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.     

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。     

Influence de variations de température sur le comportement à l'oxydation du zirconium. Comparaison avec le cas du cobalt

Influence of temperature variations on the oxidation behaviour of zirconium. Comparison with cobalt Temperature variations produced during the oxidation of a metal have an influence on oxidation cinetique depending from the particular metal. This influence is discussed for the case fo zirconium and is compared with the observations made on cobalt, because the two metals behave differently. An abrupt temperature decrease during the oxidation of zirconium yields a pronounced acceleration of the oxidation. In the case of cobalt, however, the influence of such a temperature change on the ultimate oxidation rate is negligible. These results are discussed in terms of the relative plasticities of oxide and metal.     

Oxidation of Cobalt Metal

By means of inert markers of radio-platinum, it has been shown that cobalt metal oxidizes by outward diffusion of cobalt atoms through the oxide. Oxidation rates have been measured at various temperatures and oxygen pressures and have been found to agree with the rates calculated from the Wagner equation and the authors’ values for the diffusion coefficient of cobalt in the oxide. The distribution of radio-cobalt in growing oxide layers has been accurately measured and found to be different from that predicted from the Wagner oxidation theory. Attempts have been made to measure the change of lattice parameter of the oxide with composition.     

Does order–disorder transition exist in near-stoichiometric Ti–Ni shape memory alloys?

Previous work by Honma et al. suggested that there is an order–disorder transition around 1090 °C in Ti–Ni alloys, but its existence has not been confirmed by independent investigation. In the present work, various techniques including TG-DTA, resistance measurement and TEM were employed to detect the possible signs of such a transition. We found interestingly that when strong measures were taken to prevent sample oxidation, no sign for order–disorder transition was detected; but when the sample was allowed to be oxidized during measurement, we did find a clear DTA peak at 1118 °C. With SEM-EPMA analysis this peak was identified as originating from a eutectic reaction in the oxidation-affected layer, where selective oxidation of Ti causes significant enrichment of Ni and hence drives the oxidation-affected layer into the eutectic regime. Therefore, our results disprove the existence of “order–disorder transition” in the B2 portion of the Ti–Ni phase diagram.     

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.     

An approach to modeling simultaneous high-temperature oxidation and erosion

An approach is suggested for describing the rate of degradation of alloys subjected to the combined effects of high-temperature oxidation and erosion. The basis for this approach is essentially empirical, and is drawn from observations of the kinetics and scale morphologies of alloys in laboratory tests. The two major assumptions used are that the alloy surface is always covered by an oxide layer, and that only oxide (not alloy substrate) is removed by the erosion process. The mode of erosion is not explicitly defined. The rate of erosion, that is, the amount of oxide lost in a given erosion event, is taken to be proportional to the thickness of the oxide layer. The relationships developed have been found capable of accurately describing the shapes of oxidationerosion kinetic curves, and the predicted thickness of the steady-state oxide layers remaining on the alloys agreed reasonably with experimental observations.     

Doping of polyaniline by transition metal salts: current–voltage characteristics of the ITO/polymer film/metal heterostructures

Films of emeraldine base of polyaniline (PAni) doped by various transition metal salts have been prepared, and current–voltage characteristics of the indium-tin oxide (ITO)/PAni film/metal electrode heterostructures were investigated. It was found that the electrical characteristics of the heterostructures are greatly affected by the dopant used and the metal electrode used. Different dopants resulted in different current anomalies with asymmetric current–voltage characteristics. Depending on the dopant used, the exponential and power law of the current behavior can be distinguished. Depending on the metal electrode used, two different regimes of current passing have been found at low applied voltages, namely, a nearly ohmic regime for the indium electrode, and a diode regime for the aluminum electrode. The diode regime was found to accompany by a positive charge accumulation in the film near the film/metal interface, which creates a built-in potential in the film. The amount of positive charges accumulated at the interface and therefore the value of the built-in potential can be reversibly increased or reduced by successive runs of the applied voltage in the forward or reverse direction, respectively.     

