Early‐stage oxidation behavior of Co‐rich high‐temperature alloys

The long‐term oxidation performance of an alloy is critically linked to the early‐stage oxidation behavior of high‐temperature alloys. This study investigates early‐stage oxidation behavior in terms of oxidation kinetics, scale evolution, and residual stresses developed within a scale of the commercially available cobalt‐rich alloys: HAYNES® 188, 6B, 25, and HR‐160® and a newly developed nitride‐dispersion strengthened NS‐163® alloy (HAYNES®, HR‐160®, NS‐163® are registered trademarks of Haynes International, Inc). Short‐term isothermal oxidation exposures were conducted in flowing air at 982 °C for durations of 1–50 h. Oxidation kinetics was assessed by weight‐change behavior, which showed that 188 alloy exhibited the lowest weight‐gain, while for similar times HR‐160 alloy underwent weight‐loss. SEM/EDS analysis was performed to characterize oxides formed in these alloys, while stresses developed in the oxides of different alloys were measured using synchrotron X‐ray radiation. The results in this paper clearly demonstrated the effects of alloy composition on the scale evolution and the amount of stresses developed in oxides.     

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al₂O₃. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al₂O₃ films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al₂O₃ such that peened surface samples showed significant α-Al₂O₃ growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al₂O₃ at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.     

Increasing the Upper Temperature Oxidation Limit of Alumina Forming Austenitic Stainless Steels in Air with Water Vapor

A family of alumina-forming austenitic (AFA) stainless steels is under development for use in aggressive oxidizing conditions from ~600?C900 °C. These alloys exhibit promising mechanical properties but oxidation resistance in air with water vapor environments is currently limited to ~800 °C due to a transition from external protective alumina scale formation to internal oxidation of aluminum with increasing temperature. The oxidation behavior of a series of AFA alloys was systematically studied as a function of Cr, Si, Al, C, and B additions in an effort to provide a basis to increase the upper-temperature oxidation limit. Oxidation exposures were conducted in air with 10% water vapor environments from 800?C1000 °C, with post oxidation characterization of the 900 °C exposed samples by electron probe microanalysis (EPMA), scanning and transmission electron microscopy, and photo-stimulated luminescence spectroscopy (PSLS). Increased levels of Al, C, and B additions were found to increase the upper-temperature oxidation limit in air with water vapor to between 950 and 1000 °C. These findings are discussed in terms of alloy microstructure and possible gettering of hydrogen from water vapor at second phase carbide and boride precipitates.     

Characterization and Oxidation Behavior of Rayon-Derived Carbon Fibers

Rayon-derived fibers are the central constituent of reinforced carbon/carbon (RCC) composites. Optical, scanning electron, and transmission electron microscopy were used to characterize the as-fabricated fibers and the fibers after oxidation. Oxidation rates were measured with weight loss techniques in air and oxygen. The as-received fibers are ~10 μm in diameter and characterized by grooves or crenulations around the edges. Below 800 °C, in the reaction-controlled region, preferential attack began in the crenulations and appeared to occur down fissures in the fibers.     

Oxidation Behaviour of Model Cobalt-Rhenium Alloys During Short-Term Exposure to Laboratory Air at Elevated Temperature

The alloys being used in high-temperature systems such as stationary gas turbines and aircraft engines are iron-, cobalt- and nickel-based superalloys, amongst which the latter is the most widely used for highest temperatures. However, the use of Ni-based alloys is limited to temperatures below 1,100 °C. The experimental Co–Re-based alloys are promising for high-temperature applications for service temperatures beyond 1,200 °C. The purpose of the present investigations, at this still early stage of the alloy development, is to gain a first insight into the oxidation mechanisms and to find ways to improve oxidation resistance of this class of materials. Thermogravimetric studies in combination with microstructural examinations of six model Co–Re alloys with different compositions showed the negative influence of rhenium on the oxidation resistance of Co-based alloys due to evaporation of rhenium oxide(s). Oxidation at 1,000 °C in air yielded an oxide scale, that consists of a Co-oxide outer layer on a thick and porous Co–Cr oxide and a semicontinuous and therefore non-protective Cr-oxide film on the base metal substrate. This allowed for the vaporization of rhenium oxide formed during oxidation and hence led to a loss of Re. Computer-aided thermodynamic calculations were carried out to supplement the experimental analyses and were found to reasonably predict the stability ranges of the various oxide phases observed.     

