Influence of Cr on the Oxidation of Fe3Al and Ni3Al at 500°C

The influence of chromium on the mechanical properties of the aluminides Fe₃Al and Ni₃Al has been studied extensively. In order to evaluate the role of Cr during the early stages of oxidation, Fe₃Al and Ni₃Al containing 2 and 4 at.% Cr were oxidized in dry air at 500°C for 6, 50, and 100 hr. The oxide scale on Fe₃Al consists of a layer of Fe₂O₃ mixed with FeAl₂O₄ on top of a continuous layer of (Al, Cr)₂O₃. Ni₃Al is covered with a mixed layer of (Al, Cr)₂O₃ and NiO/NiAl₂O₄ underneath a layer of NiO/NiAl₂O₄. Moreover, Cr induces the nucleation and growth of Fe₂O₃ and NiO particles at the oxide surface of Fe₃Al and Ni₃Al, respectively. This is due to enhanced cationic diffusion through the Cr-modified oxides. As a conclusion, additions of Cr up to 4 at.% are detrimental to the oxidation behavior of both aluminides at 500°C.     

Oxidation Behavior of Ni-3,6, 10 wt.% Al Alloys at 800°C

The oxidation behavior of Ni and Ni-3, 6, and 10Al alloys at 800°C in an N₂–O₂ gas mixture was investigated. The mass gain of each alloy depended on both the oxidation periods and Al content. NiO scale was formed on all alloy substrates accompanied by internal oxides of Al₂O₃. Many cavities were formed at the NiO/substrate interface at shorter oxidation times, and these cavities were found to be filled by metallic Ni(Al) from the matrix in the internal-oxidation zone by the development of internal oxides. The filling of cavities by Ni(Al) was more significant on higher Al alloys, which had a higher density of internal Al₂O₃. Once metallic Ni(Al) formed along the entire NiO/substrate interface, the oxidation kinetics became the same as pure Ni. It was concluded that pure Ni filling the cavities at the interface provided a diffusion path of Ni from the substrate to the NiO scale, and that controlled the oxidation kinetics.     

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

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

Thermal oxidation of single-crystal aluminum at 550°C

Oxygen transport during oxide growth on (110), (111), and (100) Al crystals at 550°C is investigated by¹⁸O/SIMS combined with kinetic measurements and SEM and TEM observations. Starting with an electropolished surface, the experimental evidence suggests oxide growth by oxygen anion transport via local pathways through an outer amorphous Al₂O₃ layer and oxygen incorporation at the periphery of the underlying laterally growing -Al₂O₃ islands. The kinetics, island morphology and epitaxy are sensitive to substrate orientation. This oxide growth behavior is compared with oxide formation on a sputter-cleaned and annealed (111) Al surface.     

Effect of La2O3 Particles on the Oxidation of Electrodeposited Nickel Films

Electrodeposited Ni-La₂O₃composite films with nanometer-sizeLa₂O₃ oxide inclusions werefabricated by the codeposition of nickel withLa₂O₃ particles. The comparativeoxidation behavior in air at 900 and 1000°C of nickel coated with theNi-La₂O₃ composite and films withand without nickel-plating was studied by TGA, AE,SEM/EDX, EPMA, and TEM/EDX. In general, theNi-La₂O₃ composite-coated nickelhad the slowest rate and the best resistance tothermal cycling. AE tests revealed that cracking eventsin NiO scales on Ni-La₂O₃composite-coated nickel was significantly reduced incomparison to that of the scale on nickel-coated nickel during thermalcycling at 900°C. SEM investigation showed that theLa₂O₃-free NiO scale was composedof outer coarse columnar grains and inner equiaxed ones.By contrast, the scale on the Ni-La₂O₃composite-coated nickel consisted of only fine equiaxedNiO grains. The scale on theLa₂O₃-free samples wascharacterized by cracks that originated at thescale-substrate interface and spanned the scale thickness. By contrast,no scale cracks formed at theLa₂O₃-doped NiO scale-substrateinterface, but small voids were created at the triplepoints of the grain boundaries of NiO. In the La₂O₃-doped NiOscale, segregation of La ions to the NiO grainboundaries near the scale-surface was observed by EDXmicroanalyses in the TEM. It is believed that the Laions segregated at the grain boundaries of NiO led to an increase in thecohesion between nickel oxides and in a reduction of thescaling rate and the formation of scale with fineequiaxed crystal structure by blocking the outward and lateral growth of scale. The latter was dueto the predominant outward diffusion of nickel along NiOgrain boundaries being inhibited effectively by thesegregated La ions. The mechanism of the effect of the added La₂O₃particles on the nickel electrodeposits is discussed indetail.     

