Microelectrochemical methods and scanning tunneling microscopy (STM) are powerful techniques to study localized corrosion processes on passive metals in the mu m- and nm-range. On commercial stainless steels pit nuclei are initiated due to the oxidation and dissolution of inclusions, even in chloride-free environments. Microelectrochemical techniques allow to perform in situ studies on the electrochemical behaviour of single inclusions. In presence of chlorides additional nucleation sites are activated. It is presently assumed that most of these pit nuclei also originate from small inclusions. Chlorides also accelerate and stabilize pit growth.