Kinetic and Microstructural Analyses of Chromia Forming Alloys during Cyclic Oxidation

Transcription

1 4 Å Journal of Chinese Society for Corrosion and Protection 315 Kinetic and Microstructural Analyses of Chromia Forming Alloys during Cyclic Oxidation N.K. OTHMAN, J. ZHANG, D.J. YOUNG (School of Materials Science & Engineering, University of New South Wales, Sydney 2052, Australia) Contact: Abstract: Three Fe-Cr model alloys (Cr: 9, 17, and 25 wt%) were subjected to cyclic oxidation in Ar-10%H 2-5%H 2O (p o2 = atm), Ar-20%O 2 and Ar-20%O 2-5%H 2O atmospheres at 700. In general, increasing alloy chromium levels reduced the oxidation rate. At high p o2, in the absence of water vapour, the 9%Cr alloy exhibited breakaway oxidation at the beginning of the reaction. However, for chromium levels 17%, no breakaway oxidation occurred, and the weight gain was very low. Adding water vapour to Ar-20%O 2 accelerated the oxidation of alloys containing 17% Cr or less. However, this accelerating effect was not evident for high chromium alloys. At low p o2, breakaway oxidation of alloys with chromium concentration 17% occurred after 300 cycles exposure. High chromium content alloys did not undergo significant oxidation. XRD analysis showed that for low chromium content alloys (Cr 17%), spinel and wustite were formed in the low p o2 gas, while in the high p o2 gas, hematite, magnetite and spinel were produced. In the case of high chromium alloys, only Cr 2O 3 was detected. Metallographic examination showed that the spinel was present in the form of internal precipitates. A dense chromia layer was observed at alloy chromium levels above 25%. The reasons for the difference in oxidation behaviour at different oxygen potentials, and the effect of water vapour are discussed in connection with the microstructure of the oxides. Environmental and Compositional Effects on the Hot-Corrosion Behavior of Ni-Based Alloys and Coatings B. GLEESON (Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261) Contact: bmg36@pitt.edu Abstract: Environmental and compositional factors often influence the hot-corrosion behavior of alloys and coatings used in gas turbine engine applications. This presentation examines the effects of environmental factors such as salt deposition rate, gas composition, and extent of oxidation on the hot-corrosion resistance (Type I-900 and Type II-704 ) of Ni-based alloys and coatings. It also evaluates the effects of Pt-modified b-nial alloy composition on hot-corrosion resistance. Both types of hot corrosion conditions were simulated by depositing Na 2SO 4 salt on the test samples and then exposing those samples to a catalyzed O 2:SO 2 atmosphere. It will be shown that Type I hot corrosion resistance tends to decrease with increase in salt deposition rate up to 8 mg/cm 2 ; however, no measurable hot corrosion was observed when the sample was completely buried in a salt. It will also be shown that the Type II hot corrosion resistance of Pt-modified Ni-based alloys is very sensitive to both alloy and gas composition.

