Study of the passive film on duplex stainless steels and its breakdown by Atomic Force Microscopy and Conductive-AFM

In order to characterize the local electronic properties of the passive film formed on austeno-ferritic stainless steel (Uranus 50), its electric resistance has been measured by using a Current-Sensing Atomic Force Microscope (CSAFM). The surface topography and maps of electric current for a constant voltage bias applied between the sample and the conductive AFM tip, have been simultaneously acquired with an 80-100 nm lateral resolution. The results reveal that the passive film on both phases presents different conductive properties. The passive film covering austenite grains has a high and homogenous electrical resistance (typically 10 GΩ under 1 V). In strong contrast, conductivity maps of the passive layer on the ferrite grains exhibit a high density of conductive spots (few 10-100 MΩ). This suggests that it contains numerous defects resulting from a non-stoichiometric oxi-hydroxide layer. I/V curves acquired on both phases show an asymmetric shape (different from an ohmic behaviour) and are attributed to the electrical characteristic of a passive layer. However the I/V curves measured on the passive film on austenite grains are shifted by 1 V when compared to the curves obtained on ferrite grains. The origin of such differences is discussed in terms of changes in chemical compositions and/or semi-conductive properties of the passive layer growing on austenite and ferrite phases. Besides the CS-AFM measurements, the pitting corrosion process on duplex stainless steel is studied by electrochemical-AFM in chloride solutions. The obtained results show that pits initiate at a highly electric conductive zones: the ferrite/austenite interface and inside a ferrite grains. (authors)