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 kinetics of duplex scale formation on 18/8 type stainless steel

A previous model of duplex scale growth on 18/8 stainless steels based on the formation of an impervious barrier at hemispherical alloy grain boundaries has been extended to consider the influence of a finite healing layer growth rate. The analysis shows that true primary and secondary or healing layer parabolic rate constants can best be calculated from weight gain vs. the square root of time plots. Accurate determinations of the secondary rate constant requires oxidation exposures a factor of 3 or 4 times greater than the time to form a complete healing layer at the oxide-metal grain boundary interface. Good agreement is found between the present theory and expeimental data and the model gives a simple extrapolation method for the evaluation of long-term metal losses or oxide thicknesses.     

Investigation on steam oxidation behaviour of TP347H FG Part 1: Exposure at 256 bar

The stainless steel TP347H FG is a candidate material for the final stage tubing of superheater and reheater sections of ultra supercritical boilers operated at steam temperatures up to 620°C in the mild corrosion environments of coal‐firing. A series of field tests has been conducted with the aforementioned steel in coal‐fired boilers and this paper focuses on the steam oxidation behaviour for specimens tested at various metal temperatures for exposure times of 7700, 23000 and 30000 hours as investigated by light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide, where the outer layer consists of two sub‐layers, an iron oxide layer and an iron‐nickel oxide layer; the inner layer is chromium rich chromium‐iron‐nickel oxide. Microstructure examination showed that for all these samples the varying grain size of subsurface metal affected the oxide thickness, where the larger the metal grain size, the thicker the oxidation scale. This gave the appearance of uneven inner oxides with a varying pit thickness. Comparison of the pit thickness measurement and oxide composition reveals that the oxidation rate is fast during the initial oxidation stage, but the subsequent growth of oxide from further exposure is slower due to the formation of a healing layer consisting of chromium rich oxide near original alloy grain boundaries. At a temperature region above 600°C a thin oxide rich in chromium and manganese is sometimes formed. In addition precipitation of secondary carbides in the bulk metal also occurs at this temperature region.     

The growth and structure of oxide films on Fe. I. Oxidation of (001) and (112) Fe at 200–300°C

A study has been made of the oxidation of (001) and (112) Fe at 200–300°C in 5×10^–3 Ton O₂ to determine the influence of substrate orientation and surface pretreatment. Using oxidation kinetics, reflection electron diffraction, and electron optical techniques, it has been shown that the nature of the prior oxide film has a marked effect on oxidation behavior for a given orientation. The initial faster rate for a surface covered with a 16-Å prior oxide film formed by dry oxidation at room temperature is attributed to a smaller sub grain size in this film, compared to that for a prior film formed by electropolishing. This initial rapid rate is not sustained because of oxide separation from the metal. -Fe₂O₃ formation, which occurs at higher temperatures for either surface pretreatment, is enhanced by the oxide separation.     

The metallurgical pretreatment and protective oxidation of a 9.75% Cr-Fe alloy in CO2

The variation of the structure and composition of the oxide formed in CO₂ on cold worked material with distance from the oxide-gas interface has been studied using specimens ion beam thinned for the transmission electron microscope. The results presented have been compared with those previously reported for the protective oxidation of an annealed alloy in CO₂. The oxide formed is duplex and the inner oxide which differs from the fine grained oxide on the annealed material consists of larger columnar grains. Carbon, from the oxidation reaction, is injected into the metal forming M₂C and M₂₃C₆ carbides in addition to the M₇C₃ carbides exclusively formed in the annealed alloy. The oxide-metal interface is well defined and dependent on the grain structure of the metal. This feature together with the larger grain size of the oxide may result in the reduction in the oxidation rate for cold worked alloys reported by other workers.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

Influence of chromium on the oxidation of titanium between 550° and 700°C

The oxidation behavior of Ti-Cr alloys (1, 4, 11, and 19% chromium by weight) was investigated, between 550 and 700°C, in air and in oxygen, for a maximum duration of about 800 hr. The kinetics results revealed a significant influence of the chromium content with a maximum, in the unfavorable sense, around 4%. Moreover, the analysis of kinetics curves showed that stable rate constants could be obtained only if the duration of oxidation was sufficiently long. The oxide layers were duplex with a much higher chromium content in the inner layer. The morphological investigations performed on cross-sections of the oxidized specimens showed that the chromium modifies the crystallization of the oxide layers. The adherence of the oxide layers evolves nonmonotonically as a function of chromium content for the same degree of progression of the reaction and as a function of the oxidation rate, which is itself dependent on chromium content.     

