Using of transpassive dissolution for detection of structural changes in 9Cr-1Mo creep resistant steel

The development of structural changes during long term service is a serious problem of heat resistant steels due to formation of deleterious Cr- and Mo-rich phases in the structure. Since chromium and molybdenum are easily soluble by transpassive mechanisms, electrochemical polarisation in appropriate electrolyte could be used as method for detection of such phases. Transpassive behaviour of creep resistant steel 9Cr-1Mo (P91) in different states of heat treatment/long term aging was studied in this work. Materials were studied by means of electrochemical methods, OES analysis and SEM observation. Potentiodynamic curves of tempered and aged (650 degrees C/10 000 h) samples in NaOH solution show a significant local peak in transpassivity/secondary passivity region. No peak was observed on polarisation curve of quenched sample. Formation of the local peak on polarisation curve is assumed to be caused by dissolution of Cr- and Mo-rich phases by a transpassive mechanism. This assumption was confirmed using potentiostatic etching, chemical analysis of electrolytes and microstructure observation.