Comparison of the effects of small additions of silicon or aluminum on the oxidation of iron-chromium alloys

A study has been undertaken of the oxidation behavior of Fe-26 wt.% Cr-1 wt.% Al and Fe-26 wt.% Cr-1 wt.% Si at 800° and 1000°C in oxygen, in order to determine the usefulness of the two tertiary elements in facilitating the development of the Cr₂O₃ external scale. The research has also permitted a comparison of the modes of internal oxidation of these elements, with a view to ascertaining the ease of establishment of the tertiary element oxides as healing layers at the scale/alloy interface. It has been shown that aluminum is the more effective addition in this respect, due to formation of a higher population density of internal oxide nuclei in the early stages. However, in the 1% Al alloy, the precipitates penetrate inward, to considerable depths, as continuous platelets, making development of a complete healing layer difficult. In practice, a higher aluminum concentration is necessary for the closely spaced precipitates to coalesce to form the healing layer, but the process then occurs rapidly. The initial internal oxide nuclei in the 1% Si alloy have a much smaller population density and are restricted to a location very close to the surface. Thus, a healing layer can be established, but the large interparticle spacing makes this a very slow process. Even at a higher silicon concentration, it takes a significant period to be completed. The effects are discussed and accounted for, particularly in terms of the relative stabilities of the various oxides.     

Effects of minor additions and impurities on oxidation behaviour of FeCrAl alloys. Development of novel surface coatings compositions

In the present work the effects of single or combined minor additions of Zr, Hf, Ti and C on the oxidation behaviour of Y‐containing, FeCrAl alloys have been studied. For this purpose high‐purity, model alloys with single or multiple minor alloying additions were used. The results of long term discontinuous oxidation tests and detailed kinetics studies using thermogravimetry were complemented with extensive microstructural characterisation of the formed alumina scales using SEM and STEM. Hence, the oxidation kinetics and scale spallation rates and failure modes were correlated with the oxide composition and microstructure. The results demonstrate that the frequently reported positive effect of Zr, Hf and Ti on the lifetime oxidation behaviour of FeCrAl alloys can only be fully exploited if the concentrations of the above elements are carefully adjusted and the interaction with typical alloy impurities, such as carbon, is considered.     

The internal oxidation of two-phase binary alloys beneath an external scale of the less-stable oxide

The internal oxidation of two phase binary A-B alloys by a single oxidant at high temperatures, under partial pressures sufficient to also form external scales of the less-stable oxide, is examined by means of quantitative models and compared with the corresponding behavior of single-phase alloys. It is shown that, depending on various factors, particularly on the solubility and diffusivity of the most-reactive component B in the most-noble component A, this process may or may not involve a diffusion process of the alloy components, leading to different scale morphologies. It is also concluded that even when the solubility and diffusivity of B in A are sufficiently high, so that the internal oxidation of the common type occurs, the restriction to the diffusion of B in the alloy due to its limited solubility affects the kinetics of internal oxidation, producing an increase of the rate of internal oxidation and of the critical concentration of B in the alloy required for the transition to the external oxidation of B with respect to single-phase alloys under the same values of all the relevant parameters. The lower the solubility of B in A, the larger these effects.     

Microcrack generation and its healing in the oxide scale formed on Fe-Cr alloys

Fe -base alloys containing 5, 10, and 20 wt. % Cr were oxidized in a stream of O₂ at 750 and 900°C up to 264 hr. A sulfur decoration method was applied to detect the cracks generated in the scale during oxidation. This method revealed frequent crack generation and its healing in the scale. In the case of low-Cr alloys, the cracks are filled up with newly formed Fe-rich oxide but may be regenerated during further oxidation. Cracks are generated in the scale on an Fe-20Cr alloy also, although this alloy is not so severely attacked because of rapid healing.     

Formation mechanisms of keying or pegging yttrium oxide and increased plasticity of alumina scale on FeCrAlY

Formation of Keying clusters is presented in conjuction with its effects on the adherence of the alumina scale and nonuniform stress distribution in the scale. In addition, the specific nature of point defects and various aspects of scale plasticity are considered with respect to their role on the oxidation behavior and mechanism.     