The contribution of different types of point defects to diffusion in CoO and NiO during oxidation of the metals

Starting from a general model for the defect structure of nickel and cobalt oxides, the concentrations and diffusion coefficients of the major defects in these oxides are determined as functions of temperature and oxygen pressure. At the higher oxygen pressures, the majority of cation defects are mononegative metal ion vacancies while at lower oxygen pressures appreciable concentrations of dinegative metal ion vacancies are present. It is shown that it is highly probable that the onion defects are dipositive oxygen vacancies. However, these are minority defects over the whole stability range of the oxides. It is not possible with the methods discussed here to determine their individual concentrations and diffusion coefficients. On the basis of this defect structure, a model for the oxidation of Co and Ni is proposed; the deductions from this model are compared with experimental results and good agreement is observed. It is shown that more than one type of defect contributes to the total transport through the oxide. For cobalt, these are mono- and dinegative metal ion vacancies; for nickel, dipositive oxygen vacancies also make a significant contribution.This work is part of a thesis submitted by G. J. Koel to the Twente University of Technology.     

Effect of Cobalt Content on the Oxidation and Corrosion Behavior of Ni–Fe-Based Superalloy for Ultra-Supercritical Boiler Applications

The oxidation and corrosion behavior of three model alloys with different cobalt contents (6–20 wt%) were investigated in static air and a simulated coal ash/gas environment at 750 °C. The model alloys follow a parabolic law approximately during the oxidation in static air. High cobalt level improves the oxidation resistance, however, without noticeable improvement in coal ash/gas corrosion resistance. The sample with the highest cobalt content grows the thinnest oxide layer and the fewest internal oxidation products in the oxidation test. Cobalt in the model alloys promotes the establishment of a protective chromium oxide scale during the oxidation test, but did not show much difference in restraining the inward diffusion of sulfur by increasing its content. The oxidation and corrosion products formed on the sample surface consist mainly of a protective chromium oxide film. Internal aluminum oxide particles have been found especially along the grain boundaries of the base alloy.     

The Fast Linear Growth of Magnetite on Mild Steel in High-Temperature Aqueous Conditions

Abstract

Unstressed mild steel placed in unstirred aqueous solutions of ferrous ion at 300–400° oxidises at a rapid linear rate with formation of a firm scale of magnetite. Weight-change data show that this non-protective oxidation is active in certain chloride and bromide solutions, particularly if small amounts of metallic nickel or cobalt have deposited on the steel or its incipient magnetite coating. Whereas, with protective growth of magnetite on mild steel in liquid alkaline solutions at high temperature, only about half of the oxide adheres to the metal, all of the non-protective magnetite produced under the influence of chloride and nickel adheres to the metal. This finding, together with topographical evidence that the bulk of the non-protective magnetite forms at the metal/oxide interface, leads to the conclusion that prohibitively high stresses develop in this magnetite as it grows, causing the continual development of porosity in the oxide. This cohesive failure is sometimes inadequate to relieve all growth stresses and is then accompanied (especially at 300°) by adhesive loss leading to lamination of the magnetite layer. Beside accounting for the linear oxidation rate, the porosity of the magnetite is believed to place the oxidation under control of the cathodic reaction occurring at the environment side of the magnetite layer. The efficiency of nickel and cobalt as cathodes at the magnetite surface is suggested as an explanation of the accelerating effect of these metals on the oxidation reaction. At 400° the oxidation often takes place with decarburisation and embrittlement of the underlying steel by the hydrogen released during the action. This effect is discussed in detail and the conditions for its occurrence are defined in terms of the competition for the available hydrogen between the embrittlement reaction and various processes for the dispersal of the hydrogen.     

Passivity in metals and alloys

Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues.The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys.The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys.     

