TG-mass spectrometry studies in coating design for supercritical steam turbines

Abstract

Ferritic steels in steam turbines for the power industry operate without coatings in the temperature range of 590-600°C. For higher operation temperatures the substrate has to be replaced or coated, otherwise the ferritic substrate at a temperature of 650°C develops thick oxide scales that promote sudden turbine blade failure. The advantage of the use of coatings is that coated ferritic steels are much less expensive than austenitic stainless steels or nickel base superalloys. In order to go forward to coatings design, the Thermo-Calc code was used as a base for the mass spectrometry (MS)-data. Thermogravimetry (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. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA.