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.     

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.     

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr₂O₃ scale (in Ni-34Cr alloys) mainly as S^2– species, which at high temperatures increases the V_Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr₂O₄, Cr₂O₃ oxide multilayers (in a Ni-20Cr alloy), sulphur in the S^2– state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.     

The high-temperature oxidation of cobalt-21 wt.% chromium-3 vol.% Y2O3 alloys

Alloys of Co-21 wt. % Cr-3 vol. % Y ₂ O ₃ have been prepared by a mechanical alloying method, and oxidized in oxygen at 100 Torr in the temperature range 900–1200°C. The general effects of the dispersed oxide phase are similar to those reported for nickel-base alloys: the selective oxidation of chromium to form a continuous protective Cr ₂ O ₃ scale is promoted; the rate of growth of Cr ₂ O ₃ is reduced compared to dispersoid-free alloys; the adhesion of the Cr ₂ O ₃ is greatly improved; and the scale-forming reaction is probably at the scale-metal interface in the alloys containing the dispersoid, whereas it is at the scale-oxygen interface in dispersoid-free alloys. This last point has not been positively demonstrated. The improvement in adhesion is of particular significance, since the scales on cobalt-base alloys are prone to spallation, and it has been possible to study the mechanism of adhesion in more detail. It appears that in dispersoid-free material the metal recedes from the scale-metal interface, leaving the scale supported on the tops of metal peaks but this does not happen in the alloy containing the dispersoid, either because the growth direction of the scale has been changed, or because of changes in the substrate grain size. In general, the observations support the model proposed in an early study for the oxidation of Ni-20 wt.% Cr alloys containing oxide dispersions.This work has been supported by the Naval Air Systems Command under Contract No. N00019-71-C-0079.     

Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys

This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl₂O₃-rich scale.     

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.     

Simultaneous Internal Oxidation and Nitridation of Ni–Cr–Al Alloys

A series of Ni–Cr–Al alloys was subjected to thermal cycling to 1100°C in air for up to 260 1-hr cycles. All alloys exhibited poor corrosion resistance. Repeated scale spallation led to subsurface alloy depletion in aluminum and, to a lesser extent, chromium. This caused transformation of the prior alloy three-phase structures (-Cr+-NiAl+-Ni) to single-phase -nickel solution. Destruction of the external scale allowed gas access to this metal, which was able to dissolve both oxygen and nitrogen. Inward diffusion of the two oxidants led to development of a complex internal-precipitation zone: Al₂O₃ and Cr₂O₃ beneath the surface, followed by Al₂O₃, then AlN, then AlN+Cr₂N, and, finally, AlN alone in the deepest region. This distribution is shown to reflect the relative stabilities of the precipitates and the higher permeability of nitrogen. Diffusion-controlled kinetics were in effect initially, but mechanical damage to the internal-precipitation zone led to more rapid gas access and approximately linear kinetics in the long term.     

Effect of Nanocrystallization on the Oxidation Behavior of a Ni–8Cr–3.5Al Alloy

Magnetron-sputter deposition was used to produce a Ni–8Cr–3.5Al(wt.%) nanocrystalline coating on substrates of the same alloy. Theoxidation behavior of the cast Ni–8Cr–3.5Al alloy and itssputtered coating were investigated at 1000°C in air. Complex,layered-oxide scales composed of Cr₂O₃ outer layer,mixed spinel NiAl₂O₄ and NiCr₂O₄middle layer, and -Al₂O₃ inner layer were formedon the Ni–8Cr–3.5Al nanocrystalline coating during 200-hroxidation, whereas Cr₂O₃, with some NiCr₂O₄external layer with internal Al₂O₃, formed on the castalloy. Because of the formation of this -Al₂O₃inner layer on the coating, the sputtered Ni–8Cr–3.5Al coatingshowed better oxidation resistance than the cast alloy. The effect ofnanocrystallization on oxide formation is discussed. It was indicated thatthe formation of this -Al2O3 inner layer was closely related to therapid diffusion of Al through grain boundaries in the nanocrystallinecoating and the relatively high Cr content in Ni–8Cr–3.5Al.     

The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, an-Al₂O₃ layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr₂O₃ to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al₂O₃grains. The intermediate and inner layers consist of columnarAl₂O₃ grains and equiaxedCr₂O₃ grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with thedynamic segregation model to account for the effects of reactive elements on thegrowth of Al₂O₃ scales.     

