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Advanced materials for green aviation
Published in Emily S. Nelson, Dhanireddy R. Reddy, Green Aviation: Reduction of Environmental Impact Through Aircraft Technology and Alternative Fuels, 2018
Recently, as disk temperatures have increased from ~648°C to ~704°C, hot corrosion has surfaced as a new environmental degradation mode (Gabb et al., 2009). Hot corrosion is due to the deposition of salts—multicomponent sulfates—on the disk alloy surface. Figure 5.31 shows a hot corrosion pit that has been observed in service. The pitting morphology in the figure resembles typical Type II hot corrosion, which was identified as a new corrosion phenomenon in the 1970s for marine gas turbine engines (Luthra, 1982). There is a significant reduction in fatigue life due to the formation of hot corrosion pits, as shown in Figure 5.32. One approach to mitigating the effect of hot corrosion is to apply a protective coating. The key challenge for any coating is that it must be thin and ductile so that the fatigue life of the disk is not adversely affected by the coating.
Contemporary Methods of Protection and Restoration of Components
Published in E. S. Gevorkyan, M. Rucki, V. P. Nerubatskyi, W. Żurowski, Z. Siemiątkowski, D. Morozow, A. G. Kharatyan, Remanufacturing and Advanced Machining Processes for New Materials and Components, 2022
E. S. Gevorkyan, M. Rucki, V. P. Nerubatskyi, W. Żurowski, Z. Siemiątkowski, D. Morozow, A. G. Kharatyan
Due to harsh work conditions, turbine blades are usually covered with coatings that display various properties like oxidation and hot corrosion resistance, strength maintenance at elevated temperatures, etc. Shourgeshty et al. (2016) divide high-temperature damage into three general groups in line with temperature ranges: High-temperature corrosion type II (600–850°C), where sulfates are formed from the substrate at a certain partial pressure for sublimation of sulfur trioxide. The sulfate reaction with alkali metal forms low-melting-point particles that prevent formation of a protective layer.High-temperature corrosion type I (750–950°C), includes transportation of sulfur from a deposit (sulfate base like Na2SO4) through an oxide layer into a metal substrate resulting in formation of stable oxides. After a reaction between a stable sulfide and sulfur moving through a scale, the base metal sulfides form a disastrous sequence in the molten phase due to high temperatures. Thus, formation of NiS2 (molten at 645°C) and CoxSy (lowest liquids at 840°C) can lead to serious component degradation. The most suitable materials which can resist type I hot corrosion are PtAl2-(Ni-Pt-Al) coatings (aluminide coatings modified with platinum) and MCrAlY coatings containing up to 25 wt% Cr and 6 wt% Al.Oxidation (950°C and higher), which depends on transportation of cations or anions through the structure of an oxide layer and grain boundary. In order to form a continuous oxide layer for cobalt base superalloys, Cr content should be at least 25%. To increase oxidation resistance of chromia, addition of aluminum is preferred, especially for severe and critical conditions experienced by gas turbine blades. However, when thermal cycling conditions prevail, oxide scales can spall from a substrate surface as a result of thermally induced stresses. In the event, the oxidation resistance can be improved by addition of reactive elements to alloys and coatings, such as Y, Hf, and Ce.
Effect of burnishing on HVOF coated boiler steel against hot corrosion in actual boiler environment
Published in Surface Engineering, 2023
Atul Agnihotri, Sukhminderbir Singh Kalsi, Harmesh Kansal
Burnishing is known to be a surface finishing technique in which the material is subjected to loading where the asperities are hard-pressed to fill the valleys thereby reducing the pores thus yielding a smooth surface [20]. In this study, the effect of burnishing on thermal spray coatings against the high-temperature aggressive environmental application of coal-fired boilers is critically examined. This study also aims to develop a suitable protective burnished-coated material system to enhance the life of metals and alloys operating under a hot corrosion environment. Hot corrosion is caused by the diffusion of substrate components into the coating and the passage of corrosive ambient species through the coating to the substrate. The bonding strength of the coating reduces when corrosive species interact with an underlying metal. Therefore, it is very important to reduce the porosity in the coating for better corrosion resistance [21]. Coatings with post-heat and sealing treatment often have better erosion-corrosion resistance [22]. Although many post-treatment procedures have been used, their effectiveness in preventing hot corrosion has been tested, but no research on burnishing coated specimens has been published in the open literature. Hence, burnishing as an innovative post-treatment approach is used in the research work to reduce the porosity of thermal spray coating.