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Showing papers in "Oxidation of Metals in 2003"


Journal ArticleDOI
TL;DR: In this article, the authors reviewed previous studies on iron and steel oxidation in oxygen or air at high temperatures and found that the scale structures are similar to those formed on iron, but for longer-time oxidation, because of the less adherent nature, the scale structure developed are typically much more complex.
Abstract: This paper reviews previous studies on iron and steel oxidation in oxygen or air at high temperatures. Oxidation of iron at temperatures above 700°C follows the parabolic law with the development of a three-layered hematite/magnetite/wustite scale structure. However, at temperatures below 700°C, inconsistent results have been reported, and the scale structures are less regular, significantly affected by sample-preparation methods. Oxidation of carbon steel is generally slower than iron oxidation. For very short-time oxidation, the scale structures are similar to those formed on iron, but for longer-time oxidation, because of the less adherent nature, the scale structures developed are typically much more complex. Continuous-cooling conditions, after very short-time oxidation, favor the retention of an adherent scale, suggesting that the method proposed by Kofstad for deriving the rate constant using continuous cooling or heating-oxidation data is more appropriate for steel oxidation. Oxygen availability has certain effects on iron and steel oxidation. Under continuous cooling conditions, the final scale structure is found to be a function of the starting temperature for cooling and the cooling rate. Different scale structures develop across the width of a hot-rolled strip because of the varied oxygen availability and cooling rates at different locations.

488 citations


Journal ArticleDOI
TL;DR: In this article, the isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000-1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS.
Abstract: The isothermal oxidation behavior of bulk Ti2AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS The results demonstrated that Ti2AlC had excellent oxidation resistance The oxidation of Ti2AlC obeyed a cubic law with cubic rate constants, kc, increasing from 238×10-12 to 213×10-10 kg3/m6/sec as the temperature increased from 1000 to 1300°C As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner α-Al2O3 layer and a discontinuous outer TiO2 (rutile) layer formed on the Ti2AlC substrate A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys In addition, the scales had good adhesion to the Ti2AlC substrate during thermal cycling

259 citations


Journal ArticleDOI
TL;DR: In this article, the scaling behavior of pure chromium and ODS-Cr alloys was studied at 1000°C and it was shown that the scaling rate increased with the pH2O/pH2 ratio.
Abstract: The oxidation behavior of pure chromium and ODS-Cr alloys in Ar-H2-H2O and Ar-O2-H2O was studied at 1000°C. At high oxygen potentials, the addition of H2O to the gas had negligible effect on the scaling behavior. However, at low oxygen potentials, when the pH2O/pH2 ratio was held constant, the oxidation rate increased with water partial pressure. Increasing values of pH2O/pH2 led to more rapid rates. At fixed pH2O values, the rate increased with increasing pH2. Compact scales were formed under all conditions. In addition Cr2O3 blades grew on the scale surface when pure chromium was reacted with H2O/H2 mixtures, but not in reaction with O2/H2O. These blades did not form when Y2O3 dispersion-strengthened material was reacted. A model, in which oxide growth was sustained by diffusion of chromium vacancies and adsorption of H2O on oxide exposed to low oxygen-activity gas led to the formation of hydroxyl species, explained most of the complex effects of gas composition on scale growth and blade formation. However, it failed to account for the observed increase in scaling rate with pH2 at fixed pH2O. The latter effect is ascribed to alteration of an additional contribution to diffusion from chromium interstitials.

125 citations


Journal ArticleDOI
TL;DR: In this article, the effects on the oxidation behavior of elements that are commonly present in low-activity aluminide bond coatings on single-crystal, Ni-base superalloys were examined in cyclic and isothermal exposures at 1100 to 1200°C.
Abstract: Cast model alloys, based on β-NiAl+0.05at.%Hf, were used to study the effects on oxidation behavior of elements that are commonly present in low-activity aluminide bond coatings on single-crystal, Ni-base superalloys. Single additions of Re, Ti, Ta, and Cr were examined in cyclic and isothermal exposures at 1100 to 1200°C in order to determine their effect on the oxide growth rate and resistance to scale spallation. With 1 at.% additions, all of these elements were found to be detrimental to the oxidation performance of the base NiAl+Hf alloy. Additions of Re and Cr were found to form second-phase precipitates in the alloy, which appeared to lead to scale spallation, while additions of Ti and Ta were internally oxidized and incorporated into the scale as grain-boundary segregants. These results suggest that it is necessary to minimize the levels of these types of elements that enter Hf-modified aluminide coatings by using process modifications or a diffusion barrier.

