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Studies on the Surface Chemistry of Oxide Films Formed on IN-738LC Superalloy at Elevated Temperatures in Dry Air
Abstract
The oxidation behavior of IN-738LC was studied to develop high-temperature materials for low cost and highly efficient turbine systems. The present study was undertaken to investigate the kinetics and the surface chemistry of the oxide films formed during isothermal oxidation of IN-738LC superalloy in the temperature range 1123–1223 K in dry air. The oxidation kinetics followed the parabolic law. The activation energy of oxidation was 264 kJ mol–1. The scaling process is controlled mainly by the diffusion of chromium ions through the intermediate chromia layer in the scale. The surface morphology and the oxide phases of the scale were characterized by SEM, XRD, and EDS studies. XRD analysis revealed the presence of NiO, NiAl2O4, NiCr2O4 spinel, Al2O3, and Cr2O3 on the top-scale surface. The scale surface and cross section were further characterized using X-ray photoelectron spectroscopy (XPS), which revealed the presence of NiO, Ni2O3, NiAl2O4, Al2O3, and TiO2 on the top-oxide surface. The chromia layer was found to be underneath the top scale. The chromia layer also contains NiCr2O4 and NiAl2O4 spinels along with Al2O3. Application of XPS was found to be successful to understand the oxide-scale chemistry in terms of the oxide-growth mechanism of IN-738LC at elevated temperatures.
... When the temperature rise to 1,000°C, the peak of Sr3d 5/2 in SrAl 4 O 7 shifts to a low binding energy of 132.73 eV, which is assigned to Sr3d 5/2 in SrCrO 4 [23]. Using a full-width-halfmaximum (FWHM) of 2.5 eV for envelope of Cr peak in Fig. 9b, the binding energy of Cr2p 3/2 at 579.5 eV is assigned to Cr in SrCrO 4 [23,24] [27], NiO [28], and NiAl 2 O 4 [28] species on the worn surface (Fig. 9c). It also revealed that the oxides thin films covered on the worm surfaces for there is no metallic Ni peaks appeared in XPS spectra, but Ni (Cr) solid solution peaks appeared in XRD patterns (Fig. 8). ...
... When the temperature rise to 1,000°C, the peak of Sr3d 5/2 in SrAl 4 O 7 shifts to a low binding energy of 132.73 eV, which is assigned to Sr3d 5/2 in SrCrO 4 [23]. Using a full-width-halfmaximum (FWHM) of 2.5 eV for envelope of Cr peak in Fig. 9b, the binding energy of Cr2p 3/2 at 579.5 eV is assigned to Cr in SrCrO 4 [23,24] [27], NiO [28], and NiAl 2 O 4 [28] species on the worn surface (Fig. 9c). It also revealed that the oxides thin films covered on the worm surfaces for there is no metallic Ni peaks appeared in XPS spectra, but Ni (Cr) solid solution peaks appeared in XRD patterns (Fig. 8). ...
... Between 600 and 800°C, the oxidation of Ni and Cr results in a well-distributed triboreaction film on the worn surfaces, the wear resistance is enhanced owing to synergistic effect of SrAl 4 O 7 , NiCr 2 O 4 , and Ag and formation of oxides. When the temperature rise to 1,000°C, The compound SrCrO 4 was formed on the worn surfaces due to the growth of chromium oxide of the worn surface which outward diffuse through a Cr 2 O 3 layer situated underneath the top oxide[28]. This result was verified by heating mixtures of SrCO 3 and Cr 2 O 3 in the appropriate molar ratios at *1,000°C in air in the literature[30]. ...
NiCr–Al2O3–SrSO4–Ag self-lubricating composites were prepared by powder metallurgy method and the tribological properties of composites were evaluated by a ball-on-disk tribometer against alumina ball at wide temperature range from the room temperature to 1,000 °C in air. The linear coefficient of thermal expansion was evaluated for investigation of thermal stability of composites. The tribo-chemical reaction films formed on the rubbing surfaces and their effects on the tribological properties of composites at different temperatures were addressed according to the surface characterization by SEM, XRD, and XPS. The results show that the NiCr–Al2O3 composite with addition of 10 wt% SrSO4 and 10 wt% Ag exhibits satisfying friction and wear properties over the entire temperature range from room temperature to 1,000 °C. The composition of the tribo-layers on the worn surfaces of the composites is varied at different temperatures. The synergistic lubricating effect of SrAl4O7, Ag, and NiCr2O4 lubricating films formed on worn surfaces were identified to reduce the friction coefficient and wear rate from room temperature to 800 °C. Meanwhile, at 1,000 °C, the SrCrO4 and NiAl2O4 was formed on the worn surfaces during sliding process, combining with the NiCr2O4, Al2O3, Cr2O3, Ag, and Ag2O, which play an important role in the formation of a continuous lubricating film on the sliding surface.
... In particular, many authors have put forward the hypothesis that a high concentration of titanium generates a strong diffusional activity of Cr 3+, therefore a higher oxidation kinetics [12,47,48]. ...
... It is important to note that the result that we obtained fairly agrees with those obtained by some authors who found a parabolic form of isothermal oxidation kinetics) [12,47,[49][50][51][52] for the same Inconel, but not in the same use conditions (air or not, blade or not, etc.). However, for relatively short-term and other long-term exposures, where various factors can alter the kinetics of the oxide layers, some discrepancies are seen. ...
An as-received IN738LC blade root material has been firstly characterized, in order to appreciate its structural, microstructural and morphological properties. Different observations and analysis (OM, XRD, SEM/EDS, etc.) showed that our sample is mainly constituted of γ´ precipitates in a Ni-rich matrix (γ solution), a γ/γ´ eutectic and some carbides. After that, the oxidation behavior of IN738LC superalloy has been investigated to assess the oxidized formed layers. For this purpose, isothermal oxidation experiences have been carried out at 950 and 1150 °C for various times (24h, 48h and 72h). The different microstructures of oxides and substrate obtained after oxidation tests were observed and analyzed by scanning electron microscopy (SEM/EDS). The oxidation kinetics were monitored by samples weight change with time. Composition and phase evolutions, mean crystallite size and strain appeared in this superalloy, were determined by X-ray diffraction (XRD) patterns. Last, the adhesion of the growing oxides onto the substrate was evaluated by the scratch test. The main results of these investigations reveal that microstructure parameters strongly depend on temperature and durations of the process. Moreover, the oxidation kinetics of the alloys follows a parabolic rate law both at 950 °C and at 1150 °C. From XRD and EDS analysis, the major phases seen are Cr2O3, TiO2, Al2O3, NiO and NiCr2O4, which contribute to a better oxidation resistance. The oxide scales increase with time and temperature, they are compact, dense, distributed uniformly and adherent at 950 °C/72h, while they are relatively larger and non-uniform at 1150 °C. On the other hand, the adhesion of the oxides at 1150 °C is fairly weak, and oxidation products flake off quite easily, which indicated weakening of the protection.
... The presence of TiO 2 in the external scales formed on superalloys oxidized at a high temperature was earlier noticed in several works, but for oxidation temperatures lower than 1200 • C. In some cases, TiO 2 was mixed with Al 2 O 3 , Cr 2 O 3 and spinels [17][18][19][20][21], but not with the shape and locations observed at 1200 • C [15] and at 1250 • C in the present work. In other cases, the presence of TiO 2 was effectively noticed at the outer surface of an alumina scale [22,23] or a chromia scale [24] for other oxidized alloys (RHEA, nickel-based alloys . . . ). ...
... It was earlier reported that the oxide scale grows by the outward diffusion of Cr and Ti and ingress of O [22,25], and that Ti induces a doping effect that enhances the growth rate of chromia [20,24]. It was also seen that Ti can diffuse through chromia [26,27]. ...
Six conventionally cast chromium-rich titanium-containing alloys based on cobalt and nickel with various Co/Ni ratios were considered. They were tested in oxidation in air at 1250 °C for 70 h in a thermo-balance. The mass gain curves were exploited to specify different types of kinetic constants as well as several parameters characterizing the oxide spallation occurring during cooling. The obtained results show that, the higher the Ni content, the slower the mass gain and the better the quality of the protective external chromia scale. Secondly, no dependence of the oxide spallation characteristics on the Co content was clearly noted. Globally, the isothermal oxidation behavior becomes better when Ni is more and more present at the expense of Co. Titanium seems to be playing a particular role in the process of oxidation. It notably leads to the presence of an external thin TiO2 continuous scale beyond the chromia scale. The thermogravimetry records were numerically treated to determine the parabolic constant and the chromia volatilization constant. The values of these constants evidenced a double tendency: chromia growth acceleration and chromia volatilization slow-down. These trends are to be confirmed and further investigated.
... As for the 1100 °C/5 h exposure results, continuous Al2O3 can be observed in HESA-2 (Figure 2d), while thick external (Ni, Co)O, Co, Fe, Ni and Ti-rich oxides, Cr2O3 and discontinuous Al2O3 are present in HESA-1 (Figure 2c). Although both Cr2O3 and Al2O3 can provide surface protection, Al2O3 would possess lower oxygen permeability and be more thermodynamically stable than Cr2O3, while Cr2O3 may gradually transform into the volatile CrO3 beyond 950 °C (2Cr2O3 (s) + 3O2 (g) = 4CrO3 (g)) [26][27][28][29][30]. Hence, the formation of a continuous Al2O3 layer is critical for protection against oxidation at high temperature. ...
... The measured oxide concentrations should approximately agree with the compositions indexed by XRD. The XRD analysis are shown in Figure 3, and it shows that identical type of oxides are formed at 900˝C and 1100˝C, Figure 3a [26][27][28][29][30]. Hence, the formation of a continuous Al 2 O 3 layer is critical for protection against oxidation at high temperature. ...
The present work investigates the high temperature oxidation and corrosion behaviour of high entropy superalloys (HESA). A high content of various solutes in HESA leads to formation of complex oxides, however the Cr and Al activities of HESA are sufficient to promote protective chromia or alumina formation on the surface. By comparing the oxidation and corrosion resistances of a Ni-based superalloy-CM247LC, Al2O3-forming HESA can possess comparable oxidation resistance at 1100 °C, and Cr2O3-forming HESA can exhibit superior resistance against hot corrosion at 900 °C. This work has demonstrated the potential of HESA to maintain surface stability in oxidizing and corrosive environments.
