Development,growth, and adhesion of Al2O3 on platinum-aluminum alloys

The development, growth, and adhesion of -Al₂O₃ scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the -Al₂O₃ scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the -Al₂O₃ scales. The -Al₂O₃ scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the -Al₂O₃ may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.This work was supported by the U.S. Army Research Office, Durham, under Contract Number DAHCO 4 73 C 0021.     

Influence of a mixed Al2O3+Cr2O3 superficial coating on the non-isothermal oxidation behavior of Fe and Fe-Cr alloys

The non-isothermal oxidation behavior of electrolytic-grade iron and Fe-Cr alloys in dry air has been studied using linear heating rates of 6 K/min, 10 K/ min, and 15 K/min up to a final temperature of 1273–1473 K. Some of the iron and iron-chromium alloy samples were given a surface treatment by dipping them in an aqueous solution containing both Cr and Al ions before their oxidation studies. This pretreatment has resulted in improved oxidation resistance and scale adherence as depicted by no scale rupture even after a second thermal cycle. Mass changes were recorded gravimetrically, and scales have been characterized by SEM, EPMA, and x-ray diffraction analyses.     

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al ₂ O ₃ scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the ¹⁸ O isotope as a tracer. The ¹⁸ O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al ₂ O ₃ scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated k _p 's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and ¹⁸ O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these ¹⁸ O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.     

Reactive-element effect of electrodeposited Y2O3 oxide films on the oxidation of Fe−25Cr and Fe−25Cr−10Al alloys

Thin Y₂O₃ films were deposited by the electrochemical deposition-pyrolysis process on Fe–25Cr and Fe–25Cr–10Al alloys. The influence of the films on the oxidation behavior of the alloys was studied at 850°C and 1000°C. The results showed that Y₂O₃ films remarkably decreased the oxidation rate of Cr₂O₃-forming alloys and spallation of the scales, but they did not decrease the oxidation rate of the Al₂O₃-forming alloys, although they do reduce the spallation of Al₂O₃ scales. Y₂O₃ films remarkably change the morphology of the scales on both alloys, depending on the oxidation temperatures. These results show that the reactive-element effects of Y₂O₃ films on the Cr₂O₃ formers and Al₂O₃ formers are different.     

The effect of preoxidation on the sulfidation of Ni-20Cr (2–5)Al Alloys

Ni-20Cr alloys with 2, 3.5, and 5 wt.% Al have been preoxidized up to 100 hr at 1000°C in dry H₂, in H₂/23% H₂O and in air and subsequently exposed to an H₂/5% H₂S atmosphere at 750° C. During the preoxidation treatment different types of oxide scales were formed which affect the sulfidation protection in different ways. Optimum results were obtained for alloys with 3.5 and 5 wt.% Al after 20 hr exposure to dry H₂ at 1000°C. A thin Al₂O₃ scale is formed which decreases the sulfur attack by more than one order of magnitude. Preoxidation conditions for Ni-20Cr-2Al alloys in H₂ and for Ni-20Cr-2Al and Ni-20Cr-3.5Al in H₂/H₂O were observed to be less effective. No improvement was found for preoxidation in air or for Ni-20Cr-5Al alloys preoxidized in H₂/H₂O.     

Diffusion of18O in massive Cr2O3 and in Cr2O3 scales at 900°C and its relation to the oxidation kinetics of chromia forming alloys

The lattice and grain-boundary diffusion coefficients of¹⁸O atP _{O 2}=0.1 atm and at 900°C were determined in massive Cr₂O₃ and in Cr₂O₃ scales which were grown on a Ni–30Cr alloy. The diffusion profiles were established by SIMS and analyzed considering two domains in the case of polycrystalline Cr₂O₃ (massive or scales), the first one relative to apparent diffusion and the second to grain-boundary diffusion. A ridge model is proposed for Cr₂O₃ scales to modify thef value, fraction of sites associated with the grain boundary. With such a model,f is equal to 0.0006 and 0.0005 for the scales formed during 15 hr and 165 hr, respectively. The oxygen-lattice diffusion coefficients determined in Cr₂O₃ scales are in very good agreement with those in massive Cr₂O₃. With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant equal to the experimental one. Scale growth occurs by countercurrent diffusion of oxygen and chromium, mainly by grain-boundary diffusion.     

