ICMCTF2005 Session A1-1: Coatings to Resist High Temperature Corrosion and Wear

Monday, May 2, 2005 10:30 AM in Room Sunrise

Monday Morning

Start	Invited?	Item
10:30 AM	Invited	A1-1-1 Coating Design for High Temperature Applications Using Computational Codes for Steam Oxidation Conditions F.J. Pérez (Universidad Complutense de Madrid, Spain) Ferritic steels in steam turbines for the power industry is operating without coating in the temperature average of 590-600°C. For higher operation tempratures the susbtrate need to be change or the susbtrate need to be coated, otherwise the ferritic susbtrates at temperatures of 650°C develop thick oxide scales that promotes suddent turbine blades failure. The advantage of the use of coatings is that the other substrate candidates, such as austenitic stainless steels or nickel base superalloys are much more expansive that the coated ferritic steels. In order to go forward to coatings design Thermocalc anc Dictra codes were used to predict at different pressures and temperatures, and the average of coating composition that are able to resist long period of operation times. The coating design is based on the reaction of the steam onto the coating surface and the interdiffusion among the susbtrate and the coating. The influence of different elements such as Si, Cr, Al, Fe, Mn, W are established, joinint with the phases formed and the interdifussion species after operation time. A description of the algorithm and the "coating maps" are provided.
11:10 AM		A1-1-3 Long Exposure Steam Oxidation Testing and Mechanical Properties of Slurry Aluminide Coatings for Steam Turbine Components A. Agüero (Instituto Nacional de Tecnica Aéroespacial (INTA), Spain); S. Osgerby (NPL Materials Centre, United Kingdom); R. Muelas, A. Pastor (Instituto Nacional de Tecnica Aéroespacial (INTA), Spain) Important efforts to develop new steels or to protect high creep strength steels in order to allow operation of steam turbines at 650°C are being carried out world-wide in order to increase efficiency. Within the European Project "SUPERCOAT" (Coatings for Supercritical Steam Cycles), work has been concentrated in the development of coatings to withstand 50,000-100,000 of operation at 650°C under high pressure steam. Slurry aluminide coatings on ferritic-martensitic steels have already shown to be protective at 650°C under steam for at least 30,000 h of laboratory steam exposure under atmospheric pressure. Both high pressure laboratory testing as well as field exposure have confirmed the laboratory atmospheric pressure testing results for shorter exposures. Microstructural characterization of samples at different periods of exposures has been carried out by SEM-EDS and XRD. Moreover, physical and mechanical properties of "as coated" specimens were measured at ambient and elevated temperature employing a range of techniques. The properties that were investigated were Young´s Modulus, fracture strength and thermal cycling resistance.
11:30 AM		A1-1-4 Effect of Si in High-Temperature Oxidation Behavior of Hot-Dip Aluminized Carbon Steel C.J. Wang, S.-M. Chen (National Taiwan University of Science and Technology, Taiwan) SB450 carbon steel was coated by hot-dipping into molten bath containing pure Al, Al-7wt%Si, Al-11wt%Si respectively. Oxidation and thermal-cycling were studied at 750 and 850°C in static air. After hot-dip treatment, all the coating layers consisted of three phases, where Al, FeAl₃, Fe₂Al₅ were detected from external topcoat to the aluminide/steel substrate. The addition of Si in molten Al bath was responsible for the formation of thinner intermetallic layer and smoother interface between aluminide layer and the steel substrate. After high temperature exposure, a continuous alumina scale with some SiO₂ was detected on the surface of the aluminide layer. The thickness of aluminide layer was about 100 µm in each cycle test, while voids resulting from Kirkendall effect formed at the interface between the aluminide layer and the steel substrate. The addition of Si in the coating layer was beneficial for improving the oxidation resistances of the coated steel. Keywords: SB450 carbon steel, Hot-dip, Si, aluminide, Fe₂Al₅, Kirkendall effect.
11:50 AM		A1-1-5 Microstructure and Phase Study of the Hot-Dipped Aluminide Coatings on 430 Stainless Steels H. Ching, J.W. Lee (Tung Nan Institute of Technology, Taiwan); C.J. Wang (National Taiwan University of Science and Technology, Taiwan) The 430 stainless steels have been hot dipped in the molten Al-6.5 Si alloy from 680°C to 800°C, respectively. The aluminized steels were then heat treated at 850°C for 1 to 81 hrs. The effects of the hot dipping temperature and time on the thickness of the alloying layer have been investigated. The X-ray diffractometer and electron probe microanalyzer were adopted to explore the phase transformation and chemical composition of the aluminide coatings. The FeAl₃ and Fe₂ Al₅ phases were formed by the inward diffusion of hot dipped aluminum. Complex phase transformation caused by the inward and outward diffusion of aluminum and other components were found on the aluminide layer after heat treatment at 850°C.
12:10 PM		A1-1-6 Cyclic Oxidation Behavior of Aluminide Coating on the Co-Base Superalloy AMS 5608 J.W. Lee (Tung Nan Institute of Technology, Taiwan) Aluminization has been employed on alloys to form dense and adhesive Al₂ O₃ surface layers to prolong their service life at high temperature. Cobalt-base superalloy AMS 5608 was used to deposit high aluminum containing surface layers at 950°C for 9 hours by pack cementation process. The cyclic oxidation tests of aluminized alloys and untreated matrix were conducted at 1100°C for 196 hours. It is found that the cobalt aluminide phase ranging 60 µm in thickness was achieved. Dense Al₂ O₃ surface layer was spalled on the aluminized alloys after cyclic oxidation test at 1100 °C for 196 hours. The phase transformations and microstructure phenomena of aluminized coating layers and matrix of alloys were studied with X-ray diffractometer and electron probe microanalyzer. It was observed that cyclic oxidation resistance of superalloy was enhanced by aluminizing processes. However, the inward and outward diffusion of aluminum on the surface layer showed detrimental effect to the cyclic oxidation resistance of alloy tested for long time.