Effect of the θ–α-Al2O3 Transformation in Scales on the Oxidation Behavior of a Nickel-Base Superalloy with an Aluminide Diffusion Coating

The high-temperature oxidation behavior of a nickel-base superalloy andaluminide-diffusion coating has been investigated over the temperaturerange from 800–1100°C and analyzed by TGA, XRD, EDAX, andSEM. The -NiAl coating was formed by low-pressure gas-phasecementation at 950°C 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 1050°C during transient oxidation. The transformationcaused the anomalous behavior of the oxidation kinetics curves of thisdiffusion coating in the temperature range 800–1100°C 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.     

Diffusional transport during the cyclic oxidation of γ+β, Ni-Cr-Al(Y,Zr) alloys

The cyclic oxidation behavior of several cast +, Ni-Cr-Al(Y, Zr) alloys and one LPPS +, Ni-Co-Cr-Al(Y) alloy was examined (, fcc; , NiAl structure). Cyclic oxidation was performed by cycling between 1200°C and approximately 70°C. Oxide morphologies and microstructural changes during cyclic oxidation were noted. Recession of the high-Al phase was nonparabolic with time. Kirkendall porosity resulting from diffusional transport within the alloy was observed in the near-surface -phase layer of one alloy. Concentration profiles for Ni, Cr, and Al were measured in the -phase layer after various cyclic oxidation exposures. It was observed that cyclic oxidation results in a decreasing Al concentration at the oxide-metal interface due to a high demand for Al (a high rate of Al consumption) associated with oxide scale cracking and spalling. In addition, diffusion paths plotted on the ternary phase diagram shifted to higher Ni concentrations with increasing cyclic oxidation exposures. The alloy with the highest rate of Al consumption, and highest Al content, underwent breakaway oxidation after 500 1-hr cycles at 1200°C. Breakaway oxidation occurred when the Al concentration at the oxide-metal interface approached zero. The relationship between the Al transport in the alloy and breakaway oxidation is discussed.     

Effect of Palladium Incorporation on Isothermal Oxidation Behavior of Aluminide Coatings

A (Ni, Pd)Al coating has been prepared by low-pressure pack cementation on the nickel-base superalloy IN738 with a preplating of Pd–20 wt.% Ni alloy. The coating consists mainly of a single phase of -Pd(Ni)Al in the outer part and -Ni(Pd)Al in the inner part. The oxidation behavior of the (Ni,Pd)Al coating at 900–1100°C was studied by TGA, XRD, and SEM/EDS. Results show that the transformation of - to -Al₂O₃ is more rapid on the (Ni,Pd)Al coating than that on a simple NiAl coating. The addition of Pd to the aluminide coating facilitates the transformation of - to -Al₂O₃e oxide scale formed and accelerates the diffusion of Ti from the substrate to the coating surface simultaneously.     

Oxidation Behavior of a PVD-Processed Mg–10.6Zr Alloy

The oxidation behavior in air of a physical vapor-deposited (PVD) Mg–10.6Zr (wt. %) alloy was studied in the 325–450°C temperature range. The oxidation rate of this alloy remains low at temperatures below 375°C. However, at higher temperatures, the alloy experienced extremely high oxidation rates, which can even lead to disintegration of the sample. Oxidation is controlled by fast inward oxygen transport along the open boundaries of the alloy, leading to the formation of cracks throughout the sample, and subsequent formation of a thin MgO at crack interfaces. The MgO layer remains protective while coarsening of zirconium precipitates at the open boundaries does not take place. Thickening of Zr precipitates over a critical size induces impairment of the MgO layer at crack interfaces, facilitating inward oxygen diffusion. The volume increase resulting from the formation of new oxide at open boundaries favors decohesion of open boundaries, leading to accelerated oxidation.     