Influence of a TiO2 surface treatment on the growth and adhesion of alumina scales on FeCrAl alloys

Deposits of TiO₂ on FeCrAl alloys were obtained by surface TiO₂ slurry application or by immersion of samples in tetraisopropylorthotitanate (TIPT) solution followed by air dry which gave the thinnest coatings. Isothermal oxidation of treated samples showed strong modification compared to non‐treated ones, particularly in the temperature range of 850–925 °C where parabolic rate constants rapidly decreased when alloys were TiO₂ treated. SEM surface observation, X‐ray diffraction and ruby fluorescence showed that the presence of TiO₂ promoted the formation of α‐Al₂O₃ whereas non‐treated samples exhibited large amounts of transition aluminas. An interesting effect of the rapid change from metastable to stable α‐alumina was a strong increase of scale adhesion, determined by tensile testing, from 300–400 to 2000 J/m² for scales grown at 850 °C on Aluchrom YHfAl. This was explained not only by the change from outward to mainly inward growth but also by the volume reduction at the transition to alpha transformation.     

Some observations on the effects of sulfur and active elements on the oxidation of Fe−Cr−Al alloys

A very-low-sulfur-content industrial Fe–Cr–Al alloy has been used both as a baseline and as a charge material for laboratory metlts with variable sulfur and rare-earth additions. No significant differences in behavior were observed in cyclic oxidation tests on 1-mm-thick coupons at 1100°C, except for an excessive rare-earth content, which led to accelerated scale growth. At 1300°C, alloys without rare-earth additions developed high growth stresses in the oxide, leading to large tensile strains in the substrate. The oxide-metal interface in the low-sulfur (<2ppm) material resisted these stresses and the oxide remained adherent. However, as little as 4ppm S was sufficient to cause considerable spalling. Rare-earth additions markedly reduced growth stresses and eliminated both dimensional instability and spalling.     

Formation of Al2O3 Scales on Single-Phase RuAl Produced by Reactive Sintering

The oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated. Oxidation was conducted at 1000°C; oxide scale growth and phase formation were studied using scanning-electron microscopy (SEM) and x-ray diffraction (XRD). A dense protective scale with an Al–depleted sublayer was formed during oxidation. The oxide scale is the stable -Al₂O₃. The oxide-scale morphology shows the presence of whiskers, with a needle-like form, which suggests that the growth of the oxide scale is produced by outward diffusion of Al. At the beginning, oxidation follows a parabolic law, but, after 100 hr of oxidation; the growth rate is slower than expected from a parabolic law.     

Microstructure,adhesion and growth kinetics of protective scales on metals and alloys

Currentunderstanding of the complex interrelationships among growth kinetics, microstructure, and adhesion of protective Cr₂O₃ and Al₂O₃ scales is critically reviewed. Similarities and differences in the behavior of these two systems are highlighted. The morphology of the alloy-scale interface appears to be a critical factor. Recent ideas are advanced to interpret the effect of oxygen-active elements on the development of a tortuous interface conducive to improved scale tenacity.This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098.Deceased.on leave to Industrial Materials Research Institute, NRC, Montreal, Quebec, H4C 2K3 Canada.     

The effect of Al2O3 addition on the milling behavior of NiAl powder

The milling behavior of nickel aluminide, NiAl, powder in the presence of a fine Al ₂ O ₃ powder was investigated in the present study. The milling was carried out in an attrition mill. The size and shape of NiAl particles were not changed after milling while only NiAl powder was milled. When fine Al ₂ O ₃ powder was added to the NiAl powder, the Al ₂ O ₃ particles attached to the surface of NiAl particles during milling. As a consequence, the size of NiAl particles was reduced after milling. The shape of NiAl particles also changed. The presence of fine Al ₂ O ₂ particles enhanced the milling efficiency. The Al ₂ O ₃ particles on the surface of NiAl powder can be removed by washing repeatedly in an ultrasonic bath.     

The formation of aluminum oxide scales on high-temperature alloys

This paper is a brief review of the extensive literature relating to the formation of protective —Al₂O₃ scales on alloys at high temperature. Emphasis is placed on the proposed mechanisms of scale growth based on observations of scale morphologies and microstructures, inert-marker experiments and the distribution of oxygen isotope tracers within thermally-grown oxides. Attention is also given to the determination of ionic-transport mechanisms by electrochemical methods and to the effects of reactive elements such as yttrium in modifying ionic-diffusion processes.     