... The formation of the alloy compounds in Cr-NiO was confirmed by the appearance of new peaks at 857.4 eV (NiCr x O y ). The NiO peak shifted to the highest binding energy (BE), which may be due to the dissolution of Cr in the nickel species [36]. ...
... One route is the addition of some elements to the Ni-based alloys, such as Cr and Al. These elements can promote the formation of NiCr 2 O 4 , Cr 2 O 3 , and Al 2 O 3 protective layers [5][6][7][8][9][10]. The Ni-based alloy used in the manufacture of natural gas engine exhaust valve seats has a high concentration of Cr, Fe, W, Co, and Mo to yield the properties required for this application [11,12]. ...
This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up to 1 h. At 660 and 740 °C, only one stage was observed during the whole time studied. At this stage, the oxide layer was formed mainly by NiO + Cr2O3, following a linear oxidation law with a rate constant (Kl) on the order of magnitude of 10−6 kg/m2s and an apparent activation energy (Ea) of ~47 kJ/mol. At 860 and 900 °C, an identical first stage was observed with a transition to a different stage. In the second stage, the oxidation layer was composed of Cr2O3, and a parabolic oxidation law was followed with a rate constant (Kp) on the order of 10−8 kg2/m4s and Ea of ~128 kJ/mol. Moreover, the Ni-based alloy formed a dense and compact oxide layer after oxidation, with no apparent cavities, pores, or microcracks. Characterization techniques such as Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy were carried out to characterize the formed oxide layer.
... As miniaturization and precision of parts are demanded and structural thickness of parts is decreasing recently, high temperature oxidation and high temperature stability became important [30][31][32][33] . If Inconel alloys are exposed to high working temperature, oxidation might cause depletion of some elements, thereby reducing its high temperature strength and creep property [34][35][36] . ...
The present study investigated the effect of as-built and post heat-treated microstructures of IN738LC alloy fabricated via selective laser melting process on high temperature oxidation behavior. The as-built microstructure showed fine cell and columnar structure due to high cooling rate. Ti element segregation was observed in inter-cell/inter-columnar area. After post heat-treatment, the initially-observed cell structure disappeared, instead bimodal Ni3(Al, Ti) particles formed. High temperature (1273 K and 1373 K) oxidation test results showed parabolic oxidation curves regardless of temperature and initial microstructure. The as-built IN738LC fabricated via the selective laser melting process displayed oxidation resistance similar to or slightly better than that of IN738LC fabricated via wrought or cast process. Heat-treated SLM IN738LC, although had similar oxidation weight-gain values to those of the SLM as-built material at 1273 K, showed relatively better oxidation resistance at 1373 K. Bimodal Ni3(Al, Ti) precipitate formed in the post heat treatment changed the local chemical composition, thereby led to changes in alumina former/chromia former location and fraction on the alloy surface. It was concluded that in heat-treated IN738LC increased alumina former fraction was found, and this resulted in excellent oxidation resistance and relatively low weight-gain.
... The turning point of the mass change of the coating from mass gain to continuous mass loss is the indicator of failure. Lots of research reveal that compared with the oxidation time, oxidation temperature plays a more decisive role in the diffusion of oxygen and related reactions [74][75][76][77]. In other words, the rate of the oxidation reaction is increased with increasing oxidation temperature. ...
Spent MoSi2 heating elements, which worked in the industrial electrical furnace at high temperature, were crushed into powders and reused as coating raw materials. ZrO2 was added into spent MoSi2 powders to fabricate MoSi2-ZrO2 coatings on niobium by spark plasma sintering, and the microstructure and oxidation behavior of the coatings at 1500 and 1700 °C were explored. The results showed that cracks exist in MoSi2 coating while ZrO2-containing coatings are crack-free with the formation of (Mo,Nb)5Si3 diffusion layer after SPS, indicating good metallurgical bonding. After oxidation at 1500 °C for 20 h, the lowest mass gain is obtained by MoSi2-30ZrO2 coating. SiO2 and ZrSiO4 are formed on the composite coatings at elevated temperature, which improves the high-temperature oxidation resistance. After oxidation at 1700 °C for 60 min, the oxide scale of MoSi2-30ZrO2 coating is dense without any voids or cracks, demonstrating better high-temperature oxidation resistance than that of single MoSi2 coating.
... As is discussed above, the nanocrystalline Ni\ \Cr alloy coating can lead to the formation of finer-grained oxide scale. These finergrained oxide scales exhibits relatively good properties of high temperature plasticity and creep deformation [58,59]. Thereby the stresses in the oxide scales can be relieved through high temperature creep, instead of cracking or spalling. ...
... The proofs of spallation, or tiny black particles were observed on the specimen holders around the vertically placed specimens in 1100 1C tests, but not in 900 1C tests. In addition to spallation, another major source of weight loss is the evaporation of Cr-rich oxide layers that become thermodynamically unstable and begin to be evaporated at 1100 1C [17]. Other sources of weight loss, like decarburization and thermal etching were also observed, but their contribution to weight loss was considered not so significant. ...
Nickel-base superalloys are considered as materials for piping and structural materials in a very high temperature gas cooled reactor (VHTR). They are subjected to the environmental degradation caused by a continuous process for oxidation due to small amount of impurities in He coolant during long term operation. In the present study, the oxidation behaviors of several nickel-base superalloys such as Alloy617, Haynes214 and Haynes230 in particular, were studied at the temperature of 900°C and 100°C in air, the high purity He environment. Oxide layers were analyzed by SEM and EDX. The differences in oxidation behaviors of these alloys were mainly caused by different protective oxide layers on surface. In the case of Alloy617 and Haynes230, Cr 2O3 layer formed on the surface which is not stable at 1100°C. Therefore, the weight increased significantly due to oxidation at the initial stage, which followed by a decrease due to the spoiling and volatilization of Cr2O3 layer. On the other hand, since Haynes214 has mainly Al2O3 oxide layer on surface which is more stable and dense structure at the higher temperature, the weight gain eventually reaches to parabolic. Microstructural characteristics of internal carbides and carbide depletion zone were analyzed. With oxidation time, continuous grain boundary carbides of M23C6 type were getting thin or it disappeared partially. Especially, carbides on grain boundary disappeared entirely below oxide layer (carbide depletion zone). It was getting wide with oxidation time. For Haynes214, the size of carbide depletion zone was smaller than other alloys because Al2O3 layer acted as a diffusion layer prevented effectively to penetration of oxygen into base metal.
... The binding energies occurred at 577.1 eV, 576.1 eV are assigned to Cr in Cr 2 O 3 [32] and NiCr 2 O 4 [33], respectively. The Ni3p 3/2 peaks at 856.2 eV [34] and 854.5 eV [35] indicate the presence of NiCr 2 O 4 and NiO species on the worn surface (Fig. 10b). In Fig.10c, according to the handbook of X-ray photoelectron spectroscopy, the Al 2 O 3 is identified on the rubbing surface. ...
The effect of Al2O3 content on the mechanical and tribological properties of Ni–Cr alloy was investigated from room temperature to 1000 °C. The results indicated that NiCr–40 wt% Al2O3 composite exhibited good wear resistance and its compressive strength remained 540 MPa even at 1000 °C. The values obtained for flexural strength and fracture toughness at room temperature were 771 MPa, 15.2 MPa m1/2, respectively. Between 800 °C and 1000 °C, the adhesive and plastic oxide layer on the worn surface of the composite was claimed to be responsible for low friction coefficient and wear rate.KeywordsNiCr–Al2O3 compositesMechanical propertiesTribological propertiesElevated temperatures
... elevated-temperature crack growth, creep-fatigue interaction, design for high temperature applications and prevention of excessive oxidation are of great importance in this aspect.Superalloys were developed with a view to improve both reliability and performance in service in gas turbine environments[1]. find extensive application in aerospace industries, nuclear reactors, chemical, petrochemical, power generation plants, environmental protection systems, cryogenic applications and furnaces, where extreme temperatures, mechanical stresses and corrosive environments are encountered[2]. They are classified as nickel based, cobalt-based and iron based superalloys.The turbine system that functions at high temperatures consists of many expensive components, which must all work together without failure. ...
... An extensive number of surface science techniques have been used to date to investigate these and related issues, including for example X-ray photoelectron spectroscopy (XPS) [1], auger electron spectroscopy (AES) [2], secondary ion mass spectroscopy (SIMS) [3], energydispersive spectroscopy (EDS) [4], electron energy loss spectroscopy (EELS) [5] and scanning/transmission electron microscopy (SEM/TEM), often used to powerful effect when combined in a multi-technique approach to give a fuller picture [6,7]. ...
... Considering the reaction between NiO and -Al 2 O 3 , the possible form of Ni element could be Ni 2 O 3 , NiAl 2 O 4 , NiO, or Al 3 Ni. According to the previous reported data, the electron binding energy of Ni 2p 3/2 is about 851.7-853 eV for Ni metal [28,29], 852.6-853.7 eV for Al 3 Ni [30], 853.6-855.5 eV for NiO [31,32], 855.8-856 eV for Ni 2 O 3 [33], and 855.9-857 eV for NiAl 2 O 4 [34]. Comparing our results with these data, we could find that for the sample annealed at 100 • C, the Ni element is still in the form of NiO phase. ...
NiO/γ-Al2O3 catalysts with NiO content of 9 wt% and 24 wt% were prepared by solid state reaction method. They are annealed in air at temperatures from 100 °C to 1000 °C. Positron lifetime spectra were measured to study the microstructure variation during annealing process. Four positron lifetime components were resolved with two long lifetime τ3 and τ4, which can be attributed to the ortho-positronium lifetime in microvoids and large pores, respectively. It was found that the longest lifetime τ4 is rather sensitive to the chemical environment of the large pores. The NiO active centers in the catalysts cause decrease of both τ4 and its intensity I4, which is due to the spin-conversion of positronium induced by NiO. However, after heating the catalysts above 600 °C, abnormal increase of the lifetime τ4 is observed. This is due to the formation of NiAl2O4 spinel from the reaction of NiO and γ-Al2O3. The generated NiAl2O4 weakens the spin-conversion effect of positronium, thus leads to the increase of o-Ps lifetime τ4. Formation of NiAl2O4 is further confirmed by both X-ray diffraction and X-ray photoelectron spectroscopy measurements.