Passivity in metals and alloys

Attempted explanations of passivity in metals and alloys since the early proposals of Faraday in 1836 fall into the categories: (1) metal modification, (2) reaction velocity, (3) oxide film, and (4) adsorption. Historically the subject proved to be unusually complex; general concurrence on any one viewpoint was not achieved despite considerable effort and thought by generations of able investigators. Notwithstanding, progress is being made as the search continues.The present survey emphasizes that two basic mechanisms of passivity prevail depending on the metal and its environment. The first depends on a diffusion barrier stoichiometric oxide or other type of reaction product film; the second depends on reduced reaction kinetics introduced by a surface film of chemisorbed oxygen ranging from less than a monolayer to the dimensions of a thin non-stoichiometric metal oxide of variable hydrogen content. The latter film in part impedes rate of metal ion hydration; it is a source of passivity mostly in the transition metals and their alloys.The film of chemisorbed oxygen is more strongly bonded to the base metal than is a metal oxide. This accounts for an observed better resistance to erosion and cavitation damage of metals passive because of chemisorbed oxygen (e.g. 18-8 stainless steel) compared to metals protected by stoichiometric oxides (e.g. iron, copper, cupro-nickels, lead). Since chemisorbed oxygen can be mechanically activated to form the metal oxide by pronounced impingement, abrasion and rubbing of the metal surface, any factor that resists such activation is beneficial. Alloyed cobalt apparently acts in this way, explaining why Co-base passive alloys are appreciably more resistant to erosion, cavitation damage and fretting corrosion than the Ni- or Fe-base passive alloys.     

Oxidation of two-phase Co-54.86 wt.% Cu alloy at 700–1000°C

The oxidation in 1 atm of pure oxygen of a binary two-phase Co-Cu alloy has been studied as a simple example of the oxidation behavior of a multiphase alloy. The two-phase alloy oxidizes according to a parabolic rate law to a good approximation throughout the entire exposure period over the temperature range 700–1000°C with an oxidation rate constant greater than that for pure cobalt in the whole temperature range, and greater than that for pure copper at 900–1000°C, but lower below 900°C. The scale presents essentially the same type of layered structure at all the temperatures investigated, with an outer region composed of copper oxides, while cobalt is preferentially accumulated in the inner region of the scale, mainly in the form of CoO. A subscale formed by internal oxidation of the particles of the Co-rich phase is also present. The observed increase of the oxidation rate of the alloy in comparison with pure cobalt is attributed mainly to the presence of a high concentration of copper dissolved in CoO in the form of monovalent ions, which produces a significant modification of the concentration of defects of cobalt oxide with a consequent increase of the oxidation rate constant of the alloy if a suitable model for the defect structure of pure CoO is considered, which takes into account also the presence of a small concentration of interstitial metal ions.     

Observation of oscillating reaction rates during the isothermal oxidation of ferritic steels

Oscillating reaction rates have been observed in the steam oxidation of 21/4Cr-1MoNb and 9Cr-1Mo ferritic steels at 500–550°C. Changes in reaction rate are associated with the formation of a laminated, inner-oxide layer, made up of bands of fine and coarse-grain spinel oxide. The lowest reaction rates occur during growth of the fine-grain oxide. Coarse-grain oxide generally contains the same levels of Cr, Mo, and Si as the steel (after allowing for loss of Fe to the outer layer), while the fine-grain material contains three times these levels. Ni builds up in the metal and is present in the oxide as metallic particles (mostly associated with fine-grain oxide). A mechanism is proposed in which the highest reaction rates are controlled by diffusion of Fe ions through the oxide layer (as in normal parabolic oxidation) and the lowest rates by diffusion of Fe through the Ni-rich layer in the metal.     

