Cyclic oxidation behavior of Ni3Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025% B between 900 and 1100°C

A Ni₃Al-based alloy, the composition of which was Ni-16.0% Al-7.8% Cr-1.3% Zr-0.8% Mo-0.025%B, was cyclically oxidized in the temperature range of 900 to 1100°C in air for up to 500 hr. The alloy displayed good cyclic oxidation resistance up to 1000°C, with little scale spallation. It, however, lost cyclic oxidation resistance during oxidation at 1100°C after about 200 hr, displaying large weight losses due to serious scale spallation. NiO, α-Al₂O₃, NiAl₂O₄ and ZrO₂ were formed. The oxide scales consisted primarily of an outer Ni-rich layer which was prone to spallation, and (Al, Cr, Zr, Mo, Ni)-containing internal oxides which were adherent due mainly to the formation of (Al₂O₃, ZrO₂)-containing oxides that keyed the oxide scale to the matrix alloy.     

Oxidation kinetics of a Pb-64 at.% in single-phase alloy

The solid-state oxidation kinetics of a Pb-64 at. % In (50 wt. %) single-phase alloy were studied from room temperature to 150°C using an AES (Auger Electron Spectroscopy) depth profiling technique. The general oxidation behavior of this alloy is different from that of a Pb-3 at.% In alloy but similar to that of a Pb-30 at.% In alloy. The oxide formed on this alloy is almost pure In oxide (In₂O₃) with the possible existence of some In suboxide near the oxide/alloy interface. At room temperature, oxidation of the alloy follows a direct logarithmic law, and the results can be described by the model proposed previously by Zhang, Chang, and Marcotte. At temperatures higher than 75° C, rapid oxidation occurred initially followed by a slower parabolic oxidation at longer time. These data were described quantitatively by the model which assumes the existence of short-circuit diffusion in addition to lattice diffusion in the oxide as proposed by Smeltzer, Haering, and Kirkaldy. The effects of alloy composition on the oxidation kinetics of (Pb, In) alloy are also examined by comparing the data for Pb-3, 30, and 64 at. % In alloys.     

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 Oxidation of a Directionally Solidified Ni-Al-Cr3C2 Alloy at 1100 and 1200°C in Oxygen

The oxidation behavior of a directionallysolidified Ni-Al-Cr₃C₂ alloy ofover all composition Ni-12.3Cr-6.9Al-1.8C (wt.%) hasbeen investigated at 1100 and 1200°C under 1 atmoxygen. Experiments have also been carried out on specimens having the samecomposition but with a nonaligned structure. At1100°C, in both cases and unlike conventionalnickel-base superalloys with the same chromium andaluminum contents, aluminium was found to oxidize internallybeneath an external Cr₂O₃ scale.Although the volume fraction of the internalprecipitates was significant, they showed no tendency tocoalesce into a compact subsurface layer, but formed randomly distributed clustersin the alloy matrix. The kinetics of oxidation andmorphologies of the oxide scales were not substantiallyaffected either by thermal cycling or by the alloy microstructure. At the higher temperature,continuous Al₂O₃ scales formedbeneath thick layers of transient nickel andnickel-chromium-rich oxides; no internal precipitationof aluminum-rich oxides was observed. However, internal degradation of thedirectionally solidified specimens at 1200°C wasquite significant, due to in situ oxidation of primarycarbides. The multilayered scales formed at 1200°C spalled extensively on cooling as a consequenceof loss of contact, starting preferentially at theintersections of the Cr₂C₃ fiberswith the alloy-scale interface. The observed behaviorcan be attributed to a reduction in the availability of chromiumbecause of the multiphase structure of the alloy; this,in turn, resulted in an increase in the flux of oxygeninward, leading to internal oxidation of aluminum at 1100°C. The almost exclusive externaloxidation of aluminum becomes possible at 1200°C,probably because of an increase in the diffusivity ofaluminum in the alloy matrix.     

Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H₂/H₂O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.     

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.     

