Oxide-alloy Interface Research Articles

The effect of ceria coatings on the high-temperature oxidation of iron-chromium alloys

The effect of sputtered ceria coatings (0.2–30 nm thick) on the oxidation of Fe25Cr alloys at 1100 °C has been studied using the 18O/secondary ion mass spectrometry technique. Samples were oxidized either in 16O 2 only or sequentially first in 16O 2 and then in 18O 2 for periods up to 20 h. Oxidation rates were reduced with increasing ceria thickness up to 4 nm; there was no significant further reduction in rate for thicknesses of 4 nm or more. The secondary ion mass spectrometry sputter profiles indicated that scales were mainly Cr 2O 3; iron was present in the outer part of the oxide layer at levels of 1% or less. With increasing ceria coating thickness the maximum in the cerium sputter profile moved from the vicinity of the alloy-oxide interface towards the gas-oxide interface. When Fe25Cr was sequentially oxidized, 18O was found to have diffused towards the alloy-oxide interface. The secondary ion mass spectrometry data suggest that when ceria is located within the scale there has been a change in mechanism from predominantly cation to predominantly anion diffusion.

High-temperature oxidation of ZrAl

The oxidation of ZrAl in dry oxygen in the temperature range 800–1100 K follows a parabolic rate law. An activation energy of 86.4 kJ mol −1 was estimated. During the oxidation, aluminium diffuses from the oxide layer to the ZrAl bulk, thus forming a ZrAl 2 phase very close to the oxide-alloy interface. The excess zirconium is therefore selectively oxidized, yielding tetragonal mainly and monoclinic ZrO 2 containing no Al 2O 3.

Adherence of oxide scale formed on Ni20Cr1Si alloys with small additions of rare earth elements

The isothermal and cyclic oxidation behaviour of Ni20Cr1Si (where the composition is in approximate mass per cent) alloys with small additions of rare earth elements has been studied in air over the temperature range 1273–1523 K. The rate of oxidation was reduced by the addition of cerium. In most alloys except the alloy with the addition of 0.34 mass% Ce, the oxide scales began to spall off at about 1000 K during cooling; this was caused by the difference between the thermal contraction of the alloy and that of the oxides. The scales severely spalled again at around 500 K in the alloys oxidized above 1473 K. In these alloys a continuous layer of cristobalite was formed beneath the oxide scales. The spalling is probably due to the stress caused by the displacive transformation of cristobalite which involves a substantial volume change. The best improvement in the scale adherence was attained by the addition of 0.34 mass% Ce even in the cyclic oxidation. In this alloy, SiO 2 is formed as cristobalite in a spherical shape beneath the protective Cr 2O 3 layer. The stress generated by the transpormation of cristobalite is relaxed by the deformation of the alloy adjacent to the spherical SiO 2. The scale adherence was also improved by the superficial application of CeO 2 powder. The suppression of spalling was also observed in the alloy with the addition of yttrium, lanthanum or erbium. The good adherence of the scales is due to the prevention of void formation at the alloy-oxide interface and to the keying-on structure of scales.

High Temperature Oxidation of the Austenitic Fe–9Al–30Mn–1.0C and Duplex Fe–10Al–29Mn–0.4C Alloys

The oxidation behaviors of the austenitic Fe–8.7Al–29.7Mn–1.04C and duplex Fe–10Al–29Mn–0.4C alloys in air at elevated temperatures have been investigated by the X-ray diffraction and metallographic techniques. The results showed that the oxide nodules were formed on the surface of both alloys at 1073 and 1273 K during the early stage of oxidation. These oxide nodules grew and eventually coalesced to form continuous duplex scales. The composition and structure of the duplex scales depended on the oxidation temperature and alloy composition. For the duplex Fe–10Al–29Mn–0.4C alloy oxidized in air at 1073 K for 259.2 ks (72 h), the duplex scales consisted of an outer (FeMn)2O3 layer and an inner α-Al2O3 layer at the oxide-alloy interface. However, oxidation of the duplex Fe–10Al–29Mn–0.4C alloy at 1273 K for 259.2 ks led to the formation of an external (FeMn)3O4 and an inner α-Al2O3 layers with some internal oxides precipitated within the austenitic phase. Furthermore, the SEM micrographs showed that the oxide growth pits were formed on the austenitic Fe–8.7Al–29.7Mn–1.04C alloy after exposure in air at 1273 K for 259.2 ks. However, oxide pyramids were observed to grow on this alloy as the temperature was increased to 1473 K.

