The oxidation of Fe-19.6Cr-15.1Mn stainless steel

The oxidation behavior in air of Fe-19.6Cr-15.1Mn was studied from 700 to 1000°C. Pseudoparabolic kinetics were followed, giving an activation energy of 80 kcal/mole. The scale structure varied with temperature, although spinel formation occurred at all temperatures. At both 700 and 800°C, a thin outer layer of -Mn₂O₃ formed. The inner layer at 700°C was (Fe,Cr,Mn)₃O₄, but at 800°C there was an intermediate layer of Fe₂O₃ and an inner layer of Cr₂O₃ + (Fe, Cr,Mn)₃O₄. Oxidation at 900°C produced an outer layer of Fe₃O₄ and an inner layer of Cr₂O₃+(Fe,Cr,Mn)₃O₄. Oxidation at 1000°C caused some internal oxidation of chromium. In addition, a thin layer of Cr₂O₃ formed in some regions with an intermediate layer of Fe₃O₄ and an outer layer of (Fe,Mn)₃O₄. A comparison of rates for Fe₃O₄ formation during oxidation of FeO as well as for the oxidation of various stainless steels, which form spinels, gave good agreement and strongly suggests that spinel growth was rate controlling. The oxidation rate of this alloy (high-Cr) was compared with that of an alloy previously studied, Fe-9.5Cr-17.8Mn (low-Cr) and was less by about a factor of 12 at 1000°C and by about a factor of 100 at 800°C. The marked differences can be ascribed to the destabilization of wustite by the higher chromium alloy. No wustite formation occurred in the high-Cr alloy, whereas, extensive wustite formed in the low-Cr alloy. Scale structures are explained by the use of calculated stability diagrams. The mechanism of oxidation is discussed and compared with that of the low-Cr alloy.     

Oxidation of Fe-8Cr-14Ni-Al-Si-Mn from 700 to 1000°C

This paper reports an investigation into reducing the Cr concentration in commercial-grade stainless steels while maintaining oxidation protection at elevated temperatures. Aluminum and Si were added as partial substitute alloy elements to enhance the reduced operation protection resulting from Cr concentration reduced by approximately 50 pct of that found in stainless steels. The goal of this study was to determine the oxidation mechanism of such an Fe, Al-Si alloy: Fe-8Cr-14Ni-1Al-3.5Si-1Mn. During the initial oxidation period the protection resulted from a thin film of Al₂O₃ over an Fe and Cr spinel. Long-term oxidation protection resulted from the gradual formation of a Cr sesquioxide (Cr₂O₂) inner oxide layer. Eventually an outer oxide layer formed that was a mixed composition spinel of Cr and Mn (MnO · Cr₂O₃). The Al₂O₃, which was part of the original protective layer flaked off early in the oxide testing, and the aluminum oxide that formed later appeared as an internal oxide precipitate.     

The development of silicon-containing oxides during the oxidation of iron-chromium-base alloys

The influence of silicon on the oxidation of Fe-14% Cr and Fe-28% Cr has been studied at high temperature, with particular emphasis on the development and nature of the healing SiO₂ layer. In general, silicon is a less effective addition than aluminium to these alloys in improving oxidation resistance because SiO₂ grows at a lower rate than α-Al₂O₃. Hence, silicon is a less successful oxygen secondary getter and development of a complete healing layer of SiO₂ is less rapid than that of α-Al₂O₃ on a corresponding aluminium-containing alloy. Nonetheless, the addition of only 1% Si to Fe-28% Cr causes a marked reduction in the overall oxidation rate, particularly by facilitating development of the Cr₂O₃ scale. Precipitates of SiO₂ form at the alloy/scale interface. These grow inwards and laterally until they eventually link up to establish a continuous healing layer at the interface after several hundred hours exposure at 1000°C. Similar features are observed for Fe-14% Cr-3% Si but the healing SiO₂ layer develops after a much shorter time for Fe-14% Cr-10% Si, due to the high silicon availability. In every case, the healing layer has been shown to be amorphous SiO₂. Although this phase is very protective during isothermal oxidation, it is a site of weakness during cooling and scale spallation is very extensive from specimens where the SiO₂ is continuous, with failure occurring cohesively within that layer. Ion implantation of silicon into Fe-14% Cr and Fe-28% Cr gives a reduced oxidation rate due to facilitation of a more rapid establishment of a Cr₂O₃ scale. Similar implantation of yttrium into the ternary alloys assists in development of the silicon-containing oxide layer, possibly associated with an influence on the nucleation of the oxide precipitates in the early stages of exposure.     