116 citations


Journal ArticleDOI
TL;DR: In this article, a pack-cementation coating process was used to aluminize the surface region of a Ti-50 at.% Al alloy to TiAl3, the most promising, oxidation-resistant phase in the Ti-Al system.
Abstract: The long-term application of TiAl alloys based on the γ-phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl3, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of γ-TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl3 coating and the γ-TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl2 phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.

83 citations


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

79 citations


Journal ArticleDOI
TL;DR: In this paper, the corrosion behavior of pure Fe and Fe-Cr alloys with different Cr content in the presence of a solid NaCl deposit and water vapor at 600degreesC was studied.
Abstract: The corrosion behavior of pure Fe and Fe-Cr alloys with different Cr content in the presence of a solid NaCl deposit and water vapor at 600degreesC was studied. Results indicated that the corrosion of pure Fe was severe even in air at 600degreesC and the scale formed on the surface was compact and uniform. However, with a solid NaCl deposit on its surface, the corrosion of pure iron in air was suppressed to some extent, but the presence of water vapor in the atmosphere causes accelerated corrosion. Under the synergistic effect of NaCl and water vapor, the corrosion of pure iron is accelerated more significantly. In contrast with the known effect of Cr content on the oxidation of Fe-Cr alloys, an increasing Cr content in Fe-Cr alloys increases the corrosion rate of the alloys under the synergistic effect of solid NaCl and water vapor. A mechanism to explain the effect of water vapor and NaCl on the corrosion of pure iron and Fe-Cr alloy is proposed.

76 citations


Journal ArticleDOI
TL;DR: In this paper, the evaporation behavior of Cr2O3 in N2-O2-H2O atmospheres was investigated, and the rate of mass loss was found to be one order of magnitude higher than the rates in N 2 -O2 and N 2-H 2 - atmospheres, indicating that deterioration of the Cr2 o3 scale is likely to occur in mixed atmospheres of oxygen and water vapor.
Abstract: Stainless steels in atmospheres containing H2O form a Cr2O3 scale in the early stage of oxidation. However, the Cr2O3 scale gradually degrades with time. In order to determine the effect of H2O on the deterioration of a Cr2O3 scale, the evaporation behavior of Cr2O3 in N2–O2–H2O atmospheres was investigated. The rate of mass loss in an N2–O2–H2O atmosphere was found to be one order of magnitude higher than the rates in N2–O2 and N2–H2O atmospheres, indicating that deterioration of the Cr2O3 scale is likely to occur in mixed atmospheres of oxygen and water vapor. Volatilization of Cr2O3 is probably based on the following reactions: 1/2Cr2O3(s)+3/4O2(g)+H2O(g)=CrO2(OH)2(g). However, it is also speculated that the reaction, Cr2O3(s)+2/3O2(g)=2CrO3(g), affects the evaporation of Cr2O3 at temperatures higher than 1323 K. The evaporation rate of Cr2O3 is roughly comparable to the growth rate of the Cr2O3 scale. Therefore, a Cr2O3 scale can be degraded by the evaporation of Cr2O3.

76 citations


Journal ArticleDOI
TL;DR: In this paper, a statistical cyclic-oxidation model is presented to assess stabilized metal consumption in cyclicoxidation experiments, and applied to Ni-base superalloys, which form an α-alumina scale during oxidation above 1000C.
Abstract: A statistical cyclic-oxidation model is presented. This model gives analytical formulas to assess stabilized metal consumption in cyclic-oxidation experiments. The model is first detailed, then applied to se eral Ni-base superalloys, which form an α-alumina scale during oxidation above 1000C. A new map is introduced in order to compare the cyclic-oxidation beha ior of these alloys.

