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3684 |
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Thursday, May 22, 2014, Hall H1 2:40 PM Novel Industrial Applications |
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Effect of Al enrichment by pack cementation of FeCr coatings deposited by HVOF |
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Alessandra Bellucci* / Centro Sviluppo Materiali S.p.A., Italy Rosanna Pileggi / Centro Sviluppo Materiali, Italy Domenico Stocchi/ Centro Sviluppo Materiali, Italy Satu Tuurna/ VTT, Finland |
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In order to improve coal sustainability and reducing CO2 emissions, co - firing alternative materials with pulverised coal and using oxy fuel combustion conditions have become very important for power industry. Both these processes lead to more aggressive operative environment which accelerates metal wastage rate. Utilization of coatings for the protection of high temperature components is widely considered among innovative solutions required by advanced service conditions. To withstand such in service conditions, coatings have to fulfil multiple requirements, both in terms of oxidation/corrosion resistance, physical stability and matched coating/substrate properties. In the present paper, a development activity of FeCr alloys deposition by High Velocity Oxy - Fuel (HVOF) on grade 91 steel, a widely adopted material in coal fired systems, is described. The low porosity, uniform fine grain and flat microstructure typically presented by HVOF deposited coatings, make them particularly suitable for applications in aggressive environments, due to the absence of routes for corrosive species penetration. Moreover, HVOF process offers flexibility, cost effectiveness and possibility of application in situ. FeCr alloys can form oxide layer able to act as diffusion barrier in many environments, present high thermal conductivities and thermal expansion coefficient close to the ones of common carbon steel substrates; furthermore their chemical composition can simplify coating/ substrate interdiffusion phenomena. Low (<20wt%) and high (>20wt%) chromium alloys were deposited, and multilayer coatings have been produced by overaluminizing the HVOF coated samples by low activity pack cementation process. Aluminization parameters have been optimized to get aluminium enrichment without modifying substrate microstructure. Laboratory scale corrosion testing activity has been carried out. The role of chromium content in the alloy related to breakaway oxidation onset and the effect of overaluminizing treatment in enhancing coating anticorrosion properties was investigated. |