Interaction of Zr 2Al with oxygen at high temperatures

The oxidation of Zr 2Al in dry oxygen in the temperature range 730–875 K follows a parabolic rate law. An activation energy of 371.5 kJ mol −1 is estimated. The oxidation results in the formation of the following sequence of layers: tetragonal ZrO 2, monoclinic ZrO 2, Zr 2Al and tetragonal Zr 5Al 3. During oxidation aluminium diffuses from the oxide layer into the Zr 2Al bulk, thus forming a Zr 5Al 3 phase at some distance from the oxide-alloy interface. The excess zirconium is therefore selectively oxidized, yielding monoclinic and/or tetragonal ZrO 2 containing no alumina.

Analytical electron microscopy investigation of the oxide scale on an yttrium-implanted Ni-20wt.%Cr alloy

Analytical electron microscopy has been used to investigate the influence of yttrium ion implantation on the oxidation resistance and scale adherence on Ni-20wt.%Cr alloys oxidized in dry air at 1000°C. A multilayer scale consisting of NiO, NiCr 2O 4 and Cr 2O 3 layers is observed on both unimplanted and yttrium-implanted specimens. The yttrium is concentrated in the Cr 2O 3 layer near the alloy interface and the Cr 2O 3 has much finer grain size in the implanted alloy. The effects of yttrium may be explained by a modification of the growth mechanism caused by a decrease in the cation diffusion rate through Cr 2O 3 in the presence of yttrium and by enhanced adhesion of the scale due to an increase in bond strength at the oxide-alloy interface.

Selective oxidation of zirconium in Zr3Al

Abstract The oxidation of Zr3Al in oxygen at temperatures ranging from 740 to 1050 K results in the formation of the following sequence of layers: cubic ZrO2, monoclinic ZrO2, Zr3Al and Zr2Al. During the oxidation, aluminium diffuses from the oxide layer into the Zr3Al bulk, thus forming a Zr2Al phase at some distance from the alloy-oxide interface. The excess zirconium is therefore selectively oxidized, yielding monoclinic and/or cubic ZrO2 containing no alumina.

Fe-26Cr-21Ni-1.8Si合金の高温酸化挙動に及ぼすY添加および合金表面に付着させたY<SUB>2</SUB>O<SUB>3</SUB>の影響

The isothermal and cyclic oxidation behavior of Fe-26%Cr-21%Ni-1.8%Si alloys with and without superficial Y2O3 powder application as well as similar alloys containing 0.02% and 0.34%Y was examined in air at 1100°C.The oxide scales on the surface of yttrium-containing alloys consisted of (Cr, Fe)2O3, and had excellent adherence to the alloy substrate even in cyclic oxidation. No void formation was observed at the alloyoxide interface, and no silicon-rich inner layer was developed. Such effects of yttrium addition were recognized to be significant even in a fairly early stage of oxidation.The superficially applied Y2O3 powder on the alloy improved the scale adherence, and provided the oxidation morphology similar to that of the yttrium-containing alloy. The effect of Y2O3 powder was also observed in preoxidized specimens.

