S.E. Sadique(previously 1, now 2), M.A.H. Mollah(2), M.M. Ali(1), M.M. Haque(1), S. Basri(2), M. M. H. M. Ahmad(2) and S.M. Sapuan(2)
1Department of Materials and Metallurgical Engineering,
Bangladesh University of Engineering and Technology (BUET), Dhaka : 1000,
2Faculty of Engineering, University Putra Malaysia, 43400UPM, Serdang, Selangor D. E.
The oxidation behavior of Fe-Cr-Al alloys containing 10% chromium and aluminum in the range of 2 - 8% by weight in pure oxygen at 1 atmosphere pressure at higher temperatures under cyclic conditions (3 hour cycles) has been studied. In metallographic investigation, healing layers of Cr2O3 and of Cr2O3/a -Al2O3 could be observed after breakaway of the initial protective alloy in case of the 2%Al and 4%Al alloys but the 6%Al and 8%Al alloys reformed external a -Al2O3 scales at all the temperatures and develops a convoluted configuration. 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 aluminium diffusing outward through the bulk oxide. The scales developed on the four alloys maintain more or less good contact with the alloy and thicken more slowly in case of 2%Al and 4%Al alloys. These effects can be associated with the reduction in the rate of transport of chromium across the scale in case of 4%Al and 6%Al alloys and with formation of intergranular and internal oxides in the underlying alloy substrate.
Key Words: Oxidation Behavior, Fe-Cr-Al alloys, Cyclic Oxidation, Oxidation Morphology, Breakaway, Oxidation, Healing Layer, Metallography.
The metallurgical problems associated with the operation of high temperature machines nearly arise from the very high temperatures used for the working fluid, usually gases. Those parts of the machine in contact with the hot gases must, therefore, be made of some suitable materials which will maintain adequate strength at its working temperatures, will not oxidize or corrode appreciably at that temperature, will not become brittle and not be seriously subject to the effect of creep .
Many heat-resisting alloys are based on Fe-Cr and rely on the development of Cr2O3 scales for resistance to oxidation at high temperatures. However, such scales are often susceptible to spallation, particularly under thermal-cycling conditions. Gardiner et al reported that the oxide film formed on iron-chromium alloys less than 5 wt%Cr below 873K consisted of 2 layers of magnetite and hematite. It is well known that Fe-Cr alloys in oxygen at higher temperature formed spinel (FeCr2O4) and Cr2O3 on the inside and Fe2O3 on the outside of the scale. Thermodynamically findings shows that the scales of the lowest oxygen pressure lie closest to the metal and the scale nearest the gas phase must be of the highest dissociation pressure.
Small addition of reactive metals such as Th, Al, Y, Ce, Ca, Mg, Ti, V etc can improve oxidation resistance of high temperature alloys. It also can improve the casting quality by reducing the formation of voids, gas cavities etc and secondly, the resultant metal possessed higher strength and better heat-resisting properties as a result of its inherent fine-grained structure . Fe-Cr-Al type ferritic alloys show a great resistance to high temperature oxidation. Their high aluminium content (5%) allows the formation of an alumina refractory layer, which protects them. The increase of oxidation rate, which follows this spalling, decreases the in-service lifetime of component . Previous studies have been shown that additions of other reactive metals to such alloys have a considerable, beneficial effect on the oxidation performance. There is a reduction in the concentration of Cr in the alloy necessary to form a continuous, protective Cr2O3 layer, the growth rate of that layer, once established, is reduced, most importantly, the resistance of the scale to spallation is improved significantly .
As reported by Islam , observed the beneficial effects of these reactive metal additions. The effective partial pressure of oxygen behind the less protective Fe2O3/Fe3O4 scale is much lower than the atmospheric and hence preferential oxidation of Al or Cr will tend to occur in case of 2% and 4% Al alloys. The new element with higher reactivity enables the protective element being available at the interface region without being oxidized in the interior of the alloy.
Jedilinski and Borchardt  observed that the change of the scale growth direction from the predominant outward metal to the prevailing oxygen inward transport occurred at the rather early stages of oxidation of Fe-Cr-Al alloys. At these stages of reaction the development of unstable alumina is very plausible as already demonstrated for scale on Fe-Cr-Al alloys were analyzed by means of X-Ray.
