High temperature corrosion in gas turbines : thermodynamic modelling and experimental results

The introduction of new materials as well as the improvements in fuel quality have raised a need for re-evaluation of the hot corrosion risk in industrial gas turbines. In this study, the risk of hot corrosion was determined using thermodynamic modelling for different impurity-contents and combustion parameters. Based on these results, the parameters for the corrosion tests have been selected to investigate the response of the different materials under corrosive conditions. The corrosion risk arises if the surface temperature of a material is below the dew point of a corrosive salt and condensation can take place. The dew point of different salts has been calculated in the thermodynamic modelling, and the impurities S, Na, K and Cl were added in typical amounts for low-impurity fuels. This study has taken into account the variation of combustion parameters, such as excess air and injected water, as well as the C to H ratio of the fuel, to simulate gas and oil firing. The main influencing parameter on the dew point of the alkali-sulphates is the alkali-content in the hot gas. The amount of sulphur and chlorine as well as the combustion parameters have only a small impact. Based on this approach, the maximum surface area with a risk of corrosion on each component can be evaluated for the hot gas path. The corrosion behaviour of three base materials, IN738, CM247 and CMSX-4, and a NiCrAlY-coating, SV20, were tested between 750 and 950°C. A salt-spraying test was used with either Na2SO4 or Na2SO4/K2SO4 as a deposit in an atmosphere of air with 300ppm SO2. Both salts gave the same corrosion morphology. The addition of K2SO4 has led to an increased depth of attack and shorter incubation times. The carbides and heavy element contents, especially tungsten and molybdenum, play an important role in the hot corrosion of the investigated base materials. The carbides, in IN738 and CM247, were identified as preferential sites for the initiation of hot corrosion. Furthermore, type I corrosion has been observed for the base materials at temperatures as low as 750°C, and the mechanism proposed is characterised by an initial basic dissolution of the oxide scale through Na2SO4, followed by a change to a self-sustaining alloy-induced acidic fluxing due to the presence of W and Mo. The present study has shown a new approach to assess the risk of hot corrosion in gas turbines by the combination of thermodynamical modelling and corrosion experiments