The local breakdown of the iron passive layer and the resulting pitting corrosion were studied with a coupled scanning electrochemical microscopy/ electrochemical quartz microbalance technique (SECM/EQCM). The initiation and the propagation of a single pit on iron deposited on the quartz gold electrode were controlled through the local production of chloride anion thanks to the silver chloride reduction at the tip of the scanning electrochemical microscope. The frequency response of the quartz was correlated to the pit evolution with respect to time. After the breakdown of the passive layer, the frequency changes were then directly linked to the generation of Fe (II) during the pit growth. It was also shown that in 0.1 M NaOH solution, the local amount of Cl-necessary to initiate the pit was about 0.15 M. In addition to the presence of the oxidized Fe3O4 passive layer and corrosion products like Fe2O3 5H(2)O, beta-FeOOH, and Green Rust, Raman analysis evidenced the formation of Fe (OH)(2) inside the pit. Moreover, the influence of the pit size was also studied by varying the amount of chloride ions generated close to the iron substrate. (c) 2006 The Electrochemical Society.