High-temperature corrosion of titanium-, niobium-, and manganese-rich Fe-25Cr alloys in H2-H2O-H2S gas mixtures

The simultaneous sulfidation and oxidation of Fe-25Cr, Fe-25Cr-4.3Ti, Fe-25Cr-7.5Nb, and Fe-25Cr-9.0 Mn alloys were studied at 1023, 1123, and 1223 K, respectively, in H₂-H₂O -H₂S gas mixtures. The influences of titanium, niobium, and manganese on the transition from protective oxide formation to the formation of sulfide-rich corrosion products of Fe-25Cr alloys have been investigated. It has been found that additions of titanium and niobium can improve the scaling resistance of Fe-25Cr alloys against sulfidation in H₂ -H₂O -H₂S gas mixtures at high temperatures. However, the addition of manganese does not increase the resistance to sulfidation of Fe-25Cr alloy. The oxide Cr₂Ti₂O₇, which can suppress sulfide formation, formed on the Fe-25Cr-4.3Ti alloy. The addition of manganese to Fe-25Cr does not form more stable and protective oxides than Cr₂O₃ which formed on Fe-25Cr. Thermodynamic stability diagrams are used to explain the experimental results.     

The effect of silicon and manganese on the oxidation mechanism of Ni-20 Cr

Additions of 3% silicon or manganese to Ni-20 Cr reduced the oxidation rate, whereas additions of 1% had little effect. Three percent silicon alloys formed an inner scale of SiO₂, and 3% manganese alloys formed an inner spinel layer of essentially pure MnCr₂O₄. The experimentally determined solid-state growth rate of NiCr₂O₄ was about 1000 times slower than the growth rate for Cr₂O₃. It has been established that the protective layer on Ni-20 Cr (Nichrome alloys) is the spinel and not Cr₂O₃ as previously postulated. The mechanism for scale growth is discussed for Ni-20 Cr alloys.This work was performed at Stanford Research Institute, Menlo Park, Calif. and was supported by the National Aeronautics amd Space Administration, Contract NAS 3-11165.     

The mechanism of corrosion of Fe-9%Cr alloys in carbon dioxide

Oxide layers have been grown on Fe-9% Cr, Fe-9% Cr-0.3% Si, and Fe 9% Cr-0.6% Si alloys in carbon dioxide at 853 °K. It is known that such oxides are duplex, the outer layer being magnetite, formed by iron transport. The inner layer is Fe-Cr spinel but little is known about its growth mechanism so this has been investigated using oxygen-18 as a tracer. Oxides were grown first in C¹⁶O₂ and then in C¹⁸O₂ and the distribution of oxygen-18 in the scale measured using nuclear techniques. For all the alloys used, significant amounts of oxygen-18 were observed within the inner layer in addition to growth of¹⁸O-rich magnetite at the outer surface. The two possibilities of the oxygen-18 being present as a consequence of isotopic exchange or because new oxide had formed within the spinel layer are discussed. Our conclusion is that it is very unlikely that significant isotopic exchange had occurred in any part of the scale, and we deduce that at least a substantial amount of the oxygen-18 in the inner layer was deposited as a result of new oxide formation within that layer. The results also indicate that the location of growth sites within the inner layer differed between the alloys.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