Oxidation Mechanisms of Copper and Nickel Coated Carbon Fibers

Differential-Thermal Analysis (DTA) and X-ray diffraction analysis were applied to determine the mechanisms of high-temperature oxidation of copper- and nickel-coated carbon fibers. Both kinds of coatings were deposited by electroless plating onto the fiber surface. The as-deposited copper film was crystalline, whereas the nickel coating consisted of an amorphous Ni–P alloy. Coated fibers were heated from room temperature to 900 °C in air at 10 °C min^?1. For the copper coating, the main oxidation product formed at low temperatures was Cu₂O, while at higher temperatures was CuO. The crystallization of Ni–P took place at 280–360 °C with the formation of Ni and Ni₃P. The final compounds were NiO, Ni₂P and Ni₃(PO₄)₂. After complete oxidation of the carbon fibers, copper and nickel-oxidized microtubes were obtained. Besides, while copper reduced the temperature of the fiber oxidation, nickel coatings increased the minimum temperature needed for this reaction.     

Effect of Oxidation with Lanthanum Sol–Gel Coating on the 330Cb Carburization at 900 °C

Corrosion and carburization are the main reasons for the degradation of mechanical properties of conveyor units. In order to improve alloy carburization resistance, an adherent oxide scale can act as a carbon diffusion barrier. Oxidation of Alloy 330Cb in air at 900 °C was performed in order to grow a protective chromia scale. Nevertheless, after oxidation some scale spallation occurred during cooling. Then, argon-annealed lanthanum sol–gel coatings were applied in order to improve the oxide scale adherence. This led to a thinner chromia scale. The convoluted scale structure suggested that a mixed diffusion process was induced by lanthanum. After carburization during 22 h at 900 °C under Ar/10 % CH₄, the Cr₂O₃ scale was converted into carbides and it appears that is necessary to have a thick oxide scale to get a good protection.     

Effect of Addition of 4 % Al on the High Temperature Oxidation and Nitridation of a 20Cr–25Ni Austenitic Stainless Steel

In high temperature applications, the alumina forming austenites (AFA) have recently gained more focus. These utilise the advantageous effect of Al on oxidation resistance, and also have good mechanical properties. Two experimental alloys [20Cr–25Ni–1Mn–0.5Si–Fe (wt.%)] were prepared. To one of the alloys 3.77 wt.% Al was added. The alloys were studied in air and air/water at 700 °C and 1,000 °C, in a sulphidising/chlorinating environment at 700 °C and in a nitriding atmosphere at 1,000 °C. The time of exposure was 100 h, except for one 1,000 h exposure in air/water. At 700 °C in air and air/water, the AFA displayed lower mass gain than the reference material. After exposure in the sulphidising-chlorinating environment, the material displayed a surface alumina layer with some spallation. In air or air/water at 1,000 °C, internal aluminium nitride and alumina formation occurred, appreciably reducing the sound metal thickness. The nitridation was enhanced in the nitriding environment.     

Mechanical Properties of Carbon Fiber-Reinforced Aluminum Manufactured by High-Pressure Die Casting

Carbon fiber reinforced aluminum was produced by a specially adapted high-pressure die casting process. The MMC has a fiber volume fraction of 27%. Complete infiltration was achieved by preheating the bidirectional, PAN-based carbon fiber body with IR-emitters to temperatures of around 750 °C. The degradation of the fibers, due to attack of atmospheric oxygen at temperatures above 600 °C, was limited by heating them in argon-rich atmosphere. Additionally, the optimization of heating time and temperature prevented fiber degradation. Only the strength of the outer fibers is reduced by 40% at the most. The fibers in core of fiber body are nearly undamaged. In spite of successful manufacturing, the tensile strength of the MMC is below strength of the matrix material. Also unidirectional MMCs with a fiber volume fraction of 8% produced under the same conditions, lack of the reinforcing effect. Two main reasons for the unsatisfactory mechanical properties were identified: First, the fiber-free matrix, which covers the reinforced core, prevents effective load transfer from the matrix to the fibers. And second, the residual stresses in the fiber-free zones are as high as 100 MPa. This causes premature failure in the matrix. From this, it follows that the local reinforcement of an actual part is limited. The stress distribution caused by residual stresses and by loading needs to be known. In this way, the reinforcing phase can be placed and aligned accordingly. Otherwise delamination and premature failure might occur.     

Cyclic Oxidation Behaviour of 1Cr–0.5Mo (T11) Boiler Tube Steel and Its Weldments in Air at 900 °C

Oxidation behaviour of weldments at elevated temperature has become an object of scientific investigation. Weldments were prepared using shielded metal arc welding and tungsten inert gas processes to weld together 1Cr–0.5Mo (T11) boiler tube steels. This paper reports the oxidation behaviour of welded and unwelded 1Cr–0.5Mo (T11) boiler tube steel specimens after exposure to air at 900 °C under cyclic condition. The thermogravimetric technique was used to establish kinetics of oxidation. X-ray diffraction and scanning electron microscopy/energy-dispersive analysis techniques were used to analyse the oxidation products. The unwelded steel showed a higher oxidation rate (in terms of weight gain) than that of welded steels.     

