Scale Growth Mechanism Research Articles

Improved High Temperature Oxidation Behaviour of AISI 347 Grade Stainless Steel by Superficial Coating of CeO2.

Influence of superficially applied CeO2 coating on the high temperature oxidation behaviour of AISI 347 grade stainless steel under different rates of heating followed by isothermal holding at 1273 K in dry air is reported. The reactive oxide coating not only reduced the oxidation rate but also facilitated imporved oxide scale adherence to the alloy substrate. The coated steel could withstand a number of thermal cyclings without scale rupture and exhibited further reduction in rate in sequential exposures. Reduced reaction rate has been attributed mainly to a change over in the rate controlling process from outward migration of cations to the ingress of oxidant species. Improved scale adhesion is due to change in chemical composition and grain size of the resultant oxide scale promoted by the formation of silicon-oxide stringers into the base alloy favouring pegging mechanism. Scale growth mechanism has been substantiated by post oxidation analyses using SEM, EDS, EPMA and XRD which could illustrate the role of reactive oxide coating and the formation of various cmplex oxide phases along with NiNb2O6.

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation: effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (18O) and aluminium (26Al) self-diffusion coefficients in Al2O3 were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped α-alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr2O3.

High-Temperature Oxidation of Stainless Steels

The first stainless steels, mainly low carbon chromium-iron alloys, have been known since the beginning of this century. These steels show good resistance against wet corrosion and high-temperature corrosion. This article focuses on high-temperature corrosion, with emphasis on gaseous sulfidizing and oxidizing environments. The discussion is limited to these two gases since corrosion involving halogen-and/or carbon-containing gases involves other specific processes. The behavior of binary and ternary alloys will be successively examined, then the role of minor elements will be considered.Fundamental Mechanisms of High-Temperature Corrosion of Stainless SteelUsually, a dry corrosion process results in the formation of corrosion products, giving a simple or complex oxide or sulfide scale on a metallic substrate, separating it from the aggressive gaseous environment and, consequently, acting as a protective barrier. Scale growth is controlled by the conductivity of the reaction products which are solid electrolytes. Generally, the mechanism of scale growth is governed by outward cation or inward anion diffusion processes. This is the basis of the model originally put forward by Wagner for a single metal and subsequently developed for alloys, and particularly, for stainless steels. This one-way point-defect diffusion process is responsible for the observed parabolic scaling kinetics characterized by a parabolic rate constantkp. This model is well described in the literature.In the case of stainless steels, formation of a protective scale is required; this is possible if the oxide or sulfide products have a low diffusivity to cations or anions due to a low density of point defects in the crystal lattice. The protective characteristics of the corrosion products may be experimentally determined by measurement of their electrical conductivity, although the scales should also be effective against short-circuit transport of ions, atoms, or molecules. The best barriers consist of oxides, such as Al2O3, SiO2, and Cr2O3.

Limitations on the Use of Surface Doping for Improving High-Temperature Oxidation Resistance

For more than 50 years, scientists have studied the “magic dust” of high-temperature oxidation—certain oxygen active or “reactive” elements which, when added to alloys in small quantities, effect profound improvements in their oxidation resistance. In general, high-temperature oxidation resistance is achieved by the oxidation of one or more alloy components to form a dense, stable, slow-growing, external oxide layer, or ’scale” such as α-Cr2O3, α-Al2O3, or SiO2. When added properly, reactive elements have a beneficial effect on the formation and growth of both α-Cr2O3and α-Al2O3scales. A standard list of reactive element (RE) effects would include: (1) an improvement in scale adhesion or resistance to spallation, (2) a change in the scale growth mechanism, (3) a reduction in the oxidation rate, related to the change in mechanism, (4) a modification in the scale microstructure, and (5) in the case of alloys that form Cr2O3scales, an improvement in selective oxidation, meaning that a lower Cr concentration in the alloy is required to form and maintain an external Cr2O3scale.

On the growth mechanism of the sulphide scale on amorphous AlMo alloys

The growth mechanism of the sulphide scale on amorphous Al-Mo alloys has been studied using a marker technique. A very thin (5 nm) gold marker film was deposited onto the surface of the Al-46Mo alloy and after sulphidation for 5 hours at 1073 K the gold marker distribution in the scale has been determined by RBS. It has been found that the main part of the marker remained at the scale surface indicating that the double-layer sulphide scale on the discussed alloy is growing by the inward diffusion of sulphur.