Grain size effects on the oxidation of two ternary Cu-Ni-20wt.% Cr alloys at 700-800 °C in 1 atm O2

Two nanocrystalline two-phase Cu-Ni-Cr alloys, both prepared by mechanical alloying and containing about 20 at.% Cr but with different Ni contents (40 and 20 wt.%, respectively), have been oxidized in 1 atm O₂ at 700-800 °C. Their oxidation behavior has been compared with that of two cast alloys of the same composition, already studied previously, to examine the effects of a large reduction of the size of the individual phase grains and particles. The nanophase alloy with 40 wt.% Ni formed a flat external layer of chromia of regular thickness, while the corresponding cast alloy produced a very irregular chromia layer, often protruding deeply into the alloy, only after an initial stage of rather fast corrosion involving also copper and nickel, associated with some degree of internal oxidation. By oxidation at 700 °C the nanophase alloy with 20 wt.% Ni formed an irregular chromia layer associated with low corrosion rates. The corresponding cast alloy formed complex scales containing Cu, Ni and Cr oxides, extending into the alloy in the form of large pegs, even though a very irregular and discontinuous innermost chromia layer was still able to produce low corrosion rates. On the contrary, at 800 °C both alloys formed complex scales containing mixtures of the oxides of the three metal components. However, the scales grown on the cast alloy were much more irregular in thickness and formed large protrusions into the alloy. In spite of this, the corrosion kinetics of the nanophase 20 wt.% Ni alloy at 800 °C were more irregular and, except for an initial stage, less protective than that of the cast alloy with the same composition.     

Studies on the development of TZM alloy by aluminothermic coreduction process and formation of protective coating over the alloy by plasma spray technique

TZM alloy is a potential candidate for high temperature structural applications. However, in the preparation of this alloy by conventional melt-casting route, difficulties are encountered in achieving homogenized alloy composition in view of high melting temperature of the alloy and presence of minor alloying components. Therefore, an alternative technique of aluminothermic co-reduction was adopted to prepare TZM alloy of composition, Mo-0.5Ti-0.1Zr-0.02 °C, wt.% by simultaneous reduction of uniformly premixed oxides of MoO₂, TiO₂ and ZrO2 by aluminium in presence of requisite amount of carbon. The as-reduced alloy was further arc melted for consolidation. Since, TZM alloy is by nature highly susceptible to oxidation at elevated temperature in air or oxygen, therefore feasibility of development of silicide type of coating over the synthesized alloy by plasma coating technique was also examined. Silicon powder coated on TZM alloy surface by plasma spray technique was finally converted into MoSi₂ coating by sintering at 1350 ^°C for 2-4 h duration under argon. A double layer coating structure was formed with two distinct phases. The inner thin layer was consisted of Mo₂Si₅ phase (~ 10 μm) followed by thick outer layer of MoSi₂ (~ 150 μm). The coating showed good adhesion strength and stable oxidation with negligible mass gain (10 g/m²) at 1000 ^°C in air.     

The oxidation of two ternary Ni-Cu-10 at.% Al alloys in 1 atm of pure O2 at 800-900 °C

The oxidation of the ternary alloys Ni-45Cu-10Al and Ni-30Cu-10Al has been studied at 800-900 °C under 1 atm O₂. The presence of 10 at.% Al reduces significantly the oxidation rate of the corresponding Cu-Ni alloys during the initial oxidation stages, even before the establishment of a complete Al₂O₃ layer. The weight of individual sample of the two ternary Ni-Cu-10Al alloys at 800 °C increases more rapidly than at 900 °C during the initial oxidation stage. As oxidation proceeds, the weight gain at 800 °C slows down to a degree that the total weight gain after 24 h oxidation at 800 °C is less than that at 900 °C. Due to a faster formation of the Al₂O₃ layer, which suppresses earlier the further oxidation of Cu and Ni, the external region of the scales grown on Ni-45Cu-10Al contain much less Cu and Ni oxides than those grown on Ni-30Cu-10Al. The transition from the internal oxidation to the selective external oxidation of the most reactive component Al in Ni-Cu-Al alloys is favored by higher values of the Al content, of temperature and of the Cu/Ni ratio.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Influence of nitridation on the oxidation of a 304 steel at 800 °C

This paper presents a study on the oxide growth on a AISI 304 chromia-forming alloy, in air at 800 °C. After the nitridation treatment was performed on the steel surface, a γ_N solid solution is detected. In this case, no nitride formation in the alloy surface could be observed. In situ X-ray diffraction has been used to follow the oxides evolution at testing temperature. At the beginning of the oxidation test, CrN is formed together with Fe₂O₃. Nevertheless, Cr₂O₃ quickly appears and leads to a protective oxide scale formation growing according to a parabolic rate law. During oxidation in situ X-ray diffraction also shows that Fe₂O₃ is transformed into FeCr₂O₄. Our results show that nitridation increases the high temperature oxidation resistance of 304 steels at 800 °C.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

Influence of nitridation and temperature cycling on oxidation of Ti-0.96 wt % Si alloy

The oxidation kinetics of prenitrided, and subsequently oxidized in either air or oxygen, Ti-0-96 wt % Si alloy have been investigated in the temperature range of 900–1100°C. The activation energies for various gas-alloy reactions are reported. Comparative kinetics data are given for oxidation of the alloy in oxygen or air without prenitridation. The oxidation resistance of nitrogen pretreated alloy is about the same as its resistance to oxidation in air. Temperature cycling during oxidation does not exert a beneficial effect on the oxidation characteristics of the alloy. X-ray and electron microprobe analyses reveal the presence of TiO₂, Ti₂O, -TiN, and -TiN in the scale and the concentration profiles of Ti, Si, O, and N in the alloy.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

The oxidation resistance of a sputtered,microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al₂O₃+TiO₂ scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.     

