Pitting corrosion of passive metals is a classic issue in corrosion field. Over the past decades, the mechanism of pitting corrosion has attracted corrosion community striving to study. However, the mechanism at the pitting initiation stage is still controversy, due to the difficulty encountered in obtaining precise experimental information with enough spatial resolution. Using transmission electron microscopy, we focus on the structure of matrix and the nature of passive film, which are closely related to pitting initiation. This work clarifies the structural origins of the pitting initiation and unmasks the mechanism of chloride-induced passivity breakdown. At the pitting initiation stage, the heterogeneous corrosion activity of inclusions/second phase in matrix, and the preferential attack of the passive film are originated from the atomic-scale structural defects. This work enriches, improves and even modifies the classical understanding on the pitting initiation mechanism.