High-Temperature Oxidation Studies of Low-Nickel Austenitic Stainless Steel. Part I: Isothermal Oxidation

The oxidation behavior of low-nickel austenitic stainless steel (LNiSS) in air at 873 and 973 K was investigated for 500 hr. The oxide scales formed during the process were examined by a wide range of experimental techniques including SEM/EDS, XRD, and EPMA, in order to determine their influence on kinetics behavior. Kinetics laws were close to parabolic at both temperatures, but the morphology of scales showed important differences with temperature. At 873 K the oxide scale was thinner, with irregular growth, intrusions, and without spallation. It was concluded that slower kinetics and advantageous scale morphology suggest that LNiSS is a suitable material for isothermal oxidation in air at 873 K. At higher temperatures, uniformly thick scales plus iron-rich nodules were observed with different composition regions. The most destructive feature was the formation of Fe-rich nodules, which were vulnerable to spalling during cooling.     

TEM Observations of the Initial Oxidation Stages of Nb-Ion-Implanted TiAl

Coupon specimens of TiAl were implanted with Nb ions at an acceleration voltage of 50 kV with a dose of 10²¹ ions m.^–2 They were then slightly oxidized during heating to 900 or 1200 K, or at 1200 K for 3.6 ksec (1 hr) in a flow of purified oxygen under atmospheric pressure. The implanted specimens and oxidized specimens were characterized and observed by AES, X-ray diffractometry, SEM, TEM, EDS, and EPMA. Implantation improves the oxidation resistance significantly by forming virtually -Al₂O₃ scales. The implanted layer is about 75 nm thick; the outer part of 30-nm thickness is -Ti phase and the rest of 45-nm thickness is amorphous. Heating to 900 K in O₂ results in partial crystallization of the amorphous layer to Ti₅Al₃O₂ (Z-phase) and to 1200 K results in oxide scales of 270 to 400 nm thickness consisting mainly of Al₂O₃. The fraction of Al₂O₃ in the scale increases toward the substrate. Oxidation at 1200 K for 3.6 ksec results in Al₂O₃-rich scales of about 400-nm thickness. The oxide grain size is very fine, about 80 nm in size, and becomes smaller toward the outer scale surface. This implies that implantation enhanced the nucleation of Al₂O₃ grains relative to the growth of TiO₂ grains. This finding and the formation of -Ti phase are thought to be responsible for the excellent oxidation resistance obtained.     

Influence of an Externally-Applied Static Charge on the Oxidation Kinetics of Copper

The kinetics of copper oxidation under theinfluence of an externally-supplied static charge ofeither kind at one of the reaction interfaces of agrowing oxide film on its subsequent thickening weredetermined in the temperature range of 523-1173 K andoxygen-pressure range of 5.06-50.66 kPa. The kineticsconformed to the parabolic rate law under all conditionsof experimentation. In the temperature range of 523-723 K, charge supply of either kind ateither of the oxide interfaces, reduced the ratescompared to normal oxidation. The reduction in rates ismore pronounced with (-)ve charge supply. In thistemperature range, Mott's in situ electrical-potentialgradient across the oxide film is identified as thepredominant driving force for migration of copper ionsduring the subsequent film-thickening process. On the other hand, in the temperature range of 873-973K, a charge supply of either kind enhanced the ratescompared to normal oxidation, where Wagner'selectrochemical-potential gradient acts as the maindriving force for ion diffusion. However, at 1073 K and1173 K, the rates were found to decrease slightlycompared to normal oxidation. The oxygen-pressuredependencies of rate constants at 623 K exhibitedrelations of the type k_P P _O2 ^1/4 for normal and k_p P _O2 ^1/8 (approximately) for oxidation witheither (+)ve or (-)ve charge supply at the oxide/oxygeninterface. However, at 873 K the oxygen-pressuredependencies of rate constants conform to k_P P _O2 ^1/6 for normal as well as for oxidationwith either (+)ve or (-)ve charge supply at theoxide/oxygen interface. The estimated activationenergies are 54 kJ/mol and 160 kJ/mol in Mott's and Wagner's parabolic ranges,respectively. It is established that migration of Cu+ions through the growing film is the rate-limiting stepunder all conditions of experimentation. This study has clearly demonstrated that changes inoxidation rates can be brought about by disturbing theinterfacial defect equilibria with anexternally-supplied static charge when no net currentflows through the oxide film. The estimated self-diffusivityvalues of Cu⁺ ions in the growingCu₂O at 873 K are also reported.     

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 of iron-nickel aluminum alloys in oxygen at 600–800° C

The oxidation behavior of iron-nickel-aluminum alloys containing 0–40 wt.% nickel and 0–30 wt.% aluminum has been investigated at 600 and 800° C. Through the construction of oxide maps it can be shown that three possible oxide morphologies may exist: Alloys containing less than approximately 5 wt.% aluminum form scales consisting of predominantly Fe₂O₃ and spinel; alloys with between 5 and 10 wt.% aluminum form Al₂O₃ scales interspersed by Fe₂O₃ nodules, and alloys with greater than 10 wt.% aluminum form predominantly -Al₂O₃ scales.     