72 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of yttrium on the transformation from gamma to theta alumina and also its subsequent transformation to α alumina has been investigated using photostimulated-luminescence spectroscopy.
Abstract: The thermally-grown alumina formed at 1000, 1100, and 1200°C on magnetron-sputtered, nanocrystalline CoCrAl coatings, with and without yttrium, has been characterized using photostimulated-luminescence spectroscopy. The measurements enable the evolution of initially-formed transient alumina to its stable, α phase to be followed, and in particular, the effect of yttrium on the transformation. Yttrium retards the transformation from gamma to theta alumina and also its subsequent transformation to α alumina. The retardation of the transformation decreases with increasing oxidation temperature until at ∼1200°C the transformation is complete within minutes. The presence of yttrium in the coatings also affects the residual stress in the thermally-grown oxide. For samples oxidized at 1100 and 1200°C the residual stress is ∼0.3 GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.

70 citations


Journal ArticleDOI
TL;DR: In this article, the porosity of these materials leads to notable differences between their high-temperature oxidation behavior and that of conventional stainless steels of similar composition, and they found that porosity is strongly influenced not only by the degree of porosity, but also by the concentration of Ni in the base metal.
Abstract: This study examines the resistance to cyclic oxidation at 900°C of two commercial P/M stainless steels—one austenitic (AISI 316L), the other ferritic (AISI 434L)—and of three duplex stainless steels of different compositions. The test results show that the porosity of these materials leads to notable differences between their high-temperature oxidation behavior and that of conventional stainless steels of similar composition. In the case of P/M materials, the resistance to oxidation and the chemical composition of the oxides formed are strongly influenced not only by the degree of porosity of each material, but also by the concentration of Ni in the base metal. In the conditions adopted in the study, ferritic stainless steel was found to provide the best high-temperature oxidation resistance.

Journal ArticleDOI
TL;DR: In this paper, tensile tests of virtually pure FeO, γ -Fe3O4, and α-Fe2O3 were performed at 600-1250°C at strain rates of 2.0×10−3-6.7 × 10−5 s−1 under controlled gas atmospheres.
Abstract: Tensile tests of virtually “pure” FeO, γ -Fe3O4, and α-Fe2O3 were performed at 600–1250°C at strain rates of 2.0×10−3–6.7×10−5 s−1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For α-Fe2O3, brittle fracture resulted at 1150–1250°C, and the fracture strain was below 4.0% at a strain rate of 2.0×10−4 s−1. Oxide of γ -Fe3O4 deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800–1100°C was considered to result from dislocation glide. Using TEM, burgers vector of dislocation observed in deformed γ -Fe3O4 was determined to be 〈110〉, and its slip system was estimated to be {111} . Oxide of FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000 and 1200°C were proportional to the fourth power of the saturated stress, indicating that the plastic deformation was affected by dislocation climb.

Journal ArticleDOI
TL;DR: In this paper, the mechanical properties of the oxide scales formed on mild steel were investigated in 4-point bend tests at 800, 900, and 1000°C in dry air, humid air, and laboratory air at different deformation rates.
Abstract: In the hot-rolling process of steels the oxide scales play a key role with regard to surface quality of the sheet. Therefore, a quantitative knowledge of the mechanical properties of oxide scales at rolling temperature can provide a significant improvement of sheet quality. In the present paper the mechanical properties of the oxide scales formed on mild steel were investigated in 4-point bend tests at 800, 900, and 1000°C in dry air, humid air (7–19.5 vol.% H2O) and laboratory air at different deformation rates. It turns out that the environment has a significant influence on scale thickness and structure as well as on adhesion of the oxide scales. The mechanical measurements show plastic-creep deformation of the oxide scales. Water vapor did not have any significant effect on the creep properties of the oxide scales. In the measurements the secondary-creep-stress values of the oxide were determined as a function of the strain rate and plotted into an Ashby map for FeO. This plot can serve for an extrapolation of the data for even higher strain rates as relevant for the hot-rolling process.