The Hot Corrosion of Fe-Mn-Al–C Alloy with NaCl/Na2SO4 Coating Mixtures at 750°C

The high-temperature corrosion behavior of Fe-30.1Mn-9.7Al-0.77C alloy initially coated with 2 mg/cm² NaCl/Na₂SO₄ (100/0, 75/25, 50/50, 25/75 and 0/100 wt.%) deposits has been studied at 750°C in air. The result shows that weight-gain kinetics in simple oxidation reveals a steady-state parabolic rate law after 3 hr, while the kinetics with salt deposits all display multi-stage growth rates. The corrosion morphology of the alloy with 100% Na₂SO₄ coating is similar to that of simple oxidation. NaCl acts as the predominant corrosion species for Fe-Mn-Al-C alloy, inhibiting the formation of a protective oxide scale. For the alloy coated with over 50% NaCl in salts, NaCl induces selective oxidation of manganese and results in the formation of secondary ferrite in the alloy substrate as well as void-layers with different densities of voids layer by layer in the secondary-ferrite zone.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

High-temperature corrosion of nickel in SO2

The reaction of nickel with SO₂ has been studied in the temperature range 650–1100°C at SO₂ pressures from 10 to 760 Torr. Reaction kinetics have been studied by thermogravimetry; the reacted specimens have been characterized by means of metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves oxidation and sulfidation except at sufficiently high temperatures and low pressures of SO₂ (e.g., 1000°C and 10 Torr SO₂) where only formation of NiO takes place. Approximately linear reaction kinetics are observed between 650 and 900°C. Reaction mechanisms are discussed, and the relative importance of oxide formation and sulfidation is interpreted in terms of the thermodynamics of the Ni-O-S system.     

The influence of ion-implanted yttrium on the selective oxidation of chromium in Co-25 wt.% Cr

Specimens of Co-25 wt.% Cr, Co-25 wt.% Cr-1 wt.% Y, and yttriumimplanted Co-25 wt.% Cr alloy were oxidized at 1000°C in 1 atm O₂. The implantation dosage ranged between 10¹⁶ to 10¹⁸ ions/cm². The unimplanted binary alloy oxidized to a duplex Co-rich scale, but the Y-containing ternary alloy formed a continuous Cr₂O₃ layer. When the implantation dosages were lower than a nominal 10¹⁸ ions/cm², the alloy failed to develop a similar continuous Cr₂O₃ layer as that observed with the Y-containing alloy. A temporarily stable external Cr₂O₃ scale was formed on the most heavily implanted specimen (1×10¹⁸ Y⁺/cm²). This Cr₂O₃ scale consisted of very fine-grained oxide, which is permeable to the outward transport of Cr and Co. Internal oxidation pretreatment of the ion-implanted specimens converting the Y metal to its oxide prior to the oxidation experiment, can enhance the development of an external Cr₂O₃ scale, but this scale is also unstable. Results suggest that the selective oxidation of chromium in an ordinarily non-Cr₂O₃ -forming alloy can be due to the reactive element oxides acting as preferential nucleation sites on the alloy surfaces, but the subsequent growth of these scales may require a continuous supply of reactive elements in the alloy.     

Hot corrosion behaviour of Nbss/Nb5Si3 in situ composites in the mixture of Na2SO4 and NaCl melts

Hot corrosion behaviour of Nb–16Si–24Ti–6Cr–6Al–2Hf (at.%) in the mixture of Na₂SO₄ and NaCl melts at 900 °C was studied. The results show that the corrosion kinetics of the alloy fit parabolic law. The oxides consist of a loose and porous outer layer and an internal oxidation zone. Outer oxides are mainly composed of TiO₂, TiNb₂O₇, Nb₂O₅, CrNbO₄ and SiO₂ while the internal oxidation zone is composed of TiO₂. Hot corrosion mechanism of the alloy in the presence of Na₂SO₄ and NaCl deposits is discussed.