Hot Corrosion of Shipboard Gas Turbine Blades
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Hot Corrosion of Shipboard Gas Turbine Blades K. J. Meisner1 · Elizabeth J. Opila1 Received: 3 April 2020 / Revised: 19 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Turbine blades removed from the first stage of a shipboard gas turbine engine for excessive degradation were characterized. Scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray diffraction, and inductively coupled optical emission spectroscopy was used to characterize corrosion deposits and features of field hardware that are not typically obtained in controlled laboratory settings. Corrosion was associated with deposits of varying compositions on the airfoil, beneath the platform, and within cooling passages. Deposits on the airfoil were primarily sodium sulfate presumably derived from seawater. Deposits below the platform and within cooling channels were crystalline aggregates of Ca, Mg, Al, and Si compounds presumably derived from dust and sand. FactSage thermochemical calculations were performed for gas turbine environments, and results are used to explain variations in deposit chemistry. The results show that solid sodium sulfate may not be retained in some gas turbine conditions, leaving the deposits rich in Ca and Mg compounds. Keywords Hot corrosion · Oxidation · Turbine blade · NiCoCrAlY · Deposits · Salt
Introduction Superalloy components and their coatings are known to be attacked by molten salt species in gas turbines. Gas turbine combustion environments result in accumulation of deposits on hardware such as turbine blades, leading to accelerated high-temperature oxidation/corrosion processes referred to as “hot corrosion” [1]. The deposits are commonly N a2SO4 and other compounds of the alkali metals and alkaline earth metals. One hypothesis is that N a2SO4 forms in gas turbine conditions due to ingested NaCl and sulfur impurities in the fuel and/or environment [1]. Equations 1 and 2 show the reaction of NaCl with SOx and water vapor in the combustion environment to form N a2SO4 and gaseous HCl [2]: * K. J. Meisner [email protected] 1
University of Virginia, Charlottesville, USA
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Oxidation of Metals
1 2NaCl(g) + SO2 (g) + O2 (g) + H2 O(g) ↔ Na2 SO4 (g) + 2HCl (g) 2
(1)
2NaCl(g) + SO3 (g) + H2 O(g) ↔ Na2 SO4 (g) + 2HCl (g)
(2)
Note that while Reactions 1 and 2 are thermodynamically favorable the kinetics are slow, especially considering the short residence times of gases in turbines. Schofield, Steinberg, and Hynes performed spectroscopic and kinetic analyses of Naand S-doped flames under varying conditions. They concluded that Na2SO4 is not a major product; other Na species such as NaOH are dominant [3–5]. Reactor experiments by Hanby [6] suggest that Reactions 1 and 2 cannot form sufficient amounts of Na2SO4 within engine residence times (≤ 16 ms). He hypothesized that N a2SO4 forms via impaction of condensed phase NaCl particulates on turbine components, which remain in place long enough to convert to the sulfate [6].
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