Journal ArticleDOI
TL;DR: In this paper, the density, size and depth of interfacial pores on Fe-40 at 1000degreesC were examined using scanning-electron microscopy and atomic force microscopy (AFM).
Abstract: Pores, or voids, at oxide-alloy interfaces are commonly observed after high temperature oxidation when the alloy does not contain a reactive element. In order to under stand the pore-nucleation and growth processes, the density, size and depth of interfacial pores on Fe-40 at. %Al as a function of oxidation time at 1000degreesC were examined. Scanning-electron microscopy (SEM) and atomic force microscopy (AFM) were used to characterize the pores after removal of the surface Al2O3 scale. The nucleation of pores was most rapid during the initial stage of oxidation where cation-transported-alumina growth dominates. Pore growth involves widening as well as deepening, where the deepening rate is slower for larger pores. Growth is accomplished by aluminum evaporation after similar to20 min or by surface diffusion before that time. Pore shape within an alloy grain stays constant and is dictated by the balance of surface and interface energies.

Journal ArticleDOI
TL;DR: In this article, the isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings, deposited on the same single crystalline Ni-base superalloy, CMSX-4, was investigated.
Abstract: The isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings, deposited on the same single crystalline Ni-base superalloy, CMSX-4, was investigated. The oxidation was studied by gravimetry, scanning-electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDS) and grazing angle X-ray diffraction (XRD). TEM samples were prepared by focused ion beam (FIB) milling. It was found that the oxide on the simple-aluminide coating grew much faster and started to spall much earlier than those on the Pt-modified coatings. This was related to the higher amount of other phases than α-Al2O3 in the oxide scale on the simple-aluminide coating. It was shown that the presence of Pt in the coating suppressed the formation of deleterious phases such as spinels in the oxide scale, but also that the surface morphology of the coating prior to oxidation plays an important role.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the spallation behavior of oxides formed during isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings on the same Ni-base, single-crystalline superalloy (CMSX-4).
Abstract: The spallation behavior of oxides formed during isothermal oxidation at 1050°C of one simple (PWA73) and three Pt-modified (RT22, SS82A and MDC150L) aluminide diffusion coatings on the same Ni-base, single-crystalline superalloy (CMSX-4) was investigated by scanning electron microscopy (SEM) and transmission-electron microscopy (TEM). It was found that the main spallation mechanism was the formation of large Kirkendall voids at the oxide–coating interface. It is believed that the void formation was caused by counter-current flow of vacancies to the diffusion of Ni away from the interface as Al is consumed by the oxide. The magnitude of the vacancy current was determined by the oxidation rate. The properties of the void-formation mechanism are discussed in view of previous data on the microstructure of the oxide scales.

Journal ArticleDOI
TL;DR: In this article, the formation of cavities at the metal-oxide interface is not due to sulfur but only to vacancy injection during the cationic growth of transient θ-alumina.
Abstract: In Part II of this study, the characterization by TGA, SEM and AFM of the oxidation behavior at 900°C of NiAl(001) samples with various sulfur concentrations is reported. The formation of interfacial cavities is observed for all samples. A constant ratio of the oxide thickness to cavity depth is found showing that the formation of cavities at the metal–oxide interface is not due to sulfur but only to vacancy injection during the cationic growth of transient θ-alumina. It is also observed that the presence of sulfur in the alloy decreases the oxidation rate of the nickel aluminide and, consequently, lowers the formation of interfacial cavities. This effect is interpreted as an indirect evidence of the control of the transient-alumina growth by the aluminium diffusion in the alloy, also advanced as an explanation for the interfacial nucleation of alpha alumina. These results are combined with those presented in Part I to propose a model that explains how sulfur, present in small quantities in the alloy, has a deleterious effect on the oxide adherence. The indirect role of the cavities formed during the growth of the transient alumina is to create transitory conditions for the rapid segregation of sulfur at the interface. The segregated sulfur remains as a vestige of the initial stages of growth after the transformation of the scale into mature alumina and weakens its adherence.

