X-ray topographic study of β-NiAl upon growth of an α-Al2O3 film

The Berg-Barrett X-ray topographic method was employed as a microstructural technique to seek correlations of the metal substructure to the morphological features of -Al₂O₃ films grown on -NiAl. An analysis of diffraction micrographs using {112} and {002} reflections from individual grains in -NiAl revealed its subgrain structure to a depth of 30 . The dimensions of these subgrains were directly related to the density of oxide ridges in the -Al₂O₃ films and to the dimensions and shapes of cavities at the NiAl-Al₂O₃ interface.     

The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr₂O₃ layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr₂O₃ layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that the-phase has transferred to the -phase at the temperature ofthe oxidation test (1150°C). Conversely, -phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the than in the -phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.     

Application of an acoustic-emission technique to the high-temperature oxidation of Fe-Cr-Al alloys

Acoustic-emission analysis combined with thermogravimetry has been used to investigate the oxidation behavior of undoped and doped Fe-Cr-Al alloys. It was demonstrated that acoustic-emission signals which were detected upon isothermal oxidation of undoped Fe-20Cr-5Al arise from buckling of the finegrain -Al₂O₃ layer. The acoustic-emission signals which were detected upon isothermal oxidation of Fe-18Cr-12Al are attributed to repeated cracking of the coarse-grain -Al₂O₃ layer. The mass-gain curve results from superimposed diffusion-controlled oxide growth and accelerated oxide growth after cracking of the oxide layer. The present study shows that acoustic-emission analysis is very useful as it complements thermogravimetric results. The advantage of acoustic-emission analysis is that it reveals cracking and spalling of the oxide layer, which is not recorded by thermogravimetry.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

Transient oxidation in Fe-Cr-Ni alloys: A Raman-scattering study

Using Raman scattering we have investigated the oxidation, in air, of the Fe-Cr-Ni stainless steels Fe-25Cr-20Ni, Fe-25Cr-20Ni-3Zr, and Fe-24Cr-3Zr (wt.%) as a function of temperature in the range 300 to 1000°C. The Raman technique is very sensitive to, and provides a clear identification of, the oxides Fe₂O₃ and Cr₂O₃. However, the technique is insensitive to NiO, FeO, and does not give a clear identification of spinels. The Fe–Cr–Ni alloys form chromia scales at temperatures greater than 800°C. At lower oxidation temperatures, transient phases are observed. With a 1-h heat treatment at 300°C, we observe the formation of an unidentified scale; we speculate that it is either amorphous or consists of disordered spinel(s). Near 400°C we begin to observe hematite (Fe₂O₃). The intensity of the Fe₂O₃ signal increases with temperature to 600°C and then decreases, being largely replaced by the signal from Cr₂O₃. The thickness of the Cr₂O₃ scale increases with temperature up to 1000°C above which spallation becomes apparent. Spinel phases also apparently persist in the scale to 1000°C.     

Strain determination in thermally-grown alumina scales using fluorescence spectroscopy

By exploiting the strain dependence of the ruby luminescence line, we have measured room-temperature residual strains in thermally-grown alumina scales. Measurements were made on two alloys Fe-5Cr-28Al and Fe-18Cr 10Al (at.% bal. Fe), oxidized between 300–1300°C. Significantly different levels of strain buildup were observed in scales on these alloys. Results on similar alloys containing a dilute reactive element (RE) are also presented. Scales formed on RE-containing alloys (Zr or Hf) could support significantly higher strains at T 1000°C. Strain relief associated with spallation thresholds is readily observed. In early-stage oxidation, the evolution of transition phases is monitored using Raman and fluorescence spectroscopies. The fluorescence technique also provides a sensitive probe of early-stage formation of -Al₂O₃. It appears that, in the presence of Cr₂O₃ or Fe₂O₃, the -phase of Al₂O₃ can form at anomalously low temperatures.     

Microstructural Evolution of Alumina Layers on an Al–Cu–Fe Quasicrystal during High-Temperature Oxidation

The oxidation of a quasicrystal with the nominal compositionAl₆₃Cu₂₅Fe₁₂ was studied around 800°Cin environmental and synthetic air by means of thermogravimetric analysis,electron microscopy, and analytical electron spectroscopy. In an earlyoxidation stage, -Al₂O₃ formed with an orientational relationship tothe quasicrystal. At the oxide–metal interface, -Al₂O₃transformed into large hexagonal shaped -Al₂O₃grains. The change in surface morphology indicated that at theoxide–gas interface -Al₂O₃ continued togrow as -Al₂O₃. Locally the metastable aluminalayer was transformed thoroughly into -Al₂O₃,which then continued to grow with a nodular morphology. On top of the oxidenodules, several at.% of Cu²⁺ were detected.     

