Growth Mechanisms Of Oxide Scales Research Articles

Water-vapor-effect on the oxidation of Fe-21.5 wt.%Cr-5.6 wt.%Al at 1000°C

Fe-21.5 wt. %Cr-5.6 wt. %Al oxidation, at 1000°C, in dry or wet oxygen shows that steam has an influence on the oxide-scale growth mechanism. Steam modifies the kinetics of early-stage oxidation. In dry oxygen, an initial fast linear regime is observed during one hour. Under wet conditions, weight-gain curves follow the same parabolic regime over the entire oxidation test. The scale structure strongly depends on the presence of steam in the gaseous environment. With dry oxygen, the scale is composed mainly ofα-Al2O3 after the initial formation ofγ-Al2O3 identified by ESCA and RHEED. The kinetics transient stage corresponds to the necessary time for the internal part of the initialγ-Al2O3 scale to transform into a continuousα-Al2O3 diffusion barrier. Under wet oxygen conditions, transient oxides are identified as (Mg, Fe) (Cr, Al)2O4, MgAl2O4 (orthorhombic), Al2O3 (hexagonal), these oxides transform into MgAl2O4 (cubic), Cr3O4, Fe2O3,α-Al2O3, with time. When water vapor does not change drastically oxidation kinetics, the induced presence of iron and chromium in the oxide scale could be responsible for weakening the protectiveness of alumina scales.

Mathematical modelling of oxide growth mechanisms measured by 18O tracer experiments

A mathematical model was developed which allows the description of the growth mechanisms of oxide scales on high temperature alloys forming by oxygen grain boundary diffusion. The model uses a finite element technique both for the calculation of oxygen grain boundary diffusion as well as tracer exchange between grain boundary and oxide grains. For checking the validity of the model, FeCrAl-based ODS (oxide dispersion strengthened) alloys were oxidised at 1100°C in a two-stage oxidation technique using 18O tracer. Oxygen isotope distributions in the so formed alumina scales, determined by SIMS or SNMS, were compared with calculated profiles. It was found, that the model, which accounts for experimentally observed changes in oxide grain size as a function of scale thickness, is able to simulate the in-scale transport processes with satisfactory accuracy. Comparison of experimental with calculated data revealed, that the assumption of grain boundary oxygen diffusion in a pure alumina scale on Fe-based ODS alloys cannot be applied to scales with a thickness of less than about 0.7 μm. This is due to the fact that in such scales the alumina contains small amounts of Fe- and Cr-containing oxides. In all cases, also for thicker scales, a small amount of outward scale growth occurs due to yttrium and titanium transport through the alumina scale.

Analysis of composition and growth mechanisms of oxide scales on high temperature alloys by SNMS, SIMS, and RBS

Composition and growth phenomena of alumina scales on a conventional wrought alloy Fe-20Cr-5Al and an yttria containing ODS alloy with the same base composition were investigated. The alloys were oxidized in a twostage oxidation process at 1000 °C in air and air containing18O-tracer. By using secondary neutrals and secondary ion mass spectrometry in combination with RBS, the in-depth concentration of main and minor alloying elements as well as the oxygen isotope distribution in the alumina scale could be determined. It is shown, that the difficulties in deriving the exact growth processes of the scales from the oxygen tracer profiles can be overcome by using the time dependance of the iron and chromium distribution in the alumina. From these results it could be derived that growth of the oxide scales formed on the ODS and the conventional alloy mainly differ in the relative contribution of aluminium diffusion. The mechanisms which lead to these differences in scale growth phenomena will be discussed.

Growth mechanisms of oxide scales on ODS alloys in the temperature range 1000–1100°C

AbstractAfter a short overview of the production, microstructure and mechanical properties of nickel‐ and iron‐based Oxide Dispersion Strengthened (ODS) alloys, the oxidation properties of this class of materials is extensively discussed. The excellent oxidation resistance of ODS alloys is illustrated by comparing their behaviour with conventional chromia and alumina forming wrought alloys of the same base composition.ODS alloys exhibit improved scale adherence, decreased oxide growth rates, enhanced selective oxidation and decreased oxide grain size compared to corresponding non‐ODS alloys. It is shown, that these experimental observations can be explained by a change in oxide growth mechanism. The presence of the oxide dispersion reduces cation diffusion in the scale, causing the oxides on the ODS alloys to grow mainly by oxygen grain boundary transport. As oxide grain size increases with time, the oxide growth kinetics obey a sub‐parabolic time dependence especially in the case of the alumina forming iron‐based ODS alloy.

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C, Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr2O3 (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr2O3 by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr2O3 scale (in Ni-34Cr alloys) mainly as S2− species, which at high temperatures increases the V‴Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr2O4, Cr2O3 oxide multilayers (in a Ni-20Cr alloy), sulphur in the S2− state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.

