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.     

Some interactions in the erosion-oxidation of alloys

The modes of interaction of erosion and high-temperature oxidation, occurring simultaneously on an alloy surface, have been studied using Ni-30Cr, MA754, Ni-20Al, and Co-22Cr-11Al-0.2Y alloys to examine the influence of chromia and alumina scale formation on the erosion of nickel and cobalt base alloys. The results have shown that, in the presence of a rapidly flowing oxidizing gas stream, the evaporation of volatile metal oxides becomes important at lower temperatures where normally it can be ignored. Otherwise, the erosion and oxidation of alloys parallels the behavior of pure metals but also introduces additional factors derived from lengthening of the period of transient oxidation and modification of concentration profiles in the alloy adjacent to the alloy/scale interface. Higher erosion intensities extend the transient oxidation behavior which adversely affects the formation of protective scales. As with pure metals, the presence of erosion and oxidation together always produced increased rates of degradation.     

The effect of Mo on the high-temperature sulfidation of Ni

The sulfidation behavior of Ni-Mo alloys containing up to 40 wt.% Mo was studied at =0.01 atm. over the temperature range of 550–800°C. The alloys included two solid solutions (Ni-10Mo and Ni-20Mo), the single-phase intermetallic compound Ni₄Mo(Ni-29Mo), and two alloys which were two-phase, Ni-30Mo and Ni-40Mo (Ni₄Mo+Ni₃Mo). The sulfidation of all alloys followed the parabolic rate law. The rate of sulfidation decreased with increasing amounts of Mo. Activation energies for sulfidation gave values of 39.1±1.0 kcal/mol. The sulfide scales were bilayered, consisting of an outer layer nickel sulfide (NiS_1+x and Ni₃S₂) and an inner, complex layer of MoS₂ plus intermetallic particles. The rate-controlling step of the sulfidation for the alloys was inward sulfur diffusion and/or outward nickel diffusion through the inner MoS₂ layer. Neither selective sulfidation nor internal sulfidation were observed. No significant difference in the sulfidation kinetics, sulfide structure, and scale constitution could be noted between single-phase alloys and two-phase alloys. The location of the markers was the interface between the inner and outer layers, indicating that the inner layer formed by inward diffusion of sulfur, and the outer layer grew by outward nickel diffusion. The inability to form a continuous protective molybdenum sulfide layer is discussed in terms of the structure of MoS₂ and changes caused by intercalation of Ni into the layered crystal structure. The decrease in sulfidation rate with increasing Mo was attributed to increasing amounts of the intermetallic compound. The increasing volume fraction of particles decreased the available diffusion area in the inner layer and provided a blocking effect.     

The Oxidation of Two Ternary Ni–Cu–5 at.%Al Alloys in 1 atm of Pure O2 at 800–900°C

The oxidation of a Ni-rich and a Cu-rich single-phase ternary alloy containing about 5at.% aluminum has been studied at 800 and 900°C under 1atm O₂. The behavior of the Ni-rich alloy is similar to that of a binary Ni–Al alloy with a similar Al content at both temperatures, with formation of an external NiO layer coupled to the internal oxidation of aluminum. The Cu-rich ternary alloy shows a larger tendency to form protective alumina scales, even though its behavior is borderline between protective and non-protective. In fact, at 800°C, after an initial stage of fast reaction during which all the alloy components are oxidized, this alloy is able to develop a continuous layer of alumina at the base of the scale which prevents the internal oxidation of aluminum. On the contrary, at 900°C the innermost alumina layer undergoes repeated rupturing followed by healing, so that internal oxidation of Al is only partly eliminated. As a result, the corrosion kinetics of the Cu-rich ternary alloy at 900°C are much faster than at 800°C and very similar to those of pure copper and of Al-dilute binary Cu–Al alloys. Possible reasons for the larger tendency of the Cu-rich alloy to form external alumina scales than the Ni-rich alloy are examined.     

