High-Temperature Oxidation Behavior of Iron–Chromium–Aluminum Alloys

The detailed oxidation behavior of Fe–10Cr alloys, containing aluminumin the range of 2–8% by weight, was studied in pure oxygen at 1 atmpressure. The investigations were performed over the temperature range950–1050°C under cyclic conditions (3-hr cycles) in each case. Thecyclic-oxidation resistance, as measured by the specific weight-gain values,was observed to progressively improve with increasing aluminum content inthe alloy. For a particular aluminum content, however, the oxidationresistance decreased with increasing temperature. Following the initialtransient-oxidation period, a healing layer of chromia was established onthe four alloys. The lower-aluminum alloys (2–4% Al) were observedto end up with Fe-rich oxide scales under the experimental conditions atall temperatures, whereas those containing aluminum in the range of6–8% formed -Al₂O₃ scales.     

Alloy-grain-size dependence of the effectiveness of silica coatings as oxidation barriers on stainless steel

In order to determine the effect of alloy grain size on the oxidation properties of silica-coated austenitic Fe-18Cr-20Ni stainless steel, both coarse-grain (100-m grain size) and fine-grain (5-m grain size) forms of the alloy were produced. A 1-m-thick vitreous silica coating was deposited by chemicalvapor deposition on the alloys, which were subjected to isothermal and cyclic oxidation in air at 900°C. The coarse-grain alloys underwent widespread oxidation below the silica coating, leading to extensive coating spallation. This was attributed to the inability of the alloy to supply a sufficient outward flux of chromium to prevent oxygen penetration through microcracks in the silica coating. Due to an abundance of chromium available at the surface of the finegrain alloy, chromia formed in the microcracks within the silica layer. As a result, the silica-coated, fine-grain alloy demonstrated superior oxidation resistance and excellent adhesion of the coating.     

钛及钛合金高温氧化行为研究

对工业纯钛TA2、TC4及Ti60合金分别在600、700、800℃进行氧化,得到氧化增重曲线.利用SEM、XRD等手段对氧化表面形貌、氧化产物构成及分布进行研究并分析了3种钛材在不同温度下的氧化机制.结果 表明:随着温度升高,TA2、TC4以及Ti60三种钛材的抗氧化能力均有所下降,且抗氧化能力由强到弱依次为Ti60...     

The Application of a Wedge-Shaped Sample Technique for the Study of Breakaway Oxidation in Fe–20Cr–5Al Base Alloys

The evolution of the breakdown of protective alumina scales on Fe–20Cr–5Alalloys has been studied by the use of taper-sectioned samples. The oxidationin air of two model alloys is compared with that of two commercial alloys,Kanthal APM and MA 956 at 1350°C. It is shown that the length of thebreakaway-oxidation region along the taper may be used to rank theperformance of the alloys and that the wedge geometry allows a detailedstudy to be made of the chemical processes involved in the degradation ofthe oxide scale just prior to breakaway oxidation.     

Internal oxidation of ternary alloys. Part II: Kinetics in the presence of an external scale

Classic kinetics equations of internal oxidation of binary alloys associated with the formation of an external oxide scale of the base metal have been simplified by applying mathematical asymptotical analysis. An analysis for the case of ternary alloys is also presented. The analysis involves simultaneous internal oxidation of the two solutes in the ternary alloys below the external oxide scale of the base metal. The kinetics equations derived are applied to calculate depths of internal oxidation zones of Ni-4 at.% Al-1 at.% Si alloys oxidized in 760 torr of oxygen at 800°C.     

Influence of Yttrium-Alloying Addition on the Oxidation of Alumina Formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into -Al₂O₃, leading to the formation of a more protective oxide scale.     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Growth of Nodular Corrosion Products on Fe–Al Alloys in Various High-Temperature Gaseous Environments

