Microstructure and fracture mode of a martensitic stainless steel steam turbine blade characterized via scanning auger microscopy and poteniodynamic polarization

The microstructure, the fracture and the resistance to pitting corrosion, in a10-2M (NaCl + Na/sub 2/SO/sub 4/) solution, of steam turbine blades, made of martensitic stainless steel (12% Cr), were analysed prior and after their utilization (during about 100 000 hours). The unused blades display an inter-granular fracture mode whereas a trans-granular one emerges in used blades. The SAM analysis of the fractured surfaces reveals that this change is concomitant with carbon and chromium redistribution from the grain boundaries to the grain interior. The potentiodynamic polarization curves and the SEM-EDS analysis of the pitting sites show that the used blades are less susceptible to pitting corrosion than the unused ones. These results are interpreted as a further qualitative evidence of an evolution of the microstructure leading to the formation of new precipitates. (author)