The oxidation mechanism of Ni3Al containing yttrium

The high-temperature oxidation behavior of Ni₃Al (Ni-13.2 wt.% Al) with and without additions of 0.5 wt.% yttrium has been studied over the range of 900–1200°C in air. None of the commonly accepted rate laws were followed by the kinetics. Although the weight gains of samples containing yttrium were consistently 10–20% greater than those without yttrium, the steady-state scaling rates were identical. A quantitative x-ray diffraction technique was used to determine the kinetics of growth of the protective alpha-alumina layer (one of several oxides formed). The alumina growth followed the parabolic rate law under all conditions studied. The rate-controlling transport process in alumina was the enhanced diffusion of oxygen down grain boundaries. The presence of yttrium as nickel-rich intermetallics promoted the formation of nickel aluminate (spinel). A marked increase in scale adherence was observed for short times. At longer times, however, the outer layer of spinel and unreacted nickel oxide spalled off along with some of the inner alumina layer. Loss of adherence was caused by a complex yttrium-aluminum oxide which formed by the solid-state reaction of yttria and alumina. The poor scale adherence on Ni₃Al was due to the formation of voids at the alloy-oxide interface. These voids concentrated the athermal stresses above the oxide-to-metal adherence strength. The voids were produced as a result of the selective oxidation of aluminum resulting from a Kirkendall effect in the substrate. During the selective oxidation process, a vacancy flux directed from the matrix to the metaloxide interface resulted in a supersaturation of vacancies. Equilibrium was maintained by the condensation of excess vacancies. The presence of yttrium as either nickel-rich intermetallics or internal oxide prevented the voids from forming. The yttrium-rich particles relieved the matrix of vacancy supersaturation by providing vacancy sinks. The chemical nature of the particles does not seem important. A necessary and sufficient condition for an effective vacancy sink appears to be the presence of an incoherent boundary between particle and matrix.This work is based on a portion of the dissertation of J. D. Kuenzly, accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering, University of California, Los Angeles, California, 1973.Supported by the Advanced Research Projects Agency of the Department of Defense under Contract No. DAHC 15-70-G-15.     

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of a-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of an₂-Ti₃Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN₂ + 20% O₂ environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of the₂ intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the ₂ phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.     

The Influence of Erosion-Induced Roughness on the Oxidation Kinetics of Ni and Ni-20Cr Alloys

Surface roughness plays a dominant role inincreasing the oxidation rate of metals and alloysduring erosion compared to the oxidation rate in staticair. Ni and Ni-20Cr were eroded at two different impact velocities (35 and 65 m/s) and for twodifferent impact angles (90° and 30°). Theeroded samples were subsequently isothermally oxidizedin static air at three different test temperatures. Theincreased oxidation kinetics in the case of Ni could beexplained on the basis of increased roughness caused byerosion prior to oxidation. In the case of Ni-20Cr, theeffect of increased roughness on oxidation was largely offset by the fact that the number ofgrain-boundary diffusion paths decreased due tocoarsening of the grains of the oxide scale.     

Resistance of Ni-Cr-Al alloys to cyclic oxidation at 1100 and 1200°C

A series of Ni-rich alloys in the Ni-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 sample weight-change data for each sample were then used to determine both a scaling constant k₁ and a spalling 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 + 10 k₂).Log K_a was then fitted to a fourth-order regression equation as a function of the Cr and Al content at the two test temperatures. The derived estimating equations for log K_a were presented graphically as iso-attack contour lines on ternary phase diagrams at each temperature. At 1100°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-30 at. % Cr-20 at. % Al, while at 1200°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-35 at. % Cr-15 at. % Al. In general, good cyclic oxidation resistance is associated with Al₂O₃ and/or NiAl₂O₄ formation. The analysis also indicated that alloys prepared by zirconia crucible melting, compared to other types of melting, had tramp Zr pickup, which significantly improved the cyclic oxidation resistance. The nature of the improvement in oxidation due to tramp Zr pickup, however, is not yet understood.     

三种β-NiAl相材料的短期热腐蚀行为比较研究

在Ni-30Al(mass%,下同)和Ni-20Cr-10Al铸态合金上进行粉末包埋渗铝,分别得到纯β-NiAl相涂层和β-(Ni,Cr)Al相涂层.在这两种渗铝试样和铸态Ni-20Cr-13Al合金(含β-NiAl、γ′-Ni3Al、α-Cr相)上涂覆了硫酸钠薄膜,进行900℃热腐蚀测试.经过10小时热腐蚀测试后,三种试样表面均生成Al2O3膜.腐蚀动力学分析表明,Cr对提高材料的抗热腐蚀性能是有益的;Ni-20Cr-10Al渗铝涂层试样的增重最低,仅为铸态Ni-20Cr-13Al合金试样增重的80%左右,Ni-30Al渗铝涂层试样增重的50%左右.     