Improved Oxide Spallation Resistance of Microcrystalline Ni-Cr-Al Coatings

Microcrystalline Ni-20Cr-3Al coatings weredeposited on Ni-20Cr-3Al substrates by unbalancedmagnetron-sputter deposition. The grain size of thecoatings was varied by using different Ar pressures.Cyclic-oxidation testing was performed at 1100°C. It wasfound that (1) an external-Al₂O₃ scale formed oncoating A (4.7 m thick, 50 nm grain size); (2) anexternal Cr₂O₃ scale and internalAl₂O₃ oxide formed on coating B (14 m thick, 500 nm grain size);and (3) an outer layer scale ofCr₂O₃ +NiCr₂O₄ and interior layer ofAl₂O₃ formed on the as-cast alloy.Extensive spallation of the Cr₂O₃+ NiCr₂O₄ scale took place on the as-cast alloy, but no obviousspallation occurred on the two coatings. Improvement ofthe spallation resistance of the scale is explained byeffective diffusional creep of the coatings and the micropegging effect of the inward-grownoxides.     

Protection of Fe-Cr-Al alloys in sulfidizing environments by means of an α-Al2O3 scale

The sulfur corrosion behavior of ferritic Fe-22.2Cr-5.5Al and Fe-10.2Cr-5.1Al (wt.%) alloys was studied in sulfur vapor and in a 10%H ₂ S -H ₂ (vol. %) atmosphere at 900°C after preoxidation of the alloys at 1000°C in oxygen to form an -Al ₂ O ₃ scale. The immunity time before onset of sulfidation attack to form a layered scale containing chromium and iron sulfides was dependent upon the -Al ₂ O ₃ scale thickness and the nature of the sulfidizing atmosphere. In pure sulfur at low vapor pressure, =8.1×10 ^–5 atm, the sulfide scale initially developed by the diffusion of metal cations through the -Al ₂ O ₃ barrier. On the other hand, the sulfide was nucleated and grew beneath the Al ₂ O ₃ barrier when the alloys were exposed in the H ₂ S -H ₂ atmosphere at =1.5×10 ^–5 atm. It was possible to demonstate by calculations from a gas-oxide-metal model for sulfur adsorption and diffusion in the solid phases that the types of initial sulfidation attack in these atmospheres were determined by sulfur threshold concentrations at the alloy-oxide and oxide-gas interfaces.     

A solid-state EMF study of the Fe-S-O and Co-S-O ternary systems

The equilibrium oxygen potentials of the two-phase equilibria Fe₃O₄/Fe_1–xS, Co_1–xS/Co₃S₄, Co₃S₄/CoS₂, Co_1–xS/CoO, and CoO/CoSO₄ were measured as a function of temperature. A solid-state emf technique using calcia-stabilized zirconia (CSZ) solid electrolyte was used. These equilibria were studied atP _SO21 atm; the equilibrium Co_1–xS/CoO was also studied atP _SO20.1 atm. Two emf cell designs were used for the measurements atP _SO21 atm andP _SO21 and 0.1 atm, respectively. The homogeneity range of FeS in equilibrium with Fe₃O₄ and that of Co_1–xS in equilibrium with CoO at 1073 K andP _SO21 atm were measured by electron microprobe analysis.     

Phase equilibria in the subsolidus region of the NiO-α-Al2O3 system between 1000 and 1920°C

Nickel spinel, Ni_1–xAl_2(1+x/3)O₄, is the only intermediate compound in the quasibinary NiO--Al₂O₃ system at temperatures between 1000 and 1920°C. The spinel equilibrated with NiO occurs at its stoichiometric composition, NiAl₂O₄, independent of temperature. An alumina rich spinel, 0.17 x 0.62, equilibrated with -Al₂O₃ increases in alumina content with increasing temperature. Aluminum oxide solubility in NiO increases from 1 mole % at 1000°Cto 3 mol % at 1800°C. Nickel oxide solubility in -Al₂O₃ was found to increase from 2 mole % at 1000°C to 3 mole % at 1920°C.     

On the transient oxidation of a Ni-15Cr-6Al alloy

Stages in the development of a protective -Al₂O₃ scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al₂O₃ layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr₂O₃. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al₂O₃ upon incorporation into the external oxide scale.     

The initial stages of high-temperature corrosion of Fe-Cr-Ni and Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

The initial corrosion reactions on a number of Fe-Cr-Ni and Cr-Ni alloys when exposed in a H₂-CH₄ gas mixture ofa _c =0.8 and 10^–30 bar at1098 K have been investigated. All alloys were studied with surfaces in boththe cold-worked and electropolished conditions. The experiments, in whichexposure times up to 400 rain were used, were carried out in a conventionalcorrosion rig and also in a small reaction chamber contained within an X-rayphotoelectron spectrometer. The use of XPS, SEM, and TEM showed thattransient -Cr₂O₃ layers formed on all alloy surfaces during the heat-up period.On the electropolished surfaces, the oxide layers developed nonuniformly withrespect to microstructure and thickness, whereas homogeneous, compact, andfine-grained -Cr₂O₃ layers formed on the cold-worked specimens. Followingan incubation period, M₇C₃ carbides nucleated on the surfaces; they grew byconsuming the less stable -Cr₂O₃ and also through diffusion of metal fromthe alloy substrate. The mode of nucleation of the M₇C₃ and its rate of growthwere strongly dependent on the character of the transient -Cr₂O₃ layers. Afterremoval of -Cr₂O₃, carbon ingressed into the substrate where M₂₃C₆ wasformed internally. These results are broadly in line with the thermodynamicstability predictions and are discussed in terms of simple models.     