Oxygen exchange in oxidation of an Fe-20Cr-10Al alloy in ∼10 mbar O2/H2O-gas mixtures at 920°C

The oxidation of an alumina forming Fe-200-10Al alloy was studied in situ. The gas phase components in isotopically labeled 10 mbar O₂/H₂O-gas mixtures were measured from a virtually closed system at 920°C. Oxidation rates and oxygen-exchange rates were measured and related to each other. From the exchange rates the dissociation rates of O₂ and H₂O were calculated. These dissociation rates were in the early stage of oxidation found to be the same as the oxidation rate. The rate of oxygen exchange with the oxide can exceed the oxidation rate. From this follows that in analysis of a two-stage oxidation in^16,16O₂/^18,18O₂ the exchange rate of O between the oxide and gas phase has to be considered. The oxides formed in H₂O containing gas had a H/O-ratio of 0.1. Vacuum annealing of the alloy before oxidation increased the oxidation rate by a factor of 2.     

Thermal Electrochemical, and Plasma Oxidation of Ti–50Zr, Cu–50Zr, Cu–50Ti, and Cu–33Ti–33Zr Alloys

The three main methods for oxidation of metallic substrates, thermal, anodic and plasma have been applied to a copper, titanium, and zirconium alloy and its corresponding binaries (Cu–33Ti–33Zr, Cu–50Ti, Cu–50Zr and Ti–50Zr). Polished polycrystalline samples of these alloys were examined before treatment, after vacuum thermal annealing at 100°C and heating in 20 mTorr oxygen at 100, 200, and 300°C. ISS depth profiles were taken of selected samples. The least-noble component oxidizes first, but at high temperatures and with plasma oxidation the noble component segregates to the surface. A comparison of the resulting structures on the ternary and binary alloys with different oxidation methods is used to explore the physico-chemical processes during oxidation. Results from these three methods are discussed in terms of physical/chemical parameters that influence the chemical nature and structure of the resulting oxides. The electrochemical processes that occur during the materials reaction with a chosen environment are used to discuss the physical and chemical mechanisms involved. Intrinsic (thermal and plasma oxidation) and extrinsic (electrochemical oxidation) electric fields are shown to influence the chemical and structural nature of the resulting oxide structures. The influence of transport phenomena is discussed.     

Oxidation Behavior and Breakdown of an Aluminide Coating on DZ40M Alloy at High Temperatures

DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both -Al₂O₃ and -Al₂O₃ at 900 and 1000°C but only -Al₂O₃ at 1050 and 1100°C.     

Long-term high-velocity oxidation and hot corrosion testing of several NiCrAl and FeCrAl base oxide dispersion strengthened alloys

Several oxide dispersion strengthened (ODS) alloys have been tested for cyclic, long-term, high gas-velocity resistance to oxidation at 1100°C and hot corrosion at 900°C. Both nominally Ni-16Cr-4Al and Fe-20Cr-4.5Al ODS alloys were subjected up to 2500 cycles, where each cycle consisted of 1 hr in a hot, Mach 0.3 combusted gas stream followed by a 3-min quench in an ambient temperature, Mach 0.3 air blast. For comparison to existing technology, a coated superalloy was simultaneously tested. The ODS iron alloy exhibited clearly superior behavior, surviving 3800 oxidation and 2300 hot corrosion cycles essentially unscathed. While the ODS nickel alloys exhibited adequate oxidation resistance, the long-term hot corrosion resistance could be marginal, since the best life for such alloys under these conditions was only 1100 cycles. However, the hot corrosion resistance of the ODS Ni-base alloys is excellent in comparison to that of traditional superalloys.     

Transient-Alumina Transformations During the Oxidation of Magnetron-Sputtered CoCrAl Nanocrystalline Coatings

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, 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 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 0.3GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.     

The influence of thermal cycling on the oxidation and oxide spallation of a 1%Cr–0.5%Mo low-carbon steel

The oxidation and oxide spallation of 1%Cr–0.5%Mo low carbon steel disks in dry oxygen was studied isothermally at 800°C (1073 K) and in thermal cycling between 800 and 600°C (1073 and 873K) followed by cooling at rates from 3 to 100°C/min. Mostly parabolic oxidation kinetics were observed. Thin scales (10 ) were more prone to spalling than thicker scales (20 ). The thickness and growth imperfections of an inner scale layer enriched in chromium, molybdenum, and silicon strongly influenced the probability of cohesive failure exceeding that of adhesive failure of the scale. Cohesive failures in the bulk scale during thermal cycling were probably nucleated at voids and microcracks produced in the initial isothermal period of scale growth. The number of segmented scale layers that became detached during cycling was governed by the number of parallel rows of voids in the scale and not necessarily by the number of cycles.     