Journal ArticleDOI
TL;DR: In this article, it is shown that fluorine can have a beneficial effect on oxidation resistance in a certain F-range which is quantified by thermodynamic calculations and by experimental investigations.
Abstract: High-temperature oxidation resistance of gamma titanium aluminides can be achieved by the formation of a continuous scale of slowly growing Al2O3. The formation of such a scale was favored by the addition of small amounts of fluorine. It is shown that fluorine can have a beneficial effect on oxidation resistance in a certain F-range which is quantified by thermodynamic calculations and by experimental investigations. It is assumed that the F-effect offers a significant potential for improvement of the oxidation resistance of technological TiAl alloys.

Journal ArticleDOI
TL;DR: In this article, a 19Cr−10Ni−1.5Si alloy with and without 0.03Y has been examined using scanning-electron microscopy, secondary-ion mass spectroscopy and transmission electron microscopy and the synergism between the beneficial effects of Y and Si is described in relation to the deoxygenating and de-sulphurizing influences of Y in steel production.
Abstract: The oxidation characteristics of a 19Cr–10Ni–1.5Si alloy with and without 0.03Y have been examined using scanning-electron microscopy, secondary-ion mass spectroscopy and transmission-electron microscopy. Y has been found to segregate to oxide grain boundaries and thereby to inhibit outward-cation diffusion and promote the internal oxidation of Si. The synergism between the beneficial effects of Y and Si is described, and the results are discussed in relation to the de-oxygenating and de-sulphurizing influences of Y in steel production.

Journal ArticleDOI
TL;DR: In this article, the oxidation kinetics of two ODS Fe-Cr-Al alloys, PM 2000 and MA 956, were studied under isothermal conditions from 1000 to 1300°C.
Abstract: The oxidation kinetics of two ODS Fe–Cr–Al alloys, PM 2000 and MA 956, were studied in oxygen and in air under isothermal conditions from 1000 to 1300°C. They both form an α-alumina scale and have good oxidation resistance, without any mass loss. Although the aluminum content in these alloys is higher than the minimum Al content necessary to ensure the growth of a continuous alumina scale, an aluminum depletion occurred in the substrate. This depletion allows the determination of aluminum diffusion coefficients in the ODS alloy. This method is very original and interesting as no Al-stable isotope is available. Moreover, the evolution of the aluminum concentration in the substrate allows one to determine the lifetime of these alloys: indeed, when the aluminum content decreases and becomes lower than a critical value, alumina can no longer form, and less-stable oxides grow very rapidly compared to alumina.

Journal ArticleDOI
TL;DR: In this paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures, and the rate constants were evaluated for the depth of attack due to hot corrosion was compared with oxidation data.
Abstract: The excellent combination of high-temperature strength and lightweight properties makes titanium-base alloys attractive for high-temperature applications in aircraft engines However, more hot corrosion of titanium alloys is a life-limiting factor, particularly when aircraft fly at low altitudes across the sea In the present paper, an attempt has been made to understand the degradation mechanism of titanium alloy, IMI 834 under hot corrosion conditions at elevated temperatures The hot corrosion studies were carried out by determining weight loss at different temperatures and in salts of pure Na2SO4, 90% Na2SO4+10% NaCl and 90% Na2SO4+5% NaCl+5% V2O5 Subsequently, the rate constants were evaluated The depth of attack due to hot corrosion was compared with oxidation data Finally, the degradation mechanism of the titanium alloy that leads to degradation of mechanical properties in aggressive environments has been discussed and suitable coatings suggested to enhance the operational life of engines by effectively preventing both oxidation and hot corrosion

Journal ArticleDOI
TL;DR: In this article, the oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated using scanning-electron microscopy and x-ray diffraction (XRD).
Abstract: The oxidation behavior of single-phase RuAl produced by powder metallurgy combined with arc melting was investigated. Oxidation was conducted at 1000°C; oxide scale growth and phase formation were studied using scanning-electron microscopy (SEM) and x-ray diffraction (XRD). A dense protective scale with an Al–depleted sublayer was formed during oxidation. The oxide scale is the stable α-Al2O3. The oxide-scale morphology shows the presence of whiskers, with a needle-like form, which suggests that the growth of the oxide scale is produced by outward diffusion of Al. At the beginning, oxidation follows a parabolic law, but, after 100 hr of oxidation; the growth rate is slower than expected from a parabolic law.

