Sulphide Diffusion in Nickel Based Superalloys
C.D. Ball1, P.M. Mignanelli2 and M.R. Bache1
1 Institute of Structural Materials, College of Engineering, Bay Campus, Swansea University, Swansea, SA1 8EN, United Kingdom.
2 Rolls-Royce plc, PO Box 31 Derby, DE24 8BJ, United Kingdom.
When exposed to a high temperature corrosive environment, nickel based superalloys may experience surface pitting and sulphide diffusion. Sulphur, pre-existing in the environment or as a bi-product of burning fossil fuels, reacts with sodium (as an atmospheric pollutant) creating sodium sulphate, which forms molten deposits on the metal surface. Combined with sodium chloride, these deposits attack the protective oxide layer allowing for sulphides to migrate within the material along grain boundaries. At the same time, alloying elements (including titanium, aluminium and chromium) are concentrated within a rich layer at the substrate surface. Continued sulphide diffusion may promote a weakened subsurface layer, in turn subject to grain dropout and the initiation of fatigue cracks.
Previous work has characterised this process of hot corrosion pitting in detail. However, the subsequent effects of longer term exposure to air, SOx and SOx plus salt atmospheres, together with the impact of intermediate cleaning processes require further attention. The present paper will address these additional factors, demonstrating the evolution of corrosion layers, associated pits and quantifying the grain boundary sulphide diffusion depth. SEM imaging, optical surface profiling and EDX analysis were all utilised to identify sulphide migration and diffusion depths.
Acknowledgements
The research was funded by the EPSRC Rolls-Royce Strategic Partnership in Structural Metallic Systems for Gas Turbines (grants EP/H500383/1 and EP/H022309/1). The provision of materials and technical support from Rolls-Royce plc is gratefully acknowledged. Mechanical testing was performed by Swansea Materials Research & Testing (SMaRT) Ltd.