Protection of Sintered Steels against Corrosion in Neutral Media with an N-M-1 Inhibitor

The effect of an N-M-1 inhibitor (a salt of cyclohexylamine and C₁₀ to C₁₆ aliphatic acids) on the corrosion of sintered powder steels 13 and 141 (with the 14 to 17% porosity) in distilled or tap water at 20 to 80°C, as well as in 0.05 M Na₂SO₄ solution, is studied by gravimetric and electrochemical methods. The protective concentration of the inhibitor is lower in distilled, than in tap water or sodium sulfate solution. The protective action decreased with an increase in temperature. In tap water or sodium sulfate solution, the corrosion-inhibiting effect is weaker for 13 than for 141.     

High-temperature oxidation behavior of a Ni-Cr-Al-Fe-Y alloy

A study was conducted to examine the isothermal oxidation behavior of a wrought Ni-Cr-Al-Fe-Y alloy in air at temperatures in the range of 950–1150°C. Oxidation kinetics were determined from weight-change measurements. Analytical electron microscopy, scanning electron microscopy, and x-ray diffraction were used to characterize the morphology, structure, and composition of the oxide scale. Overall oxidation of the alloy was found to follow parabolic kinetics. Under steady-state conditions, the oxidation reaction appeared to be controlled by diffusional transport in an adherent Y-modified -Al₂O₃ scale with an activation energy of about 400 KJ/mol. Yttrium was found to preferentially segregate to grain boundaries of -Al₂O₃ which maintained a fine columnar grain structure about 0.05–0.2 m in size. Based upon the results obtained, it was suggested that the role of Y was to promote the formation of a thin layer of -Al₂O₃ with improved mechanical strength.     

Effects of Chromium on the Oxidation Performance of β-FeAlCr Coatings

-FeAl coatings containing various Cr contents of 6.5–45 wt.%were produced with a closed-field, unbalanced magnetron sputter (CFUMS)deposition technique. Cyclic oxidation tests at 1100°C in air for100 1-hr cycles and isothermal exposures at 1000°C in pure O₂ for100 hr were carried out with the coatings and an as-cast FeAlspecimen. All of the coatings showed good scale-spallation resistanceduring cyclic oxidation and the coating with 6.5 wt.% Cr exhibited thelowest oxidation rates in both cyclic and isothermal oxidationexposures. After oxidation, fine-grain ridge-type oxide scales formed onthe coatings, while the oxide scale formed on the cast FeAl showed alarge quantity of -Al₂O₃ blades and large interfacial voids on thebase–alloy surface. The transformation from to -Al₂O₃was accelerated due to the presence of Cr in the coatings. The fasttransformation considerably reduced oxidation rates, suppressed fastoutward Al diffusion for the growth of a -Al₂O₃ scale, and preventedthe formation of interfacial voids that played a major role in causing thescale spallation.     

Effect of cold work on the oxidation resistance of2\tfrac{1}{4} Cr-1 Mo steel

The effect of cold work on the oxidation rate of 21/4 Cr-1 Mo steel in pure oxygen at 1 atm pressure at temperatures ranging from 400 to 950C has been studied for short exposure periods (max. 4 hr). The specimens had been cold worked up to 90% by cold rolling. The results indicate negligible effect of cold work on the oxidation kinetics up to 700C, beyond which there is general reduction in oxidation. The effect was pronounced at 900C. The increased resistance to oxidation has been attributed to the faster diffusion of chromium in the cold-worked material compared to the annealed one, leading to the formation of a chromium-rich spinel which helps in slowing down the oxidation of the alloy. The findings have been corroborated by the examination of all samples oxidized at 900C by optical, EPMA, SEM, and EDAX analyses.     