The effect of an applied electric field on the oxidation of aluminum in the temperature range 50–400°C

This paper describes some oxidation studies of evaporated aluminum films. Resistance marker measurements were carried out between 230 and 400° C and it was concluded that the oxide grows by metal transport. From the sign of its Seebeck coefficient, the oxide was deduced to ben-type. The effect of applying an electric field across the growing oxide layer on aluminum was also investigated. A porous platinum layer evaporated onto the oxide surface was used as one electrode, the underlying metal being the other electrode. At all temperatures between 50 and 400° C the same field effect was observed. When the oxygen-oxide interface was biased negative with respect to the aluminum, an enhancement of the oxidation rate was achieved. These results have been interpreted in terms of the Mott-Cabrera theory.     

Oxidation kinetics of cobalt and cobalt—iron alloys at high temperatures

The oxidation kinetics for cobalt and two cobalt—iron alloys alloys, CoFe and CoFe₃, were determined between 1073 and 1280 K in 1 atm. of oxygen. The oxidation of cobalt and CoFe follows a parabolic law, while the oxidation of CoFe₃ is better described by a direct logarithmic law. The oxidation rates for the cobalt—iron alloys alloys are lower than those expected from combining oxidation kinetics of pure iron and pure cobalt, thus suggesting a significant cobalt enrichment occurs at the oxide surface. This cobalt enrichment is observed using energy dispersive analyses. The logarithmic kinetics of CoFe₃ during oxidation is discussed, and a model is proposed to explain this behavior.     

The influence of plasma spraying regime and initial substance injection location on the structure of deposited coatings

Atmospheric pressure non-equilibrium plasma spray technology has been developed, tested and successfully employed for deposition of catalytic and tribological coatings. Aluminum hydroxide based coatings doped with carbon particles up to 50 μm of thickness were deposited on the stainless steel substrates.Results of experimental investigation on structural characteristics of plasma sprayed coatings in dependence of prevailing external factors are presented. The effect of plasma source operating regime and injection location of precursor dispersed particles (DP) on the morphology and properties of metal oxide coatings was investigated. It has been determined the relation between microstructure of coatings and arc current in plasma source.Significant differences in size and shape of coating grains were observed during deposition process when metal oxide dispersed particles were placed into the reactor channel and directly into the plasma torch discharge chamber together with the reacting gas. The plasma spray pyrolysis process has occurred in some particular regimes which have been successfully applied in the synthesis of micron- and submicron-sized uniform shaped coatings with narrow size pores and controlled surface morphologies.     

Kinetics of high temperature oxidation of chromium rich HfC reinforced cobalt based alloys

Mostly known to improve the high temperature oxidation resistance of superalloys, hafnium may also form carbides. Several per cents of Hf allow developing a dense carbide network to mechanically strengthen alloys. Here, the high temperature oxidation behaviour of three HfC containing cobalt alloys was characterised at all steps of a thermogravimetry test: heating, isothermal stage and cooling, compared with two Co–Cr–C model alloys. The five alloys were heated in synthetic air, maintained at 1200°C during 50 h and then cooled. The mass gains were plotted versus time or according to (m×dm/dt?=?Kp–m×Kv) to specify the isothermal kinetic constants, or versus temperature to determine how oxidation acts during heating and oxide spallation occurs during cooling. Compared to the ternary alloys, the oxidation of the HfC reinforced alloys starts earlier but leads to lower mass gains during heating, the isothermal oxidation is faster and oxide spallation occurs later.     

Effect of Oxide Growth Strain in Residual Stresses for the Deflection Test of Single Surface Oxidation of Alloys

Residual stresses developed in FeCrAlY and Ni₈₀Cr₂₀ alloys have been predicted considering growth strain and creep strain in oxide layer and creep strain in alloy or metal. Such stresses, a net compressive stress developed in oxide scales and a net tensile stress developed in alloy strip, produce deflection of a single surface oxidized specimen during high temperature isothermal oxidation. Stresses generated in these alloys and oxide scales were compared with creep deflections. Introducing oxide growth strain in the oxide scales increase the oxide stress value during the initial oxidation stage, during which creep analysis lacks prediction. Oxide stress reaches maximum value at certain oxidation time in the initial oxidation stage. After that oxidation time relaxation of oxide stress occurs considerably in later oxidation stage.