Thermogravimetric Study of Oxidation-Resistant Alloys for High-Temperature Solar Receivers

Three special alloys likely to be suitable for high-temperature solar receivers were studied for their resistance to oxidation up to a temperature of 1050°C in dry atmospheres of CO₂ and air. The alloys were Haynes HR160, Hastelloy X, and Haynes 230, all nickel-based alloys with greater than 20% chromium content. The oxidation rate of specimens cut from sample master alloys was followed by thermogravimetry by continuously monitoring the weight change with a microbalance for a test duration of 10 h. The corrosion resistance was deduced from the total weight increase of the specimens and the morphology of the oxide scale. The surface oxide layer formed (scale) was characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy and in all cases was found to be chromia. Oxidation was analyzed by means of parabolic rate law, albeit in some instances linear breakaway corrosion was also observed. For the temperature range investigated, all alloys corroded more in CO₂ than in air due to the formation of a stronger and more protective oxide scale in the presence of air. At 1000°C, the most resistant alloy to corrosion in CO₂ was Haynes 230. Alloy Haynes HR160 was the most oxidized alloy at 1000°C in both CO₂ and air. Hastelloy X oxidized to a similar extent in CO₂ at both 900°C and 1000°C, but in air, it resisted oxidation better at 1000°C than either at 900°C or 1000°C.     

Oxidation and Hot Corrosion Behaviors of Service-Exposed and Heat-Treated Gas Turbine Vanes Made of IN939 Alloy

Oxidation and hot corrosion tests were conducted on service-exposed and heat-treated IN939 alloys at 830, 930 and 1030 °C for testing times up to 800 h. The degradation behaviors were studied using optical and scanning electron microscopes. The oxidation results showed no tangible weight change in the samples at 830 °C. At 930 °C, after initial weight gain, the oxidation samples showed weight loss; whereas a continuous weight loss was observed at the higher temperature of 1030 °C. In the hot corrosion tests, however, a large weight loss occurred in the samples even at 830 °C, indicating an effect of fuel impurities on the high-temperature behavior of the alloy. SEM observations revealed that the main features of oxidation and hot corrosion of the alloy were internal oxidation of aluminum and depletion of chromium in the regions beneath the surface scales.     

High Temperature Oxidation of the Al3Mg2 Complex Metallic Alloy

High temperature air oxidation of the Al₃Mg₂ complex metallic alloy was investigated on powder samples and bulk polycrystals in the temperature range 350–420 °C by thermogravimetric measurements, SEM and TEM. Oxidation at 420 °C on the polycrystalline samples comprised three successive phases characterised by linear kinetics laws. The first stage corresponded to the formation of a porous non adherent nanocrystalline MgO scale having a cauliflower morphology. A first acceleration in the kinetics law was ascribed to the nucleation and growth of MgAl₂O₄ crystallites which form a thin (~10 nm) film at the interface between the substrate alloy and the MgO top layer. A new linear regime was observed in the oxidation process corresponding to diffusion of magnesium through the grain boundaries of the spinel film and through the porous MgO layer. Finally, fragmentation and cracking of the scale leads to a further acceleration followed by a new linear regime.     

Oxidation Studies on Base Metal,Weld Metal and HAZ Regions of TIG Weldment in 2.25 Cr–1Mo (T22) Boiler Tube Steel Under Cyclic Conditions

A weldment was made using the TIG welding process to weld together 2.25Cr–1Mo (T22) boiler tube steel. Oxidation studies were then conducted on different regions of the TIG weldment, i.e. base metal, weld metal and heat-affected zone (HAZ) specimens, by exposure to air at 900 °C under cyclic conditions. The thermogravimetric technique was used to study the kinetics of oxidation. Scanning electron microscopy/energy dispersive of X-ray (SEM/EDX) analysis and X-ray diffraction (XRD) techniques were used to analyse the oxidation products. The HAZ showed more weight gain than that of base metal and weld metal due to the formation of a scale that was low in Cr-rich oxide (as confirmed by EDX), with cracks and spallation.     