Alloy-grain-size dependence of the effectiveness of silica coatings as oxidation barriers on stainless steel

In order to determine the effect of alloy grain size on the oxidation properties of silica-coated austenitic Fe-18Cr-20Ni stainless steel, both coarse-grain (100-m grain size) and fine-grain (5-m grain size) forms of the alloy were produced. A 1-m-thick vitreous silica coating was deposited by chemicalvapor deposition on the alloys, which were subjected to isothermal and cyclic oxidation in air at 900°C. The coarse-grain alloys underwent widespread oxidation below the silica coating, leading to extensive coating spallation. This was attributed to the inability of the alloy to supply a sufficient outward flux of chromium to prevent oxygen penetration through microcracks in the silica coating. Due to an abundance of chromium available at the surface of the finegrain alloy, chromia formed in the microcracks within the silica layer. As a result, the silica-coated, fine-grain alloy demonstrated superior oxidation resistance and excellent adhesion of the coating.     

High-Temperature Oxidation of Ti-48Al-2Nb-2Cr and Ti-25Al-10Nb-3V-1Mo

The short-term oxidation behavior of a-TiAl alloy (Ti-48Al-2Nb-2Cr) was compared andcontrasted to that of an₂-Ti₃Al base(Ti-25Al-19Nb-3V 1Mo) alloy. Oxidation ofTi-25Al-10Nb-3V-1Mo was found to occur at a moderate rate at 800°C, in aN₂ + 20% O₂ environment. A largeincrease in the oxidation rate occurred above thistemperature. This large weight increase was attributedto a breakdown in the protective oxide scale on the surface of the₂ intermetallic alloy, therebypermitting rapid diffusion of oxygen and nitrogen to thesurface of the intermetallic. The oxidation rate of thisalloy at 1200°C was not significantly higher thanthe oxidation rate at 1000°C. In contrast, theoxidation rate of Ti-48Al-2Nb-2Cr remained low up to1200°C. At this temperature, a significant increasein oxidation was observed and was attributed to acceleratedoxygen diffusion through the ₂ phaseand increased solubility of oxygen in the gamma phase ofthe intermetallic microstructure. This weight increaseoccurred despite the fact that at 1200°C, theintegrity of the oxide layer formed on the surface ofthis alloy was maintained. The results of this studyillustrate the need for developing protectiveenvironmental coatings tailored to the individualintermetallic alloy.     

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

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

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

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

A Microstructural Study of Preferential Oxidation at the Grain Boundaries of Ni–Cr Alloys

The oxidation behavior of alloy grain boundaries in model Ni–Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni–10Cr and Ni–20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni–10Cr did not exhibit oxidation, whereas the alloy formed a thick (60 m) oxide layer which formed inwardly. The grain boundaries in this alloy showed a passivating influence at the adjacent regions and retarded oxide formation. An examination of the Ni–20Cr cross section revealed preferential oxidation to a depth of 65 m at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni–Cr alloys changes as a function of Cr content.     

Oxidation Resistance of Ion-Implanted γ-TiAl-Base Intermetallics

A series of alloy elements, including the detrimental, neutral, and beneficial elements for bulk alloying into -TiAl base intermetallics, i.e., V, Cr, Y, Er, Nb, and W, has been implanted into a -Ti–50Al intermetallic in order to explore the mechanism of high-temperature oxidation resistance for the ion-implanted intermetallic. The oxidation resistance was investigated under cyclic-oxidation conditions at oxidation temperatures from 800 to 1000°C for 200 hr in air. At a lower oxidation temperature of 800°C, the V-ion implantation has a detrimental effect on the oxidation resistance of Ti–50Al, while a neutral and beneficial effect was observed for Er-, Y- and Cr-, Nb-, W-ion implantation, respectively. At 900°C, V-, Er-, Y-, and Cr-ion implantation all showed a neutral effect, whereas Nb- and W-ion implantation apparently improved the oxidation resistance. With increasing oxidation temperature to 1000°C, Y- and Cr-ion implantation kept the neutral effect, and the beneficial effect of Nb-ion implantation disappeared gradually. The oxidation behavior of ion-implanted -TiAl base intermetallics is different from that of bulk-alloyed materials due to the two alloying methods, although the effect of the alloy elements on the oxidation resistance has not essentially changed in the -TiAl base intermetallics.