The scanning electrochemical microscope (SECM) is used to generate localized corrosion at passivating iron surfaces by using the tip to generate Cl- ions. Use of the SECM allows the rapid establishment of a locally aggressive chemical environment at a preselected site on the iron surface. The susceptibility for passive layer breakdown and corrosion initiation was examined as a function of the time between the start of the passive layer growth and the formation of Cl- ions. The breakdown of the passive layer was found to depend strongly on the passivation potential and the site of Cl- formation on the iron surface. In addition to generating Cl- ions, the SECM tip was simultaneously used to detect large iron ion concentration fluctuations as corrosion began. Current fluctuations at the tip were observed and ascribed to precursors to the passive layer breakdown.