The corrosion behavior of Fe-Al alloys in H2/H2S/H2O atmospheres at 700–900°C

The corrosion behavior of five Fe-Al binary alloys containing up to 40 at. % Al was studied over the temperature range of 700–900°C in a H₂/H₂S/H₂O mixture with varying sulfur partial pressures of 10^–7–10^–5 atm. and oxygen partial pressures of 10^–24–10^–2° atm. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants decreased with increasing Al content. The scales formed on Fe-5 and –10 at.% Al were duplex, consisting of an outer layer of iron sulfide (FeS or Fe_1–xS) and an inner complex scale of FeAl₂S₄ and FeS. Alloys having intermediate Al contents (Fe-18 and –28 at.% Al) formed scales that consisted of mostly iron sulfide and Al₂O₃ as well as minor a amount of FeAl₂S₄. The amount of Al₂O₃ increased with increasing Al content. The Fe 40 at.% Al formed only Al₂O₃ at 700°C, while most Al₂O₃ and some FeS were detected at T800°C. The formation of Al₂O₃ was responsible for the reduction of the corrosion rates.     

The influence of grain-boundary segregation of Y in Cr2O3 on the oxidation of Cr metal

The oxidation behavior at 900°C of pure Cr and Cr implanted with 2×10¹⁶ Y ions/cm² was studied. The kinetics of oxidation were measured thermogravimetrically and manometrically. The mechanisms of oxide growth were studied using¹⁸O-tracer oxidation experiments, and the composition and microstructure of the oxide scales were characterized by TEM and STEM. Segregation of Y cations at Cr₂O₃ grain boundaries was found to be the critical factor governing changes in the oxidation behavior of Cr upon the addition of Y. In the absence of Y, pure Cr oxidized by the outward diffusion of cations via grain boundaries in the Cr₂O₃ scale. When Y was present at high concentration in the scale, as when Cr implanted with 2×10¹⁰ Y ions/cm² was oxidized, anion diffusion predominated. It is concluded that strain-induced segregation of Y at grain boundaries in the oxide reduced the cation flux along the grain boundaries. The rate of oxidation was reduced because the grain-boundary diffusivity of cations became lower than the grain-boundary diffusivity of the anions, which then controlled the rate of oxidation. Changes in the relative rates of Cr³⁺ and O^2– transport, as well as a solute-drag effect exerted by Y on the oxide grain boundaries, resulted in changes in the microstructure of the oxide.     

Oxidation Behavior of Ni-3, 6 wt% Al Alloys at 800 °C in Atmospheres Containing Water Vapor

The oxidation behavior of Ni, Ni–3Al, and Ni–6Al alloys at 800 °C in air + H₂O was investigated. The oxidation kinetics of Ni and the alloys in air + H₂O were very similar, but the mass gains of Ni and each alloy were smaller in air + H₂O than in air. Oxidation products formed on Ni-3 and 6Al alloys consisted of an outer NiO scale and internal Al₂O₃ precipitates. The growth rates of both NiO and the internal oxidation zone were much smaller in air + H₂O. The NiO scale formed in air + H₂O was duplex in structure with outer porous and inner dense layers. The outer porous layer consisted of fine powder-like NiO particles. A thicker metallic Ni(Al) layer formed at the NiO/alloy interface in air + H₂O, caused by extrusion of Ni from the substrate due to volume changes accompanying the internal oxide formation. Formation of the metallic Ni layer appeared to be the reason for the similarity between the oxidation kinetics of both Ni and the alloys in air + H₂O.     

