatures results in the formation of relatively complicated scale morphologies which change subtly with increasing additions of the second element and its characteristics. The various factors that can influence the oxidation behavior of such alloys are assessed and correlated with the oxidation kinetics and scale morphology types. For very dilute alloys the increase in oxidation rate compared with that of the corresponding pure metal (nickel or cobalt) is largely due to doping of the external oxides. However, once the solubility limit of the second metal in this oxide is exceeded, additional increases in second metal content of the alloy can either increase further or decrease the oxidation rate. The exact behavior depends on the relative interplay of factors such as internal oxide formation and coalescence, blocking effects of incorporated internal oxide or pores in the scale, short-circuit paths through the scale, doping, and the relative diffusion rates of the two metals in the scale. Probable rate-determining steps for oxidation of different alloy composition ranges are proposed.