... The proofs of the spallation, or tiny black particles were observed on the specimen holders around the vertically placed specimens in 1100 o C tests, but not in 900 o C tests. In addition to the spallation, another major source of weight loss is the evaporation of Cr-rich oxide layers which become thermodynamically unstable and begin to be evaporated at 1100 o C [17]. Other sources of weight loss, like decarburization and thermal etching were also observed, but their contribution to the weight loss was considered not so significant. ...
The oxidation characteristics of Alloy 617, a candidate structural material for the key components in the very high-temperature gas-cooled reactor (VHTR), were investigated. High-temperature oxidation tests were conducted at 900 and 1100°C in air and helium environments and the results were analysed. Alloy 617 showed parabolic oxidation behaviour at 900°C, but unstable oxidation behaviour at 1100°C, even in a low oxygen-containing helium environment. The SEM micrographs also revealed that the surface oxides became unstable and non-continuous as the temperature or the exposure time increased. According to the elemental analysis, Cr-rich oxides were formed on the surface and Al-rich discrete internal oxides were formed below the surface oxide layer. After 100h in 1100°C air, the Cr-rich surface oxide became unstable and non-continuous, and the matrix elements like Ni and Co were exposed and oxidized. Depletion of grain boundary carbides as well as matrix carbides was observed during the oxidation in both environments. When tensile loading was applied during high-temperature oxidation, the thickness of the surface oxide layer, the internal oxidation, and decarburization were enhanced because of the increase in diffusion of oxidizing agent and gaseous reaction products. Such enhancement would have detrimental effects on the high-temperature mechanical properties, especially the creep resistance of Alloy 617 for the VHTR application.
... dry air. The oxidation kinetics was measured continuously in a thermogravimetry set-up consisting of a microbalance (Sartorius, model LA230P, ±0.01 mg), a vertical furnace with temperature-controlling accessories, and a computer for continuous data acquisition. The high-temperature-oxidation set-up used in the present study is described elsewhere (Seal et al . 2001. ...
Nanomaterials have attracted considerable interest with numerous technological developments in the last decade. Nanomaterials exhibit different physicochemical properties compared with their bulk counterparts because the diameters of the nanoparticles are less than the Bohr exciton radius. Cerium oxide based materials have been extensively studied for various technological applications. In the present study, the application of nanocrystalline cerium oxide for improvement of high-temperature-oxidation resistance of stainless steel has been studied. The role of coating of nanocrystalline cerium oxide towards improvement of high-temperature-oxidation resistance has been investigated and compared with that of the micrometre-sized cerium oxide particles. It was observed that nanocrystalline ceria improved high-temperature-oxidation resistance of AISI 304 stainless steel to a large extent compared with the micrometre-sized ceria coating. Nanocrystalline ceria coating decreased the isothermal parabolic rate constant of oxidation by more than two orders of magnitude compared with that of the bare alloy. The resistance to oxide scale spallation was also found to improve with the coating of cerium oxide nanoparticles. Secondary ion mass spectroscopy (SIMS) study of nanocrystalline ceria-coated and oxidized specimens revealed the presence of nanoceria at the outermost oxide surface, indicating a change in the oxide scale growth mechanism from outward cation diffusion to inward oxygen diffusion in the bare alloy at high temperature in dry air. The oxide scale morphology was studied using a scanning electron microscope (SEM) while a focused ion beam (FIB) technique was used to study the oxide-alloy interface. X-ray photoelectron spectroscopy (XPS) study of nanocrystalline ceria showed the presence of Ce3+ and Ce4+ oxidation states. It is proposed that the presence of the Ce3+ oxidation state in nanocrystalline ceria improves the oxidation resistance of stainless steel, and the related mechanisms are discussed.
... The oxidation kinetics was measured continuously in a thermogravimetry set-up which consisted of a microbalance (Sartorius; Model LA230P; ±0.01 mg), a vertical furnace with temperature controlling accessories, and a computer for continuous data acquisition. The high temperature oxidation set-up is described in details elsewhere (Seal et al., 2001). The parabolic oxidation kinetics plots of mass gain versus time were made. ...
Cerium oxide has been investigated to be an effective coating material for high temperature applications for various alumina- and chromia-forming alloys. The present study investigates the use of microemulsion method to obtain monodispersed, non-agglomerated nanocrystalline ceria particles in the range of 5nm using sodium bis(2-ethylhexyl) sulphosuccinate (AOT) as a surfactant. Furthermore, the use of non-agglomerated nanocrystalline ceria particles to develop improved high temperature oxidation resistant coatings on AISI 304-grade stainless steel was investigated. It was found that non-agglomerated nanocrystalline ceria particles were more effective in improving the oxidation resistance than the agglomerated nanocrystalline particles.
... The oxidation kinetics were measured continuously in a thermogravimetric set-up consisting of a microbalance (Sartorius, model LA230P, ±0.01 mg), a vertical furnace with temperaturecontrolling accessories and a computer for continuous data acquisition. The details of the high-temperature oxidation set-up used in the present study were described previously [25,31]. Scale morphology of the oxidized samples coated with NC and 2, 20 and 40 LDN was studied using a JEOL T-300 SEM with an acceleration voltage of 5 kV. ...
Nanocrystalline ceria (NC) and La3+-doped nanocrystalline ceria (LDN) particles synthesized by the micro-emulsion method were coated onto AISI 304 stainless steel (SS) in order to study their high-temperature oxidation resistance properties at 1243 K in dry air for 24 h. Results were compared with those for micro-ceria (MC) coatings. The coated samples showed 90% improvement in oxidation resistance compared to uncoated and MC coated steels as seen from SEM cross-sectional studies. X-ray diffraction (XRD) analysis showed the presence of chromia in both NC and 20 LDN (NC doped with 20 at.% La3+) samples which is absent in uncoated steels. From secondary ion mass spectrometry (SIMS) depth profiles, Fe, Ni depletion zones were observed in LDN-coated samples, indicating diffusion through the oxide layer. The role of oxygen vacancies in the nanoceria coatings on the formation of protective chromia layers is discussed.
To investigate the effects of air annealing on the microstructure, components, and mechanical properties of ceramic composite coatings, Al2O3–Cr2O3–ZrO2 composite coatings were prepared on silicon substrate using radio frequency magnetron sputtering at room temperature, and then air-annealed in a temperature range of 450–850 °C for 30 min. The results indicated that the phase-structure and superficial characteristics, including morphology and surface roughness, were not visibly altered in the annealed coatings up to 600 °C; the elemental component distributions remained uniform. The improvement in the mechanical properties was attributed to the growth of oxide grains. There were no significant changes in the components of Al, Cr, Zr, and O in the annealed coatings. However, an increase in the Cr component and a decrease in the Zr component occurred on the coating surface; the overall structure of the composite coatings possessed a favorable heat resistance. Upon annealing at 750 °C, the thermally-driven formation of uniform and refined nanoparticles on the coating surface was responsible for the effective enhancement of the mechanical properties. Furthermore, annealing at 850 °C induced the enlargement of the precipitated Cr2O3 nanoparticles and the generation of micro-defects, resulting in a drastic morphological evolution, an evident increase in the surface roughness, and a significant decrease in the mechanical properties. This study provides new perspectives on designing novel thermal barrier coatings and understanding the role of high temperature air annealing on the microstructural transformation.
Based on the advantages of nitric acid pressure process (NAPL) for laterite nickel ores, this study innovatively proposes nitric acid pressure leach liquor crystallization pyrolysis-mixed metal oxides separation process. Firstly, the feasibility and the possible reactions of mixed nitrate pyrolysis were confirmed by thermodynamic analysis. Subsequently, the optimal pyrolysis temperature was experimentally investigated. Then, the pyrolysis process of combined nitrate was thermodynamically analyzed, and the phase composition of mixed metal oxides was determined by X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), and Scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS). The Pourbaix diagram shows that using NaOH as leaching agent can selectively separate aluminum from mixed metal oxides, and the variables influencing aluminum's leaching rate are investigated. The experimental results are as follows: the optimal pyrolysis temperature of mixed nitrate crystals is 600 ℃; the pyrolysis products are mainly oxides with the formation of NiAl2O4 and MgAl2O4 spinel phases; the aluminum leaching rate is 81.9% under the optimal alkali leaching conditions. The presence of spinel phase is not favorable for aluminum leaching. After leaching, an enriched residue of 15.8% Ni, 1.9% Co and 0.19% Sc was obtained.
Oxidation behaviours and the interactions between friction and oxidation of Ni-based composites with in-situ carbides reinforcement were investigated at 800℃. Oxidation kinetics obey parabolic and parabolic-linear law. Oxide layers present hierarchy with three sublayers. I and II sublayers contain NiO, TiO2 and NiTiO3 etc.; III sublayer is O embrittled alloy layer. The pores in oxide layer result to transformation of oxidation kinetics from parabolic law to linear law. The worn oxide layers show the similar three-layered structure, comparable composition and faster thickening. Ultra-high flash temperature on worn surface induced by friction accounts for the formation and development of oxide layer.
The oxidation properties of a family of novel polycrystalline Ni-based superalloys with varying Ti:Nb ratio have been studied, which has shown a correlation between increasing titanium content and accelerated oxidation kinetics. High-resolution characterization of microstructure and chemistry was carried out on the resultant oxide layers using SEM/EDX and Atom Probe Tomography to precisely locate Ti segregation within chromia scales, in order to identify a potential mechanism to explain this correlation. Despite some spread in the data, levels of Ti dissolved in the chromia oxide scales showed little correlation with the nominal Ti concentration or the oxidation properties of each alloy, indicating that oxidation rates are not reliant on dopant levels within bulk chromia. A number of grain boundaries within the oxide scale were targeted for APT analysis, as these are known to act as short-circuit diffusion paths. Segregation of Ti, Nb and Ta to oxide grain boundaries was observed, suggesting that ion transport rates may instead be mediated by dopants present at grain boundaries rather than dissolved within the bulk.