The oxidation of incoloy 800 in moist air containing hcl(g) at 800°C

The oxidation of a commercial high nickel alloy, Incoloy 800 (32Ni-21Cr-46Fe) has been studied in a slowly-flowing environment of air containing 500 vpm HCl (g) and 13,000 vpm H 2O (g) at a working temperature of 800°C for times up to 250 h. Severe spalling of the oxide corrosion products were observed upon cooling and this behaviour was compared to that observed after oxidation in HCl-free moist air. The effect of temperature cycling between the working temperature and ambient was also studied. Gravimetric, metallographic, electron probe micro-analysis and X-ray diffraction results are presented along with observations of morphological detail revealed using scanning electron microscopy. The results indicate that apart from having a serious effect on oxide adhesion, the presence of a halogen-rich region at the alloy-oxide interface significantly affects the transport of metal ions, principally chromium, iron and titanium to the oxidising surface.

Characterization of the oxide-metal interface at the surface of a stainless steel

Abstract Abstract The alloy-oxide interface from 20 at.% Cr-25 at.% Ni-1 at.% Nb stabilized steel oxidized at 850°C in CO2/l% CO has been examined on both sides using scanning Auger electron spectroscopy. The interface which was exposed using a sputter ion-plating technique consists of a silicon-rich chromium sesquioxide. When the oxide separates it does so at this interface within the silicon-rich layer.

The oxidation behavior of the nimonic alloy PE16 in carbon dioxide at 700?800�C

The oxidation behavior of the nimonic alloy PE16 in carbon dioxide has been examined, at 700–800° C, for periods up to 10,250 hr duration. At all temperatures the oxidation kinetics were pseudoparabolic. The chromium-rich and titanium-bearing oxide scale was adherent, except at 800° C, when ∼10% spalled. Intergranular oxidation beneath the outer scale resulted in the formation of alumina and to a lesser depth, titanium oxide. The penetration increased parabolically with time and also with temperature, the activation energy being 50 kcal/mole. After oxidation at all temperatures the carbon profiles across the oxidized alloys were determined by nuclear microprobe analysis and indicated three distinct regions. From the gas interface carbon was picked up increasingly in the oxide scale, with a peak concentration (0.1–0.34 wt. %) at the oxide-alloy interface. The carbon level then fell sharply and to the depth of the titanium-bearing intergranular oxide the alloy was decarburized. At this juncture carbon had entered the alloy to a maximum concentration of 0.23–0.50 wt. % and a depth which increased both with temperature and exposure. Carburization is attributed to a crevice corrosion mechanism.

The early stages of development of ?-Al2O3 scales on Fe-Cr-Al and Fe-Cr-Al-Y Alloys at high temperature

The development ofthe oxides on Fe-14%Cr-4%Al, Fe-27%Cr-4%Al, and similar alloys containing 0.008% Y, 0.023% Y, and 0.8% Y has been investigated during the early stages of oxidation in 1 atm oxygen at 1000 and 1200°C. In all cases, a steady-state α-Al2O3layer is established rapidly, after some initial formation of transient oxides rich in iron and chromium. For the yttrium-free alloys the steady-state situation is achieved more rapidly for the higher chromium-containing alloy and at the higher temperature. The amount of transient oxide formed is also determined by the specimen surface topography since the development of the α-Al2O3 layer is less rapid at the base of alloy asperities than at a flat alloy-oxide interface. Following establishment of the complete α-Al2O3layer, the oxide develops a convoluted oxide morphology at temperature, due to high compressive growth stresses in the oxide. These arise following reaction between oxygen ions diffusing inward down the oxide grain boundaries and aluminum ions diffusing outward through the bulk oxide. This results in lateral growth of the oxide and plastic deformation and movement of the alloy in a direction parallel to the alloy-oxide interface. The addition of yttrium to the alloys promotes the selective oxidation of aluminum. Also, the yttrium is incorporated into the growing oxide where it changes the mechanism of growth, reducing the production of the high compressive growth stresses and thus the development of the convoluted oxide morphology.