Transition from internal to external oxidation for binary alloys in the presence of an outer scale

The critical concentration required for the transition from internal to external oxidation of the most reactive component B of a binary A-B alloy for the case of simultaneous formation of an outer AO scale is calculated by extending Wagner's original analysis, which excluded the presence of an AO oxide. It is shown that the formation of an outer AO scale tends to produce an increase in the critical concentration of B, essentially as a result of a decrease of the enrichment of B in the internal oxidized region. Examples of the calculation of some parameters are presented to examine the effect of the different factors on the properties of internal oxidation and on its transition to the formation of an external scale of the oxide of the less noble component.     

Metallic yttrium additions to high temperature alloys: Influence on Al2O3 scale properties

In the present study, the effect of adding yttrium to alloys is investigated. The microstructure of the cast Fe-25Cr-4Al-0.5Y alloy used in the study shows that the vttrium is present in different shapes and sizes as the intermetallic phase, (Fe, Cr)₄(Al, Y), previously unreported in the literature. Upon oxidation in dry oxygen in the 1100–1200 °C temperature range, a columnar, fine-grained (0.5–1 m) -Al₂O₃ scale is formed which grows predominantly by inward oxygen grain-boundary transport. The intermetallic phase, during incorporation into the oxide scale, is converted into Y₃Al₅O₁₂, the chromium and iron from the intermetallic diffusing back into the metal matrix. The Y₃Al₅O₁₂ phase saturates the oxide scale with yttrium, which segregates to oxide grain boundaries. The microstructural features of the oxide scale resemble those of the scale formed on the yttria-dispersed alloy we investigated earlier. The improved adherence of the oxide scale is a consequence of yttrium doping, which facilitates the formation of a fine-grained scale in which oxide growth stresses can be relieved by diffusional plastic flow. Further, yttrium suppresses Al transport in the oxide scale and prevents Al₂O₃ nucleation within the scale, a process which can generate compressive stresses in the scale. The yttrium doping in the oxide scale is somewhat more efficient when it is present as a dispersoid in the metal.     

Observations of nodule growth during the oxidation of pure binary iron-aluminum alloys

Oxidation studies were carried out in oxygen at 800°C, on a series of pure binary iron-based alloys with between 1.9 and 9.8 wt. % aluminum. The results are presented in conjunction with the existing literature and these permit the development of a classification of scale morphologies based on alloy composition. Alloys with less than about 2.4 wt. % aluminum form bulky stratified scales composed of Fe₂O₃ and Fe₃O₄ with FeAl₂O₄ and Al₂O₃ at the scale-metal interface. Alloys with between 2.4 and 6.9 wt. % form an external Al₂O₃ scale but this is interspersed with iron oxide nodules that penetrate the alloy substrate. Only alloys with greater than 6.9 wt. % aluminum form completely protective Al₂O₃ scales. Models based on oxide nucleation are presented for the growth of bulky scales and also the iron oxide nodules.     

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.     

Internal oxidation of ternary alloys. Part I: Kinetics in the absence of an external scale

A treatment of the kinetics of internal oxidation of ternary alloys in the absence of an external scale is presented. The analysis involves simultaneous internal oxidation of the two solutes in the ternary alloys. A comparison with measurements on Ni-4 at.% Al-1 at.% Si alloys at 800°C reveals a good agreement between theory and experiment.     

Oxide scale formation on Al containing Ni–Cr‐based high temperature alloys during application as flame tube material in recirculation oil burners

