Effect of Al on High-Temperature Oxidation of Cr–W Alloys
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- Doğan, Ö.N. Oxid Met (2008) 69: 233. doi:10.1007/s11085-008-9095-0
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The effect of Al on the high temperature oxidation behavior of Cr–10 wt.%W alloy was investigated using a cyclical oxidation test at 1,000 °C in dry air. First, Al was added into the Cr–W alloy as an alloying element up to 8 weight percent. Although alloying with Al reduced the spalling, it did not eliminate it. Secondly, Al was applied to the surface using an aluminizing process. Forming an Al–Cr layer on the Cr–W alloy reduced oxidation rate significantly and eliminated spalling completely.
KeywordsChromium–tungsten alloysHigh-temperature oxidationAluminizing
There is an increasing demand for new materials with higher strength, creep resistance, corrosion resistance, thermal-fatigue resistance, and wear resistance in energy systems to increase generation efficiency, to reduce environmental pollution, and, in some cases, to make new energy-generating technologies economically feasible. New technologies such as ultra super-critical steam plants, integrated-gasification-combined cycle, pressurized fluidized-bed combustion, oxy-fuel turbines, and solid-oxide fuel cells are being developed to meet this demand. One common barrier in the development of these different technologies is the insufficiency of existing materials. Even the highly alloyed and high-cost Ni-base superalloys do not have the desired properties at the temperatures that some parts of the future energy generating systems will be exposed. Therefore, there is an urgent need to develop structural alloys with desirable properties using elements with high melting point (refractory metals).
Refractory metals such as tungsten, molybdenum, chromium, tantalum, and rhenium have high strength at room temperature and the decrease in strength with increasing temperature is gradual compared to the conventional structural alloys such as steels and superalloys . Furthermore, the strength of chromium can be increased with alloying and other strengthening techniques . While the Cr–W alloys are very attractive strength wise, they have limitations with room-temperature ductility and elevated temperature oxidation. The reasons behind the low room-temperature ductility of Cr are not very well understood. Whether low room-temperature ductility arises from intrinsic (due to high Peierls stresses in the BCC structures) or extrinsic (due to grain-boundary segregation of impurities) properties is a matter of debate. There is theoretical and experimental work supporting both points of view [3–12]. Oxidation resistance of some refractory alloys, e.g. Cr and Mo, is low because they form volatile oxide surface layers at elevated temperatures in oxidizing atmospheres [1, 13]. Chromium forms Cr2O3 scale when exposed to oxidizing environments at elevated temperatures; however, the scale spalls under cyclical conditions or significantly oxidized to volatile CrO3 above 800 °C [14–15]. Some type of surface modification or coating technique is needed in order to use these alloys in oxidizing environments at elevated temperatures. Nitridation of Cr and its alloys in air was studied by several researchers [14–17]. Recently, the effect of additions of Fe, La, and MgO to Cr was shown to be effective in reducing the extent of Cr2N formation .
In this study, the effect of Al on the oxidation behavior of Cr alloyed with 10 weight percent W was studied in dry air at elevated temperatures. Aluminum was applied in two ways: as an alloying addition and as a surface coating (aluminizing).
Chemical composition of the experimental alloys (in wt.%)
Oxidation specimens with 25 mm diameter and 2 mm thickness were cut from the ingots. The specimens had a 6.4 mm diameter hole for hanging on a holder. Surfaces of the specimens were ground using 600-grit SiC abrasive paper. After dimensions were measured, the specimens were cleaned in ethanol and weighed before being placed in the furnace.
A thermogravimetric instrument was used for the continuous oxidation experiments. After obtaining a vacuum, dry air was introduced and flowed through the specimen chamber. The flow rate was sufficiently slow so it would not affect the microbalance. The specimens were heated to 800 °C at 5 C/min. and held at 800 °C for 99 h. They were cooled at 5 C/min until this rate was too fast for the furnace to keep up. The rest of the cooling to room temperature was uncontrolled. The mass change was recorded continuously using a data acquisition system. The buoyancy effect over the temperature range which was determined by a dry run (without samples) was subtracted from the data.
Cyclical-oxidation experiments were performed in a tube furnace with a controlled atmosphere. These tests were conducted in a flow of dry air at 1,000 °C. The specimens were placed on a quartz rack in the furnace tube. The specimens were taken out of the furnace at irregular intervals and weighed on a microbalance. They were replaced in the furnace after weighing.
The oxidized specimens were characterized using optical microscopy, scanning-electron microscopy (SEM), X-ray diffraction (XRD), and microchemical analysis using wave-length-dispersive (WDX) and energy-dispersive (EDS) spectroscopy.
Results and Discussion
Base Material Microstructure
As-solidified Cr–10W alloy has a dendritic microstructure of a bcc phase. This phase is a solid solution of W in Cr. The addition of W to Cr increases the lattice spacing in bcc Cr and provides solid-solution strengthening. The addition of up to 8 weight percent Al to Cr–10W did not change the single phase character of this alloy.
Continuous Oxidation of the Cr–10W Alloy
Equation 1 represents a faster oxidation kinetics compared to the previous work summarized in an article by Caplan et al. . This previous work was conducted on unalloyed Cr specimens in oxygen.
The significance of the evaporation of the oxide scale was not deduced from this experiment. However, the stability diagram of the Cr–W–N–O system (Fig. 5) suggests that oxidation of Cr2O3–CrO3 does not take place at 800 °C.
Cyclical Oxidation of Cr–W alloys
Effect of Alloying with Aluminum on the Oxidation Behavior of Cr–W Alloys
Effect of Aluminizing on the Oxidation Behavior of Cr–W Alloys
It should be mentioned that instead of having an aluminum–chromium oxide phase with different Al and Cr ratios in the scale as described above, possibility of having a fine mixture of Cr2O3 and Al2O3 also exists. The XRD results were not conclusive to make this distinction.
These observations suggest that there is significant diffusion of Al, N, and O into the substrate material during the exposure. Outward diffusion of Cr and W is very limited if any.
Continuous oxidation of the Cr–W alloys follows a parabolic rate in dry air at 800 °C. Observations suggest that a mixed-oxide layer composed of Cr2O3 and WO3 forms first upon exposure of fresh alloy surface to dry air. After initial formation of the mixed oxide layer, oxidation reactions take place above and below this layer simultaneously. Above the initial oxidation layer, the Cr2O3 layer grows by diffusion of Cr atoms from the substrate alloy to the scale surface. Below the initial scale, low oxygen activity and high nitrogen activity promotes Cr2N formation.
The oxidation rate of Cr and Cr–10W specimens is dominated by spalling of the oxide scale in large pieces during the cyclical tests in dry air at 1,000 °C.
The oxidation resistance of the Cr–W alloys in dry air at 1,000 °C improves significantly by alloying with Al due primarily to the reduced spalling which takes place in the form of fine particulates instead of large pieces.
Aluminizing improves the oxidation resistance of the Cr–10W alloys significantly. The aluminized specimens show no sign of spallation during the cyclical oxidation test in dry air at 1,000 °C.
I would like to thank Keith Collins and Steve Matthes for the SEM observations and microchemical measurements, David Smith for the XRD results, Glen Soltau for aluminizing the oxidation samples, Ed Argetsinger for arc melting Cr–W alloys. I would also like to thank Dr. Kyei-Sing Kwong for generating the relevant predominance diagrams and Dr. Gordon Holcomb for reviewing the manuscript. All of the above personnel are with the NETL. In addition, I would like to acknowledge the work on the continuous oxidation experiments of Julie Flores of University of Texas at El Paso during her internship on the Mickey Leland Energy Fellowship Program.