2 316 Æ «29 High-temperature Oxidation Resistant Coatings Composed of YSZ Particles Packaged by Nano-Al 2 O 3 Film Y. D. HE, C. REN, K. ZHANG (Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and Technology Beijing, , Beijing, China) Contact: htgroup@mater.ustb.edu.cn Abstract: The alloy performance to resist high-temperature oxidation is mainly determined either by the formation of oxide scales or by the deposition of oxide coatings on the surface of alloys. In the former case a protective oxide scale, such as Al 2O 3, Cr 2O 3 and SiO 2, can be selectively formed by adjusting the composition of alloys. Even if the oxide scale on the surface of alloys is spalled, new oxide can grow continuously, so the oxidation of alloys can be inhibited effectively. In the latter case, theoretically, depositing oxygen diffusion barrier oxide coatings, such as Al 2O 3, Cr 2O 3 and SiO 2, can prevent alloys from oxidation as well. However, the integrity of these oxide coatings can not maintain for a long time, owing to the thermal stress produced by the difference of the expansion coefficients between the oxide coatings and alloy substrates. Consequently, the ability to resist high-temperature oxidation by depositing such oxide coatings should be disappeared. Therefore, an oxide coating possessed both properties to act as an excellent oxygen diffusion barrier and match a suitable expansion coefficient with alloy substrate, is the basic conditions for the coating to maintain the ability to resist high-temperature oxidation of alloys. Based on above-mentioned idea, a novel composite oxide coating has been developed to obtain two kinds of functions, to act as an oxygen diffusion barrier and to adjust the expansion coefficient of coating as required. Figure 1 shows the schematic diagram of such novel oxide coating. This coating is composed of ZrO 2-8%Y 2O 3 (YSZ) particles packaged by nano-al 2O 3 film. The nano-al 2O 3 film has a bubble-like structure, each YSZ particle is packaged in an Al 2O 3 bubble. Owing to YSZ is a good conductor for oxygen ions, so oxygen diffusion in this composite coating is determined by the walls of Al 2O 3 bubbles or the nano-al 2O 3 film. If the thickness of all walls of Al 2O 3 bubbles in the direction perpendicular to the surface of alloy is over a critical value, the Al 2O 3 bubbles can act as an excellent oxygen diffusion barrier. Owing to YSZ has a high thermal expansion coefficient, so the thermal expansion coefficient of coatings can be easily increased by enhancing the ratio of YSZ to Al 2O 3 in these coatings. Fig.1 Schematic diagram of coatings composed of PSZ particles packaged by nano-al 2O 3 film These composite coatings have been prepared by an improved sol-gel method. Firstly, YSZ particles packaged Al 2O 3 gel film were painted or deposited by electrophoresis on the alloy surface. Then, the specimens were treated by thermal pressure filtration to get primary coatings without cracks. Finally, the primary coatings were sintered in microwave furnace to obtain compact coatings. It is demonstrated from the result of cycling oxidation experiment that both of the hightemperature oxidation resistance and oxide spallation resistance are increased obviously with the ratio of YSZ to Al 2O 3 in the coatings, which are much superior to that of MCrAlY coatings. Therefore, these novel coatings can be used to protect various kinds of alloys from high-temperature oxidation, and

3 4 Å Journal of Chinese Society for Corrosion and Protection 317 can be also served as the interlayer in the thermal barrier coating system. Key words: YSZ-Al 2O 3 composite coating, high-temperature oxidation, oxide spallation Acknowledgments: The authors acknowledge the Chinese National Natural Science Foundation (Grant ) for financial support. Environmental Protection of Gamma Titanium Aluminides by Coatings C. LEYENS (Technical University of Brandenburg at Cottbus Chair of Physical Metallurgy and Materials Technology, Cottbus, Germany and DLR - German Aerospace Center Institute of Materials Research, Cologne, Germany) Contact: Leyens@tu-cottbus.de Abstract: Titanium aluminides have attracted significant attention for automotive and aerospace applications since materials properties have been strongly improved over the last years, and engineers have become more familiar to design with a material that shows limited ductility and damage tolerance. Part of the reason for slow introduction into service is the complexity of materials processing which is attributed to a strong dependency of component properties on alloy chemistry and microstructure. Moreover, titanium aluminides can suffer from substantial degradation caused by interaction with the environment at elevated temperatures, typically beyond 750. The paper will provide an overview of the research and development activities on coatings for protection of gamma titanium aluminides as part of the European Framework Programme project INNOVATIAL led by the author. The paper addresses different coating development strategies followed in this project, such as intermetallic coatings, nitride coatings as well as thermal barrier coatings. It will be shown, that some of the candidates provide good protection against oxidation up to 900. The oxidation results indicate that in addition to coating chemistry the deposition process plays a major role in improving properties. Here, a new HIPIMS ( High Power Impulse Magnetron Sputtering) process has demonstrated its capability to provide coatings with very good adhesion and high density that can effectively protect titanium aluminides against environmental attack. Effect of Water Vapor on the Oxidation of Pt-modified Ni-22Al Alloys S. HAYASHI, M. AUCHI, T. NARITA, B. GLEESON (Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Japan) Abstract: Cyclic oxidation behavior of two-phase γ+γ -Ni-22Al-5 and -10 Pt alloys in air with and without 30 vol.%h 2O at 1000 was investigated. The oxidation of 5 and 10 Pt alloys in air was initially rapid and followed by a slow steady-state stage. The oxidation during this steady-state stage was followed the parabolic rate law, and the parabolic rate constants for the two different Pt content alloys were similar. However the mass change of 5 Pt alloy during the initial rapid oxidation stage was much higher than that of 10 Pt, and it resulted in higher oxidation mass gain on 5 Pt alloy after 500 cycles. Similar oxidation behavior, initial rapid followed by slow steady-state, was observed when the alloys were oxidized in air+30%h 2O, however the initial oxidation mass change in the wet atmosphere was much higher for both alloys than in air. Interestingly, the oxidation rate constants for both alloys during the steady-state