Magnetic, surface, and corrosion properties of Ni-Fe-Pd thin films

Films in the Ni-Fe-Pd ternary system were evaporated and characterized with regard to magnetic properties, resistance to atmospheric corrosion, and surface composition. It was found that Pd behaves like Ni magnetically and results in little magnetic dilution. High moments can be maintained at high Pd concentrations. Low coercive forces exist in a trough from Ni0.8Fe0.2 to Fe0.7Pd0.3 in the ternary composition field. The magnetoresistance was found to be significantly less than for Ni0.8Fe0.2 (Permalloy) in all other composition regions. Pd addition at low concentrations caused the alloy atmospheric corrosion to increase and then decrease for higher Pd addition. The surface of these alloys is enriched in Fe and O while the oxide-alloy interface shows significant Pd enrichment. The oxide thickness increases with the addition of Fe.

Failure and oxidation phenomena in Fe-35Ni-18Cr-2Si heating-element wire

The oxidation behavior of an Fe-35Ni-18Cr-2Si alloy commonly used as a resistance heating-element material has been studied in the temperature range 1000–1300°C. Techniques employed included thermogravimetry at constant temperatures, measurements of chemical changes in the protective oxide film, and metallography. The oxidation behavior was found to approximate a parabolic law and data obtained gave an estimate of the activation energy for oxidation of 125 kJ/mole (30 kcal/mole) at around 1000°C and 339 kJ/mole (80 kcal/mole) at higher temperatures. The concentration of elements in the protective oxide was found to be both temperature and time dependent. Chromium became concentrated at the expense of nickel and iron at the lower temperature ( 1000° C) were found to have oxidized to the same depth. This can be explained on the basis of a depletion of chromium having occurred at the oxide-alloy interface, and that on scaling, insufficient chromium was present to reform a protective film.

The annealing and re-oxidation of oxidized Cu [sbnd]Ni alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu 10%Ni and Cu 24%Ni at 800°C and Cu 80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques. Oxidation of Cu 10%Ni and Cu 24%Ni produces external scales consisting of a thick, inner Cu 2O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu 80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu 2O and oxygen. With Cu 80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface. In Cu 10%Ni and Cu 24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu 2O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu-80%Ni, although to a lesser extent than for the copper-rich alloys. During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu-80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.

The annealing and re-oxidation of oxidized Cu [sbnd]Ni alloys

A study has been made of the effects of an intermediate, isothermal annealing treatment in argon on the oxidation kinetics in dry oxygen of Cu 10%Ni and Cu 24%Ni at 800°C and Cu 80%Ni at 1000°C using a semi-automatic microbalance. Changes in scale morphology and composition have been investigated using various techniques. Oxidation of Cu 10%Ni and Cu 24%Ni produces external scales consisting of a thick, inner Cu 2O layer and a thin, outer CuO layer, together with nickel-rich internal oxide in the adjacent alloy. Oxidation of Cu 80%Ni yields an external scale consisting of a thick, inner NiO layer and a thin, outer CuO layer, together with a few nickel-rich internal oxide particles in the adjacent alloy. During annealing, the outer CuO layers on the three alloys dissociate to give Cu 2O and oxygen. With Cu 80%Ni only, further dissociation to give copper metal takes place after long annealing times. In all cases, the annealing treatment leads to a reduction in the cation vacancy gradient across the scale and a reduction in the total vacancy level in the scale due to the reduction in oxygen activity at the oxide-gas interface. In Cu 10%Ni and Cu 24%Ni, internal oxide formation during annealing stimulates dissociation of the Cu 2O scale near the oxide-alloy interface to give copper metal and oxygen. Similarly, internal oxide formation stimulates dissociation of the NiO layer on Cu 80%Ni, although to a lesser extent than for the copper-rich alloys. During re-oxidation, the kinetics are partly determined by the extent of scale thinning during the prior annealing treatment, giving more rapid weight gains than expected from those recorded during the initial oxidation period. Nevertheless, particularly for Cu 80%Ni, where the scale thinning is relatively small, the reduction in the cation vacancy gradient across the scale and the reduction in the total vacancy level in the scale do result in a reduced oxidation rate compared with the rate recorded during the initial oxidation period.