The Oxidation Behavior of TBC with Cold Spray CoNiCrAlY Bond Coat

Cold gas dynamic spray (CGDS) has been considered a potential technique to produce the metallic bond coat for TBC applications, because of its fast deposition rate and low deposition temperature. This article presents the influence of spray processes for bond coat, including air plasma spray, high velocity oxy-fuel, and in particular CGDS, on the oxidation performance of TBCs with a Co-32Ni-21Cr-8Al-0.5Y (wt.%) bond coat and an air plasma sprayed topcoat. Oxidation behavior of the TBCs was evaluated by examining the coating microstructural evolution, TGO growth behavior, and crack propagation during thermal exposure at 1050 °C. The relationship between the TGO growth and crack propagation will be discussed.     

Oxidation Kinetics and Scale Morphology of a Palladium-Modified-Aluminide Coating at High Temperature in Air

Oxidation of Metals - The isothermal oxidation behavior of a (Ni,Pd)Al coating was investigated in air over the temperature range of 800–1150°C by means of TGA, XRD, SEM/EDS. The...     

High Temperature Oxidation of Micro-alloyed Steel and Its Scale Adhesion

The present work investigated the high temperature oxidation behaviour of the micro-alloyed steel and the adhesion of thermal oxide scale to its steel substrate. Oxidation testing was conducted at 815 °C in oxygen without and with 17.9% v/v water vapour. The oxidation kinetics in the two atmospheres were parabolic with similar rate constants, i.e. 1.13 × 10^?9 and 1.17 × 10^?9 g² cm^?4 s^?1 for the sample oxidised in oxygen without and with water vapour, respectively. The XRD peaks for wustite, magnetite, Ti-doped magnetite and titanium carbide were detected for the sample oxidised in oxygen. For the sample oxidised in the humidified atmosphere, Ti-doped magnetite was dominantly observed, additionally with titanium carbide. A tensile testing machine equipped with an optical lens was used to monitor scale failure during straining. For the sample oxidised for 1 min, the strain initiating the first spallation of the steel oxidised in the humidified oxygen was 1.74 ± 0.14%. This strain was higher than the strain initiating the first spallation of the steel oxidised in oxygen which was 1.00 ± 0.04%, indicating the improved adhesion of scale formed in the atmosphere containing water vapour. Mechanisms of water vapour effect on scale adhesion are discussed.     

Oxidation and Hot Corrosion Behavior of Plasma-Sprayed MCrAlY–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr₂O₃ and CoNiCrAlY–Cr₂O₃ coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na₂SO₄–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10^?2 and 1.36 × 10^?2 mg/cm² h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr₂O₄ spinel on the surface, while as-deposited Cr₂O₃ in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr₂O₃ coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr₂O₃ coating was found to be more protective than the NiCoCrAlY–Cr₂O₃ coating under hot corrosion condition.     

Oxidation of Uncoated and Aluminized 9-12% Cr Boiler Steels at 550-650?°C

In this paper, oxidation behavior of 9-12% Cr steels P91 and HCM12A is studied in air and in a mixture of air and water vapor. Comparison is made between these steels in uncoated condition and coated with aluminum diffusion coating by a slurry method. Oxidation tests were carried out at 550, 600, and 650 °C for a discontinued duration of 1000 h; every 250 h the specimens were slowly cooled to room temperature and weighed. SEM + EDS and XRD characterization were performed after 500 and 1000 h. The results showed that oxidation rate of uncoated P91 and HCM12A was significantly higher in the mixture of air and water vapor than in air. Oxidation resistance of the studied materials improved substantially when they were aluminized.     

Cobalt Ferrite in YSZ for Use as Reactive Material in Solar Thermochemical Water and Carbon Dioxide Splitting, Part II: Kinetic Modeling

The kinetics of 10 wt.% cobalt ferrite (CoFe₂O₄) in 8 mol.% yttria-stabilized zirconia, synthesized via the co-precipitation method and formed into a porous structure, are investigated in support of simulating the performance of a solar thermochemical reactor. Kinetic parameters for the thermal reduction (T-R) of CoFe₂O₄ at temperatures of 1325–1500°C were investigated by thermogravimetry. A nonlinear best fit of a uniform conversion model was used to determine kinetic parameters from experimental data. In the temperature range of 1375–1450°C, the activation energy and preexponential term were found to be 386 ± 13 kJ mol^?1 and 8.8 × 10⁹ ± 2.0 × 10⁸ min^?1, respectively, while increasing at higher temperatures. Simultaneous thermogravimetric analysis and differential scanning calorimetry studies showed an increase in the reaction rate of T-R upon the onset of melting (1440°C). Oxidation studies of the material using CO₂ yield an activation energy and preexponential term of 52.1 ± 6.8 kJ mol^?1 and 2.86 ± 0.2 min^?1, respectively, which is in good agreement with past work. The reaction order for CO₂ was determined to be 0.750 ± 0.08. The reaction kinetics for oxidation using CO₂ were best described by a 3-D diffusion Jander model.