The Oxidation of NdFeB Magnets

The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335–500°C. The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe₂O₃ and an inner layer of Fe₃O₄ but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an -Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The -Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar -Fe grain boundaries.     

Oxidation of A12O3 continuous fiber-reinforced/NiAl composites

The 1200°C and 1300°C isothermal and cyclic oxidation behavior of Al₂O₃ continuous fiber (Saphikon)-reinforced/ NiAl composites were studied. Oxidation resulted in formation of Al₂O₃ external scales in a similar manner as scales formed on monolithic NiAl. The isothermal oxidation of an Al₂O₃/NiAl composite resulted in oxidation of the matrix along the fiber/matrix interface near the fiber ends. This oxide acted as a wedge between the fiber and matrix, and, under cyclic oxidation conditions, led to further oxidation along the fiber lengths and eventual cracking of the composite. The oxidation behavior of composites in which the Al₂O₃ fibers were sputter coated with nickel prior to processing was much more severe. This was attributed to open channels around the fibers which formed during processing most likely as a result of the diffusion of the nickel coating into the matrix.     

Oxidation behavior of 20%Cr/25%Ni/Nb stabilized stainless steel in CO2 environments

The oxidation of 20wt.%Cr/25wt.%Ni/Nb steel in 50 ton CO₂ at 1073 K has been studied in situ using X-ray photoelectron spectroscopy to determine the chemical composition of the oxide initially formed. The surface composition of the first formed oxide is shown to be iron rich, containing quantities of chromium and manganese, whilst analysis of the bulk oxide indicates that the majority of the oxide scale is a spinel of type MnCr_2–xFe_xO₄. The formation of a chromia layer, which has been suggested to form first on these steels, was not observed. An examination of the oxide morphology using scanning electron microscopy revealed the presence of particle mounds varying in size from <0.5 m to 3 m in diameter and embedded in the surface oxide. Other techniques, including scanning Auger microscopy and energy dispersive X-ray analysis, have been employed to determine the composition of these particles, and suggestions for their origin have been offered.     

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.     

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.     

Corrosion Protection of Low-Nickel Austenitic Stainless Steel by Yttrium and Erbium-Ion Implantation Against Isothermal Oxidation

The beneficial effect of ion-implanted yttrium and erbium on the oxidation behavior of low-nickel austenitic stainless steel (LNiSS) at 973 K has been investigated up to 500 hr of oxidation test in air. The resulting oxide scales were examined by a wide range of experimental techniques, including SEM/EDS, XRD, and EPMA. The results indicate that both reactive elements have similar effects. The most significant effects have been to significantly reduce the corrosion rate and to improve the oxide scale adhesion. It is concluded that ion implantation is a powerful tool as surface-modification process introducing reactive elements in the top surface.     

Quantification of Aluminum Outward Diffusion During Oxidation of FeCrAl Alloys

The outward flux of aluminum during the oxidation of four similar alumina-forming alloys is determined using two types of experiments. One is measurement of the total thickness of the oxide and the ratio of the thickness of the characteristic equiaxed layer to the total oxide thickness. The second is measurement of the new oxide formed along the grain boundaries of the oxide upon re-oxidation. The former provides information about the ratio of outward diffusion of aluminum to inward diffusion of oxygen during oxidation. The latter experiment directly quantifies the outward flux of aluminum as a function of oxide thickness. Both the outward aluminum flux and the ratio of inward to outward diffusional fluxes are found to vary with the minor concentrations of reactive element alloying additions. Specifically, Y in solution in the alloy is found to limit outward aluminum diffusion more than Zr in solution, with Y₂O₃ limiting aluminum diffusion more than Zr, Y, and ZrO₂.     

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti₂AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti₂AlC had excellent oxidation resistance. The oxidation of Ti₂AlC obeyed a cubic law with cubic rate constants, k_c, increasing from 2.38×10^-12 to 2.13×10^-10 kg³/m⁶/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner -Al₂O₃ layer and a discontinuous outer TiO₂ (rutile) layer formed on the Ti₂AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti₂AlC substrate during thermal cycling.     

Effects of Chromium on the Oxidation Performance of β-FeAlCr Coatings

-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O₂ for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al₂O₃ blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al₂O₃was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al₂O₃ scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation.     

Oxidation Behavior of Nanocrystalline Al Alloys Containing 5 and 10 at.% Ti

The oxidation behavior of mechanically alloyed(MA) Al-Ti alloys containing 5 and 10 at.% Ti wereinvestigated at 500-600°C under 1 atm of oxygen. Ateach temperature, alloys oxidized linearly during the initial stage and later followed the parabolicrate law. During the initial stage, the oxidation ratesof nanocrystalline (50 nm) Al-Ti alloys were fasterthan those of conventional (200 nm) alloys. It is suggested that more grain boundaries innanocrystalline alloys provide more nucleation sites foroxides, so that the oxide scales grew faster as denseprotective layers. During the parabolic stage, the nanocrystalline alloys had greater oxidationresistance than conventional alloys because of the denseprotective layer. Oxide scales on both alloys consistedof a mixture of -Al₂O₃ andTiO₂ in the outer layer and-Al₂O₃ near the alloy as aprotective layer.     