Distribution and transport of yttrium in alumina scales on iron-base ODS alloys

The oxidation behaviour of yttria-containing ODS alloys of the type MA 956 and PM 2000 was investigated at 1100 and 1200°C. Main emphasis was placed on studying the growth mechanism of the protective alumina scales and the distribution and transport of yttrium in these scales. For this purpose alumina scales formed during long time air exposure as well as during two-stage oxidation using 18O-tracer were analysed by SIMS and TEM. It was found, that the yttrium, which is initially present in the alloy in form of a thermodynamically stable yttria dispersion, is incorporated in the alumina scale where it forms precipitates on the grain boundaries and diffuses to the oxide surface. This behaviour of the yttrium is believed to be responsible for the modification of the growth phenomena of the oxide scales compared to those of yttria-free alloys. Large yttria containing dispersions, which are sometimes present in the bulk alloy due to improper mechanical alloying, are incorporated in the alumina scale and have a negative effect on long time scale integrity.

A study of the growth mechanism of the chromia scale formed on nimonic 86

The growth mechanism of the chromia scale formed in oxygen at 1 atm in the temperature range 785–985°C on Nimonic 86 (24.7% Cr, 10.2% Mo, 0.019% Ce, 0.011% Mg, 0.051% C, bal. Ni) has been studied using oxygen-18 as a tracer. The oxidation times ranged from 184 to 291 min. The location of the oxygen-18 was determined by means of charged-particle nuclear techniques. The scale grew primarily by oxygen transport, though the occurrence of scale buckling indicates that there was some cation diffusion. The cerium content of the scale was much greater than that of the alloy; as there is an optimum cerium concentration for maximum scale adhesion it is suggested that the scale buckling will expose alloy which contains less than this optimum amount.

Analysis and modelling of transport processes in alumina scales on high temperature alloys

The oxidation behaviour of a number of FeCrAl-based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 1100°C and 1200°C. The main emphasis was placed on the effect of alloy yttria content on scale composition and structure. Studies of three model alloys with various alloy yttria contents in the range 0.02–0.7% showed an increase in the oxide growth rate with increasing yttria content. Alumina scales on alloys with 0.17% and 0.7% yttria grow almost exclusively by oxygen diffusion, whereas the scale on the alloy with 0.02% yttria exhibits a significant contribution from cation diffusion. By using SIMS depth-profiling and SIMS-imaging it was found that the oxygen diffuses through the alumina scale via oxide grain boundaries. TEM-studies revealed that the yttria is incorporated in the scale grain boundaries as precipitations and as a segregation layer. This behaviour of the yttrium is believed to be responsible for its influence on scale growth mechanisms. A mathematical model has been developed which describes the alumina scale growth by oxygen diffusion through grain boundaries. The model accounts for the observed increase in aluminium oxide grain size in scale growth direction.

Transport properties of alumina scales on the β-NiAl intermetallic

The effect of implanted yttrium on the growth mechanism of alumina scales on β-NiAl intermetallic was studied as a function of temperature (1273–1473 K) using a two-stage oxidation method with 18O oxygen isotope as a tracer. The distribution of the tracer across the scale was determined by secondary ion mass spectrometry (SIMS) and the scale morphology by scanning electron microscopy (SEM). It has been shown that the presence of yttrium in the surface layer of the substrate strongly influences the transport processes in the growing scale by suppressing the inward diffusion of oxygen.

Kinetics and Mechanism of Telluride Scale Growth on Iron.

The reaction rates between iron and tellurium were measured in the temperature range of 873~1,023 K at the tellurium vapor pressure of 350 Pa and in the tellurium vapor pressure range of 66.7~1,000 Pa at 923 K. The electron probe micro-analyzer, marker experiment and X-ray diffractometer were used to clarify the mechanism of the telluride scale growth. The reaction rate between iron and tellurium obeyed the parabolic rate law and the platinum marker was observed at the phase boundary between iron and telluride scale. The telluride scale consisted of an inner and an outer layers; the former was β-iron telluride grown in parallel with a- and b-axes, and the latter consisted of δ-iron telluride above 980 K and δ- and δ′ -iron tellurides below 980 K. By comparing the activation energy for the reaction between iron and tellurium at constant tellurium vapor pressure with those for the diffusion of iron in β δ- and δ′-iron tellurides at constant composition, it was found that the rate-determining step of the rea...