4 318 Æ «29 stage significantly decreased in the atmosphere with water vapor. Particularly, oxidation mass gain on the 5 Pt alloy was not observed after about 100 cycles of oxidation. Thin single layer Al 2O 3 scale was formed on both 5 and 10 Pt alloys in air after 4 cycles of oxidation, and after 500 cycles it changed to a thin duplex scale, with an outer spinel and inner Al 2O 3 layer. A similar scale structure was observed on the 10 Pt alloy, even when the alloy was oxidized in air+h 2O, but the Al 2O 3 layer that formed during short time exposure was thicker in air+h 2O than in air. Moreover, NiO formation was also observed above the substrate γ phase in air+h 2O. Thick multilayered oxide scale, with a thick outer NiO and inner NiAl 2O 4+Pt mixture layers, was developed after just 4 cycles of oxidation on 5 Pt oxidized in air+h 2O. Between the inner mixed oxide layer and substrate, thin continuous layers of NiAl 2O 4 and Al 2O 3 layer were formed as the innermost layers. The thickness of these continuous layers increased with increase in oxidation cycles, but change in the thickness of outermost NiO was not observed after 500 cycles of oxidation. These results indicate that water vapor significantly affects the oxidation of low aluminum Ni-based alloys. In particular, water vapor increases the critical Al content in the alloy for exclusive Al 2O 3 scale formation. In this presentation, these effects of water vapor will be discussed in more detail. On the Selective Oxidation of Novel Electrodeposited Nanocomposites Distributing Cr or/and Al Nanoparticles X. PENG (State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences) Contact: xpeng@imr.ac.cn Abstract: Using an electrochemical deposition method, nanoparticles of Cr or/and Al were assembled into a nanocrystalline metal template to fabricate novel metal-matrix nanocomposites. The nanocomposites would be easier to develop protective scales of chromia or alumina during high temperature exposure, based on a selective oxidation model proposed for these types of nanocomposites. That is, the nanoparticles in the near-surface zone of the nanocomposites would convert into chromia or alumina islands at the onset of oxidation. Then, the nano length scale distribution of the nanoparticles, together with the nanograined metal-matrix structure, would favor a rapid linkage of the oxide islands to form a continuous layer of chromia or alumina. Using the oxidation model, we can explain the oxidation behavior of various nanocomposite systems such as Ni-Cr, Ni-Al, Ni-Cr-Al, Cu-Ni-Cr, etc., by comparing to those of conventionally coarse-grained alloys of similar composition and the electrodeposited composites dispersing conventional micron-sized particles instead of equivalent weight of nanoparticles. Oxidation and Coatings for Power Generation: New Super Ultra Critical Steam Turbines F. J. PÉREZ (Grupo de Investigación de Ingeniería de Superficies y Materiales Nanoestructurados U. C. M. N Facultad de Ciencias Químicas Universidad Complutense de Madrid Madrid Spain) Contact: fjperez@quim.ucm.es Abstract: The steels with chromium contents between 9% and 12% wt are used for power plants with

