In situ scanning probe microscopy (SPM) is used to characterize passivation of steel alloys [DIN 1.4301 and 1.4529 (DIN is the German characterization number, comparable to AISI numbers)] as well as pure iron in 0.01 M sulfuric acid and boron buffer solution (pH 8.4), respectively It is shown, that using d(ln i)/ds spectroscopy semiconducting properties of the growing passive layer can be investigated.