Ramanathan , Rhys-Jones et al  and Rapp and Pieraggi  summarized various explanations to account for the beneficial effects of reactive metal additions for the growth a chromia protective scale on pure Cr or on an alloy of Fe-, Ni-, Co-base such as
The oxidation of Fe-14Cr-4Al and Fe-27Cr-4Al alloys proceeded by outward diffusion of O2- ions along grain boundaries occurring simultaneously has been reported by Golightly et al . There is a widespread technological need for new high-temperature, oxidation-resistant alloys. Since Fe-Cr alloys generally provide the basis of Fe-base materials used in several high temperature applications where the formation of protective oxide scales is required, the evaluation and mechanistic interpretation of any beneficial effects imparted by additions of reactive elements to such alloys would be most useful. Although studies upon the oxidation resistance of binary Fe-Cr alloys have rather plentiful, but sufficient information of Al additions to these alloys have not been so much available. The more economical design of machine parts, which are, used at high temperatures such as aircraft gas turbines need more information about the properties on the oxidation resistance at high temperatures. The purpose of this paper is to study high temperature oxidation seems to be rightly directed towards tackling the problem of scale adhesion and its stabilization on the surface of the component part.
Mild steel scrap in the form of bar, Ferro-Chrome (containing 65% Cr) in the form of lump and aluminum in the form of ingot were used as raw materials in the production of the alloys for the investigation. The alloys were prepared in a high frequency induction furnace. They were cast in form of rod about 2.5 cm in diameter, 30 cm in length. The surface preparation of the specimens involved abrading on successively finer grades of SiC papers of finess from 3 to 2/0 grit sizes. Proper care was taken to avoid overheating of the specimen during cutting and drilling operations. The cyclic oxidation kinetics were followed by oxidizing the samples in a horizontal electrically heated furnace followed by direct weight change measurements (after cooling for at least 20 minutes) in a manual type balance.
The following tests were performed on the oxidized specimens:
It was performed under ordinary light to illustrate any prominent macroscopic feature on the specimen surface.
The oxidized specimen surfaces were analyzed on a x-ray diffractometer JDX 8P, x-ray analytical Equipment. The different phases were identified by the resulting X Ray patterns as separate peak using standards ASTM data.
The structure and morphology of the scale in cross-section were examined extensively under the optical microscope. The oxidized samples were mounted on a slow heat setting solid mounting medium (plastic materials) and quick setting solid mounting medium (Quick power) using a chemical hardener. Specimens were positioned and supported on edge in a small cast iron mould and liquid resin carefully pored around the specimen and allowed harden. After that, specimens were ground down on a dry SiC paper followed by wheel polishing with velvet cloths and finally hand polishing was completed in velvet clothes thus minimizing "pull out" of the scale.
Gravimetric runs on samples of various compositions were performed within the temperature range of 9500C 10500C. It is apparent that most of the alloys exhibit rates, which are approximately parabolic. Some of the runs were repeated to check the reproducibility of the data; generally fair agreement was found. The general oxidation trend for each alloy was almost identical. A protective scale initially forms and suffers breakaway oxidation within a few cycles as displayed by an increase in the oxidation rates and subsequently in fact a second protective oxide forms which reduces the oxidation rate to a more or less steady state value.
A graph of specific weight gain value vs. percentage of aluminum is shown in Fig. 1 for all the three temperature levels (950oC, 1000oC, and 1050oC). It will be noticed that the oxidation rate, as measured by specific weight gain, declines gradually reaching a minimum value with 8% Al. This is also evident from Fig. 2; Fig. 3 and Fig. 4 represented oxidation kinetics of the alloys at 950oC, 1000oC, and 1050oC respectively. In Fig.3 the oxidation kinetics of the binary alloy-both isothermal and cyclic (hypothetical-have been included along with the cyclic data for the modified alloys at 1000oC. It will be noticed that even the slope of the kinetics curve is much lower than that for the binary Fe-10Cr alloy (at 1000oC) i. e. the oxidation rate of the modified alloy with the lowest Al-content in even lower than that for the binary Fe-10Cr alloy. Clearly, the alloys containing higher Al have oxidation rates much lower that the 2%Al alloy. In general, the onset of initial spallation has been found to be delayed with increasing Al-content. The individual spall particles are noticed to be progressively finer with the increase of Al-content in the base alloy and a gradual decrease in the amount of spall particles. They were grayish black in color and are in the form of large thin flakes being magnetic in nature for 2%Al and 4% Al alloys. In contrast, 6% and 8% Al containing alloys forms a very fine powder particles being creamy brown in color and nonmagnetic in nature. It will be noticed that the oxidation rate in terms of specific weight gain values increase with the higher temperatures for each of the alloys. It also appears that 2% Al alloys are found to be oxidized completely so that no parent metal can be observed at higher temperatures.