The flame tube is an important functional component of burners using the concept of the flame tube stabilised combustion. Under typical combustion conditions the material of the flame tube is exposed to high temperatures (≥900 °C) and to corrosion attack by the combustion gases. Furthermore as the burners are generally operated intermittently, the material suffers from extreme temperature and atmosphere changes. For flame tubes, a lifetime of approximately 8000 h is desired. Predominantly metallic high temperature materials are used. The scope of the present work was to test—under application conditions and for maximum material temperatures exceeding 900 °C—alternative high temperature alloys for use as tube material. The corrosion resistance of the austenitic Ni–Cr‐based alloys (601, 602 CA, 617 and 693) has been investigated in a burner rig at maximum material temperatures of 950 and 1000 °C and with exposure times from 50 to 3000 h. The chromium content of the alloys was between 20 and 30 wt% and that of aluminium between 1 and 3.4 wt%. Metallographic cross‐sections of samples of the alloys were analysed by electron microprobe yielding information about the microstructure and composition of the oxides in the surface zone and variations during exposure time. This study focuses on the observed specific effects of the alloying element aluminium on the development of the oxide scale and on the lifetime of the alloys. At the alloy surface after 500 h exposure time a chromium oxide scale had formed with aluminium oxides underneath predominantly along grain boundaries. For the alloys with the lower aluminium content, the aluminium oxides built up an open network but not a closed layer. For the alloy with the highest aluminium content (alloy 693) after 50 h two different characteristic microstructures at the surface were found. In one case, the grains at the surface were covered with chromium oxide on top and the remaining grain surface was completely enclosed by aluminium oxides. In the other case, the aluminium oxide formed a thin layer directly below the chromium oxide scale. After 500 h exposure time, a significantly thinner chromium oxide scale and massive internal chromium oxides were observed. Catastrophic corrosion, formation of internal oxides and aluminium nitrides started even after 500 h. It will be demonstrated that the early breakdown of alloy 693 is linked to the aluminium oxides which act as a barrier constricting the diffusion of chromium from the alloy matrix towards the surface. Under the conditions of extreme temperature changes given in the burner the aluminium oxide layer on its part did not provide corrosion protection.     

Influence of the nature of alloying elements on the adherence of oxide films formed on titanium alloys

In order to study the influence of aluminum, chromium, and silicon on the adherence of oxide layers to the metallic substrate for binary alloys of titanium, adhesion measurements were carried out and the results were correlated with the oxidation behavior of these materials. Ti-Al, Ti-Cr, and Ti-Si alloys were prepared and oxidized at temperatures of 550–700°C in air and in oxygen, for oxidation times from a few hundred to several thousand hours. The reaction kinetics were followed using continuous thermogravity or daily weighing of samples. For Ti-Al and Ti-Si alloys, a slight decrease in the adherence of oxide layers to the substrate was observed for the lowest aluminum contents (1.65 and 3%) and silicon content (0.25%) as compared with the behavior of unalloyed titanium; at higher levels, the adherence increased with the alloying element content. For Ti-Cr alloys, the addition of chromium increased the adhesion of oxide layers to the substrate for all cases; however, the change in adhesion with chromium content was not monotonic.     

Stability of protective oxide films on Ti-base alloys

Thermodynamic calculations are performed to estimate isothermal sections of Ti-Al-O, Ti-Si-O, and Ni-Al-O phase diagrams. Very small aluminum levels (<10^–10 at. %) are needed to stabilize alumina on Ni-Al alloys. However, much higher aluminum (50%) and silicon (40%) levels are needed to stabilize alumina and silica on Ti-Al and Ti-Si alloys, respectively. These calculations suggest that the mechanism of formation of the protective oxide films on titanium-based alloys is radically different from that on nickel-based alloys. The aluminum levels needed to form a continuous film of alumina on nickel-based alloys are dominated by kinetic factors. On the other hand, thermodynamic factors appear to dominate the alloy compositions needed to form protective films of alumina and silica on titanium-based alloys. Further work is needed to evaluate any possible role of kinetic factors.     

Oxidation kinetics of Y‐doped FeCrAl‐alloys in low and high pO2 gases

A model Fe‐20Cr‐5Al‐0.05Y alloy was oxidized in Ar‐20%O₂ and Ar‐4%H₂‐7%H₂O at 1200–1300 °C. Two‐stage oxidation experiments using oxygen isotope tracers showed that inward oxygen diffusion was predominant in both gases, but more isotope exchange was observed in the H₂/H₂O gas reaction. The alumina scales formed in both gases were composed of columnar grains, the lateral size of which increased linearly with depth beneath the scale surface. Thermogravimetric measurement of oxygen uptake revealed kinetics which were intermediate to parabolic and cubic kinetic rate laws. A model based on grain boundary diffusion control coupled with competitive oxide grain growth accounts satisfactorily for the results when the requirement for a divergence‐free flux within the scale is imposed. This treatment shows that the oxide grain boundary diffusion coefficient is lower when H₂O is the oxidant. It is concluded that hydrogen slows the grain boundary diffusion process by altering the nature of the diffusing species.     