Effect of Si and Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Additions on the Oxidation Behavior of Ni–<Emphasis Type="Italic">x</Emphasis>Al (<Emphasis Type="Italic">x</Emphasis> = 5 or 10 wt%) Alloys at 1150 °C

The effect of Si and Y₂O₃ additions on the oxidation behavior of Ni–xAl (x = 5 or 10 wt%) alloys at 1150 °C was studied. The addition of Y₂O₃ accelerates oxidation rate of alloys, especially growth rate of NiO, but improves adherence of the scale to the substrate. The addition of Si facilitates the selective oxidation of Al, suppresses the formation of NiO and therefore reduces the critical Al content to form continuous layer of alumina scale. Higher Al content decreases the oxidation rate of alloys in binary Ni–Al alloys and increases the oxidation rate of alloys in ternary Ni–Al–Si alloys. The effect of third-element Si is more significant and beneficial than that of Al content in ternary Ni–Al–Si alloys.     

Oxygen Transport during the High Temperature Oxidation of Pure Nickel

The high temperature oxidation of nickel has been investigated in air under atmospheric pressure in the temperature range 600–900°C. The oxidation kinetic curves deviate from the parabolic law for temperatures over 800°C. The observation of scale morphologies and the use of two stage oxidation experiments under ¹⁶O₂/¹⁸O₂ atmospheres showed that oxygen transport through the NiO scale had to be taken into consideration during the oxidation process. Despite the main outward diffusion of Ni species through the oxide scale, the inward oxygen diffusion at lower temperatures (<800°C) or the oxygen transport, probably as molecular species, via pores or micro-cracks were found to play a major role in the formation of duplex oxide scales, made of small equiaxed oxide grains at the metal/oxide interface overgrown by larger columnar grains at the gas/oxide interface. Oxygen diffusion coefficients into thermally grown NiO scales were determined and compared to the values of Ni diffusion coefficients from the literature.     

Oxidation behavior of a Ni3Al PM alloy

The oxidation behavior of a Ni₃Al powder-metallurgical (PM) alloy doped with boron was investigated by means of discontinuous isothermal tests in the temperature range of 535° to 1020°C for exposures of up to 150 hr. The oxidation kinetics were characterized by a sharp decrease in the oxidation rate at about 730°C which is associated with a change in the oxidation mechanism. Below 730°C, the scale exhibited an outer NiO layer and an internal-oxidation zone consisting of a fine dispersion of alumina in a diluted Ni-Al solid solution. Between these two layers a very thin layer of nickel could be observed. Above 730°C, a three-layered scale was observed consisting of an outer NiO layer, an intermediate layer that depending on temperature consisted of a mixture of nickel and aluminum oxides or NiAl₂O₄, and an inner layer of Al₂O₃, which accounts for the higher oxidation resistance. Oxidation at the higher temperatures resulted in extensive void formation at the scale/metal interface which led to poorly adherent scales. It is worth noting that at the early oxidation stage the scale was characterized by planar interfaces. Roughening of the air/scale and, specially, the scale/metal interfaces after long exposures at the low-temperature range or after short times at higher temperatures could be related to the formation of the inner Al₂O₃ layer at the grain boundaries which favor oxygen penetration through the grain interior.     

Oxidation Behavior of Ni3Al and Fe3Al: II. Early Stage of Oxide Growth

Oxidation treatments of Ni₃Al and Fe₃Al were performed at room temperature in 0.2 atm O₂ for 5 min and at 300 and 500°C in air for 5 min, and 6, 50, 100, and 200 hr. The oxides were analyzed by XPS, AES, and SEM. A model explaining the initial stages of oxide formation is suggested. At room temperature and 300°C, islands of Al₂O₃ and NiO combined with NiAl₂O₄ formed on Ni₃Al. At 500°C, the Ni oxides grow laterally and cover the Al₂O₃ islands. Islands of Al₂O₃ and Fe₂O₃ mixed with Fe(Fe, Al)₂O₄ formed on Fe₃Al at room temperature. At 300 and 500°C the scale is composed of an outer layer rich in Fe oxides and an inner layer rich in Al oxides. During long time exposure, islands of Fe₂O₃ and Fe(Fe, Al)₂O₄ formed at the surface by diffusion of Fe cations through the alumina layer. The oxide growth on Fe₃Al reaches a steady-state regime after formation of the continuous alumina layer. At 300°C, the oxide formed on Fe₃Al is thicker than on Ni₃Al, whereas it is reverse at 500°C.     