The metals and alloys are prone to high-temperature oxidation and hot corrosion under severe environmental conditions. The degradation phenomenon is likely to occur by various mechanisms of deterioration such as creep damage, microstructural degradation, high-temperature fatigue, embrittlement, carburization, hydrogen damage, graphitization, thermal shock etc. In order to combat the degradation, thermal sprayed coatings can be deposited on the surface of the metal which can alter the surface properties of the metal. In the present study, WC-12%Co D-gun sprayed coating is investigated to know the corrosion phenomenon in actual coal fired boiler environment. The investigation consists of hot corrosion cyclic study at 1100 °C under cyclic conditions. The exposed samples were examined using scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS), X-ray diffractometer (XRD), cross-sectional analysis by X-ray mapping. It was found that the deposition of tungsten carbide-cobalt (WC-12%Co) coating has increased the oxidation resistance, however, the coating was partially consumed after 875 h of corrosion. The corrosion rate for WC-12%Co coated SS 304 and SS 316 was calculated in terms of MPY (mils per year) and found to be 99.41 and 47.72 respectively for 875 h.
Ni-Cr-Y 2 O 3 nanocomposite coating, with Y 2 O 3 nano-particles finely dispersed, was prepared by cathode plasma electrolytic deposition from environmental-friendly nickel-trivalent chromium sulfate electrolyte directly. The coating is dense and uniform with typically molten morphology, possessing a nanocrystalline structure with an average grain size of about 30–50 nm. Owing to the synergy effect of the fine-grained structure of the coating and the reactive-element effect of the addition of Y 2 O 3 nano-particles, a thin, compact, uniform and continuous Cr 2 O 3 oxide scale can be formed during 200 h oxidation at 750 °C. Such Cr 2 O 3 oxide scale provides effective protection to the inner alloy and substrate.
The present study was carried out to investigate the kinetics and the surface chemistry of the oxide layers formed on the IN-738LC super alloy during high-temperature oxidation at 950 °C in air from 1 to 260 h. Oxidation kinetics were studied by mass gain measurement. The oxide layers were characterized by field emission scanning electron microscope, elemental distribution map, energy-dispersive spectroscopy as well as x-ray diffractometry (XRD). The oxidation kinetics followed the parabolic law. The XRD analysis revealed that the oxide scale contained mainly NiO, Ni (Cr, Al)2O4, Al2O3, TiO2 and Cr2O3. The oxide structure, from the top surface down to the substrate, was clarified by elemental map distribution studies as Ni-Ti oxides, Cr-Ti oxides, Cr2O3 oxide band, Ni-Co-Cr-W oxide and finally a blocky Al2O3 region. The oxidation scales were composed of three distinct layers of the outer and mid layers enriched by TiO2 and Cr2O3, NiCr2O4 oxide, respectively, and the innermost layer was composed of Al2O3 and matrix alloy. The depleted gamma prime layer was formed under the oxidation scales due to the impoverishment of Al and Ti which were induced by the formation of Al2O3 and TiO2.
The CO2 reforming of CH4 to H2 over a catalyst is an effective method for renewable energy generation. In this study, SBA-15 and CNT were chosen as supports for the Ni-based catalysts prepared by the impregnation method. The FESEM images demonstrated that NiO particles were rhombic and well distributed on the SBA-15 surface. The XRD patterns showed that the chemical state of Ni changed after the reforming reaction; the main crystals on the fresh and spent Ni/SBA-15 were found to be NiO and Ni⁰. The catalytic performance of Ni/SBA-15 in CO2/CH4 reforming was found to be superior to that of Ni/CNT. The results of the TGA and BET analysis demonstrated that spent Ni/SBA-15 (at 600 °C) showed no catalytic decay as an insignificant amount of coke was deposited on the catalyst supports. Moreover, Ni-based bimetallic catalysts were studied for the reforming reaction, and the activity of the catalysts with respect to metals was observed to follow a particular order: Cu-Ni > Mg-Ni > Co-Ni > Ca-Ni > Mn-Ni. The Cu-Ni/SBA-15 catalyst exhibited higher catalytic activity at a reaction temperature of 650 °C as compared to the others; the H2 yield (40%) was not decreased as the reaction time increased, and the conversion of CO2 and CH4 is 77% and 75%, respectively.
Isothermal and cyclic oxidation behaviors of pure and yttrium-implanted nickel were studied at 1000 C in the air. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the micro-morphology and structure of oxide scales formed on nickel substrate. It was found that Y-implantation greatly improved the anti-oxidation ability of nickel both in isothermal and cyclic oxidizing experiments. Laser Raman Microscopy was also used to study the stress status of oxide scales formed on nickel with and without yttrium. The main reason for the improvement in anti-oxidation of nickel was that Y-implantation greatly reduced the growing speed and grain size of NiO. This fine-grained NiO oxide film might have better high temperature plasticity and could relieve parts of compressive stress by means of creeping, and maintained ridge character and relatively lower internal stress level, hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate. The actual existing form of yttrium in oxide can be Y2O3 and NiY2O4 spinel nanometric particles, and even be Y3+ ions segregated at NiO grain boundaries.
Isothermal oxidation behaviors of pure and yttrium-implanted nickel were studied at 900°C in air. SEM and TEM were used to examine the oxide scales formed on nickel substrate. Acoustic emission (AE) technique was used to monitor the cracking and spalling of oxide film in isothermal oxidizing stage and subsequent air-cooling stage. AE signals were analyzed in time and number domain according to the related oxide fracture model. It was found that Y-implantation greatly lowered the isothermal oxidizing rate of nickel and improve the anti-cracking and anti-spalling properties of NiO oxide film. The main reason for the improvement was that Y-implantation greatly reduced the grain size of NiO and increased the high temperature plasticity and creeping ability of the oxide film. Meanwhile, Y-implantation reduced the size and number of Ni/NiO interfacial defects, hence remarkably enhanced the adhesion of protective NiO oxide scale formed on nickel substrate.
The isothermal and cyclic oxidizing kinetics of Co-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000 degrees C in air by thermal-gravimetric analysis (TGA). Scanning electronic microscopy ( SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of the oxide film after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on formation of Cr-2 O-3 film. Laser Raman spectrum was used to examine the stress changes within oxide films. It was found that lanthanum implantation remarkably reduces the isothermal oxidizing rate of Co-40Cr and improves the anti-cracking and anti-spalling properties of Cr-2 O-3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduces the grain size and internal stress of Cr-2 O-3 oxide and increases the high temperature plasticity of oxide film. Lanthanum mainly exists win the outer surface of Cr-2 O-3 oxide film in the forms of fine La-2 O-3 and LaCrO3 spinel particles.
Isothermal oxidation and cyclic oxidation behavior of pure and yttrium-implanted nickel was studied at 1000°C in air. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) were used to examine the morphology and structure of oxide films formed on nickel. It was found that yttrium-implantation greatly improved the anti-oxidation ability of nickel. Acoustic emission (AE) technique was used in situ to monitor the cracking of oxide films in oxidizing and air-cooling stages. Laser Raman spectrometer was also used to study the stress status of oxide scales formed on nickel with and without yttrium. The main reason for the improvement of anti-oxidation of nickel was that yttrium-implantation greatly reduced the growing speed and grain size of NiO, and this fine-grained NiO oxide film might have better high temperature plasticity and could relieve part of compressive stress by means of creeping. In the meantime, yttrium-implantation reduced the size and number of interfacial defects, hence remarkably enhancing the adhesion of protective NiO oxide scale formed on nickel substrate.
Oxidation behavior of chromium with superficial ceria sol-gel coating was studied at 1000°C in air with the purpose of clarifying the related mechanism and possible application of new type of rare earth-doped ceramic protective coatings. Scanning electron microscopy(SEM) and transmission electron microscopy(TEM) were used to examine the morphology and microstructure of oxide films formed on chromium. Laser Raman spectrometer and X-ray diffraction(XRD) spectrometer were also used to study the stress status in oxide films formed on Cr with and without CeO 2 coating. In addition, depth distribution of element Cr, O and Ce in oxide film was examined by secondary ion massive spectrum(SIMS). Results show that nano-CeO 2 coating reduces the growing speed and grain size of Cr 2O 3 film. The parabolic oxidizing parameters for pure Cr and CeO 2-coated Cr were 2.6 × 10 -6 mg 2/(cm 4 · s) and 7.4 × 10 7 mg 2/(cm 4 · s), respectively. This fine grain-sized Cr 2O 3 film relieved part of the internal compressive stress by means of high temperature creeping and maintained ridge character. The nano-CeO 2 coating changed the film growing mechanism from predominant Cr 3+ cation outward diffusion to O 2-? anion inward diffusion. XRD and Raman stress testing results indicate the stress declination effect due to nano-CeO 2 application; meanwhile, the discrepancy between the two testing results was analyzed in detail concerning to the growing mechanism change of Cr 2O 3 film formed on chromium.
Isothermal oxidation behavior of pure nickel and its lanthanum ion-implanted sample were studied at 900 °C in air. The surface morphology and microstructure of NiO films formed on both samples were examined by scanning electronic microscopy(SEM) and transmission electronic microscopy(TEM). Laser Raman spectrometer and X-ray diffraction spectrometer(XRD) were used to study the stress status in NiO films formed on La-free and La-implanted nickel. Secondary ion massive spectrum(SIMS) was used to examine Ni, O and La element depth distribution in oxide films. Results showed that La-implantation remarkably reduced the growing speed and grain size of NiO film. Meanwhile it changed the oxide film growing mechanism from predominant Ni2+ cation outward diffusion to O2- anion inward diffusion. XRD and Raman testing results showed the stress declination effect due to La-implantation, while the discrepancy between the two testing results was fully analyzed regarding to the heterogeneous stress distribution in oxide film and the rare earth effect during the film growing process.
The isothermal and cyclic oxidation behaviors of pure chromium and the chromium coated with CeO 2 were studied at 900°C in air. The morphology and microstructure of the oxide films were then observed by means of SEM, TEM and high-resolution electronic microscopy (HREM). Moreover, the cracking and spalling of the oxide films were monitored on line by using the acoustic emission (AE) technique, and the distribution of AE signals in the time and number domains were respectively analyzed according to the related oxide cracking model. The internal stresses of the oxide films coated on Cr surfaces with and without ceria were finally measured using a Laser Raman spectrometer. The results show that (1) The ceria coating greatly improves the anti-oxidation ability of Cr in isothermal and cyclic oxidation conditions; (2) The main reason for the improvement of anti-oxidation ability of Cr is that the ceria addition greatly decreases the growing speed and grain size of Cr 2O 3 films; (3) The fine-grained Cr 2O 3 film is of better high-temperature plasticity and can relieve a part of the internal compressive stress by means of high-temperature creeping, thus resulting in obvious ridge character and relatively low internal compressive stress level; and (4) The adhesion of Cr 2O 3 film on Cr substrate greatly improves as the size and number of film/substrate interfacial defects decrease due to the ceria coating.