The influence of aluminum additions on the oxidation of Co-Cr alloys at 1000 and 1200°C

The isothermal oxidation of Co-Cr-Al alloys, containing 10–30 % Cr and 1 or 4.5% Al in 1 atm flowing oxygen at 1000 and 1200°C has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis, and scanning electron microscopy. The addition of 1 % Al to Co-10% Cr and Co-15% Cr has little effect on the over-all oxidation rate, although there is increased internal oxidation and the outer-inner scale thickness ratio is decreased. The oxidation rate is controlled largely by Co²⁺ ion diffusion out through the entire scale, with oxygen gas transport across voids, spinel blocking effects and doping in the inner layer probably playing subsidiary roles. With Co-15 %-Cr-1%Al, limited healing by Cr₂O₃ increases progressively with time at the alloy-oxide interface. An addition of 1 % Al to Co-30 %Cr assists the formation of an initially protective Cr₂O₃-rich surface layer by internally oxidizing, thereby allowing more of the chromium to diffuse to the surface and form an external scale. This Cr₂O₃ layer tends to lift and crack open, enabling CoO-rich scales to form on the exposed alloy. Co-15%Cr-4.5% Al produces a protective -Al₂O₃ layer on certain surface regions, sometimes with an overlying Cr₂O₃ layer and internal -Al₂O₃ particles in the underlying alloy. In other regions, rapidly growing CoO-rich nodules develop from the outset, or after early lifting and fracture of the -Al₂O₃ scale. Generally, the presence of 28% Cr and 4.5% Al is sufficient to ensure an external scale of -Al₂O₃, the chromium acting as an oxygen getter. If such scale fractures, healing is very rapid.     

The influence of platinum on the maintenance of α-Al2O3 as a protective scale

The influence of externally located platinum on the isothermal stability of -Al₂O₃ scales formed at high temperatures has been examined. It has been observed that a nickel-base alloy forms an external scale of -Al₂O₃ during oxidation at 1200°C, but this scale breaks down isothermally, enabling a faster-growing Cr₂O₃-rich scale to develop. However, in the presence of platinum metal alongside the specimen in the furnace hot zone, the breakdown of the -Al₂O₃ scale is postponed for a substantial period of time. It appears that platinum, as the volatile species PtO₂, is incorporated into the growing -Al₂O₃ scale where it either influences the stress relief mechanism at temperature or reduces oxidation growth stress generation and thus significantly enhances the isothermal stability of the scale.     

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.     

The Influence of the Alloy Microstructure on the Oxidation Behavior of Ti–46Al–1Cr–0.2Si Alloy

The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe ₂-Ti₃Al phase, being faster for the extrudedmaterial with coarse ₂-Ti₃Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-₂-Ti₃Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance.     

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.     

Influence of prior-austenite grain size on the oxidation behavior of 9 wt.% cr-1 wt.% Mo steel

The effect of grain size on the oxidation of 9 wt.% Cr-1 wt.% Mo steel (with prior-austenite grain sizes of 90, 210, and 360 m) has been studied at 973 K in air. After the initial stages of oxidation (up to 60 min), the fine-grain specimen (90 m) suffered heavy spallation. A similar spallation took place after 120 min in the case of the specimen with intermediate grain size (210 m), and after 360 min in a coarse-grain (360 m) specimen. This increase in the duration for the onset of pronounced spallation with grain size has been attributed to the smaller area of the grain boundaries which are the locations of higher growth stresses leading to spallation. This dependence of spallation characteristics on the alloy grain size has been confirmed by the incidence of higher acoustic-emission activity during AE monitoring. Scanning electron microscopy (SEM) has confirmed the occurrence of pronounced spallation of the oxides formed in the areas adjoining the grain boundaries.     