To sum up the following general observation can be made:
A general observation of the transverse cross-sections from oxidized samples was that spalling during cooling became more severe as the Al content of the alloy decreased. It has been done to examine properties such as scale density, adherence to the metal, and the nature of the metal/oxide interface. As observed in Fig. 5 and Fig. 6, it is evident that the rapidly oxidizing specimen shows a usual duplex scale with a continuous subscale layer of Cr2O3 and Cr2O3/a -Al2O3 at the scale base in 2% Al and 4% Al alloys respectively whereas a -Al2O3 scale in the 6% Al and 8% Al alloys as shown in Fig. 7 and Fig. 8. The substrate surface of the specimen shows convoluted structure and the presence of void as usual.
The development of the convoluted morphology resulted in extensive detachment of the oxide from the alloy as per cent Al increases, creating cavities beneath the oxide ridges. The coarsening of the convoluted oxide morphology was the result of a progressive increase in the length of the oxide comprising individual oxide ridges. This occurred as a consequence of lateral growth of both the oxides, which remained in contact with the alloy and that, which, as part of an oxide ridge, had become detached.
Comparison of the weight gains for the four alloys after a given period of oxidation must take into consideration the markedly different oxide morphologies, which were developed. Although parabolic kinetics were generally observed for all compositions and temperatures, it is apparent from the structural features of the scales that other that the other factors besides diffusive transport mechanisms must be considered in attempting to understand the oxidation of FeCr alloys. Comparison of the weight gains for the three alloys after a given period of oxidation must take into consideration the markedly different oxide morphologies, which were developed.
The influence of the Al additions is marked both in the initial and latter stages of the oxidation processes, affecting oxide nucleation, scale adhesion and the rate of oxide growth. During the initial stages of oxidation, Al additions promote the rapid formation of an oxide film; i.e. oxide nucleation processes are enhanced. In the case of Al-containing materials this can be attributed to the rapid formation of Al oxide nuclei on the surface of the alloys, causing a greater total number of nuclei and hence decreasing the internuclear distances. Consequently, complete surface coverage by oxide is more rapidly attained that in the base materials.
As reported by Wood et al , binary Fe Cr alloys containing Cr in the range of about 14 25% at 900 11000C immediately forms a continuous protective scale of upon the surface. When a more reactive element is present, this Cr2O3 scale is formed even with much lower percentages of Cr (10-13%). In case of the first two alloys, it is apparent from the X-ray diffractometer data that the scale is of Cr2O3 since a strong peak for Cr2O3 is noticeable. This external Cr2O3 scale suffers breakaway as it proceeds Fe begins to oxidize as depletion of Cr in the bulk alloy has already taken place. This is why the oxidation rate after breakaway increases to a high values corresponding to that for the formation of Fe2O3/Fe3O4. In the course of time, Cr2O3 may combine with FeO to form spinel, FeCr2O4, and also a -Al2O3 may combine with both FeO and Cr2O3 to form Fe-Cr-Al spinel, Fe(Cr, Al) 2O4. These internal oxide particles will ultimately coalesce together to form a more or less continuous subscale layer through which the cationic diffusion is relatively slow and the oxidation rate, therefore, settles down to a more or less steady state value. The photomicrographs of 2% Al and 4% Al alloys as shown in Fig.5 and Fig. 6 support the X-ray data in which a spinel layer in the outer region and subscale formation at the alloy/oxide interface is the principal rate-determining factor for further oxidation. A paradoxical behavior as observed in 2% Al alloy at 10500C reveals that the specimens are completely oxidized which has been proved under the optical microscopic examination, probably due to gradual loss of the Cr2O3 by volatilization and it is found to consist of Fe3O4 upon magnetic test and a dark blackish color.
Golightly et al  and R A Rapp and B Pieraggi  indicate that binary Fe-Cr alloys containing Cr of about 14-28% along with a more reactive element in the range of 4-5% initially forms an a -Al2O3 scale. The oxidation kinetics of 6% Al and 8% Al alloys represent that an external protective scale forms initially which suffers breakaway within a short period. Subsequently, a second or a third protective scale has been observed which level down the rates of oxidation to a more or less steady state. It is apparent from X-ray diffractometer data that a number of strong peaks for a -Al2O3 have been observed. Also the spall particles released from the specimens are in the form of a fine powder, non-magnetic in nature, creamy brown in color and small in quantity. Microstructures of the scales at 1050oC for the above alloys as shown in Fig. 7 and Fig. 8 show that the scale surfaces bear marks of convoluted growth with void formation at the interface region and cracked here and there. The convolutions are believed to be the result of the development of compressive stresses due to lateral growth of the oxide scale  and also due to the differential contraction of the oxide scale and metal during thermal cycling. These two factors are supposed primarily to be responsible for the formation of cracks in the scale through which contacts between the hot alloy surface and the oxidizing environment occurs.
