Local electrical characteristics of passive films formed on stainless steel surfaces by current sensing atomic force microscopy

A current sensing atomic force microscope was used to study the topography and the local electronic properties of the passive film formed on a duplex ferrite-austenite stainless steel (Uranus 50). Comparison of current maps with topography AFM images reveals that the passive film covering austenite and ferrite phases exhibits different properties. On freshly formed passive film, a high and homogenous resistance (typically 10 GΩ for 1 V) characterises the film on austenite grains while current maps of the passive layer covering the ferrite grains show a high density of spots (few 100 MΩ for 1 V). Besides the current maps, local I-V curves acquired on austenite show wider band gap energy than the ones obtained on ferrite grains. Finally, the conductivity difference in passive films covering ferrite and austenite grains is discussed.