Abstract
Stainless steel compressor blades coated with erosion-resistant coatings undergo abrasion during service, and on cooling, salt aerosol deliquescence leads to the formation of thin electrolyte films on the blade surface, resulting in a galvanic system in the vicinity of the damage due to the coating being usually nobler than the substrate. Corrosion resulting from this galvanic interaction is seen as pits, which can lead to fatigue cracks.
This study focused on evaluating the localized corrosion damage observed on UNS S17400 stainless steel compressor blades removed from active service and developing an accelerated laboratory testing procedure that permits the rapid replication of similar damage. The accelerated exposure testing was effected by utilizing aggressive solution chemistry and ozone which acts as a surrogate for all oxidizers found in natural environments. Testing was initially carried out in a compact chamber in order to determine effective levels of these parameters in order to extend the experiments to modified standard lab-accelerated procedures in fog-exposure chambers. The ASTM G85 A2 acidified salt spray technique was modified to investigate the effects of cyclic corrosion testing on pristine samples and the nature and the extent of resulting corrosion morphology.
Finally, the electrochemical separation of anode and cathode that results from the galvanic interaction was leveraged to extend a model to predict the maximum pit size that can develop on the stainless steel surface once the knowledge of the environmental conditions are known. The results from the model when compared to the pitting morphology are expected to help rationalize the pitting morphology observed on the blades removed from service as well as the samples tested under lab-accelerated conditions.
