The local breakdown of the iron passive layer and the resulting pitting corrosion of iron were studied for massive material and for a thin film on an electrochemical quartz crystal microbalance (EQCM). The initiation and the propagation of a single pit on iron were controlled through the local production of chloride anion thanks to the silver chloride reduction at the tip of the scanning electrochemical microscope (SECM). Experiments were performed in borate buffer and in KOH solution, in order to investigate the influence of the solution pH. The influence of the iron potential and of the amount of chloride anion necessary for the breakdown of the passive layer were also investigated. 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.