Oxidation Behavior of Cast Ni3Al Alloys and Microcrystalline Ni3Al+5% Cr Coatings with and without Y Doping

Ni₃Al+5% Cr and Ni₃Al+5% Cr+0.3% Y (wt.%) microcrystalline coatings were produced using a close-field, unbalanced magnetron-sputter deposition (CFUMSD) technique. Isothermal and cyclic-oxidation tests were carried out to assess the oxidation resistance of the coatings. The results showed that Al₂O₃ formed on the coatings as the main oxidation products, with the formation of - and -Al₂O₃ scales at 900 and 1200°C, respectively. The spallation resistance of the Al₂O₃ scales formed on the coatings was superior to the oxide scales formed on cast Ni₃Al. After oxidation, interfacial voids were observed on the oxide–metal interface of the cast alloy while no voids were found on the coating surfaces. On the basis of the enhancement of Al diffusion, because of the high density of grain boundaries in the coatings, oxidation mechanisms were proposed.     

Oxygen exchange in oxidation of an Fe-20Cr-10Al alloy in ∼10 mbar O2/H2O-gas mixtures at 920°C

The oxidation of an alumina forming Fe-200-10Al alloy was studied in situ. The gas phase components in isotopically labeled 10 mbar O₂/H₂O-gas mixtures were measured from a virtually closed system at 920°C. Oxidation rates and oxygen-exchange rates were measured and related to each other. From the exchange rates the dissociation rates of O₂ and H₂O were calculated. These dissociation rates were in the early stage of oxidation found to be the same as the oxidation rate. The rate of oxygen exchange with the oxide can exceed the oxidation rate. From this follows that in analysis of a two-stage oxidation in^16,16O₂/^18,18O₂ the exchange rate of O between the oxide and gas phase has to be considered. The oxides formed in H₂O containing gas had a H/O-ratio of 0.1. Vacuum annealing of the alloy before oxidation increased the oxidation rate by a factor of 2.     

High-temperature oxidation of Al-deposited stainless-steel foils

The oxidation resistance of Al-deposited Fe–Cr–Al foils containing small amounts of La and Ce was assessed by a cyclic oxidation test with temperature varying between room temperature and 1323 K to 1423 K in static air. (1) The Al content of Fe–Cr–Al–La, Ce foils can be increased by depositing an Al layer from the vapor phase. The deposition of a 1-m-thick Al layer on both sides of the 50-m-thick foil is equivalent to a 1.5 mass% increase in the Al content. The deposited Al diffuses into the foil during heat treatment. The uniform distribution of Al is obtained by heating at 1273 K for 18 ks. (2) After the initial transition stage the oxidation follows the parabolic law until breakaway sets in. The scale consists mainly of -Al₂O₃ during the parabolic period. (3) The increase in the Al content by more than 5 mass% by the Al-deposition remarkably improves high-temperature oxidation resistance (smaller parabolic rate constant and longer protection time). (4) The Al-deposited foils have better oxidation resistance than the conventional foils with the same contents of Al and rare-earth elements. This is attributable to the different nature of the initially formed oxide on the Al-deposited foil. (5) The so-called rare-earth element effect was also observed for the Al-deposited foils. Predominant diffusion of oxygen through the Al₂O₃ scale and vacancy-sink mechanism are applicable to the present results.     

Water-vapor-effect on the oxidation of Fe-21.5 wt.%Cr-5.6 wt.%Al at 1000°C

Fe-21.5 wt. %Cr-5.6 wt. %Al oxidation, at 1000°C, in dry or wet oxygen shows that steam has an influence on the oxide-scale growth mechanism. Steam modifies the kinetics of early-stage oxidation. In dry oxygen, an initial fast linear regime is observed during one hour. Under wet conditions, weight-gain curves follow the same parabolic regime over the entire oxidation test. The scale structure strongly depends on the presence of steam in the gaseous environment. With dry oxygen, the scale is composed mainly of-Al₂O₃ after the initial formation of-Al₂O₃ identified by ESCA and RHEED. The kinetics transient stage corresponds to the necessary time for the internal part of the initial-Al₂O₃ scale to transform into a continuous-Al₂O₃ diffusion barrier. Under wet oxygen conditions, transient oxides are identified as (Mg, Fe) (Cr, Al)₂O₄, MgAl₂O₄ (orthorhombic), Al₂O₃ (hexagonal), these oxides transform into MgAl₂O₄ (cubic), Cr₃O₄, Fe₂O₃,-Al₂O₃, with time. When water vapor does not change drastically oxidation kinetics, the induced presence of iron and chromium in the oxide scale could be responsible for weakening the protectiveness of alumina scales.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.