The phenomena of multiple breakaway and repeated protective scale formation, as observed at higher temperature in the oxidation of the above alloys is due to the failure of the initial external protective scale definitely produces depletion of the alloy-content in the interface region which mat even reach the minimum level of the alloying element necessary for the reformation of the protective scale, but eventually replenished to a higher level of alloy content to enable the formation of a second protective scale. This particular behavior is widely known as healing.
It has been postulated that the generation of a complete protective scale of alumina on the alloy surface requires the aluminium-content in the alloy to be above a certain critical values, which is a function of temperature. For example, an Fe 4.9% Al alloy forms iron oxides and spinel below 570�C and alumina above 570�C. In the present study, it is observed that the 6%Al alloy did form such a protective scale whereas this was not possible with the 4%Al alloy. This finding confirms the same fact that the minimum level of Al necessary for the formation and maintenance of a -Al2O3 scale on the alloy surface is about 5% in the present instance as well.
The rate of oxidation decreases with the increase of Al-content as shown in Fig. 1 due to rapid oxidation of base metal in the lower Al alloys. It is evident from the observation of Mosely et al  that oxides of base metal (iron) initially grow more rapidly outwards from the metal surface but ultimately the surface becomes covered by a layer of the more stable oxides (alumina) and oxidation of base metal ceases in higher Al alloys.
The size of the individual spall particles is observed to be progressively finer and smaller with the increase of Al-content. As the rate of oxidation increases, oxide layers being thick and stratified and coarsen the spall particles in lower Al alloys. Distinctive color of the spall particles and of the oxidized specimens is likely due to the compositional changes in the oxide layers. As per cent Al increases in the alloy, oxidation of Fe and Cr almost ceases and Al begins to oxidize preferentially. This is why, the spall particles released from the 6%Al and 8%Al alloys has been observed to be creamy brown in color (most identical to the appearance of a -Al2O3 particles).
Al additions have been shown to mark effects upon the oxidation characteristics of Fe-10Cr at all temperatures. The scale growth process results in the generation of stresses and a convoluted scale configuration is established. Considerable deformation of the alloy substrate and eventual fracture of the oxide accompany lateral growth of the scale. For the modified Fe-10Cr alloys containing 2-8 percent aluminum, a general reduction in the rate of oxidation in terms of specific weight gain values was noticed with increasing aluminum content. The amount of spallation decreased with increasing aluminum content. The individual spall particles become smaller in size and progressively finer accompanied by a change in color from blackish to creamy brown.
Tendency towards the formation of a -Al2O3 scale was definite and conspicuous with the increase of aluminum content in the alloy. Scale thickness reduced progressively with increasing aluminum content marked by an almost complete cessation of spallation with the 8% Al alloy.
The oxide nucleation processes occurring on the alloy surface is favored by Al due to the provision extra oxide nuclei of Al oxide. A decreased internuclear distance is achieved, thus causing time required by alloy to form a protective film to be shortened. Al markedly enhances the adhesion of the Cr2O3. Failure of scale was inhibited either through the immediate reformation of an external protective scale or through the internal oxidation and subsequent formation of a sub-scale layer. A minimum level of aluminum, depending upon temperatures, exists for formation of a complete protective scale on the alloy surface.
The authors express thanks to the Lab. Assistants of the Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology. Appreciation is extended to Dr M S Islam and Dr M M Haque for their encouragement during the study.
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Fig.1 Effect of Al-content and temperature upon sp. wt. gain (48-hour basis) of the modified Fe-10Cr alloys.
Fig. 2 Influence Al-content on the oxidation kinetics of the modified Fe-10Cr alloys (2-8%) at 9500C.
Fig. 3 Comparison of cyclic oxidation kinetics of the modified Fe-10Cr alloys with 2-8%Al at 10000C
Fig. 4 Effect of Al-content on the oxidation kinetics of the modified Fe-10Cr alloys with 2-8%Al at 10500C
|Fig.5: Optical cross-section of oxide scale on an Fe-10Cr-2Al alloy specimen after exposure of 51 hours (17 3-hour cycles) at 950� C, x 300.||Fig.6: Metallographic cross-section of oxide scale on an Fe-10Cr-2Al alloy Specimen oxidized for 48 hours(17 3-hour cycles) at 1000� C, x 1200.|
|Fig.7: Microstructure of oxide scale on an Fe-10Cr-6Al alloy after 51 hours exposure (119 3-hour cycles) at 1050� C, x 1200.||Fig.8: Microstructure of oxide scale on an Fe-10Cr-8Al alloy specimen specimen oxidized for 19 3-hour cycles at 1050� C, x 1200.|
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