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al ₂ O ₃ scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the ¹⁸ O isotope as a tracer. The ¹⁸ O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al ₂ O ₃ scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated k _p 's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and ¹⁸ O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these ¹⁸ O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.     

On the reaction mechanism of nickel with SO2+O2/SO3

High-purity nickel has been reacted with 96% O₂+4% SO₂ at 700–900°C. The reaction has been studied at 700°C as a function of the total gas pressure (0.06–1 atm) and at 1 atm as a function of temperature (700–900°C). The reaction mechanism changes with the effective pressure of p(SO₃) in the gas. When NiSO₄ (NiO + SO₃ = NiSO₄) is formed on the scale surface, the scale consists of a two-phase mixture of NiO + Ni₃S₂; in addition, sulfur is enriched at the metal/scale interface. A main process in the reaction is rapid outward diffusion of nickel through the Ni₃S₂ phase in the scale; the nickel reacts with NiSO₄ to yield NiO, Ni₃S₂, and possibly NiS as an intermediate product. When NiSO₄ cannot be formed, the scale consists of NiO, and small amounts of sulfur accumulate at the metal/scale interface. It is proposed that the reaction under these conditions is primarily governed by outward grain boundary diffusion of nickel through the NiO scale, and in addition, small amounts of SO₂ migrate inward through the scale—probably along microchannels.     

The use of SIMS to investigate the incorporation of oxygen in CoO scales

SIMS was used to locate O¹⁸ in a thick oxide scale grown on a cobalt specimen oxidized first in O¹⁶ and subsequently in O¹⁸. Analyses were performed in depth profile from the oxide-gas interface and in step scan across a polished cross section. SIMS was found to be well suited to this type of oxygen tracer study. Eighty percent of the O¹⁸ was located in the inner quarter of the scale with a maximum value of 70% 10–15 from the metal-oxide interface. The distribution of the remaining O¹⁸ was essentially uniform in the outer 3/4 of the scale at an average value of 4.6%. There was, however, a layer 150 nm thick highly enriched in O¹⁸ at the oxide-gas interface. The O¹⁸ in the outer 3/4 of the porous scale is probably the result of normal exchange processes. The form of the distribution near the metal-oxide interface cannot be explained at this time.     

The Effect of Cr on the Oxidation of Ni–10 at% Al in 1 atm O2 at 900–1000°C

The oxidation of three Ni–xCr–10Al alloys with a constant Al content of 10 at% and containing 3, 5, and 10 at% Cr was investigated at 900–1000°C in 1 atm of pure oxygen and compared to the behavior of Ni–10Al. At both temperatures, an external NiO scale overlying a zone of internal-oxide precipitates formed on Ni–10Al and Ni–3Cr–10Al: in addition, a discontinuous Al₂O₃ layer formed at the front of the internal oxidation for Ni–3Cr–10Al. An exclusive external scale of Al₂O₃ formed at most places on Ni–5Cr–10Al at 900°C, while, at some sites, the same alloy formed an outer NiO layer overlying an internal oxidation zone. The scales formed on Ni–5Cr–10Al at 1000°C were complex, but eventually a protective Al₂O₃ layer developed either at the alloy surface or beneath a region containing a mixture of different oxides. Finally, an exclusive external Al₂O₃ layer formed on Ni–10Cr–10Al at both temperatures. Thus, the addition of sufficient chromium to Ni–10Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum under a constant Al content. A possible mechanism for the effect of chromium on the oxidation of Ni–10Al is discussed on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most-reactive component in binary alloys.     

