Abstract Pitting corrosion of mild steel in its normal nonpassivated condition (neutral chloride solutions) was studied using a scanning microprobe potentiometer. This apparatus measures potential distributions on a corroding metal surface and records the information in the form of a potential contour map as a function of time. Using these maps along with scanning electron microscopy, pit development was studied. Three stages in the pitting process were recognized: (1) pit initiation, (2) pit coalescence, and (3) pit propagation. Pit initiation is shown to depend on the presence of metallurgical heterogeneities in the steel surface. The most effective initiation sites are sulfide inclusions, their behavior being dependent on their size and also the density of neighboring sulfide inclusions. Sulfides which are anodic with respect to the matrix initiate micropits which develop primarily by metal dissolution. Following the initiation stage, pit coalescence may occur resulting in the formation of macropits. I...