5 4 Å Journal of Chinese Society for Corrosion and Protection 319 advanced steam oxidation conditions. They present good creep properties as 9% Cr steels and good oxidation resistance, at the temperatures range between In the last years numerous investigation in development of coatings has been realized with the aim the protected them against the oxidation in order to allow operation of steam turbines at 650. In this study, Al-Hf protective coatings were deposited by CVD-FBR on ferritic steel HCM-12A followed by a diffusion heat treatment, have shown to be protective at 650 under steam for at least 3000 hours of laboratory steam exposure under atmospheric pressure. Morphology and composition of coatings were characterized by different techniques, such as scanning electron microscopy (SEM), electron probe microanalysis, and X-ray diffraction (XRD). On the other hand, volatile species by mass spectrometry (MS) and thermogravimetric measurements (TG) during the initial stages oxidation of the P92 ferritic steel without and of Al and Al/Si coating by chemical vapour deposition in fluidized bed reactors (CVD-FBR) at 650 in Ar+20%H 2O for 150 h were studied. In order to go forward to coatings design the Thermocalc code is used as base for the MSdata. TG-MS experiments were conducted in a closed steam loop in order to obtain information about the oxyhydroxides formation as reaction between coatings and steam. From those results the role of the different coating element could be established and optimized for the coating durability. An oxidation mechanism based on the TG-MS results is given. The morphology/composition and structure of the oxidized samples were also studied using SEM/EDS and XRD techniques. Key words: steam oxidation, mass spectrometry, volatile species and protective coatings. iron aluminides Continuous Monitoring of Oxygen Chemical Potential on the Surface of Growing Scales and Its Application to High Temperature Oxidation of Metals T. MARUYAMA, K. AKIBA, M. H. BIN ANI, M. UEDA, K. KAWAMURA (Department of Metallurgy and Ceramics Sciences Graduate School of Science ande Engineering, Tokyo Institute of Technology S8-11, O-okayama, Meguro-ku, Tokyo , Japan) Contact: maruyama@mtl.titech.ac.jp Abstract: There are two kinetic regimes in high temperature oxidation of metals and alloys. One is the parabolic kinetics in which the ionic diffusion through oxide scales is the rate-determining step. The other is the linear kinetics in which rate determining process is the supply of oxidant through the gaseous boundary layer. The oxygen chemical potential is almost the same as that in the atmosphere in the former case, but in the later case, the oxygen chemical potential is much lower than that in the atmosphere. The oxygen chemical potential at the surface of scale is a good measure of the protective character of the scale. The novel technique was developed for continuous monitoring of the oxygen chemical at a surface of a growing oxide surface in high temperature oxidation of metals and alloys. The technique was based on an electrochemical oxygen concentration cell constructed with a stabilized zirconia, of which electromotive forces is generated by the difference in oxygen chemical potentials between the surface of scales and the bulk atmosphere. During the oxidation of nickel and cobalt at 1373 K in Ar-21%O 2 gas, the oxygen chemical potential at the surface of formed NiO scale and CoO scale were slightly lower than that in the atmosphere. It is because the growth of these oxide scales are mainly rate-limited by the diffusion of the constituent ions in the scales. In the case of FeO scale formed at 1373 K in Ar-1 %CO-5 %CO 2 gas, the oxygen chemical potential at the surface was a much lower than that in the atmosphere and the measured potential was a little larger than that of Fe/FeO equilibrium. This technique was applied to the oxidation of Fe-Cr alloys with various Cr contents at 1073 K in air.

6 320 Æ «29 The oxygen chemical potential at the scale surface was close to that in air if the protective Cr 2O 3 scale was formed, but in the case that the scales were the less protective iron oxides and iron-chromium spinel, the oxygen chemical potential at the scale surface was much lower than that in air. This technique provides the quick determination of the formation of protective external Cr 2O 3 oxide in the high temperature oxidation of alloys. Thin Oxide Film Growth during Final Annealing of Stainless Steels A. GALERIE 1, M-T. TRAN 1, A. LOUCIF 1, Y. WOUTERS 1, S. GONZALES 2 (1. SIMaP Laboratory, Grenoble INP, BP 75, Saint Martind Hères, France; 2. ArcelorMittal Stainless Europe, Research Centre, BP 33, Isbergues, France) Contact: alain.galerie@ltpcm.inpg.fr Abstract: After the cold rolling sequence of the production line, stainless steels strips present highly disordered structure and have to be annealed to recover their crystalline structure. This operation takes place by passing the moving strip through a furnace heated by combustion of natural gas with little air excess. Although the time at high temperature is short (< 1 min), the formed surface oxide is thick enough to modify the surface aspect of the steel, making necessary subsequent chemical pickling. The present work explores the oxidation step of various stainless steels during final annealing in order to better understand the mechanisms involved and make further improvements in the pickling sequence. Several austenitic and ferritic grades were tested in a laboratory device simulating the industrial annealing: same temperature (900 for 430 non-stabilized ferritic, 1060 for 441 stabilized one, 1120 for 304 and 316L austenitics), same atmosphere (73% N 2, 16% H 2O, 8% CO 2, 3% O 2) and various durations between 15 s and 3 min). Oxide characterization by XRD, Raman spectroscopy, GDOS and photoelectrochemistry detected initial corundum-type phase (Fe,Cr) 2O 3 with Cr/Fe ratio increasing with time and temperature. Surface enrichment in spinel phase (Mn,Cr) 3O 4 was observed to increase with time. Minor element oxides were also detected and analyzed. The kinetic results showed linear oxide thickening (constant rate) due to slow oxygen transfer at the oxide-gas interface. In addition, mass determination evidenced high chromium (VI) vaporization during the first instants at high temperature, decreasing with the concurrent increase of surface spinel phase. The influence of Ti, Nb and Mo minor additions in the steels is also described and discussed.