Journal ArticleDOI
TL;DR: In this article, the internal oxidation of the most reactive component C of ternary A-B-C alloys was examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components.
Abstract: The internal oxidation of the most-reactive component C of ternary A-B-C alloys by a single oxidant is examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components. The precipitation of the most-stable oxide leaves behind a matrix composed of a binary alloy of the two less-reactive components, whose composition affects the solubility and diffusivity of the oxidant within the region of internal oxidation, with an effect on the reaction kinetics. Approximate relations between these properties are proposed and used to predict the kinetics of internal oxidation of C under the assumption of parabolic rate law. The results obtained for the ternary alloys are compared with the behavior of binary A-C and B-C alloys with the same C content. A new important factor in establishing the difference between the internal oxidation in ternary A-B-C alloys and in binary A-C and B-C alloys under a fixed gas-phase oxygen pressure and C content is the ratio between the concentrations of A and B in the bulk ternary alloy.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the hot-corrosion properties of a CoNi-Cr-Al-Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating in air at 900°C and in molten 75 wt.% NaCl+25 wt% Na2SO4 at 850°C.
Abstract: The oxidation and hot-corrosion behavior of a Co–Ni–Cr–Al–Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating were investigated in air at 900°C and in molten 75 wt.% NaCl+25 wt.% Na2SO4 at 850°C. The results show that the enamel coating possesses excellent hot corrosion resistance in the molten salt, in comparison with the HVOF-sprayed Co–Ni–Cr–Al–Y coating alone. In the hot-corrosion test, breakaway corrosion did not occur on the samples with the enamel coating and the composition of the enamel did not significantly change. The oxidation resistance of the Co–Ni–Cr–Al–Y coating, which had good adhesion, was also improved by the enamel coating.

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the kinetics of oxidation of copper powders in oxygen and in dry and humid air using thermogravimetric analysis (TGA) and found that the extent of oxidation grew linearly with time until the weight-based thickness of the oxide film reached 0.13-1.22 nm, depending on the temperature.
Abstract: The kinetics of oxidation of copper powders in oxygen and in dry and humid air was investigated using thermogravimetric analysis (TGA). The extent of oxidation grew linearly with time until the weight-based thickness of the oxide film reached 0.13–1.22 nm, depending on the temperature. Between 30 and 90°C there was little difference between the kinetic curves observed in air and in oxygen, respectively. Higher humidity of the air resulted in an increased oxidation rate. Following the initial linear segment, the oxidation kinetics could be best described in terms of a logarithmic rate law between 30 and 45°C and in terms of a power law between 60 and 90°C. The activation energy for the initial linear stage was (44±2) kJ and for the subsequent oxidation (102±12) kJ. Delayed increases in oxidation rate were observed with a ca. 0.1-μm powder around 100°C, with a ca. 1-μm powder around 320°C, and with a < 10μm powder around 360°C. A three-stage model consisting of an initial linear stage, parabolic growth culminating in cracking of the oxide film, and subsequent re-start of the parabolic growth, gave good agreement with the experimental data. Whenever the powder is relatively uniform and the distribution of film-cracking times among the powder grains is narrow, e.g., within 23% of the median cracking time, an increase in the oxidation rate of the entire sample can be observed.

Journal ArticleDOI
TL;DR: In this paper, the effect of sulfur impurities on the adherence of the thermally-grown oxide on the boundary layer in thermal barrier-coating systems was investigated. And the authors improved the understanding of the deleterious effect of impurity on adherence of thermally grown oxide.
Abstract: The aim of this study was to improve the understanding of the deleterious effect of sulfur impurities on the adherence of the thermally-grown oxide on the boundary layer in thermal-barrier-coating systems. In Part I, the sulfur segregation on the free surface of NiAl(001) and at different interfaces between metal and transient alumina scales has been characterized by AES, XPS and LEED. The sulfur diffusion coefficient in the alloy has been determined (D = 0.15 exp(−218,000/RT) cm2/s). It is by three orders of magnitude larger than the nickel and aluminum self-diffusion coefficients. It has also been observed that the sulfur de-segregates upon Al enrichment of the metallic surface. The saturation of the metallic surface with an amorphous alumina layer formed at room temperature blocks the segregation of S. However, in the initial stages of oxidation where the transient θ-alumina grows by cationic transport and inject vacancies at the interface, S segregates at the interface between the alumina thin films and the metallic substrate.

