Development of localized corrosion damage on low pressure turbine disks and blades. III: Application of Damage Function Analysis to the prediction of damage

Integral damage functions for Type 403 stainless steel (SS) in borate buffer solution containing chloride ion have been experimentally determined. The pits are of an open morphology, thereby rendering effective determination of the damage functions using optical microscopy. Extensive pit coalescence caused the measurement of the damage functions to be less accurate than desired and, in some instances, rendered their measurement impossible. Accordingly, the Deterministic Extreme Value Statistics (DEVS) protocol of Damage Function Analysis (DFA) is adopted as a convenient means of presenting pitting damage in terms of the growth of the deepest pit in the population. A similar analysis is reported for A470/471 steel, where an insufficient population of shallow, pan-shaped pits precludes DFA, which requires a large population. In this case, too, DEVS is used to extrapolate maximum pit depth to a longer time (750 h) after calibrating the model on shorter term data (24 h and 240 h). The experimental maximum pit depth distribution for 750 h is in good agreement with that predicted from the calibrated DEVS, thereby demonstrating the veracity of the method. The maximum pit depth distributions on Type 403 SS and A470/471 steel surfaces do not display the increasing trend with increasing applied potential and chloride concentration. The large depth distribution and/or a large ohmic potential drop inside a pit under a thick adherent layer of corrosion product, probably have "buried" the effects of applied potential and chloride concentration.