The oxidation of chromium- and nickel-implanted nickel at high temperatures

The isothermal oxidation behaviour in oxygen of nickel, implanted with nickel and chromium ions, has been studied in the range 950–1150°C using kinetic and electronoptical techniques. Implantation with nickel ions has a significant and lasting effect on the oxidation kinetics and on the morphology and grain configuration of the NiO scale. The oxide grains are generally smaller, more facetted and more mismatched for the implanted surfaces and there is more extensive stress generation during oxide growth. After an initially slower rate, the oxide on the implanted surfaces develops at a faster rate than that on the unimplanted surfaces, particularly at the higher temperatures. Implantation with chromium ions inhibits the initial development of the NiO scale. However, subsequently, the oxidation rate is more rapid for the implanted surfaces and increases progressively with increasing chromium ion dose. The results can be accounted for in terms of doping of the NiO and the much decreased grain size of the oxide. It is concluded that short-circuit grain boundary diffusion processes, as well as bulk lattice diffusion of Ni²⁺ ions, are important in the oxidation of nickel at high temperatures.     

CO2 High Temperature Corrosion and Its Prevention of Chromia Forming Fe-base Alloys

High temperature corrosion of chromia forming Fe-base alloys by CO_2 produces not only oxidation but also carburisation. The corrosion kinetics in CO_2-rich gas is found to be increased compared with that in air or oxygen. As a result, higher alloy chromium levels are required to achieve protective chromia formation in CO_2. Corrosion reaction mechanisms in CO_2 are examined and the internal carburisation of alloys in low carbon activity CO_2 gases are analysed based on the variation of p_(O_2) at the interface of oxide and metal. Carbon penetration through chromia oxide scale has been revealed by atom probe tomography. The strategies to resist CO_2 corrosion are reviewed by alloying of Si and/or Mn, forming additional diffusion barrier layers, and by adding sulphur to modify oxide grain boundaries to reduce carbon diffusion along the grain boundaries.     

Grain boundary diffusion in Cr-doped NiO and the oxidation of Ni-Cr alloy

We have measured the diffusion of ⁶³Ni radiotracer into polycrystalline NiO, nominally doped with 0.1% Cr (Cr/Ni ratio) in the temperature range 600 to 900°C. The experiments show that Cr doping increases diffusion of Ni in the oxide lattice, but decreases diffusion of Ni along grain boundaries provided that the grain boundary Cr/Ni ratio is sufficiently large (greater than about 1%). This is believed to be due to the formation of immobile Cr-vacancy pairs which block fast boundary diffusion. The oxidation rate of Ni 0.1% Cr alloy is, however, slightly faster than that of pure Ni at 700°C, at which temperature grain boundary diffusion should be dominant. This apparent discrepancy between oxidation rates and diffusion studies, it is argued, is due to the complicating effects of low Cr mobility, a duplex film structure and inward oxygen transport during oxidation of the alloy.     

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.     

单晶与多晶NbSi2在1023K氧化行为的对比

选择NbSi2的单晶、放电等离子烧结(SPS)多晶和电弧熔炼多晶样品在1O23K下氧化,研究了微裂纹、孔隙和晶界对NbSi2氧化行为的影响．结果表明,含有大量微裂纹的电弧熔炼多晶样品经3h氧化后完全粉化,而单晶和SPS样品经89h氧化亦未出现粉化．无论是单晶还是多晶样品,氧化产物均为Nb2O5和SiO2,生成的Nb2O5易剥落,样品的氧化动力学曲线均呈直线规律,证明NbSi2的氧化过程是一种本征无保护条件下氧元素与基体元素直接反应的过程．多晶样品的氧化速率明显高于单晶样品的氧化速率,分析表明晶界和孔隙相当于增加了氧化反应的有效面积,提出了相应模型．     

Influence on the oxidation kinetics of metals by control of the structure of oxide scales

An explanation of the deviation from the parabolic law is the treatment which considers both shortcircuit and lattice diffusion in the oxide scale. In this study we examine how the oxidation kinetics are influenced by changing the structure of the scale of copper oxide in order to confirm the role of short-circuit diffusion in determining the oxidation rate. In addition we explain the oxidation kinetics of copper and nickel by using a model of the scale structure which includes recrystallization and grain growth. Results are as follows: (1) The nucleation and growth behavior of oxide have a direct effect on the structure and in turn the oxidation kinetics due to short-circuit diffusion. (2) A modified treatment is valid in the region where volume diffusion and short-circuit diffusion play an important role in which it is necessary to consider the scale structure such as the grain size distribution and the boundary width. (3) When recrystallization takes place it is necessary to consider the model of a two-layered scale structure which is different in properties and morphology. (4) In this region the rate curves are S-shaped when oxide recrystallization takes place and exhibit a transition from a parabolic to an nth-power relationship (n>2) when grain growth takes place.This research was performed at the University of Tokyo in partial fulfillment of the requirements for the degree of Doctor of Engineering.     

