The General Electric Company, under contract to the Department of Energy, is conducting the Long Term Materials Test at the GE Malta Pressurized Fluidized Bed Combustor (PFBC) test facility. The primary objective of this program is to determine the corrosion resistance of candidate gas turbine vane and blade base alloys and protective coating systems in the effluent from the PFBC. Test specimens are exposed to hot combustion gases at temperatures of 788°-816°C (1450°-1500°F) and velocities of 9 m/s (30 ft/s). Corrosion resistance of the various materials is being evaluated at three metal temperatures: 788°-816°C (1450°-1500°F) uncooled, and 593°C (1100°F) and 704°C (1300°F) using air-cooled hollow pins. In addition, airfoil-shaped specimens are exposed to the PFBC effluent in a high velocity test section at 732°C (1350°F) and 244-274 m/s (800-900 ft/s). This paper only presents results on materials tested in the low velocity test section.

Results of the first 1000-hours of exposure indicate that unprotected nickel and cobalt-base gas turbine blade and vane alloys are susceptible to hot corrosion attack at all three metal temperatures. MCrAℓY (M=Fe, Ni, Co) overlay coatings and aluminide diffusion coatings in general show good corrosion resistance at 816°C (1500°F), but exhibit susceptibility to pitting attack at metal temperatures in the range of 593°-704°C (1100°-1300°F).

The corrosion results agree in many respects with the corrosion observed on CoCrAℓY coated blades in gas turbines used for ship propulsion after extended operation at low power levels. Blade temperatures of 593°-732°C (1100°-1350°F) are associated with low power operation.

The 593°-704°C (1100°-1300°F) corrosion observed in the Malta PFBC test, however, includes an added dimension because the coal and dolomite feedstock to the PFBC contain both sodium and potassium as contaminants, whereas the corrosion of marine turbine blades at low temperatures is associated with sodium alone since it is the principal alkali impurity in sea salt. There is evidence to suggest that the potassium in the coal combustion products enhances the tendency for the occurrence of low temperature corrosion. In addition, iron-based and aluminum-rich coatings also appear to be susceptible to low temperature corrosion attack in the PFBC environment. This has not been observed in oil-fired gas turbines.

Thus, there are some unique features associated with the low temperature corrosion observed in the Long Term Materials Test.

Subject
Coatings, Materials, Gas turbines, Nickel based alloys, Potassium, Corrosion attacks, Aluminides, Sodium, Metals, Pitting, Coal, Corrosion resistance, Deposit corrosion
© 1984 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1984
Association for Materials Protection and Performance (AMPP)
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