A study of the relation between non-metallic inclusions in carbon steels and its pitting behavior is presented. Using a hybrid Nano-Auger microscope , a comprehensive characterization of MnS inclusions and their bulk/metal interfaces was performed. Both bulk and surface sensitive techniques were employed to obtain chemical and structural information at nanometer length scales. We show that MnS inclusions are complicated multiphase defects with surface properties distinct from the bulk. The data from this study are used to provide insight into the observed electrochemical behavior during the localized corrosion of steel.