Densification of zirconia-based anti-corrosion coatings for application in waste-to-energy plants

Müller, Dirk (2017): Densification of zirconia-based anti-corrosion coatings for application in waste-to-energy plants. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Corrosion of functional parts (e.g. boiler tubes) within waste-to-energy (WTE) plants significantly reduces their efficiency with respect to maintenance costs. Currently, nickel based alloy claddings, several millimeters thick, are the state of the art as anti corrosion coating. Another approach is to utilize thermally sprayed multilayer coatings with a zirconia based top coat, e.g. yttria stabilized zirconia (YSZ). Lab scale experiments and in situ tests within a WTE plant were conducted by using a multilayer coating of a nickel based bond coat and an YSZ top coat, applied onto a steel substrate. For lab scale experiments WTE plant operation conditions were simulated within a tube furnace through an atmosphere of N2-HCl-O2, enriched with additional volatile gas phases emerging out of an equimolar KCl-K2SO4-ZnCl2-ZnSO4 salt mixture, which was placed beneath the sample in the high temperature zone of the furnace at 500 - 700 °C. Under an atmosphere composition of N2 - 2.1 vol. % HCl - 0.9 vol. % O2, a partially reduced porosity of the zirconia top coat can be achieved, where former pores are filled up with zirconia. The addition of ZrO2 powder into the salts and the use of additional sodium salts enhance the densification of the zirconia top coat. The zirconia top coat of the in situ experiments at 450 °C and 700 °C also show a partial densification, whereas the pores are filled up with newly formed metal (chromium, nickel, iron) oxides. Since the formation of low melting eutectic salt melts on the surface of those boiler tubes is a common process within WTE plants, the performance of YSZ under these conditions was evaluated, too. Multilayer coated samples were impregnated by a salt solution and heat treated under various mixtures of argon, nitrogen, gaseous hydrochloric acid, sulfur dioxid and / or oxygen at 700 °C. No densification was observable, but an element migration of mainly iron from the steel substrate into the coating and nickel and chromium from the bond coat into the steel. Furthermore, YSZ beads were embedded in an equimolar mixture of KCl-K2SO4-ZnCl2-ZnSO4. At temperatures of 600 °C and 700 °C under an atmosphere of N2 - 2.55 vol. % HCl - 0.45 vol. % O2, a dissolution and recrystallization of the zirconia was observed. Both processes, the densification of the zirconia top coat after a heat treatment under an HCl-O2 bearing atmosphere and the ability of a zirconia recrystallization within salt melts enables the zirconia based coating to act as a barrier against aggressive gases, hence, to represent a possible anti corrosion coating for steel tubes within WTE plants.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: zirconia, corrosion, waste-to-energy plant, hot corrosion
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 6. Februar 2017
• 1. Bericht­erstatter:in: Heuss-Aßbichler, Soraya