In situ ECSTM investigation on formation and breakdown of passive film for polycrystalline stainless steel

The dynamic behavior of surface topography for the stainless steel in 0.5M H 2SO 4+ 0.02M NaCl was traced in situ by electrochemical scanning tunneling microscope (ECSTM). The results showed a different response for the integrated passivated domain and pit-prone location on the surface when a given potentials was applied to the sample. The roughness parameter Sa for both domains appeared a maximum when controlled potential in the transition passivation region. In the potential region of passivation, passivated grains began to grow orderly on the integrated domain, and Sa decreased with increasing potential. While on the pit-prone domain, metastable micro-pits were observed when a passive potential was controlled at 0.5 V and 0.8 V. The impedance behaviors for steel sample in the different potential regions of passivation were examined by electrochemical impedance spectroscopy (EIS) and discussed based on the ECSTM results. At 0.2 V potential, the flawless surface showed the greatest impedance, and at 0.5 V, the impedance droped due to the initiation of metastable micro-pit. When controlled potential at 0.8 V, the surface roughness decreased considerably on the integrated domain, but the total impedance showed smaller resulted from the development of micro-pit in the preference location