Abstract
In the absence of protective scales, nickel-base superalloys have an extremely limited hot corrosion incubation period before increased rates of attack are experienced. This paper reports on the nickel-base superalloys: CMSX-4, CM247LC DS and IN6203DS subjected to 550 °C hot corrosion exposures of durations ranging from 0 to 800 h, during which none of the superalloys developed a fully protective scale. The aim of the research was to identify the incubation period of each superalloy and this was achieved by means of surface roughness evaluations. A metrology exercise was performed on the cross section of test specimens which produced Cartesian data points which were subsequently converted to Ra and Rz data. Statistical analysis of the results suggested the incubation period lasted approximately 400, 500 and 200 h, respectively, for each superalloy. It was concluded that refractory metal phases within the microstructure were associated with the relatively short IN6203DS incubation period. This paper demonstrates that monitoring the changes in surface roughness provides a plausible method to identify the transition from incubation to propagation when studying 550 °C hot corrosion attack.
Highlights
A power generating industrial gas turbine (IGT) comprises three sections: compressor, combustion and turbine [1, 2]
Hot corrosion can be considered a subset of deposit-induced corrosion [1, 2] with the former recognised in aero, marine and IGTs, while the latter is found in many combustion-based power generation systems
For this form of hot corrosion, a reaction between the deposits, the S O3 and the oxides of the superalloy may occur which leads to a low melting point deposit forming [5]
Summary
A power generating industrial gas turbine (IGT) comprises three sections: compressor, combustion and turbine [1, 2]. An accumulation of deposits in the solid state, providing SO3 is present within the hot gas stream, may trigger an attack [1, 2] For this form of hot corrosion, a reaction between the deposits, the S O3 and the oxides of the superalloy may occur which leads to a low melting point deposit forming [5]. It can be NiO reacting with S O3 to form N iSO4 which further reacts with the Na2SO4 to form a Na2SO4:NiSO4 sulphate system with a lowest melting point of 671 °C [1]. For types I and II hot corrosion with a liquid sulphate, a fluxing mechanism dissolves any protective scale from the component surface [1, 2, 4, 5, 7, 8, 10, 11], thereby allowing rapid attack of the underlying superalloy as sulphur diffuses inwards [3, 4, 7]
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