Transient Oxidation of a γ-Ni–28Cr–11Al Alloy

γ-NiCrAl alloys with relatively low Al contents tend to form a layered oxide scale during the early stages of oxidation, rather than an exclusive α-Al₂O₃ scale, the so-called “thermally grown oxide” (TGO). A layered oxide scale was established on a model γ-Ni–28Cr–11Al (at.%) alloy after isothermal oxidation for several minutes at 1100°C. The layered scale consisted of an NiO layer at the oxide/gas interface, an inner Cr₂O₃ layer, and an α-Al₂O₃ layer at the oxide/alloy interface. The evolution of such an NiO/Cr₂O₃/Al₂O₃ layered structure on this alloy differs from that proposed in earlier work. During heating, a Cr₂O₃ outer layer and a discontinuous inner layer of Al₂O₃ initially formed, with metallic Ni particles dispersed between the two layers. A rapid transformation occurred in the scale shortly after the sample reached maximum temperature (1100°C), when two (possibly coupled) phenomena occurred: (i) the inner transition alumina transformed to α-Al₂O₃, and (ii) Ni particles oxidized to form the outer NiO layer. Subsequently, NiO reacted with Cr₂O₃ and Al₂O₃ to form spinel. Continued growth of the oxide scale and development of the TGO was dominated by growth of the inner α-Al₂O₃ layer.     

High-Temperature Oxidation Behavior of SIMP Steel at 800 °C

In this work, the high-temperature oxidation behavior of SIMP and commercial T91 steels was investigated in air at 800 °C for up to 1008 h. The oxides formed on the two steels were characterized and analyzed by XRD, SEM and EPMA. The results showed that the weight gain and oxide thickness of SIMP steel were rather smaller than those of T91 steel, that flake-like Cr₂O₃ with Mn_1.5Cr_1.5O₄ spinel particles formed on SIMP steel, while double-layer structure consisting of an outer hematite Fe₂O₃ layer and an inner Fe–Cr spinel layer formed on T91 steel, and that the location of the oxide layer spallation was at the inner Fe–Cr spinel after 1008 h, which led the ratio between the outer layer and the inner layer to decrease. The reason that SIMP steel exhibited better high-temperature oxidation resistance than that of T91 steel was analyzed due to the higher Cr and Si contents that could form compact and continuous oxide layer on the steel.     

Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe-25Cr-35Ni-2.5Al-XNb alloys at 1000°C and 1100°C

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-_XNb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the high-temperature oxidation resistance. A bilayer oxide scale with a Cr₂O₃-rich outer layer and Al₂O₃-rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al₂O₃ scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb₂O₅) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al₂O₃ scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr₂O₄, Cr₂O₃, and Fe₂O₃ outer layers and Al₂O₃ inner layer.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Cyclic and isothermal oxidation behavior on some Ni-Cr alloys

Additions of 3 wt.% Mn and 3 wt.% Si were made to Ni-20Cr. These alloys, along with Ni-20Cr and Ni-40Cr were oxidized for 100 1-hr cycles at 1100°C and 50 1-hr cycles at 1200° C. Oxidation behavior was judged by sample weight and thickness change, metallography, x-ray diffraction, and electron microprobe analysis. These tests showed that Ni-40Cr and Ni-20Cr-3Si were about the same and were the most oxidation-resistant alloys. Ni-20Cr-3Mn was not as oxidation resistant, especially at 1200° C. Ni-20Cr was far less oxidation resistant than any of the other alloys. The Ni-40Cr and Ni-20Cr-3Si relied on a protective layer of Cr₂O₃ for their oxidation resistance. A SiO₂ layer was noted beneath the Cr₂O₃ layer on the Ni-20Cr-3Si, but had apparently only a second-order effect. The source of improved protection of the Ni-20Cr-3Mn was apparently the formation of a relatively adherent MnCr₂O₄ layer at the metal-oxide interface.     