Some conclusions on the 2nd international symposium on high-temperature corrosion, Les Embiez, May 22–26, 1989

It would be too ambitious to give an overview of all the ideas and results presented by authors of 145 oral communications and posters. Instead, I will attempt to answer the question, What are the main trends in high-temperature corrosion studies pursued by the many experts in the world? The answer will be useful for developing new research toward the goal of improving the performance of materials in hot gaseous environments. A division of these 145 oral communications and posters into the ll topics of this 2nd International Symposium on High-temperature Corrosion of Advanced Materials and Coatings shows the present research emphasis (see Table 1). In addition to the papers shown in Table l, two plenary lectures were presented concerning topics 1 and 5. The majority of papers were presented for topics l, 2 and 4. The next most prevalent topics were 3 and 5-7. In sharp contrast, topics 8 and above all 9 attracted a minimum of authors. Similarly, topic l0 was poorly represented despite the effort of the Advisory Scientific Committee to enlarge the field of hot gaseous corrosion for different systems producing energy, such as thermal and nuclear plants. In fact, most of the papers treated problems of the aerospace industry. A round table or a panel discussion at future conferences may promote a useful discussion among different experts in the chemical, nuclear and metallurgical industries. The first plenary lecture of G. H. Meier presented a review of the recent advances in fundamental research on high-temperature corrosion. The transition from internal to external oxidation of binary alloys and the effects of transient oxidation and third-element additions was considered. The author showed that the classic concept for calculating the critical solute content to form an external oxide scale must be modified for alloys in which the thermodynamic stability of the lowest oxide of the matrix element approaches that of the solute oxide and/or the oxygen diffusivity in the alloy is very small. Recent developments in the understanding of the hightemperature corrosion of alumina-forming alloys were also reviewed. The second plenary lecture of R. Mevrel was a critical and prospective review of high-temperature corrosion-resistant coatings. Four main aspects were developed. The first analyzed the degradation mechanisms of protective metallic coatings at high temperatures. The understanding of this degradation requires more fundamental studies on the growth mechanism of oxide scales and on the role of minor addition elements such as palladium and platinum. The second problem concerns the mechanical properties of coated systems which determine the adhesion of a coating on the substrate. The third point concerns the present trend to use ceramic coatings in turbine engines. For about 15 years these ceramic coatings were successfully used on sheet metal combustor components. However, two problems must be solved to exploit these ceramic coatings on turbine blades; their reliability and their hightemperature corrosion resistance in isothermal, and above all, thermal cycling conditions. The fourth part of Dr. Mevrel's paper compared alternative techniques for depositing MCrAIY coatings; electrolytic codeposition, and electrophoresis, which would permit a more economical deposition of MCrAIY coatings. In addition to both of these principal lectures which emphasized the need to pursue certain fundamental research efforts, I would like to emphasize three other fields of fundamental interest. The first is the continued study of the beneficial

State of the art on high-temperature corrosion-resistant coatings

A high-temperature protective coating must meet several criteria: provide adequate environmental resistance, be chemically and mechanically compatible with the substrate, be applicable. Comprehensive reviews on high-temperature coatings have appeared regularly since the early 1970s. Our purpose is not to recapitulate the material covered therein but rather to focus on recent trends, and point out some research perspectives. Historically the development of high-temperature protective coatings has been linked with the evolution of demanding applications such as super-alloy components in gas turbine engines; the searches for better performance (higher inlet temperatures, longer lifetimes, etc.) and for cost-saving solutions (use of contaminated low-grade fuels) have been the main incentives for developing the different coatings now available in production: simple and “modified” diffusion coatings, overlay coatings, thermal barriers. In recent years research and development activity has been concentrating on the following points. 1. (a) Degradation mechanisms in high-temperature corrosion of metallic coatings; basic studies on the growth mechanisms of oxide scales, for example are still required, in particular to understand the role of addition elements such as platnum and palladium. 2. (b) Alternative techniques for depositing MCrAlY coatings; electrolytic codeposition and electrophoresis, for example, have been developed at the laboratory stage and these permit the deposition of MCrAlY coatings with claimed economic and technical advantages over processes already in production. 3. (c) Thermal barrier coatings; ceramic coatings have been applied to sheet metal combustor components for about 15 years; only recently have they been used in the turbine section. Two challenges remain though to exploit these coatings on turbine blades: improve their reliability and, in the case of stationary gas turbines, their hot corrosion resistance. Both structural and mechanical approaches are required to determine, in particular, the role of microstructure, microcracking, porosity, residual stresses, oxidation of the bond layer in the degradation mechanisms of these coatings. 4. (d) Mechanical properties of coated systems; the intrinsic mechanical properties of coating materials are still poorly described and the lack of information hinders the adequate modelling of the behaviour of coating-substrate composite systems. In parallel, an increasing activity is noted concerning the design and development of high-temperature coatings for protecting materials other than superalloys, for instance ceramic composites and titanium-based alloys.

Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys

The oxidation behavior in air of two ODS alloys, MA 754, a chromia former, and MA 956 an alumina-scale former, has been compared with that of conventional wrought model alloys with similar compositions. The main effects on scale properties of both oxide types due to oxide dispersions were found to be improved adherence, decreased growth rates, and enhanced selective oxidation. In addition to metallography, X-ray diffraction, energy dispersive X-ray analysis of the scales, and studies of scale morphology, the detailed growth mechanisms of the oxide layers were studied using an18O tracer technique. The results show that the oxides on the conventional alloys grow by both metal and oxygen transport, and that the addition of oxide dispersions suppresses the outward scale growth. This change in growth mechanism is a possible explanation for the observed improved scale adherence, decreased growth, and enhanced selective oxidation in the yttria-containing alloys.

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.

The use of 18O as a tracer to study the growth mechanisms of oxide scales

A nuclear technique for the determination of 18O concentration as a function of depth is described. Examples are given of the application of this technique to studies of growth mechanisms of oxide scales.

The use of 18O as a tracer to study the growth mechanisms of oxide scales☆

The use of 18O as a tracer to study the growth mechanisms of oxide scales☆