High temperature corrosion of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two nickel-base alloys, Ni-20Cr and Ni-15Cr-8Fe, in the form of wire specimens, have been exposed to 100 mbar of sulfur dioxide between 550 and 850°C. For Ni-20Cr, an outer Cr₂O₃ layer formed only at the beginning of the reaction, but very quickly Ni₃S₂ grew preferentially at the exterior by outward diffusion of nickel. The reaction rate is regulated by an external interfacial process. A barrier effect was noted near 645°C associated with the formation of NiCr₂O₄; a new acceleration takes place above 680°C. The external growth of Ni₃S₂ is attributed to the low radius of curvature of the samples. For Ni-15Cr-8Fe, the reaction mechanism is rather similar, except that no barrier effect occurred. A protective Cr₂O₃ layer formed above 800°C in both cases.     

Influence of Powder Particle Size on the Oxidation Behavior of a PM Ni3Al Alloy

The influence of particle size on the oxidationbehavior of Ni₃Al prepared by powdermetallurgy (PM) was investigated in the temperaturerange of 535 to 1020°C for exposures up to 200 hr.Four alloys were obtained, each one processed with a differentpowder particle size fraction (<25, 25-50, 50-100,and 100-200 m). For temperatures below 730°C,the scale consists of an outer NiO layer, a thindiscontinuous intermediate nickel layer, and an internaloxidation zone. The lowest oxidation rate corresponds tothe material with the smallest particle size. Thisresults from its higher grain-boundary density; the boundaries act as easy-diffusion paths foraluminum leading to the rapid formation of a continuousinner alumina layer. At temperatures above 730°C, athree layered scale is observed consisting of an outer NiO layer, an intermediate layer that,depending on temperature, consisted of a mixture ofnickel and aluminum oxides orNiAl₂O₄, and an inner layer ofAl₂O₃, which accounts for thehigher oxidation resistance. The oxidation attack is characterized byintrusions of the scale into the alloy, the intrusionnumber increasing as the particle size decreases. It isassumed that oxide particles and impurities present at the original particle boundaries facilitatealumina growth along these regions. Thus, the lowestoxidation rate for the highest temperature rangecorresponds to the largest particle-sizematerial.     

Oxidation resistance of aluminized coatings on a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Although a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy has good high-temperature mechanical properties, it does not have adequate oxidation resistance for prolonged exposure to high surface temperatures. Thus the oxidation behavior of several aluminized coating systems on this alloy in flowing air at temperatures of 900 to 1100°C under isothermal and thermal cycling conditions has been investigated. Attempts to produce an oxidation-resistant system by direct aluminizing have not been successful since removal of carbide fibers results in a porous coating which gives little protection to the alloy. The deposition of a layer of nickel or a Ni-20%Co-10%Cr-4%Al alloy on the eutectic prior to aluminizing gives improved isothermal oxidation resistance for prolonged exposure to high surface temperatures. Thus the eutectic alloy substrate occur during thermal cycling. A more successful system has been produced by depositing a thin layer of platinum on the eutectic alloy prior to aluminizing. Protective -Al₂O₃ scales are formed and maintained during isothermal and thermal cycling oxidation at 900 and 1000°C. Similar scales are developed at 1100° C although these do break down during thermal cycling. However, surface -Al₂O₃ scales are able to re-form rapidly, thereby preventing excessive oxidation of the coating.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. I: Oxidation Kinetics and Morphology at 800°C

The oxidation of nickel-matrix/aluminum-particle composite coatings was studied using thermogravimetric (TG) analysis in air at 800°C for up to 100 hr. Long-term oxidation behavior was investigated with furnace exposures up to 2000 hr. The coatings were applied to nickel substrates by the composite electrodeposition technique and vacuum heat treated for 3-hr at 825°C prior to oxidation testing. The heat-treated coatings contained a two-phase (Ni)+(Ni₃Al) microstructure and the overall coating composition was approximately 7 wt.% Al. Also examined were uncoated nickel substrates and bulk Ni–Al alloys containing 6.2, 9.0, and 14 wt.% Al. For all samples, mass-gain kinetics were obtained from thermogravimetric (TG) experiments and furnace exposures and the composition and morphology of the oxidation products were examined using optical microscopy, scanning-electron microscopy (SEM), electron-probe microanalysis (EPMA), and X-ray diffraction (XRD). An outer NiO layer and an inner -Al₂O₃ layer formed on the composite-coating surface. The addition of a small amount of Si (about 1–2 at.%) was found to have little effect on Ni–Al composite-coating oxidation behavior. The Ni–Al coatings behave similarly to bulk + (Ni₃Al) or single-phase (Ni₃Al). In addition, at lower temperatures, such as 800°C, the coatings benefit from a small grain size that enhances Al diffusion to the surface to form the protective alumina layer. Based on oxidation kinetics and morphology, a critical Al content of about 6 wt.% was found, below which internal oxidation and higher oxidation mass gains were observed.     