Diffusion of cations in chromia layers grown on iron-base alloys

Diffusion of the cations Cr, Fe, Mn, and Ni in Cr₂O₃ has been investigated at 1173 K. The diffusion measurements were performed on chromia layers grown on the model alloys Fe-20Cr and Fe-20Cr-12Ni in order to consider effects of small amounts of dissolved alien cations in Cr₂O₃. The samples were diffusion annealed in H₂-H₂O at an oxygen partial pressure close to the Cr₂O₃/Cr equilibrium. For all tracers the lattice-diffusion coefficients are 3–5 orders of magnitude smaller than the grain-boundary diffusion coefficients. The lattice diffusivity of Mn is about two orders of magnitude greater than the other lattice-diffusion coefficients, especially in Cr₂O₃ grown on Fe-20Cr-12Ni. The values of the diffusion coefficients for Cr, Fe, and Ni are in the same range. Diffusion of the tracers in Cr₂O₃ grown on different alloys did not show significant differences with the exception of Mn.     

The Sulfidation Behavior of Several Commercial Ferritic and Austenitic Steels

The sulfidation behavior of C-steel, 1Cr-0,5Mosteel, 12Cr-1Mo-0.25V steel, 18Cr-10Ni-Ti steel, thebinary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr, and pure Cr wasinvestigated between 400 and 700°C in a94Ar-5H₂-1H₂S gas mixture. All steels sulfidize according tocomplex kinetics which, after a period with decreasingrate, can be approximated by a linear rate law. Thescale of the three ferritic steels consists of two layers, an outer outward-growing one of FeSwith traces of dissolved Cr and an inner, inward-growingone, which contains in addition to Fe the alloyingelements Cr and Mn. Most of the outer FeS layer is separated from the inner layer and can be splitinto several partial layers, the number increasing withincreasing sulfidation time and temperature. The scaleon the austenitic 18Cr-10Ni-Ti steel differs insofar as that of the ferritic steels as theouter FeS layer contains some Ni and that a third layerof the spinel FeCr₂S₄ is formedbetween the outer and the inner layer. This intermediatelayer is responsible for the lower sulfidation rate of this materialcompared with that of the ferritic steels. The scale ofthe binary Fe-Cr alloys is similar to that of theaustenitic steel. From AE-measurements it can be deduced that the separation of the outer FeSlayer occurs during isothermal sulfidation and isaccompanied by an increase in the AE event rate. Theseparation is a consequence of the formation and growth of pores in the region close to the inner/outerlayer interface and the development of compressivegrowth stresses in the outer FeS layer. While detachmentof the FeS layer on the ferritic steels was already observed at 400°C, the austenitic steelshowed a similar separation of the FeS layer only at600°C. The detached FeS layer is obviously rathergas tight. Differences in the sulfur partial pressure ofthe bulk gas and the gas in the cavity between theinner and separated outer layer lead to a reduction ofFeS at the inner surface of the detached FeS layer. TheFe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outerFeS surface. This process not only shifts the detachedFeS layer continuously away from the core of thespecimen but offers also the possibility of healing cracks in the separated FeS layer. This scaledetachment does not stop scale growth. After scaleseparation the total sulfidation reaction consists of atleast seven partial reactions: phase-boundary reaction at the outer surface, diffusion of iron ionsand electrons outwards in the detached FeS layer,formation of H₂S at the inner surface of thedetached layer, gas diffusion in the cavity, formationof FeS on top of the porous inner layer, gas diffusionin the channels of the porous inner layer, FeS formationat the metal/scale interface. When the new FeS layer ontop of the porous inner layer exceeds a critical thickness, the detachment of the FeSlayer from the inner porous layer repeats. This processcan take place several times, leading to an outer FeSpartial scale, split into several layers, which are separated by relatively large cavities andkept together only locally by FeS bridges. The overallreaction rate is controlled by the phase-boundaryreaction at the outermost FeS surface.     

Roles of Manganese in the High-temperature Oxidation Resistance of Alumina-forming Austenitic Steels at above 800?°C

A new family of alumina-forming austenitic (AFA) stainless steels is under development for uses in aggressive oxidizing conditions. This paper investigates the effect of manganese additions on the oxidation kinetics and alumina scale formation in two series of AFA steels, i.e., Fe–20Ni–14Cr–2.5Al and Fe–18Cr–25Ni–3Al base. At 800?°C in dry air, the oxidation resistance was moderately degraded with additions of larger than 1 wt% Mn in the AFA steels based on Fe–14Cr–20Ni–2.5Al. At 900?°C in air with 10?% water vapor, however, additions of Mn in these AFA steels based on Fe–18Cr–25Ni–3Al would significantly destabilize the alumina scale formation and degrade the oxidation resistance. Our analysis revealed that additions of Mn stimulated formation of the coarse spinel CrMn_1.5O₄ and Cr₂O₃ oxide and destroyed the continuity of the protective alumina scales, thus worsening the oxidation performance. In addition, it was found that there exists an upper limit for the Mn additions which is decreased with the increase of the service temperatures and presence of aggressive oxidizing agents.     