Oxidation of Ni–Al-Base Electrodeposited Composite Coatings. II: Oxidation Kinetics and Morphology at 1000°C

The oxidation behavior of nickel-matrix/aluminum-particle composite coat–ings was studied using thermogravimetric (TG) analysis and long-term furnace exposure in air at 1000°C. The coatings were applied by the composite-electrodeposition technique and vacuum heat treated for 3 hr at 825°C prior to oxidation testing. The heat-treated coatings consisted of a two-phase mixture of (Ni)+ (Ni₃Al). During short-term exposure at 1000°C, a thin -Al₂O₃ layer developed below a matrix of spinel NiAl₂O₄, with -Al₂O₃ needles at the outer oxide surface. After 100 hr of oxidation, remnants of -Al₂O₃ are present with spinel at the surface and an inner layer of -Al₂O₃. After 1000–2000 hr, a relatively thick layer of -Al₂O₃ is found below a thin, outer spinel layer. Oxidation kinetics are controlled by the slow growth of the inner Al₂O₃ layer at short-term and intermediate exposures. At long times, an increase in mass gain is found due to oxidation at the coating–substrate interface and enhanced scale formation possibly in areas of reduced Al content. Ternary Si additions to Ni–Al composite coatings were found to have little effect on oxidation performance. Comparison of coatings with bulk Ni–Al alloys showed that low Al -alloys exhibit a healing Al₂O₃ layer after transient Ni-rich oxide growth. Higher Al alloys display Al₂O₃-controlled kinetics with low mass gain during TG analysis.     

The hot corrosion of directionally solidified Ni-Cr-Nb-Al (γ/γ′-δ) eutectic alloys

The hot-corrosion resistance of two /- eutectic alloys, Ni-23.1 Nb-4.4 Al and Ni-19.7Nb-6Cr-2.5 Al, has been studied using three techniques: (1) a salt-coated test, in which the specimens are coated with a thin layer of Na₂SO₄ and oxidized; (2) the Dean rig test, in which salt is evaporated into the gas stream and allowed to condense continuously on the specimen; and (3) a sulfidation/oxidation test, in which the specimens were presulfidized for a short time in a H₂/H₂S mixture and then oxidized. The salt-coated test suggested that these alloys are generally quite resistant to sodium-sulfateinduced corrosion, although localized severe attack was sometimes encountered. The morphology of the corrosion products suggested that the mechanism of the attack was sulfidation/oxidation, and that salt fluxing mechanisms were relatively unimportant. In the Dean rig test it was shown that sodium chloride greatly increased the severity of attack, as has been shown earlier for alumina-forming superalloys.     

The influence of yttrium additions on the oxidation resistance of a directionally solidified Ni-Al-Cr3C2 eutectic alloy

Isothermal oxidation of a directionally solidified Ni-Al-Cr₃C₂ eutectic alloy results in development of an external -Al₃O₃-rich scale. However, this scale breaks down after relatively short times at temperature and a less protective Cr₂O₃-rich scale is formed, together with substantial internal oxide in the alloy. In an attempt to maintain the external -Al₂O₃-rich scale and prevent damaging subscale oxidation, modified yttrium-containing directionally solidified alloys have been developed. The oxidation resistance of these alloys at 1000 and 1100°C in flowing air has been investigated and found to be considerably better than that of the corresponding yttrium-free alloy. At both temperatures an external -Al₂O₃-rich scale is produced and is retained for much longer periods than on the yttrium-free alloys during isothermal and thermal cycling oxidation. Some scale breakdown does occur during thermal cycling at 1100°C, but -Al₂O₃ is able to re-form as the surface oxide. However, although external -Al₂O₃-rich scales are retained for long periods on these alloys, some oxide penetration into the alloy beneath these scales does occur where coarse carbide fibers intersect the alloy surface. This is associated with relatively poor scale integrity at these intersections.     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

High-resolution SIMS and analytical TEM evaluation of alumina scales onβ-NiAl containing Zr or Y

High-resolution SIMS and TEM have been used to evaluate growth processes and interfacial segregation occurring in -Al₂O₃ scales grown at 1200°C on -NiAl containing zirconium or yttrium.¹⁸O/SIMS shows that the extent of aluminum diffusion occurring during -Al₂O₃ growth is reduced by the presence of these alloying elements, which are seen by SIMS imaging as oxide particles within the scale. STEM/EDS of the same oxide scales show that zirconium and yttrium also segregated to the oxide-alloy interface to the extent, respectively, of 0.15 and 0.07 of a monolayer and to oxide grain boundaries (0.2 monolayer). The complementary information provided by SIMS, TEM, and STEM provides a better understanding of the role of reactive elements in modifying scale-growth processes.