The influence of titanium doping on the electric properties of amorphous alumina (Al2O3) films prepared by sol–gel technology was investigated. EDX mapping analysis indicates that the titanium cations are uniformly distributed in the film as Al2−xTixOy. The XPS analysis reveals that the titanium exists in the form of TiO2. The breakdown strength of the doped film can be much enhanced by the titanium doping. The dielectric strength of undoped Al2O3 films is 3.4 MV/cm, while the breakdown strength of the 10 % Ti-doped Al2O3 films is 5.3 MV/cm. Compared with the undoped Al2O3 films, the soft breakdown of Ti-doped Al2O3 films is gradually suppressed with the increasing of the titanium doping amount. Especially, there is hardly any soft breakdown observed for 10 % Ti-doped Al2O3 films.
The corrosion behavior and corrosion kinetics of FGH95 alloy at different temperatures were investigated by TGA, XRD, SEM and EDS. The results show that FGH95 alloy has good hot corrosion resistance characteristics at 650 £, and its corrosion kinetic curve obeys a parabolic rule. When the temperatures increase up to 700 and 750 £, the hot corrosion resistance of FGH95 alloy decreases and corrosion is serious. XRD, SEM and EDS indicate that the oxidized layer is mainly composed of NiO, Ni 3 S 2 and Cr 2 O 3 , and there exists a little amount of NiCr 2 O 4 and NaCl at 700 and 750 £, respectively.
Isothermal oxidation experiments at 1100°C in air were carried out to evaluate the protective capability of a new rare earth oxide coating realized by electrodeposition onto a Ni-base single crystal superalloy. A subsequent heat treatment of the RExOy coating already allowed the establishment of a very thin and discontinuous inwardly grown alumina scale. Under isothermal conditions at 1100°C in air a fully parabolic regime installed from 25h leading to parabolic rate constants of 2.5 10-7 mg2.cm-4.s-1 after 200h, similar to those of conventional β-NiAl coatings. The initial, transition and parabolic regimes were ascribed to the major development of NiAl2O4/Al2O3 mixed oxides by in situ high temperature X-ray diffraction (HT-XRD). No major transient alumina was observed. The α-Al2O3 scale intensity increased with increasing oxidation time, in particular with respect the rare earth oxide coating signal. The scanning electron microscopy (SEM) images showed an oxide system consisting on a top NiAl2O4 oxide and a bottom α-Al2O3 scale underneath the RExOy
coating. Alumina grew within the substrate surface. After 500 and 1000h of oxidation, very scarce nodules grew between the alumina and the rare earth oxide deposit. Despite the thermodynamic calculations suggested a REAlOy perovskite at the alumina-RExOy interface, this was not observed experimentally either by XRD or scanning electron microscopy (SEM).
Several advanced air separation unit (ASU) designs being considered for use in coal gasification rely on the use of solid state mixed ionic and electronic conductors. Nicrofer-6025HT, a nickel-based alloy, has been identified as a potential manifold material to transport the hot gases into the ASUs. In the current study, isothermal oxidation tests were conducted on Nicrofer-6025HT in the temperature range of 700–900 °C for up to 24 h. The evolution of oxide scale was evaluated using SEM, XRD, and XPS. The composite surface oxide layer that formed consisted of an outer chromia-rich scale and an inner alumina scale. For the longer times at the higher temperatures evaluated, a NiCr2O4 spinel phase was located at the interface between the alumina and chromia. Based on the experimental results a four-step oxidation model was proposed.
Ni-based alloys are widely used in combustion process, and their catalytic activities are strongly influenced by factors such as the nature of support, active phase, metal loading, catalyst preparation method and pretreatment. The isothermal and cyclic oxidizing kinetics of Ni-40Cr alloy and its lanthanum ion-implanted samples were studied at 1000degC in air by thermal- gravimetric analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) was used to examine the oxide film's morphology and structure after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on formation of Cr2O3 film. Laser Raman spectrum was used to examine the tress changes within oxide films. It was found that lanthanum implantation remarkably reduced the isothermal oxidizing rate of Ni-40Cr and improved the anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide, increased the high temperature plasticity of oxide film. Lanthanum mainly existed in the outer surface of Cr2O3 oxide film in the forms of fine La2O3 and LaCrO3 spinel particles.
This electronic document Isothermal oxidation kinetics of pure nickel and its lanthanum ion-implanted sample are studied at 900degC. Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) are used to examine the surface morphology and microstructure of oxide films. Laser Raman spectrometer and X-ray diffraction spectrometer (XRD) are used to study the stress level in oxide films formed on La-free and La-implanted Ni. Secondary ion massive spectrum (SIMS) is used to examine Ni, O and La element distribution in depth in oxide films. Results show that La-implantation remarkably reduces the growing speed and grain size of MO film; Meanwhile it changes the oxide film growing mechanism from predominant cation outward diffusion to anion inward diffusion. The fine-grained La-containing NiO film can relieve part of internal stress via high temperature creeping, and results in heterogeneous stress distribution in depth. XRD and Raman testing results show the stress declination effect due to La-implantation, and discrepancy between the two testing results is analyzed regarding to the rare earth effect during the film growing process.
Oxidation resistant fourth and fifth generation Ni base superalloys were developed. The fourth generation superalloys TMS-138A were modified by increasing the content of Al and Cr and deceasing Mo. Designed alloys were cast as single crystal and examined in cyclic and isothermal exposures at 1100 degrees C and creep rupture test. Under high temperature/low stress condition, 0.5 wt-% of Al addition was effective to improve the oxidation resistance but degrade creep property. Cr addition and Mo and W reduction were effective to improve the oxidation resistance and display creep property almost equal to the original TMS-138A. The fourth generation superalloy TMS-138 and fifth generation superalloy TMS-173 were also modified. By adding Cr and removing Mo and W, their oxidation resistances were dramatically improved. Thus it was found that it is possible to improve the oxidation resistance of Ru containing SC superalloys with keeping the high temperature strength by the increase in Cr and decrease in Mo and W.
Isothermal oxidation behavior of chromium with and without nanon sol-gel CeO2 coating is studied at 1 000°C in air. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the surface morphology and microstructure of the oxide films. It is found that ceria coating greatly improves the anti-oxidation property of chromium. Laser Raman spectrometer and X-ray diffraction spectrometer (XRD) are also used to study the stress level in oxide films formed on ceria-coated and ceria-free Cr. Secondary ion mass spectrometer (SIMS) is used to examine Cr, O and Ce element distribution in depth in oxide films. Results show that nano-ceria application greatly reduces the growth speed and grain size of Cr2O3 film, and this fine grained Cr2O3 film probably has better high temperature plasticity, i. e., oxide film relieves part of the compressive stress by means of creeping. Meanwhile, CeO2 changes the oxide film growth mechanism from predominant cation outward diffusion to anion inward diffusion. XRD and Raman testing results both show the stress declination effect due to nano-CeO2 application, and their discrepancy in the rare earth effect is analyzed.
Isothermal oxidation behavior of chromium with and without nanometric sol–gel CeO2 coating is studied at 1000°C in air. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the surface morphology and microstructure of their oxide films. It is found that ceria coating greatly improves the anti-oxidation property of chromium. Laser Raman spectrometer and X-ray diffraction spectrometer (XRD) are also used to study the stress level in oxide films formed on ceria-coated and ceria-free Cr. The difference in oxidation behavior is mainly attributed to the fact that ceria greatly reduces the growth speed and grain size of Cr2O3 film, and this fine grain-sized Cr2O3 film probably has better high temperature plasticity, i.e. oxide film can relieve parts of compressive stress by means of creeping. XRD and Raman testing results both show the stress declination due to nano-CeO2 application, and their deviation is analyzed concerning to the rare earth effect.
The isothermal and cyclic oxidizing kinetics of Co–40Cr alloy and its lanthanum ion-implanted samples were studied at 1000°C in air by thermal-gravity analysis (TGA). Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM)) were used to examine the oxidized film's morphology and the structure after oxidation. Secondary ion mass spectrum (SIMS) method was used to examine the binding energy change of chromium caused by La-doping and its influence on formation of Cr2O3 film. Laser Raman spectrum was used to examine the tress changes within oxidized films. It was found that lanthanum implantation remarkably reduced the isothermal oxidizing rate of Co–40Cr and improved the anti-cracking and anti-spalling properties of Cr2O3 film. The reasons were that the implanted lanthanum reduced the grain size and internal stress of Cr2O3 oxide, increased the high temperature plasticity of oxidized film. Lanthanum mainly existed in the outer surface of Cr2O3 film in the forms of fine La2O3 and LaCrO3 spinel particles.
Isothermal and cyclic oxidation behaviors of chromium samples with and without nanometric CeO2 coating were studied at 900 °C in air. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM) were used to examine the morphology and microstructure of the oxide film. It was found that ceria coating greatly improved the oxidation resistance of Cr both in isothermal and cyclic oxidizing experiments. Acoustic emission (AE) technique was used in situ to monitor the cracking and spalling of oxide film, and AE signals were analyzed in time-domain and number-domain according to related oxide fracture model. Laser Raman spectrometer was also used to study the stress of oxide film formed on Cr with and without ceria. The improvement in oxidation resistance of chromium is believed mainly due to that ceria greatly reduced the growth speed and grain size of Cr2O3. This finegrained Cr2O3 oxide film might have better high temperature plasticity and could relieve parts of the compressive stress by means of creeping and maintained ridge character and relatively lower level of internal stress. Meanwhile, ceria application reduced the size and number of interfacial defects, remarkably enhanced the adhesive property of Cr2O3 oxide scale formed on Cr substrate.