Journal ArticleDOI
TL;DR: In this paper, an AlTiN coating with and without CrN and NbN diffusion barriers was used to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti 46.7Al−1.9W−0.5Si, in an H2/H2S/H 2O atmosphere at 850°C.
Abstract: Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H2/H2S/H2O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10−11 g2/cm4/s. A multi-layered scale developed: an outer rutile (TiO2) layer, a continuous layer of α-Al2O3 beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl3 and TiAl2) between the scale and substrate.

Journal ArticleDOI
TL;DR: In this article, the authors examined void generation at scale-alloy interfaces during the oxidation of binary alloys as a result of matter transport through the scale and of diffusion of the metal components in the alloy.
Abstract: Void generation at scale–alloy interfaces during the oxidation of binary alloys as a result of matter transport through the scale and of diffusion of the metal components in the alloy is examined under a number of simplifying assumptions. In particular, it is assumed that the scale–alloy interface does not displace with time and that all the metal vacancies at this interface condense there to form voids, which amounts to calculating the maximum rate of interface void production. The analysis is developed both for the case of the parabolic rate law and for a general kinetics behavior, using the experimental information concerning the kinetics of weight gain. Application of this treatment to the oxidation of FeAl at 1000°C, taking into account the gradual transformation of the initial form of alumina into the stable alpha form, predicts the presence of a maximum in the overall void volume, in qualitative agreement with experimental observations. However, the overall volume of voids measured is much smaller than the values calculated under various conditions, suggesting that an effective mechanism of vacancy annihilation must be present at the scale–alloy interface.

Journal ArticleDOI
TL;DR: In this article, the kinetics of the coupled internal oxidation of the two most reactive components in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most noble component are examined using a number of simplifying conditions which allow to develop an approximate analytical treatment.
Abstract: The kinetics of the coupled internal oxidation of the two most-reactive components in the scaling of ternary alloys under oxidant pressures below the stability of the oxide of the most noble component are examined using a number of simplifying conditions which allow to develop an approximate analytical treatment. The precipitation of the two oxides may occur either at a single front or at two different fronts of internal oxidation. The former case corresponds to a unique solution for all the parameters involved in the process. On the contrary, the existence of two fronts of internal oxidation yields a finite range of possible solutions for the oxidation kinetics as well as for all the other relevant parameters. Even though the present treatment does not allow to predict which solution will be adopted by a real system, it is possible to set limits to the values of the parameters yielding physically-acceptable solutions. After considering a general case, the treatment is applied to a real system already examined experimentally.

Journal ArticleDOI
TL;DR: In this article, the authors used a 30nm field-emission Auger spectroscopy probe to study the segregation of sulfur to a growing oxide-metal interface and found that sulfur started to segregate to the interface only after a complete layer of α-Al2O3 developed there, its concentration then increased slowly with further oxidation until reaching a level close to half a monolayer.
Abstract: Using a 30-nm field-emission Auger spectroscopy probe, the segregation of sulfur to a growing oxide–metal interface was studied. The interfaces were formed by the oxidation of a Fe–40at.% Al alloy at 1000°C for various times. Both the oxide and the alloy sides of the interface were examined after spalling the surface Al2O3 layer in ultra-high vacuum. Results were compared with similar studies performed using conventional AES and related to scale development and the interface microstructure. Sulfur started to segregate to the interface only after a complete layer of α-Al2O3 developed there, its concentration then increased slowly with further oxidation until reaching a level close to half a monolayer. Higher amounts were observed on interfacial-void surfaces, where Al and S cosegregated. The study showed that sulfur segregation to oxide–alloy interfaces depended on the type of interface, indicating possible relationships between segregation energies and interface microstructure.