Effects of composite oxide scale on oxidation resistance of superalloy K273

A Si-containing K273 superalloy was made using intermediate frequency induction funace in the study.In the testing of oxidation resistance,the oxidation process of the alloy specimens during the testin...     

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.     

The cyclic oxidation resistance of cobalt-chromium-aluminum alloys at 1100 and 1200° C and a comparison with the nickel-chromium-aluminum alloy system

A series of Co-rich alloys in the Co-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100° C and 200 1 -hr heating cycles at 1200°C, The specific weight-change data for each sample were then used to determine both an oxide growth constant, k₁, and a spoiling constant, k₂, for each alloy, using the regression equation W/A=k ₁ ^1/2 t^1/2— k₂t±. These in turn were combined to form an oxidation attack parameter, K_a, where K_a=(k ₁ ^1/2 +10k₂). These attack parameters, along with X-ray diffraction results, were then compared with K_a values determined for comparable Al and Cr compositions in the Ni-rich Ni-Cr-Al system. The K_a and X-ray diffraction results indicated that initially, if the Cr and Al contents in both systems are high enough, protective -Al₂O₃ and aluminate spinel(s) are formed. However, in the long run, when the scales eventually start to fail, mainly CoO is formed in the Co-Cr-Al system and mainly NiO in the Ni-Cr-Al system. The Ni-Cr-Al system is considered more oxidation resistant since CoO leads to massive spalling and sudden drastic weight loss, while NiO fails in a more gradual, predictive manner. Both sets of alloys were melted in zirconia crucibles and the resultant Zr pickup increased the cyclic oxidation resistance of the alloys in both systems.     

Effect of niobium additions on CO2-enhanced oxidation of titanium-aluminum intermetallic alloys

In a recent study, CO₂ has been reported to enhance the oxidation rate of binary titanium-aluminum alloys. The detrimental effect of CO₂ was not, however, observed in a ternary alloy containing niobium. In this paper, possible explanations for these observations are examined. First, results from the literature regarding the effects of niobium in improving the resistance of titanium-aluminum alloys are briefly reviewed. Second, a thermodynamic analysis which offers a possible explanation for the beneficial effect of niobium in eliminating the CO₂-enhanced oxidation of titanium-aluminum alloys is presented.     

High-temperature oxidation of copper-manganese alloys

The oxidation behavior of copper-manganese alloys (2–35 wt. % Mn) in pure oxygen at 760 Torr was investigated at 100° intervals between 550 and 850°C. Gravimetric measurements of the oxidation kinetics have been combined with microstructural studies of the reacted samples in order to evaluate the reaction mechanisms. The scales formed on Cu-2Mn, Cu-5Mn, Cu-10Mn are always composed of three different layers; in any case manganese is present only in the inner layer. The scales formed on Cu-20Mn and Cu-35Mn are composed of two layers, both containing manganese, with a more Cu-rich outer layer. In all the samples internal oxidation in combination with external scale formation is observed.     

The transition from internal oxidation to continuous-film formation during oxidation of dilute Ni-Si alloys

The oxidation of Ni-2.05Si and Ni-4.45Si was studied in oxygen over the range of 600°–1000°C for 18 hr. The oxidation kinetics did not follow a parabolic rate law, bur rather a power law of the form (w)ⁿ=kt was followed. The value of n ranged from 2.7 to 4.9 for Ni-2.05Si and from 3.0 to 6.4 for Ni-4.45Si. The low-silicon alloy exhibited extensive internal oxidation, whereas the higher-silicon alloy did not internally oxidize. In general, NiO containing little or no silicon formed as an exterior layer on both alloys. The internal oxidation zone in Ni-2.05Si was highly irregular in thickness, and in some areas there was no internal oxidation. The higher-silicon alloy formed a continuous layer of a silicon-rich oxide. X-ray diffraction did not detect silica (amorphous), and no evidence of Ni₂SiO₄ was observed, although EDAX analysis suggests that small amounts of the silicate might have formed. Theaverage thickness of the internal oxidation zone was found to agree well with calculated values based on oxygen solubility and diffusivity data. No enrichment of silicon occurred in the internal oxidation zone. Calculated values, 0.033 and 0.038 (depending on the model used), of the mole fraction of silicon required for the transition from internal oxidation to continuous silica film formation agreed well with experimental data obtained in both this study and with others reported in the literature.