The oxidation kinetics of IN-738LC at 1173 K in dry air up to 1500 hr followed parabolic law. Surface morphology and the oxide phases present in the scale were characterized by SEM, XRD, EDS, FIB, and XPS. FIB investigation exhibited a compact and adherent oxide layer. XRD analysis revealed the presence of NiO, NiAl2O4, NiCr2O4 spinel, and Al2O3 on the top scale surface formed at 1173 K in dry air. Extensive XPS analyses revealed the presence of Cr2O3, CrO2, and CrO3 on the top scale surface formed on IN-738LC after 10 hr of exposure. The presence of TiO2, Al2O3, Cr2O3, NiO, and NiAl2O4 and NiCr2O4 spinels along with the oxides of Ta at the top surface of the scale was observed after 100 hr of oxidation. The TiO2 content was high on the surface and the entire scale cross section was composed mostly of Cr2O3, NiO, TiO2, and Al2O3 after 100 hr of exposure to dry air at 1173 K. The concentration of Al2O3 on the surface of the oxide scale was found to increase after 100 hr of exposure and remained constant thereafter. After 300 and 1500 hr of exposure, the surface oxide was mainly Al2O3 along with oxides of Ni, Ti, and Cr. The oxide scale cross section consisted mostly of Al2O3 along with other oxides such as Cr2O3, NiO, and TiO2. The oxide-scale composition was found to vary significantly with the duration of exposure to dry air at 1173 K.
The 4th and advanced generation Ni-base single crystal superalloys, which contain large amounts of refractory metals for strengthening and platinum group metals, e.g., Ru, for TCP-phase prevention, show excellent high-temperature strengths. However, these alloying elements seem to decrease high-temperature oxidation resistance. In this study, Ni-base superalloys with various amounts of Ta, Re and Ru were examined in isothermal and cyclic exposures at 1373K to investigate the effect on the oxide growth rate and resistance to scale spallation. Structures of the oxide for the alloys were analyzed by XRD, SEM and EDX, and the oxidation kinetics is discussed. Ru and Re were found to degrade the oxidation resistance by the vaporization of their oxide. Ta-rich oxide in the spinel layer affects to stabilize ruthenium and rhenium oxide in the scale and improve the oxidation resistance of Ru-containing Ni-base superalloys.
Fiber media composed of Fe–Cr–Al–Y alloy are being used increasingly as materials for high-temperature applications for their excellent oxidation resistance. The oxidation kinetics of Fe–Cr–Al–Y alloy fiber medium as a heat-resistant material for high-temperature applications was studied in dry air at 1073, 1188, 1255, and 1318 K. The oxidation process followed the parabolic kinetic law. The alumina-scale growth was found to be influenced by short-circuit diffusion and the presence of stresses related to oxide-scale growth. The surface of the oxide scale formed on the fiber medium was analyzed using X-ray photoelectron spectroscopy, which revealed that the outer surface of the oxide scale formed on the fiber medium composed of 12-m diameter Fe–Cr–Al–Y alloy fibers, consisted of -Al2O3, -Al2O3, and Cr-oxide. The metastable -Al2O3 subsequently partially transformed into the more stable -phase following a time-temperature-transformation relationship. The surface morphology and the cross section of the oxide scale formed on the fiber medium in the temperature range 1073–1318 K in dry air, have been studied by scanning-electron spectroscopy (SEM) and focused-ion beam, respectively.
The fourth-generation nickel-based single-crystal superalloys, which contain large amounts of refractory metals for strengthening
and platinum group metals for topologically close-packed phase prevention, show excellent high-temperature strength. However,
these alloying elements seem to decrease high-temperature oxidation resistance. In this study, nickel-based superalloys with
various amounts of tantalum, rhenium, and ruthenium were examined in isothermal and cyclic exposures at 1,100°C to investigate
the effect on the oxide growth rate and resistance to scale spallation. Ruthenium and rhenium were found to degrade the oxidation
resistance by the vaporization of their oxide. Tantalum-rich oxide in the spinel layer acts to stabilize ruthenium and rhenium
oxide in the scale. The addition of hafnium and yttrium is effective in improving the oxidation resistance of ruthenium-containing
nickel-based superalloys.
Morphology changes in the oxide layer during the oxidation process of nickel alloys at 1100 °C was described under simplifying assumptions as a sequence of local phase equilibria. The oxidation behaviour was modeled for binary Ni60Al40 and Ni80Al20 alloys as well as for a ternary alloy with the composition Al19.4Cr19.7Ni60.9 typically used as a bond coat alloy in thermal barrier coatings for superalloys. The formation of NiAl2O4 in equilibrium with Al2O3 at the boundary with a bond coat as well as of NiO in equilibrium with NiAl2O4 in a top layer on binary Al–Ni alloys was predicted. It was proved that mixed oxides (Al,Cr)2O3 as well as mixed spinel phase Ni(Al,Cr)2O4 appear with the Al19.4Cr19.7Ni60.9 ternary alloy in bottom and upper zones of thermally grown oxide layer, respectively. The stronger disposition to oxidation of Cr doped alloys compared to binary Al–Ni alloys was established. It was shown that alumina can form only on the surface of β rich grains.
High temperature materials degradation or protection of Fe–Cr alloys are often related to the nature of their oxide scale formation. Breakdown of passive oxide films leads to localized corrosion. Various alloying elements are often incorporated in these alloys to prevent high temperature oxidation. The addition of selected alloying elements is cumbersome and not always cost effective. In this article, we investigate the role of rare earth oxide coatings on high temperature corrosion prevention of both low and high Cr steel. An in situ high temperature oxidation setup has been built to study the oxidation kinetics of both coated and uncoated low and high Cr steels under ambient pressure and dry air. Reduction in scale growth kinetics is observed in the presence of coating. While scanning electron microscopy and x-ray diffraction are employed to study the structure and morphology of the oxide films, x-ray photoelectron spectroscopy and Auger electron spectroscopy are used to study the surface chemistry of the oxide layer. This article relates some of these data to explain the nature of scale growth kinetics (linear or logarithmic or parabolic) observed in both low and high Cr steels. © 1999 American Vacuum Society.
The oxidation behavior of an oxide-dispersion strengthened (ODS) NiAl has been studied between 900 and 1100°C in air. The dispersoids of mostly Al2O3 in fine-grained β-NiAl were incorporated by mechanical alloying (MA) in an argon atmosphere and hot pressing. It was found that excessive amounts of dispersoids and voids within the matrix had serious negative effects on the oxidation resistance of β-NiAl, by allowing for a more rapid formation of oxide scales and by providing fast diffusion paths for oxygen. Below the thin surface oxide scales consisted of α-Al2O3, NiAl2O4 and Ni2O3, an internal oxidation zone was formed deep into the matrix. No metastable transient aluminas were formed during oxidation. The oxide ridge structure began to evolve after oxidation at 1100°C at the oxide–gas interface.
The feasibility of processing less-expensive alternative coatings to platinum aluminide was examined. Three approaches were followed: (1) enhancement of nickel-aluminide coatings by application of sol-gel derived two-phase-glass (TPG) overlayers, (2) evaluation of TPG coatings on bare IN 738LC, and (3) substitution of Pt with a less expensive platinum group metal (palladium). Accordingly, IN 738LC coupons were tested with several coatings including TPG, aluminide coatings (platinum aluminide, palladium aluminide, and conventional nickel aluminide), and TPG overlayers on the aluminide coatings. Isothermal-oxidation, cyclic-oxidation, and hot-corrosion tests were conducted at 900°C for 500 h to evaluate the coatings. The results showed that the TPG by itself provided superior protection compared to the platinum-aluminide coatings under both oxidation and hot-corrosion conditions. The TPG coating also showed promise as an overcoat on aluminide coatings.
The process of internal nitridation of the three commercial single-crystalline nickel-base superalloys CMSX-2, CMSX-6, and
SRR99 has been studied in air and oxygen-free nitrogen atmospheres at 800 C to 1100 C using thermogravimetric techniques
supplemented by extensive microstructural examinations. Non-protective oxide formation, particularly cracking and spalling
at edges or curved surfaces, enables nitrogen to penetrate into the alloy leading to the precipitation of stable Ti and Al
nitrides. The high-temperature corrosion behavior of the superalloys studied is strongly affected by compositional differences
between dendritic and interdendritic areas due to segregation resulting in an inhomogeneous internal precipitation zone. Furthermore,
the stability of the strengthening γ′ phase (Ni3(Al, Ti, Ta)) in front of the growing internal-nitridation zone was observed to depend clearly on the alloy composition. Therefore,
the near-surface area of the alloys can be weakened by γ′ depletion and by embrittlement resulting from internal-nitride precipitation. The results obtained on the nickel-base superalloys
are discussed, taking into account the results of a computer-based simulation of internal-corrosion processes. Furthermore,
results on Ni-base model alloys of the system Ni-Cr-Al-Ti provided information on the role of the alloy composition. It was
found that a higher Cr concentration seems to increase the nitrogen solubility and diffusion in Ni-base alloys.
The oxidation behavior of a nickel-cobalt base alloy containing 19% chromium and 3% aluminum was studied in the temperature range of 760–1200°C. The oxidation process at a given temperature was interrupted after 4, 16, and 64 hr, and every 100 hr thereafter, and the specimens were air-cooled, weighed, and subsequently reheated. The weight gain followed the parabolic law, and the activation energy was 58.8 kcal/mole. The oxide scale consisted of a chromium-rich continuous outer layer and aluminum-rich discontinuous internal oxides at both low and high temperatures. At an intermediate temperature, about 1090°C, the internal oxides became continuous, accompanied by a reduction in oxidation rate. The experimental results are discussed and compared with the information available in the literature.
The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr2O3 (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr2O3 by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.
The oxidation of alloys in 0.1 atm of oxygen has been studied at temperatures of 1000°, 1100°, and 1200°C. Twenty‐one alloys with varying chromium [2–30 weight per cent (w/o)] and aluminum (1–9 w/o) contents were examined. It was found that all of the alloys initially underwent a period of transient oxidation before steady‐state conditions were established. The transient period of oxidation usually did not exceed 1 hr and was characterized by rapid conversion of thin surface layers of the alloys to oxides with the subsequent formation of continuous layers of one of the following oxides: , , or . Steady‐state conditions were established with the formation of these continuous oxide layers, and oxidation occurred by three different mechanisms which were characterized by the growth of an external layer of over a subscale of and Al2O3, the growth of an external layer of over an subscale or the growth of a continuous, external layer of .