Oxide phase development upon high temperature oxidation of γ‐NiCrAl alloys

The amount of each oxide phase developed upon thermal oxidation of a γ‐Ni‐27Cr‐9Al (at.%) alloy at 1353 K and 1443 K and a partial oxygen pressure of 20 kPa is determined with in‐situ high temperature X‐ray Diffractometry (XRD). The XRD results are compared with microstructural observations from Scanning Electron Microscope (SEM) backscattered electron images, and model calculations using a coupled thermodynamic‐kinetic oxidation model. It is shown that for short oxidation times, the oxide scale consists of an outer layer of NiO on top of an intermediate layer of Cr₂O₃ and an inner zone of isolated α‐Al₂O₃ precipitates in the alloy. The amounts of Cr₂O₃ and NiO in the oxide scale attain their maximum values when successively continuous Cr₂O₃ and α‐Al₂O₃ layers are formed. Then a transition from very fast to slow parabolic growth kinetics occurs. During the slow parabolic growth, the total amount of non‐protective oxide phases (i.e. all oxide phases excluding α‐Al₂O₃) in the oxide scale maintain at an approximately constant value. The formation of NiCr₂O₄ and subsequently NiAl₂O₄ happens as a result of solid‐state reactions between the oxide phases within the oxide scale.     

Oxidation resistance of sputtered Ni3(AlCr) nanocrystalline coating

Isothermal and cyclic oxidation resistance at 1000°C in air were investigated for a cast Cr-containing Ni₃Al-base alloy and its sputtered nanocrystalline coating. The results indicated that both the cast Ni₃Al alloy and its sputtered coating exhibit excellent isothermal oxidation resistance as a result of the formation of Al₂O₃ scales. However, the cast alloy possesses very poor cyclic oxidation resistance because of the spallation of the initially formed Al₂O₃ scale during cooling and subsequent formation of NiO. On the contrary, the sputtered Ni₃(AlCr) nanocrystalline coating exhibits very good cyclic oxidation resistance due to the significant improvement of the adhesion of Al₂O₃.     

The influence of reactive-element coatings on the high-temperature oxidation of pure-Cr and high-Cr-content alloys

The influence of various reactive-element (RE) oxide coatings (Y₂O₃, CeO₂, La₂O₃, CaO, HfO₂, and Sc₂O₃) on the oxidation behavior of pure Cr, Fe–26Cr, Fe–16Cr and Ni–25Cr at 900°C in O₂ at 5×10^–3 torr has been investigated using the¹⁸O/SIMS technique. Polished samples were reactively sputtercoated with 4 nm of the RE oxide and oxidized sequentially first in¹⁶O₂ and then in¹⁸O₂. The effectiveness of each RE on the extent of oxidation-rate reduction varied with the element used. Y₂O₃ and CeO₂ coatings were found to be the most beneficial, whereas Sc₂O₃ proved to be ineffective, for example, for the oxidation of Cr. SIMS sputter profiles showed that the maximum in the RE profile moved away from the substrate-oxide interface during the early stages of oxidation. After a certain time the RE maximum remained fixed in position with respect to this interface, its final relative position being dependent on the particular RE. The position of the RE maximum within the oxide layer also varied with the substrate composition. For all coatings¹⁸O was found to have diffused through the oxide to the substrate-oxide interface during oxidation, the amount of oxide at this interface increasing with increasing time. The SIMS data confirm that for coated substrates there has been a change in oxidegrowth mechanism to predominantly anion diffusion. The RE most probably concentrates at the oxide grain boundaries, generally as the binary oxide (RE) CrO₃. Cr³⁺ diffusion is impeded, while oxygen diffusion remains unaffected.     

Kinetics and mechanism of high-temperature oxidation of dilute cobalt-chromium alloys

Kinetics of oxidation of Co-Cr alloys containing 0.4%–15% Cr was studied as a function of temperature (1273–1573 K) and oxygen pressure (4 × 10²–10⁵Pa). The oxidation process was found to be approximately parabolic and faster than that for pure cobalt. The scales are double-layered and consist of a compact outer CoO layer and a porous inner layer containing CoO slightly doped by chromium and spinel CoCr₂O₄. The oxidation mechanism was investigated by means of platinum markers and the¹⁸O isotope. The scale on the alloys containing less than 1% Cr grows exclusively by outward diffusion of cobalt, while that on the alloys containing more chromium—with a significant contribution of inward oxygen transport from atmosphere. This transport is not a lattice diffusion, but proceeds presumably through microfissures resulting from the secondary process of perforating dissociation of the outer scale layer.