High temperature corrosion of Fe-Cr-Mn-alloys in H2-H2S and H2-H2O-H2S gas mixtures

The sulfidation of Fe-20% Cr-30% Mn, Fe-25%Cr-20%Mn and Fe-25% Cr was studied at 700°C in H₂-H₂S and the oxidation and sulfidation in H₂-H₂O-H₂S after preoxidation in H₂-H₂O. The sulfidation rate is strongly increased for the Mn-containing alloys, layers of (Mn,Cr)S and (Mn,Fe)Cr₂S₄ are formed. Also the oxidation rate is enhanced compared to Fe-25% Cr by formation of MnCr₂O₄ instead of Cr₂O₃. The sulfidation after preoxidation leads to internal and external sulfidation of the Mn-containing alloys. With increasing oxygen pressure p(O₂) = 10^?26…10^?22 atm. of the H₂-H₂O-H₂S mixtures the sulfidation is suppressed, for the higher oxygen pressure 10^?23 and 10^?22 atm. fast oxidation prevails under formation of MnCr₂O₄. Manganese cannot increase the sulfidation resistance of alloys, in spite of the stability and low degree of disorder of its sulfide, since the mixed sulfide (Mn,Cr)S is formed which has a high degree of disorder, high diffusivities and high growth rate according to the doping effect of trivalent Cr³⁺.     

Oxidation of Fe-3 wt.% Cr alloy

The oxidation behavior and the oxide microstructure on Fe-3 wt. % Cr alloy were investigated at 800°C in dry air at atmospheric pressure. Two distinct oxidation rate laws were observed: initial parabolic oxidation was followed by nonparabolic growth. The change in the oxidation kinetics was caused by microchemical and microstructural developments in the oxide scale. Several layers developed in the oxide scale, consisting of an innermost layer of (Fe,Cr)₃O₄ spinel, an intermediate layer of (Fe,Cr)₂O₃ sesquioxide, and two outer layers of Fe₂O₃ hematite, each with different morphologies. Wustite (Fe_1–xO) and distorted cubic oxide (-(Fe,Cr)₂O₃) were observed during the iniital parabolic oxidation only.     

Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600–700°C under 25?MPa were carried out for up to 1000?h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni–Fe/Ni–Cr spinel oxide, Ni/Co hydroxide and TiO₂ and an inner layer of a mixture of NiCr₂O₄ and Cr₂O₃ while the innermost layer is made up of Cr₂O₃. The MoO₃ can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.     

High Temperature Corrosion of Chromium–Manganese Steels in Sulfur Dioxide

This work was aimed at explaining the corrosion mechanism of commercial Cr–Mn steels at 1073, 1173 and 1273 K in the atmospheres containing oxygen and sulfur. Three steels were selected for the investigations, two single-phase austenitic steels (Cr17Mn17 and Cr13Mn19SiCa) and a two-phase austenitic-ferritic steel Cr15Mn19. On all studied steels triplex scales were formed. The inner very thin, fine-grained part of the scale contained manganese, chromium and iron sulfides and oxides, the intermediate layer was built mainly of the MnCr₂O₄ spinel while MnO was the predominant constituent of the outer scale layer. According to the gravimetric measurements, after an initial incubation period, the oxidation of steel follows a parabolic rate law. Thermodynamic and kinetic aspects of the formation of oxide-sulfide and oxide layers were discussed. Oxidation was accompanied by depletion of the subscale region of the metallic core in manganese, which is the austenite former. Consequently austenite transformed into ferrite.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.