Nano-scale chemical analyses of passivated surface layer on stainless steels

A surface analysis technique using scanning transmission electron microscopy (STEM) combined with a unique needle shape specimen is proposed. The unique needle shape specimen is effective to reduce matrix convolution as well as improve peak to background ratio of spectroscopy. To reveal the performance of the proposed technique, HNO₃ passivated surface layer on a type 436L ferritic and a type 304 austenitic stainless steels were analyzed. The results demonstrated that the proposed technique is able to quantify the distribution of alloying elements in the passivated surface layer approximately 3 nm in thickness, namely, the partitioning behaviour of alloying elements, i.e., Cr, Ni, Mo and Si in the passivated surface layer is revealed and agreed with that confirmed by Auger Electron Spectroscopy analysis. This proposed technique has potentialities for the analysis of nano scale local inhomogeneity, for example, pitting initiation process.