Improvement in the Oxidation Resistance of an Al-Deposited Fe–Cr–Al Foil by Preoxidation

The oxidation kinetics of conventional Fe–20Cr–5Al (in mass %) foil, Al-deposited foil and Al-deposited and preoxidized foil was studied at 1373 K in air. All the foils were 50-m thick and contained minor additions of rare-earth elements. The oxide scales were observed with SEM and TEM combined with EDS and were characterized with X-ray diffractometry and electron diffraction. The deposition of Al onto the foil from the vapor phase improves oxidation resistance. The details regarding this matter were reported elsewhere. The combination of the Al deposition and the subsequent preoxidation at 1173 K for 90 ks in air further increases the oxidation resistance, i.e., the smallest parabolic rate constant among the three kinds of foils, and excellent scale adherence. Preoxidation enhances the growth of -Al₂O₃, which transforms to -Al₂O₃ during subsequent oxidation. However, such -Al₂O₃ grains are much larger than those formed on the conventional foil of similar chemical composition. Small closed voids and small spinel-type, oxide particles appear in -Al₂O₃ grains with the progress of oxidation. The former is explained in terms of the volume decrease accompanying the phase transformation and the latter by the low solubility of Fe in -Al₂O₃.     

Constitution and Crystallography of Thin Thermal-Oxide Layers on varepsilon -Fe2N1-x: A HREM Investigation

Oxide layers formed on varepsilon -Fe₂N_1-x wereinvestigated with X-ray photoelectron spectroscopy, X-ray diffraction, andin particular with high-resolution transmission electron microscopy. Priorto oxidation, the varepsilon -Fe₂N_1-x substrates wereeither exposed to air at room temperature, or subjected to asputter-cleaning pretreatment, or to a sputter cleaning with an additionalannealing pretreatment. The samples were oxidized at 400 or 573 K in pureO₂ at 8.0x10^-2 Pa or at 10⁵Pa. All oxide films contained magnetite ( -Fe₂N_1-x)as a major constituent. On samples that were sputter-cleaned and annealedprior to oxidation as well as on the air-exposed sample, wüstite,(Fe₁-O) was observed between the Fe₃O₄and -Fe₂N_1-x. On the basis of HREM, thiswüstite phase, which on -Fe does not develop at temperaturesbelow 843 K, was concluded to have a preferred crystallographic orientationwith respect to the supporting -Fe₂N_1-x grains. Nospecific orientation relation was found between Fe₃O₄ and -Fe₂N_1-x. The implications of the developmentof Fe_1-O for the evolution of the oxide film arediscussed.     

Selective oxidation of Fe-Cr alloys

The phenomenon of selective oxidation is briefly reviewed with particular reference to the Fe-Cr system. The results of experiments with a binary Fe-13%Cr alloy and a commercial steel, carried out over the temperature range 400–800°C in various hydrogen-water vapor mixtures are reported. Modes of growth are postulated and the protective qualities of the films shown to be dependent more upon morphology and structure than upon composition. Although protective, -Cr₂O₃ films can be grown on the binary alloy, the presence of 0.7%Mn in the commercial steel results in the preferential formation of a nonprotective Mn-Cr spinel. In addition to -Cr₂O₃ and the Mn-Cr spinels other oxide phases are identified and their effects considered.     

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.     

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co ₃O₄ formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (10 ) and the spinel comprised small grains (3 ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co₃O₄.     

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

Low-temperature oxidation of Fe-24 wt.%Cr

Ferritic polycrystalline Fe-24 wt.% Cr was oxidized in pure oxygen at 190 T490° C and pressures in the range 5.3×10^–2–13.3 Pa for periods of up to 5 hr. The reaction proceeded in three stages. An initial period of accelerating rate was accompanied by oxide island nucleation and growth. Following island coalescence the rate was approximately logarithmic at low temperatures and somewhat slower than parabolic at high temperatures. Rate control during this period was thought to be due to mass transport through the oxide grain boundaries left by the island impingement process. During these first two stages the oxide formed was -M₂O₃ with possibly some spinel. The final stage of reaction involved the appearance of -M₂O₃ on the outer oxide surface and a substantial slowing of the oxidation rate due to the low diffusivity in this phase.