Low-temperature formation and oxidation resistance of ultrafine aluminide coatings on Ni-base superalloy

Ultrafine aluminide coatings were successfully produced on Ni-18Fe-17Cr superalloy at 540-600 °C in a modified pack-aluminizing process. Repeated ball-impacts accelerated the formation of the aluminide coatings by a surface refining process, resulting in atomic diffusion occurring at a relatively low temperature. The effects of the operation temperature and the treatment duration on the formation of the coatings have been investigated. The coatings possessed a two-layer structure. The top layer, approximately 5 µm in thickness, exhibited equiaxial coarse grains and was dominated by NiAl₃, with small amounts of Fe₂Al₅ and CrAl₅. The bottom layer showed high density, homogeneous, ultrafine grains with diameters approximately 30-50 nm. High-temperature oxidation tests were carried out at 1000 °C. The oxidation kinetics and microstructure of the oxide scale were studied. The experimental results indicated that the coatings greatly enhanced the high-temperature oxidation resistance of Ni-18Fe-17Cr superalloy.     

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.     

Failure and oxidation phenomena in Fe-35Ni-18Cr-2Si heating-element wire

The oxidation behavior of an Fe-35Ni-18Cr-2Si alloy commonly used as a resistance heating-element material has been studied in the temperature range 1000–1300°C. Techniques employed included thermogravimetry at constant temperatures, measurements of chemical changes in the protective oxide film, and metallography. The oxidation behavior was found to approximate a parabolic law and data obtained gave an estimate of the activation energy for oxidation of 125 kJ/mole (30 kcal/mole) at around 1000°C and 339 kJ/mole (80 kcal/mole) at higher temperatures. The concentration of elements in the protective oxide was found to be both temperature and time dependent. Chromium became concentrated at the expense of nickel and iron at the lower temperature (<1100°C) and was found to evaporate out of the oxide at higher temperatures and longer times, thereby increasing both the nickel and iron concentrations. On the basis of changes in the chemical composition of the oxide film, together with scaling, the method of failure of heating-element wire by the development of hot spots can be explained. At failure both long-term (low temperature 1000°C) and shorter term tests (temperature > 1000° C) were found to have oxidized to the same depth. This can be explained on the basis of a depletion of chromium having occurred at the oxide-alloy interface, and that on scaling, insufficient chromium was present to reform a protective film.     

Internal-Carburizing Behavior of Ni-V,Ni-Cr,and Ni-3Nb Alloys

Ni-2V, Ni-5V, Ni-12V, Ni-10Cr, Ni-20Cr, andNi-3Nb alloys were carburized in 1.5 v/oC₃H₆ (bal. H₂) over therange 700-1000°C. Carburization of Ni-5V, Ni-12V,Ni-20Cr, and Ni-3Nb obeyed the parabolic rate law. Ni2V and Ni-10Cr, however, formed onlythin carbide scales upon carburizing. Carburizationrates decreased with increased vanadium content fromNi-5V to Ni-12V for all exposure conditions. V4C3 formed throughout the reaction zones of Ni-12V.Cr₃C₂ formed in the surfaceregions and Cr₇C₃ formed withinthe interior of Ni-20Cr. NbC precipitated in Ni-3Nbunder all conditions. The precipitate morphology changed with temperature and distance from thegas-metal surface. V₄C₃ andCr₃C₂ particles were generallysmall and spheroidal near the surface of Ni-12V andNi-20Cr, respectively, increasing slightly in size with distance from the surface and withincreasing temperature. The vanadium and chromiumcarbides formed intergranular networks toward thereaction fronts. The NbC precipitates were generallylarge and became Widmanstatten at increasing distancewithin the carburized zone of Ni-3Nb. Expressions forthe diffusion coefficient of carbon in nickel from themeasured permeabilities and carbon solubility data were determined. Solubility products weredetermined for all of the carbides formed and found tobe large in comparison with the product of theactivities of the precipitate elements. Wagner's theory of internal oxidation was shown to be anapproximation to the carburization kinetics attemperatures of 900°C or higher.     