High Temperature Corrosion of Iron Chromium Alloys by Tellurium.

The reaction rates between iron-chromium alloys (1.17, 5.65 and 11.96 a/0Cr) and tellurium were measured in the temperature range of 8731, 023 K at 350 Pa of tellurium vapor pressure, and in the tellurium vapor pressure range of 66.7600 Pa at 923 K. The electron probe microanalysis, marker experiment and X-ray diffractometry were employed to clarify the mechanism of the telluride scale growth. The reaction rates between iron-chromium alloys and tellurium obeyed the parabolic rate law. It was observed that the telluride scale consists of the inner, the intermediate and the outer layers ; chromium is concentrated in the inner layer which may grow by the inward diffusion of tellurium ; the intermediate layer consists of mainly β-iron telluride, and the outer layer consists of both δ- and δ'-iron tellurides below 980 K, and δ-iron telluride alone above 980 K. The intermediate and outer layers grow by the outward diffusion of iron in iron tellurides. The protection effect by chromium may be explained by the fact that chromium telluride covered over the surface of iron-chromium alloy interrupts the outward diffusion of iron. The reaction between iron-chromium alloy and tellurium can be classified into three regions from the view point of the activation energy. From the comparison of the activation energy of the reaction between iron-chromium alloy and tellurium with that for the diffusion of iron in iron tellurides, the rate-determining step for the scale formation for each region was discussed.

特集 原子層制御から原子制御へ ４． 分子線成長法による原子層の成長制御

Recent advancements in molecular beam epitaxy are reviewed with special emphasis on the control down to a monolayer. Extensive studies have succeeded in shedding light on atomic scale growth mechanisms, which enabled the development of novel growth technologies. Though submicron scale terraces are now realized, much effort is still necessary for meeting the requirements from advanced quantum effect devices.

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

The influence of certain reactive elements on the oxidation behaviour of chromia- and alumina-forming alloys

The influence of implanted yttrium and lanthanum on the oxidation behaviour of Co25Cr1Al chromia former as well as of β-NiAl alumina former has been studied under isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr 2O 3 scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results, on the one hand, from promotion of fine-grained scale formation and on the other, from changing the mechanism of scale growth from the outward diffusion of cations to the inward diffusion of anions. The influence of implanted yttrium (but not lanthanum) on the growth mechanism and adherence of alumina scale is analogous: the scale adherence is greatly improved but its growth rate is only slightly reduced. Lanthanum, on the other hand, greatly improves the protective properties of the scale but does not, in practice, affect its adherence to the substrate. As in the case of chromia formers, it has been found that the presence of reactive element atoms in the surface layer of the substrate considerably influences the mechanism of alumina scale growth by changing it from predominant inward diffusion of anions to prevailing outward diffusion of cations. It is believed that the influence of an implanted reactive element on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and in various interfacial phenomena.

Mechanism of scale growth on liquid aluminium

Metal loss is an unavoidable consequence of the high production melting of aluminium and its alloys. Losses must be minimised, for reasons of economy and to ensure optimum quality of cast and wrought products. Aluminium losses during melting and casting result primarily from the formation of dross, a mixture of oxide and melt. Results are presented from an initial study aimed at reducing melt loss through a knowledge of the mechanism by which dross is formed. Work has centred on an understanding of the initial stages in oxide scale growth, a study of growth kinetics, and observations of the subsequent breakdown of these initial scales to form dross. It was found using TEM that a thin surface oxide of γ-alumina forms rapidly on melts of commercially pure aluminium at 750°C providing a highly effective barrier confining the molten aluminium. This thin surface oxide reduces scale growth to a low level before the onset of breakaway oxidation. Localised failure of this protective oxide film results in exudations forming on the melt surface, the size and number of which increase with exposure time. These exudations would appear to be the onset of dross formation. In parallel studies of the characteristics of wetting of aluminium to alumina it is shown that the reported non-wetting is a result of the presence of this thin alumina film on the melt surface. Once broken, wetting of the alumina takes place and it is shown that this accounts for the exudation of aluminium through the surface oxide and hence dross formation.MST/990