Ni-20Cr合金の高温酸化におよぼす酸素分圧の影響

The Ni-20Cr alloy was oxidized at 1100°C at various oxygen partial pressures, ranging from 10−12 to 10−1 atm, which were controlled by Ar-O2 mixtures. A large difference was observed between the oxidation behavior in a high PO2 (≥10−3 atm) range and that in a low PO2 (≤10−5 atm) range. Therefore, the detailed experiments were carried out in PO2 of 10−1 and 10−5 atm, the former being typical in high PO2 and the latter in low PO2. The weight gain-time curves showed that the growth rate of scale in low PO2 lay between the parabolic and linear law, while that in high PO2 was close to the parabolic law, which indicates that the scale formed in low PO2 is less protective than that in high PO2. It was found, on the other hand, that the oxide scales formed in low PO2 were more adherent to the alloy than those formed in high PO2. In high PO2 a large amount of spalling was observed on cooling even in short time oxidation. The oxide scales formed in low PO2 was uniform in thickness and rather porous, while those formed in high PO2 was non-uniform in thickness and dense. In low PO2 there were found a number of small voids at the oxide-alloy interface, but in high PO2 large voids were found, which suggest the occurence of plastic deformation of oxide and alloy by induced stress in the growing scale. The above difference in oxidation behavior can be ascribed to the difference in growth mechanism of oxide scale due to the high and low PO2.

Breakaway oxidation of Fe10%Cr and Fe20%Cr at temperatures up to 600°C

In the oxidation of Fe10%Cr and Fe20%Cr in air at 600°C, nucleation is followed by a period when growth rates are extremely low, and this ends in a transition to a fast linear rate. Microstructural and micro-analytical studies of the thin protective oxide (2–50nm) indicate that it is a duplex layer of Fe 2O 3 and M 3O 4 of rather variable thickness and composition. The heterogeneity of the oxide layer results from differences in diffusion rates in both the oxide and alloy. The transition to higher oxidation rates arises because of a build-up of compressive growth stresses which disrupts the protective layer. Protection cannot be re-established because during failure the alloy close to the alloy-oxide interface is deformed, causing growth of a non-coherent chromium-containing oxide.

The influence of yttrium additions on the oxide-scale adhesion to an iron-chromium-aluminum alloy

The oxidation behavior of an Fe-27%Cr-4%Al alloy and similar alloys containing 0.023% and 0.82% Y in 1 atm oxygen at 1200°C has been examined. The oxide formed on the yttrium-free alloy develops a highly convoluted configuration, apparently resulting from lateral growth of the oxide. The latter leads to oxide detachment from the alloy at temperature and extensive spalling during cooling. It is postulated that lateral growth results from the formation of oxide within the existing oxide layer by reaction between oxygen diffusing inward down the oxide grain boundaries and aluminum diffusing outward through the bulk oxide. Additions of yttrium to the alloy apparently prevent the formation of oxide within the oxide layer, the oxide-forming reaction occurring as the alloy-oxide interface. Thus lateral growth is prevented and spalling during cooling does not occur. Secondary advantages conferred by the addition of 0.82% Y to the alloy are the prevention of void formation at the alloy-oxide interface, the avoidance of alloy grain growth during oxidation, and the creation of an oxide “keying” or “pegging” effect.

Auger analysis of thin oxide films on Pb–In alloys

The composition and stability of thin oxide films (?35 Å), thermally grown on Pb–In alloy substrates at room temperature and various oxygen pressures, were studied quantitatively by Auger electron spectroscopy. The results indicate that all oxide films are enriched in indium in varying amounts, depending on the alloy composition and oxygen pressure. For alloys contaning more than 10 at.% In, the surface layer of the oxide consists of pure In2O3 only; for alloys of lower In concentrations a mixture of PbO and In2O3 is formed. The mixed oxide appears to be unstable since the Auger data monitored during vacuum storage are indicative of the reduction of PbO with attendant formation of additional In2O3 occuring near the alloy–oxide interface.