Formation and growth of spinel and Cr2O3 oxides on 20% Cr-25% Ni-Nb stainless steel in CO2 environments

The nature of the first-formed oxide on 20% Cr/25% Ni/Nb stabilized steel during exposure to CO₂ at high and low temperatures has been determined by surface analytical techniques. These results together with a consideration of gas/solid interactions show that the oxide produced may be determined by kinetics or thermodynamic factors, and a diagram is presented to show that rhombohedral Cr₂O₃ or spinel may be the oxide first formed. Under most standard conditions, a mixed spinel oxide is formed initially, and the subsequent growth of a duplex oxide is analyzed in terms of a solid-state reaction in which the spinel oxide is reduced to Cr₂O₃ at the metal/oxide interface. Diffusion control of growth by either spinel or Cr₂O₃ is incorporated in new equations describing the kinetics of oxidation, and weight-gain predictions are tested against experimental observations.     

The effect of alloy grain size on the transient oxidation behavior of an alumina-forming alloy

In the early stages of alloy oxidation, diffusion of solute through the metal to the surface is important in determining the composition of the oxide scale that forms during the transient stage. Rapid solute diffusion to the interface will promote the formation of a protective scale, thereby suppressing the formation of base-metal oxide. The effect of alloy grain size on the formation of the transient oxide scale has been studied using a very fine grained NiCrAlY alloy produced by plasma spraying. The long-term oxidation behavior of this alloy was found to be independent of the grain size of the underlying alloy. However, the short-term, transient oxidation rate was found to decrease with decreasing alloy grain size. This is attributed to the rapid grain boundary transport of Al and Cr to the oxide/metal interface which promoted the formation of Cr₂O₃ and Al₂O₃.     

EFFECTS OF ACTIVE ELEMENTS Y AND Ce ON HIGH TEMPERATURE OXIDATION OF Fe-25Cr-40Ni ALLOY

The effects of small amount(≥0.05%)of active elements Y and Ce in Fe-25Cr-40Ni al-loy have been investigated on the kinetics of early stage of high temperature oxidationas well as composition and microsructure of oxide film by ion backscattering and slowpositron beam.The results show that Y and Ce can reduce evidently the high tem-perature oxidation rate of FeCrNi alloy at early stage and can effectively enhance theCr_2O_3 formation and retard the oxide formation of Fe and Ni in the surface layer thusthe microstructure of the oxide film is improved.Y and Ce enter the oxide film andconcentrate within several tens nanometres from surface of the outer layer.The mech-anism of oxidation resistance for Ce is different from Y.Ce can decrease the densityof vacancy defects in oxide layer,so the cations are restrained from diffusion outwardthrough vacancies.While Y can control the outward diffusion of cations through va-cancies at the early statge of oxidation(shorter than 12 min),aften that restrained thecations from diffusion outward along grain boundaries up to 60 min oxidation.     

The low-temperature oxidation of calcium by water vapor

The oxidation kinetics of calcium in water vapor have been studied over the temperature range 25–300°C. There is a change in the form of the oxidation kinetics with temperature, from essentially linear at temperatures below 150°C to logarithmic at 300°C. This is coupled with a change in the manner of growth of the oxide layer as well as the chemical composition of the reaction product. In addition, the oxidation rate decreases with temperature, reaching a minimum at about 150°C. At temperatures below 150°C oxidation appears to be a result of the formation of cracks or fissures in the oxide film. Above 150°C no single oxidation mechanism can be deduced.     

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.     

Kinetics of oxidation of ferrous alloys by super-heated steam

The kinetics of the oxidation of ferrous alloys in steam (10–60 kPa) at 450–550°C have been studied by measuring both the rate of hydrogen emission and the amount of metal oxidized. Excellent agreement has been found between the amount of metal oxidized calculated from both the total mass of hydrogen produced in the reaction and the thickness of the oxide layer formed; rate constants calculated from the rate of hydrogen emission, the mass of hydrogen produced as the reaction proceeds, and the oxide formed agree within experimental error. The rate of oxidation of a 9%Cr-1%Mo alloy at 501°C was found to be independent of the partial pressure of the steam. For this alloy, the activation energy agreed with literature values obtained at higher temperatures and pressures. The effect of the chromium and silicon content on the oxidation rates is compared. The rate constants are compared with theoretical calculations, assuming that the rate is determined by diffusion of iron in the magnetite lattice. For the 9%Cr-1%Mo alloy, the parabolic rate constant and activation energy are in excellent agreement with values calculated using Wagner's theory. The experimental rate constants are greater for the alloys containing smaller amounts of chromium; diffusion of iron along magnetite grain boundaries may be the dominant mechanism.