The mechanisms for nodular corrosion-product development were investigatedin various high-temperature gaseous environments. Fe–Al alloys, with5–20 wt.% Al, were exposed in both oxidizing and sulfidizing[p(S2)=10^–4 atm, p(O₂)=10^–25 atm] atmospheres at 700°Cfor times up to 100 hr. The corrosion kinetics were monitored by theuse of a thermogravimetric balance and the morphological developmentthrough light-optical and scanning-electron microscopies,energy-dispersive spectroscopy, electron-probe microanalysis,and quantitative-image analysis. Under both conditions, theelimination of nodule formation was observed by increasing thealuminum content of the alloy, above 5 and 7.5 wt.% Al for oxidizingand sulfidizing environments, respectively, which promoted the growthand maintenance of a continuous surface scale of alumina. For thosealloys that were observed to develop nodular corrosion products, theirmorphological appearance was similar in nature regardless of thecorroding species. The nodules typically consisted of an outeriron-rich product, either sulfide or oxide, that was randomly dispersedacross an alumina scale. Samples from the oxidizing atmosphere displayeda single growth-rate time constant from the kinetics data, suggesting thatthe nodule growth mechanism was by the simultaneous or codevelopment oftwo different (Fe and Al) oxides from the onset of exposure. Measurementof nodule planar diameter and depth of penetration into the alloyindicated that growth occurred through diffusional processes. Kineticsdata from the development of sulfide nodules in the reducingenvironment revealed a different type of mechanism. Multiplegrowth-rate time constants were found due to the localized mechanicalfailure of an initially formed surface scale. At early times in thesulfidizing atmosphere, a low corrosion rate was recorded as acontinuous-alumina scale afforded protection from excessive productdevelopment. However, with the mechanical failure of the scale, sulfurwas able to attack the underlying substrate through a short-circuitdiffusion mechanism that resulted in rapid weight gains from nonprotective,iron sulfide growth. The sulfide morphologies observed were very complex ascontinued growth of the nodule did not solely depend upon the diffusingspecies through the previously formed corrosion products, but also,continued mechanical failure of the oxide scale. It is suggested that thedifference in development mechanisms between the two environments may liein the relative growth rates of the nonprotective, Fe-base corrosionproducts formed.     

Oxygen solubility and a criterion for the transition from internal to external oxidation of ternary alloys

Smith's model is expanded in order to derive expressions to quantitatively describe the oxygen-solubility behavior in ternary alloys as a function of alloy composition. Multicomponent-diffusion theory is used to establish a criterion for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least-noble metal in a ternary alloy. The oxygen-solubility model and the criterion are applied to the oxidation of Ni–Cr–Al alloys in 76 torr of oxygen at 1100 and 1200°C, predicting the minimum Al concentrations required to form a protective Al₂O₃ scale. It shows that sufficient Cr additions would significantly reduce the oxygen solubility and also alter the oxygen distribution in the ternary alloys, avoiding the oxygen supersaturation necessary for the onset of internal oxidation. These two factors make it easier to establish the protective Al₂O₃ scale.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

Oxidation of γ-Ni−Al−Si alloys at 1073 K

The formation and development of oxides in Ni–4Al and Ni–4Al–xSi (at.%, x=1, 3, 5) alloys at 5–9×10^–6 and 1 atm oxygen pressure at 1073 K have been studied. The oxidation rate increased with an increase of silicon content in the alloy at the early stage of oxidation, but decreased after longer time exposure due to formation of an intermediate layer composed of NiO and spinel (NiAl₂O₄ and Ni₂SiO₄) between the top NiO layer and the internal-oxidation zone. This intermediate layer became a barrier for releasing stress, generated by the volume expansion associated with oxidation of solute atoms, resulting in high dislocation density and severe distortion in the internal-oxidation zone for the Ni–Al–Si alloys. In Ni–4Al alloy where no complete intermediate-layer formation occurred, stress was easily released by an enhanced vacancy gradient, and therefore an enhanced vacancy-injection rate into the alloy, resulting in a higher oxidation rate than the situation where a sample was oxidized at an oxygen pressure associated with the dissociation of NiO.     

The oxidation of two Fe−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Fe–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800°C under low oxygen pressures, similar to those prevailing in environments of the coal-gasification type. The reaction produced only an internal oxidation of niobium to form two niobium oxides (NbO₂ and Nb₂O₅) and in some cases a double Fe–Nb oxide. The kinetics of this reaction were very slow at 600°C but rather fast at 700 and 800°C. A peculiar feature of the internal oxidation of these alloys is that the distribution of the internal oxides follows closely that of the Nb-rich phase in the original two-phase alloys. This behavior, as well as the lack of formation of external scales of niobium oxides, is mainly a result of the limited solubility of niobium in iron and of the consequent presence of two metal phases in the alloys.     

The oxidation of Ni-Nb alloys at low-oxygen pressures at 600–800°C

The oxidation of two Ni–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600° C and 10^–20 atm O₂ at 700 and 800° C, these pressures being less than the dissociation pressure of nickel oxide. The scales formed on both alloys at 600 and 700° C show only a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb₂O₅ or/and NbO₂), which formed from both the metal phases present, i.e., Ni₈Nb and Ni₃Nb. Only small, or even no, Nb depletion was observed in the alloys close to the interface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800° C produced a continuous external scale of niobium oxides without internal oxidation. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Phase stability,oxidation, and interdiffusion of a novel Ni−Cr−Al−Ti−Si bond-coating alloy between 900 and 1100°C

