The effect of a sulphur rich service environment and process cleaning on crack initiation, propagation and failure mechanisms

Due to increasing operational temperatures within the High Pressure Turbine module, components are experiencing hot-corrosion degradation which can compromise safe service life. Sulphides form as a result of the operational environment and their diffusion into the substrate alloy leads to material loss and the initiation of cracking. At set intervals engine modules are removed from wing and are disassembled for inspection and repair. As part of this process they undergo a cleaning process. This project was defined to identify how this cleaning process interacts with the subsequent re-introduction into service and re-exposure to the hot-corrosion mechanisms. This thesis identifies the effects of surface corrosion and sulphur diffusion on fatigue crack initiation, propagation and final failure mechanisms under a range of different environmental conditions. It is demonstrated that sulphides remain within the substrate following typical cleaning procedures and that the sulphide diffusion rate is directly linked to the thickness of oxide formed on the substrate surface. There was no apparent effect from various combinations of salt and SO2 gas environment on the crack propagation response. Therefore, fatigue behaviour was dominated by initiation mechanisms. Whilst all hot-corrosion environments sampled led to a reduction in low cycle fatigue strength relative to “baseline” performance (i.e. isothermal fatigue conducted in high temperature air) the incorporation of a cleaning procedure helped prolong the fatigue life when compared to exposed, un-cleaned specimens. The correlation between salting/exposure, surface corrosion and sulphide diffusion will be described.