The influence of superficially applied CeO[sub 2] coatings on the isothermal-oxidation behavior, preceded by nonisothermal heating, as well as cyclic-oxidation behavior of three grades of austenitic stainless steel (AISI-316. -321, and -304), in dry air is reported. The superficial coating had a thickness of 2.1 [mu]m. The linear heating rate employed was 6 K min[sup [minus]1] up to a maximum temperature of 1423 K, and the isothermal holding temperature was 1273 K. The results clearly depict that CeO[sub 2] coatings not only reduced the rates of scale growth for all three varieties of steel but also imparted improved scale adhesion to the respective alloy substrates, as evident from the fact that the coated steels could withstand a number of thermal cycles without scale rupture. In the bare condition, 321-grade steel exhibited the best performance. However, in the presence of the coating, the improved performances of 316 and 321 grades were almost identical, whereas the 304 variety showed improvement only in the first cycle of exposure. The kinetics results have been substantiated by postoxidation analyses of the alloy/scale combinations by SEM, EDS, EPMA, and XRD techniques to reveal the role of rare-earth-oxide coatings on the observed behavior. 37 refs., 14 figs., 1 tab.
The present paper deals with the thermo-mechanical fatigue-oxidation interaction through a quantitative study of matrix oxidation in IN738LC, a conventionally cast high chromium low carbon nickel-base superalloy used for land based turbine blading. It is shown that the TMF cycling strongly influences the oxidation process in this superalloy. The study of oxidation under out-of-phase TMF loading in IN738LC has shown that: (1) the mechanical strain range influences the general matrix oxidation of the alloy; (2) the introduction of a hold time in TMF cycling considerably enhances the oxidation process; (3) increasing the amount of mechanical strain range under TMF loading accelerates the average oxidation and depleted zone growth rates according to a power law relationship; (4) the oxidation kinetics for the alloy under TMF condition seems to be linear, caused by oxide fracturing and/or spalling which results in a nonprotective accelerated oxidation process.
Demand for lower fuel costs is driving turbine designers on all fronts to look for ways to increase firing temperatures to achieve higher operating efficiencies. Historically, hot corrosion attack has been the dominant environmental issue for industrial gas turbine engines. Now, efforts to raise firing temperatures create the need for materials that can endure exposures where both hot corrosion and oxidation predominate. Two single-crystal superalloys have been developed in response to these market demands. By providing a blend of hot corrosion and oxidation resistance, CMSX-11B and CMSX-11C alloys directly address the need for better components in turbine engines that burn fossil fuels. This article discusses alloy design and manufacture, mechanical properties, and results of hot corrosion and oxidation tests.
It is the objective of this paper to compare the thermal stability characteristics and isothermal oxidation behavior of an aluminide coating and a Pt-aluminide coating of the same Al content on a Ni-base superalloy. Addition of Pt to an aluminide coating was found to improve its thermal stability as well as its capability for selective oxidation of Al resulting in a purer scale of slower growth rate. This was correlated with the greater diffusional stability of the Pt-aluminide coating restricting the transport of substrate elements into the outer coating layers.
Iron aluminides (Fe-Al) are intermetallic compounds that possess a number of advantages including low materials cost, conservation of Ni and Cr, relatively low densities, and good corrosion and oxidation resistances. The authors have studied the high temperature oxidation and sulfidation behaviors of Fe-Al, including isothermal and cyclic oxidation kinetics, scale structure and spallation tendency, and the reactive elements effects (REE) in an effort to use these materials in industries. This paper reports their results on the structures and formation mechanisms of the scales formed on Fe-Al during oxidation. The scales formed on alloys after exposure to high temperatures often have a complex structure and composition. It is known that when some alloys were oxidized in air at high temperatures, nitrides were formed underneath the oxide scale. It was reported that the oxidation of TiAl alloys in an ambient atmosphere led to the nitration of the base metal beneath the oxide scales. After a long exposure time a multilayered scale was formed which consisted of metal, Ti{sub 2}AlN, TiN, TiO{sub 2} and Al{sub 2}O{sub 3}. Cr was oxidized in air at 1,200 C, forming a Cr{sub 2}O{sub 3} scale with a layer of Cr{sub 2}N beneath the oxide. When binary Cr alloys containing Ti,Zr, or Nb were oxidized in air at 1,150 C, a layer or internally distributed Cr{sub 2}N formed beneath the oxide scales. However, the structures and formation mechanisms of these types of scales are not clear. The authors have also found that the scales formed on Fe-Al at 1,100 C in air have a complex structure containing nitrides. This paper reports the authors' studies on this subject.
The oxidation of Cr and Fe‐Cr alloys which form
scales involves at least two processes: (i) the solid‐state, diffusive transport of ionic species, with parabolic kinetics, and (ii) the oxidation of
to a volatile species, probably
, with linear kinetics. The effects of this second reaction cannot be neglected, either in interpretation of short‐term oxidation
results, or in predicting long‐term oxidation behavior. In this paper equations are derived which describe the oxidation behavior
of a system in which both diffusive and gas/oxide interface processes occur simultaneously. According to the model, the oxide
scale grows to a limiting thickness which is determined by the parabolic rate constant and the surface‐reaction rate constant;
as the limiting thickness is approached, a transition from parabolic to linear kinetics takes place. The model is applied
to the oxidation of Cr and Fe‐Cr alloys, and good agreement with experimental data is found.
A review of principles governing alloy oxidation is presented together with the means by which practical oxidation-resistance alloys have been developed. The reactions involved are complex and involve acquiring data from many fields. Wagner developed the basic diffusional models for the formation of oxide scales, but these need to be expanded using detailed physical and metallurgical studies of the alloy phases, oxides and their defects, as well as the mode of scale formation and the structural relationships between alloy and oxide scale. At this time the engineering solutions have outstripped fundamental understanding, but an attempt is made to show how the development of oxidation theory has contributed to this effort by a term now referred to as 'informed empiricism'.
The surface of a polycrystalline Ni/Cr alloy has been examined using Auger electron spectroscopy (AES), ion scattering spectroscopy (ISS) and X-ray photoelectron spectroscopy (XPS) before and after a 1 h, 400 °C anneal in air. The use of these three techniques provides depth-sensitive information as a result of their different sampling depths. The outermost surface layer contains mostly O and some Ni, and the near-surface region consists primarily of NiO. Using sputter depth profiling coupled with AES and XPS, a Cr-rich region is observed between 130 and 175 min of sputtering which consists of two layers. The upper portion of this region contains Ni and Cr metals as well as a mixture of CrO and Cr2O3. The lower portion of this region contains a larger Cr oxide concentration compared with the upper layer, and the Cr oxide consists mostly of CrO and CrO3. Below this region only Ni metal, Cr metal, and a small amount of Cr oxide species are present. The Cr-oxide-rich layer may be responsible for the passivation of Ni/Crsurfaces toward oxidation.
The initial stages of oxidation of steel P91 were studied in a UHV system at oxygen partial pressures ranging from 10−8 mbar up to 10−5 mbar. Experiments were conducted at 600–650°C for heating times 5–120 min. The oxide scales were analyzed by means of Auger electron spectroscopy (AES), scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The very initial stages up to growth of the first few oxide layers correspond to a complex surface situation and are influenced by Cr-nitride formation, P segregation and SiO2 formation. After the first monolayers of oxides have grown the process becomes simpler and different oxidation kinetics accounts for the further growth. The microstructure of steel P91, tempered martensite along with chromium carbide precipitates leads to preferential chromium-rich oxide formation along martensite laths. The results are also discussed in view of diffusion data taking into account material transport by bulk and fast diffusion paths.
Several commercially available Ni-base superalloys were exposed isothermally in air at temperatures between 750 and 1000C and also under cyclic conditions at 1000C. The kinetics of oxidation were determined and the scales were analyzed by electron microscopy and X-ray diffraction. Thin adherent chromia-rich scales formed on the alloys at 750C after 1000 hr. Although Waspaloy showed the lowest weight gain in this test, it also showed the deepest internal corrosion due to oxidation of the grain-boundary carbides. At temperatures up to 1000C the external scales were also chromia-rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile (TiO2) grains at the surface as well as forming TiO2 and TiN internally. The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxides below the chromia scale, although Astroloy when oxidized isothermally at 1000C developed a semicontinuous internal layer of alumina due to its higher Al content. Under cyclic conditions Astroloy formed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.
Coatings, obtained by preoxidation of Incoloy 800H at low PO2 show good sulphidation resistance due to the higher chromia content in the oxide scale. Yttrium-ion implantation of Incoloy 800H has also a beneficial effect on sulphidation, if preoxidation is applied. The reason for this is presumably the segregation of yttrium to grain boundaries of the oxide. Furthermore, the oxidation kinetics of Incoloy 800H are independent of the partial pressure of the oxygen. Mechanical testing of the preformed oxide scale/substrate combinations in air at 600C by means of constant-extension-rate experiments shows that preoxidation at low partial pressures of oxygen leads to earlier scalecracking.
This study assessed the oxidation behavior of three commercial alloys in airand low partial pressures of oxygen roman (PO
2. The kinetics ofoxidation in air were compared with values obtained in an atmosphere of lowroman PO
2. The low partial pressure of oxygen was of the order of10–16 atm at 930C and was generated using an H2/4% H2Omixture. The nature of the corrosion products, structure, morphology, andcomposition were assessed and characterized using scanning electronmicroscopy (SEM), X-ray mapping, and scanning Auger microscopy (SAM). Theresults were compared with those obtained for 99.99% pure chromium. Parabolickinetics were exhibited by all of the alloys, with the overall kp valuesbeing of the order constant for chromia-forming alloys. Large variations inthe morphologies of the oxide scales were observed as a result of oxidationin the high and low roman PO
2 environments.