A novel, low-expansion experimental Ni–Cr–Al–Ti–Si bond-coating alloy was investigated in the as-cast state concerning its phase stability, oxidation resistance in air, and interdiffusion with single-crystal IN-100 at 900, 1000, and 1100°C. Isothermal oxidative thermogravimetry was employed up to 500 hr. Interdiffusion was compared to a commercial Ni–Co–Cr–Al–Y alloy on IN-100. Oxidized Ni–Cr–Al–Ti–Si specimens and diffusion couples were characterized by metallography, SEM, EDX, XRD, and XRF. The Ni–Cr–Al–Ti–Si alloy provides good oxidation resistance in air at least up to 1000°C. The alloy is an alumina former. Due to its coarse microstructure, other oxides (e.g., rutile) may form and considerably dominate the oxidation behavior. The kinetics of oxidation were correlated with temperature, formation of phases, and morphology of oxides. Interdiffusion fluxes between Ni–Cr–Al–Ti–Si and IN-100 were mainly directed to the superalloy. They were faster than in Ni–Co–Cr–Al–Y/IN-100 diffusion couples.     

High-temperature oxidation kinetics of Cu-Si alloys containing up to 4.75 wt.% Si inp_{{\text{O}}_{\text{2}} } =0.01 atm and pure CO2

The oxidation of Cu-Si alloys (containing up to 4.75 wt. % Si) in =0.01 atm from 800 to 1000°C has been investigated using thermogravimetry and other techniques. A 0.04% Si alloy followed a parabolic oxidation law with a rate similar to that of pure Cu. As the Si concentration increased the rate decreased and became irregular owing to SiO₂ particles or flakes at the alloy-scale interface. It is considered that sintering of SiO₂ particles and rupture of the sinter because of contraction during sintering are responsible for the irregular kinetics. A SiO₂ layer forms directly on the 4.75% Si alloy which oxidizes uniformly. The SiO₂ was always amorphous. In pure CO₂ a similar pattern of amorphous SiO₂ particles, flakes, and layers occurs.     

Influence of aluminium content on retarding hydrogen transport in Fe–Al binary alloys

The electrochemical hydrogen permeation measurement was utilized to determine the effective diffusivity, permeation rate and apparent solubility of hydrogen in a series of Fe–Al binary alloys at 25°C. The experimental results reveal that both the hydrogen effective diffusivity and permeation rate in these alloys are significantly dropped when Al content is more than 15–20 at.%. The hydrogen apparent solubility in these alloys is reduced with the increment of Al content, even though their lattice parameters and strain fields in these alloys are increased. For these Fe–Al binary alloys, the retardation factor of hydrogen is mainly dependent upon Al content. Both oxide film and the degree of structural order are the major retardation factors on hydrogen transport in Fe–Al binary alloys.     

Phase stability and fcc/hcp martensitic transformation in Fe–Mn–Si alloys: Part II. Thermodynamic modelling of the driving forces and the MS and AS temperatures

This paper presents the second part of a study of the fcc/hcp relative phase stability and the fcc↔hcp martensitic transformation (MT) in the Fe–Mn–Si system. In part I, an experimental database was built up using dilatometric measurements, which covers the composition range in which the fcc↔hcp MT is detected. This new information is analysed in the present work using models for the molar Gibbs energy (G_m) of the various phases. Hcp is a metastable phase in the system, but we show that its properties can be inferred from selected pieces of experimental data, using the concept of T₀ temperature. The assessed G_m functions of the ternary fcc and hcp phases are used to evaluate the so-called resistance-to-start-the-transformation energy (RSTE). According to our results the RSTE in this system vary smoothly with composition, and we account phenomenologically for that using low-order polynomials in the atomic fractions. Finally, the optimum G_m and RSTE functions are also used to calculate the M_S and A_S temperatures for arbitrary compositions in the ternary system. The extensive comparisons between calculations and measurements presented in this work show a very good agreement, which adds to the credibility of the present approach.     

Observations on the oxidation behavior of Ni-Cr alloys containing minor additions of group III,IV, or V elements

The corrosion of a number of experimental ternary Ni-15 wt. % Cr-0.5wt. %X alloys (where X= Y, La, Ce, Sm, Th, U, Zr, or V) has been assessed in pure oxygen at 900° C for periods of exposure up to 450 hr, and compared with the behavior of an Ni-15% Cr control alloy. It has been established that while all of the alloys oxidize in accordance with a protective, approximately parabolic regime, considerable differences in oxidation rates are exhibited. In particular, additions of La, Ce, and Th bring about progressively enhanced rates of oxidation relative to the binary alloy, whereas Y, Sm, V, U, and Zr effect increasingly reduced rates of oxidation in the order given. The rate constant for the Th-bearing alloy is about two orders of magnitude larger than that for the alloy containing Zr. Such differences in behavior seem to be associated with subtle variations in the morphology/composition of the corrosion products from alloy to alloy. The observations are considered in the light of earlier published literature concerning the effects of rare-earth and other reactive elements on the oxidation behavior of Ni/Cr alloys.This work has been carried out with the support of Procurement Executive, Ministry of Defence.