STEM Analysis of the Transient Oxidation of a Ni-20Cr Alloy at High Temperature

High-spatial scanning transmission electronmicroscopy (STEM) has been used to study the developmentof transient scales on a commercially available Ni-20Cralloy. The samples were examined usingelectron-transparent cross sections of the metal and oxide foroxidation times between 0 and 30 min in a furnace at950°C in laboratory air. The samples were polishedto 1 m diamond before oxidation, producing a recrystallized grain structure within 100 nm ofthe surface. Upon oxidation, the initial scale consistedalmost exclusively of chromia. However, at themetal-oxide interface, thin layers of silica and alumina were detected. At longer oxidation times,(>5 min), localized thickening of the silica layerwas observed. With increased oxidation time, (>25min), these regions spread along the metal-oxideinterface until an almost continuous silica layer hadformed. The silica layer was present at much shorteroxidation times than reported by other workers, however,this may be because of the thin layer being undetectable using microprobe techniques. The scale formedwas found to be adherent, although the alloy containedsulfur and did not contain reactive elements. However,sulfur was not found to segregate to the metal-oxide interface possibly because of the presence ofthe amorphous silica layer.     

Mechanism of isothermal oxidation of the intel-metallic TiAl and of TiAl alloys

The oxidation behavior of Ti36Al, Ti35Al-0.1C, Ti35Al-1.4V-0.1C, and Ti35 Al-5Nb-0.1C (mass-%) in air and oxygen has been studied between 700 and 1000°C with the major emphasis at 900°C. Generally an oxide scale consisting of two layers, an outward- and an inward-growing layer, formed. The outward-growing part of the scale consisted mainly of TiO₂ (rutile), while the inward-growing part is composed of a mixture of TiO₂ and -Al₂O₃. A barrier layer of Al₂O₃ on TiAl between the inner and the outer part of the scale was visible for up to 300 hr. Under certain conditions, the Al₂O₃ barrier dissolved and re-precipitated in the outer TiO₂ layer. This shift leads to an effect similar to breakaway oxidation. Only the alloy containing Nb formed a longlasting, protective Al₂O₃ layer, which was established at the metal/scale interface after an incubation period of 80–100 hr. During this time, Nb was enriched in the subsurface zone up to approximately 20 w/o. The growth of the oxide scale on TiAl-V obeyed a parabolic law, because no Al₂O₃ barrier layer formed; large Al₂O₃ particles were part of the outward-growing layer. A brittle 2-Ti₃Al-layer rich in O formed beneath the oxide scale as a result of preferential Al oxidation particularly when oxidized in oxygen. Oxidation in air can lead also to formation of nitrides beneath the oxide scale. The nitridation can vary between the formation of isolated nitride particles and of a metal/Ti₂AlN/ TiN/oxide, scale-layer system. Under certain conditions, nitride-layer formation seemed to favor protective Al₂O₂₃ formation at the metal/scale interface, however, in general nitridation was detrimental with the consequence that oxidation was generally more rapid in air than in oxygen.     

Oxidation Mechanism of Ni—20Cr Foils and Its Relation to the Oxide-Scale Microstructure

The oxidation behavior of Ni—20Cr foils of 100- and 200-m thickness wasstudied in air between 500 and 900°C. Simultaneously, the morphology,microstructure, and composition of the oxide layers were determined byscanning and transmission electron microscopies. Depending on thetemperature, the oxide layer differed significantly. The scale formedat all temperatures was complex, with an outer NiO layer having columnargrains, and an inner layer of equiaxedNiCr₂O₄+NiO+Cr₂O₃ grains. At low temperatures (500 and 600°C),the chromium content was insufficient to form a continuousCr₂O₃ layer, while such a continuous layer formed at theinner interface at oxidation temperatures of 700 to 900°C. At 600°C,internal oxidation of chromium occurred in the substrate. The oxidationmechanism is described taking into account these morphologies and theoxidation kinetics. The observation of no significant differences betweenthe oxidation behavior of thin strips and thick materials is related to thelimited exposure times of the study.