NiAl2O4 spinel was formed by solid state reaction. Its electrical conductivity was measured in the temperature range of 680–940 C and under various oxygen-rich environments, as well as under reducing conditions. From the temperature dependence of the conductivity, the activation energies for conduction increase for decreasing oxygen partial pressures. From the partial oxygen pressure dependence, the defect structure of the material was analysed. The conductivity change with respect to P
O2 can be attributed to singly and doubly ionized nickel vacancies. The chemical diffusivity of the oxide was determined by conductivity relaxation upon abrupt change in P
O2 in the surrounding atmosphere. The oxygen chemical diffusion coefficient is of the order of magnitude of 10–4 cm2 s–1.
The oxidation of nine ternary iron-chromium-manganese alloys was studied at 900C in an oxygen partial pressure of 26.7 kPa. The manganese concentration was set at 2, 6, and 10 wt. %, and chromium at 5, 12, and 20 wt. %. The scales formed on the low-chromium alloys consisted of (Mn,Fe)2O3, -Fe2O3, and Fe3O4. These alloys all exhibited internal oxidation and scale detachment upon cooling. The scales formed on the higher-chromium alloys were complicated by nodule formation. Initially, these scales had an outer layer of MnCr2O4 with Cr2O3 underneath, adjacent to the alloy. With the passage of time, however, nodules formed, and the overall reaction rate increased. This tendency was more marked at higher manganese contents. Although these alloys contained a high chromium content, the product chromia scale usually contained manganese. It was concluded that the presence of manganese in iron-chromium alloys had an adverse effect on the oxidation resistance over a wide range of chromium levels.
Oxidation testing of heat-resistant alloys is described. The testing procedure utilized weight-gain measurements using one specimen, which was withdrawn and weighed at intervals of 1 week, for 10–18 weeks. The specimen was placed in a porcelain cup during exposure and covered upon cooling to retain spalled oxide. Weight gain was used to determine the kinetics of oxidation and was extrapolated to 3000 hr. The specimen was withdrawn at the end of the exposure, weighed, cathodically descaled, and reweighed. The ratio of oxygen ions to metal ions was determined for each alloy and test temperature. This ratio approaches the stoichemetric ratio for Fe3O4 or Cr2O3. The ratio for each test is used to convert weight gain to weight loss. The amount of adherent oxide was determined as well as the total amount of oxide, leading to an expression for oxide adherency. The oxidation resistance of Fe-Cr-Ni alloys increased with increasing Cr and Ni, with Cr being the most critical element. Additions of Si, Al, or Ce were shown to extend the usefulness of Fe-Cr-Ni alloys.
The oxidation behavior of a new type of wrought Ni–Fe–Cr–Alsuperalloy has been investigated systematically in the temperature range of1100 to 1300C. Results are compared with those of alloy 214, Inconel600, and GH 3030. It is shown that the oxidation resistance of the newsuperalloy is excellent and much better than that of the comparisonalloys. Scanning electron microscopy (SEM), electron probe microanalysis(EPMA), and X-ray diffraction (XRD) experiments reveal that the excellentoxidation resistance of the new superalloy is due to the formation of adense, stable and continuous Al2O3 and Cr2O3 oxide layer at hightemperatures. Differential thermal analysis (DTA) shows that the formationof Cr2O3 and Al2O3 oxide layers on the new superalloy reaches a maximum at1060 and 1356C, respectively. The Cr2O3 layer peels off easily, and thesingle dense Al2O3 layer remains, giving good oxidation resistance attemperatures higher than 1150C. In addition, the new superalloypossesses high mechanical strength at high temperatures. On-site testsshowed that the new superalloy has ideal oxidation resistance and can beused at high temperatures up to 1300C in various oxidizing andcorrosion atmospheres, such as those containing SO2, CO2 etc., for longperiods.
Several commerical single-crystal superalloys (CMSX-2, CMSX-3, CMSX-4, CMSX-6, SRR 99) and some laboratory versions of one of them (CMSX-4) with various Y additions were investigated concerning their cyclic-oxidation resistance in air at 1000 and 1150C. The investigations also included two materials (CMSX-6, SRR 99) with an RT-22 coating. Weight changes and acoustic emission were recorded up to 1000 cycles and scales, coatings, and substrates were characterized by metallography, SEM, and microprobe in postexperimental investigations. The best cyclic-oxidation behavior and excellent resistance to spalling even at 1150C were shown by a laboratory version of CMSX-4 containing between 10 and 60 ppm Y. While at 1000C interdiffusion can be taken as tolerable for the coated alloys, there is rapid degradation of the coating by interdiffusion at 1150C.
The scales formed on the superalloys IN 939 and IN 738 LC at 700, 900, and 1100C in air have been investigated by optical, microprobe analysis and x-ray diffraction measurements. Both alloys form very similar scales. The main components are an outer TiO2 layer, an intermediate Cr2O3 layer with dissolved Ti, and an inner layer of (Ti, Nb, Ta)O2 with rutile structure. Beneath the scale an internal corrosion zone is formed that contained Al2O3 directly beneath the external scale and TiN further into the substrate.
The chemistry at scale/metal interfaces was studied using scanning Auger microscopy after removal of the scale in ultra-high vacuum using an in situ scratching technique. Al2O3 and Cr2O3 scales formed between 900C and 1100C on Fe-18 wt.% Cr-5 wt.% Al and on Ni-25 wt.% Cr alloys, respectively, were investigated. The adhesion of these scales was determined qualitatively by way of micro-indentation and scratching on the surface oxide. All of the alumina scales fractured to the same degree to expose the metal surface, regardless of the oxidation temperature. The chromia-forming alloy on the other hand, developed more adherent scales at lower oxidation temperatures. About 20 at.% sulfur was found at the metal surface in all cases, and its presence was not only detected on interfacial voids, but also on areas where the scale was in contact with the alloy at temperature. Results from this study clearly demonstrated that sulfur as an alloying impurity does segregate to the scale/alloy interface. However, for alumina scales and chromia scales, the effect of this segregation on oxide adhesion is noticeably different.
The high-temperature oxidation behavior of a nickel-base superalloy andaluminide-diffusion coating has been investigated over the temperaturerange from 800–1100C and analyzed by TGA, XRD, EDAX, andSEM. The -NiAl coating was formed by low-pressure gas-phasecementation at 950C for 3 hr. It was found that the formation of-Al2O3 from -Al2O3 on the -NiAl coating resulted in asharp decrease in the parabolic rate constant kp by one order ofmagnitude at 1050C during transient oxidation. The transformationcaused the anomalous behavior of the oxidation kinetics curves of thisdiffusion coating in the temperature range 800–1100C withinthe first 100 hr. A change in the morphology of scales also occurredwith the transformation. A growth stress was characterized by theformation of convoluted scales, which were observed on the surfaceafter oxidation. The oxidation mechanism of this -NiAl diffusioncoating is described.
A study of the high-temperature oxidation and Na2SO4-induced hot corrosion of some nickel-base superalloys has been accomplished by using ESCA to determine the surface composition
of the oxidized or corroded samples. Oxidation was carried out at 900 or 1000°C in slowly flowing O2 for samples of B-1900, NASA-TRW VIA, 713C, and IN-738. Oxidation times ranged from 0.5 to 100 hr. Hot corrosion of B-1900
was induced by applying a coating of Na2SO2 to preoxidized samples, then heating to 900° C in slowly flowing O2. Corrosion times ranged from 5 min to 29 hr. For oxidized samples, the predominant type of scale formed by each superalloy
was readily determined, and a marked surface enrichment of Ti was found in each case. For corroded samples, the transfer of
significant amounts of material from the oxide layer to the surface of the salt layer was observed to occur long before the
onset of rapidly accelerating weight gain. Some marked changes in surface composition were observed to coincide with the beginning
of accelerating corrosion, the most striking of which were a tenfold decrease in the sulfur to sodium ratio and an increase
in the Cr(VI) to Cr(III) ratio.
The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 degrees C, Sn, SnO, and SnO2, and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnO, Raman bands. The thin him becomes homogeneous after annealing treatments above 550 degrees C, where only the SnO2 cassiterite phase is detected.
It has become common practice to employ, as a binding energy reference for x‐ray photoelectron spectroscopy studies on nonconductive materials, the C(1s) spectra of the ubiquitous (adventitious) carbon that seems to exhibit an instantaneous presence on all air exposed materials. Despite this commonality, surface scientists, including many practitioners, have expressed substantial concerns about the validity of this approach. A detailed discussion of the method is presented including consideration of the types of materials and the electronic energy states involved, e.g., Fermi edges, vacuum levels, etc., and the couplings that must exist for the referencing method to be correctly applied. A number of other surface environments for which the carbon referencing method may be fallacious are also presented. This leads to a consideration of the electron spectroscopy for chemical analysis results for different types of adventitious species and how the presence of some of these may confuse the use of the method. In this regard, we will also discuss the use of other methods to establish binding energy scales, such as Fermi edge coupling and select doping (e.g., the Au dot approach). © 1995 American Vacuum Society
A commercial, single crystalline, nickel-base superalloy, René N6, was cyclically oxidized in 1200 °C air to evaluate the impact of indigenous sulfur impurities on the oxidation behavior. The sulfur concentrations of the samples were adjusted to levels in the sub-ppm range by heat treatment in hydrogen. Correlation between the residual sulfur levels and the degree of scale spallation are shown. Reduction of sulfur content to <0.1 ppm is shown to prevent any spalling of scale from the metal surface. Higher sulfur levels may be tolerated depending on oxidation time and temperature.Auger microscopy is used to demonstrate that extensive segregation (up to 27 at.%) of sulfur occurs at the metal/scale interface. In vacuum, debonding of the scale is shown to be difficult even when high concentrations of sulfur are present. The possible importance of humidity in the spallation process is discussed.
For two nickel-base superalloys (wrought Udimet 500 and cast IN-738LC), creep and low cycle fatigue tests were performed at 850°C in air or in an environment simulating industrial conditions (namely salt deposits of Na2SO4 or an Na2SO4−10%NaCl mixture). The kinetics were determined by measurements of external scale thickness, internal oxidation depth and thickness of the γ′-depleted zone. The results were compared with similar tests carried out on unstressed specimens. This comparison shows that creep does not affect oxidation kinetics in the creep rate range (10−6−10−8s−1) studied. On the contrary, it appears that fatigue stresses affect growth kinetics and growth mechanisms. A large increase in oxidation kinetics affects the external scale growth as well as the internal oxidation depth and the extent of the γ′-depleted zone.