Mechanism of scale growth on liquid aluminium

Mechanism of scale growth on liquid aluminium

Growth mechanism of chromia on hydrogen annealed chromium using imaging SIMS and18O as tracer

Chromium specimens were annealed in hydrogen to reduce the level of sulphur impurity and thus improve the oxide adhesion. The growth mechanism of the chromia scale in 0·1 atm oxygen at 950°C was investigated by oxidising sequentially in natural oxygen and then in gas enriched in 18O; the distribution of 18O in the scale was determined using imaging secondary ion mass spectroscopy (SIMS). From preliminary results it was found that substantial oxygen diffusion occurred. This differs from the growth mechanism of chromia on vacuum annealed material which, as far as solid state transport is concerned, mainly involves cation transport. The theory that sulphur promotes cation diffusion via grain boundaries in chromia and that removal of the sulphur favours oxygen diffusion is supported by the results. From examination of the distribution of the oxygen isotopes at the scale/gas interface, it is shown that their distribution is not uniform, from which it can be inferred that the growth of oxide at that interface was not uniform during oxidation in the 18O-rich gas.MST/967

The influence of Y on the structure and growth mechanism of alumina scales

AbstractSmall additions of yttrium to NiAl result in a decreasing oxygen transport through the Al2O3 scale on AiAl at 1270 K, as has been shown by nuclear reaction analysis (18O(p, α)15N). A higher concentration, e.g. 0.5 wt%, has the opposite effect. Yttrium segregates to the grain boundaries in the alloy, and an enhanced Y concentration has been observed by means of EPMA in the oxide scale above the grain boundaries of the alloy. Transition aluminium oxides are shown to be formed in the initial stage at 1170 K with (glancing angle) x‐ray diffraction. Only α‐Al2O3 is formed after oxidation for several minutes at higher temperatures (1470 K). The rate of transformation of transition Al2O3 to the stable α‐Al2O3 modification depends on the oxidation temperature. The transition is shown to be retarded by the presence of Y. The primary transport processes in the Al2O3 scales are short‐circuit diffusion of oxygen and/or aluminium. At 1170 K, while transition oxides are still present, Al transport predominates. At higher temperatures, when the oxide has the α‐Al2O3 structure, oxygen transport is enhanced. This would also explain the retardation of oxygen transport by the addition of Y, since Y was found to prevent the formation of the most stable form of Al2O3.

Static sims — A technique for surface chemical characterisation in basic and applied surface science

The application of static SIMS in characterising the chemistry of surfaces in basic and applied surface science is briefly demonstrated. First the ability of SSIMS to monitor the chemical structure and coverage of the adsorbed state is demonstrated with reference to CO and ethene adsorption on single crystal metals. The use of time-of-flight SSIMS to monitor the chemical state of the surface of a organic gas detector phthalocyanine is outlined. Finally a microfocussed liquid gallium beam is used to probe the mechanism of localised oxide scale growth on a steel alloy used in nuclear reactor construction.

The effect of certain reactive elements on the oxidation behaviour of chromia‐ and aluminaforming alloys

AbstractThe influence of implanted yttrium and lanthanum on the oxidation behaviour of Co‐45Cr and Co‐25Cr‐1Al chromia formers as well as of β‐NiAl and Co‐25Cr‐9Al alumina formers has been studied in isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr2O3‐scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results from the promotion of fine grained scale formation and consequently the elimination of outward diffusion of chromium. The influence of implanted yttrium (but not lanthanum) on the growth rate and adherence of alumina scale is analogous. In this case, however, both these effects result from elimination by reactive element addition of the inward diffusion of oxygen. It may be then concluded that the influence of implanted reactive elements on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and not in various interfacial phenomena.It has been found also that the protective properties of alumina scale are considerably improved by elimination of grain boundaries from the substrate, this effect being stronger than that resulting from reactive element addition.Beneficial influence of implanted reactive elements should be expected only when the protective scale layer is formed from the very beginning of the reaction, directly on the initial surface of the material exposed to the oxidizing atmosphere.