The influence of alloying elements on the development and maintenance of protective scales

Some of the important principles that determine the establishment, growth and long-term maintenance of protective Cr₂O₃, Al₂O₃ and SiO₂ scales on hightemperature iron-, nickel- and cobalt-base alloys are reviewed and discussed. Emphasis is placed on the effects of alloying elements and other additions, such as third elements and reactive elements or oxide dispersions, on each of these processes. Particular attention is paid to transport processes in the scales and the importance of short-circuit paths. Some of the important parameters that influence the long-term mechanical stability of such scales are considered and evaluated.     

Evaporation of Cr2O3 in Atmospheres Containing H2O

Stainless steels in atmospheres containing H₂O form a Cr₂O₃ scale in the early stage of oxidation. However, the Cr₂O₃ scale gradually degrades with time. In order to determine the effect of H₂O on the deterioration of a Cr₂O₃ scale, the evaporation behavior of Cr₂O₃ in N₂–O₂–H₂O atmospheres was investigated. The rate of mass loss in an N₂–O₂–H₂O atmosphere was found to be one order of magnitude higher than the rates in N₂–O₂ and N₂–H₂O atmospheres, indicating that deterioration of the Cr₂O₃ scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr₂O₃ is probably based on the following reactions: 1/2Cr₂O₃(s)+3/4O₂(g)+H₂O(g)=CrO₂(OH)₂(g). However, it is also speculated that the reaction, Cr₂O₃(s)+2/3O₂(g)=2CrO₃(g), affects the evaporation of Cr₂O₃ at temperatures higher than 1323 K. The evaporation rate of Cr₂O₃ is roughly comparable to the growth rate of the Cr₂O₃ scale. Therefore, a Cr₂O₃ scale can be degraded by the evaporation of Cr₂O₃.     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

Chromium transport through Cr2O3 scales I. On lattice diffusion of chromium

Cr specimens preoxidized at 1100–1300°C to give Cr₂O₃ scales with varying thicknesses and microstructures have been treated at temperature in high vacuum. During the high vacuum treatment the specimens lose weight due to outward Cr transport through the Cr₂O₃ scales. The initial rate of weight loss gradually diminishes, but eventually the weight loss reaches a linear rate. Concurrently the Cr₂O₃ scale exhibits grain growth and densifies. It is concluded that the mode of outward chromium transport gradually changes during the high vacuum treatment: from lattice and grain-boundary diffusion and possibly vapor transport along microcracks during the initial stage to lattice diffusion only for the densified scales. It is concluded that chromium diffuses by an interstitial type mechanism. Self-diffusion coefficients of Cr in Cr₂O₃ at the Cr-Cr₂O₃ phase boundary have been calculated from the linear rates of chromium transport for different defect structure situations.     

Chromium transport through Cr2O3 scales. II. Changes in scale morphology during high vacuum treatment of oxidized chromium specimens

Chromium specimens oxidized at 1200 and 1300° O to give Cr₂O₃ scales with varying thicknesses have been high vacuum annealed for extended periods at temperature. During the high vacuum anneal chromium is transported through the scale and evaporates from the scale surface. Initially the rate of chromium evaporation decreases with time as a result of recrystallization and densification of the scale. On extended high vacuum treatment the rate of chromium evaporation again increases and major changes in scale morphology takes place. The outer scale surfaces develop hollows in the oxide grains while the grains protrude from the scale at the inner surfaces. The morphological changes are interpreted in terms of differences in diffusion rates along grain boundaries and through the lattice and resultant variations in surface energy along the surfaces.     

High-temperature corrosion of Co-15Mo alloys containing ternary additions of Al in mixed-gas environments

The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H ₂ -H ₂ O-H ₂ S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al ₂ O ₃ formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al ₂ O ₃ in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al ₂ O ₃.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al _0.55 Mo ₂ S ₄,and Al ₂ O ₃ in the inner layer.     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

Effect of Y2O3 additions on the plasticity of sintered Cr2O3

We have carried our constant strain-rate compression tests on polycrystalline Cr₂O₃ and Cr₂O₃ doped with 0.09 wt. % Y₂O₃ to establish whether there exists an effect of Y₂O₃ on the plasticity of Cr₂O₃. This study is motivated by previous work on the oxidation of alloys containing reactive-element additions. In that work, it has been observed that the addition of oxygen-active elements, such as Y to alloys that form Cr₂O₃ or Al₂O₃ oxide layers upon exposure at high temperature, strongly enhances the adhesion of the oxide layer to the base alloy as compared with alloys without reactive-element additions. We have found that at 1200°C (1) chromia exhibits limited plasticity at high temperatures, and (2) the presence of Y in the oxides does not enhance plasticity compared with addition-free oxides.