In this paper, an ion micro dispenser (IMD) was used to initiate a single pit by generating chloride anions above a 316L stainless steel electrode in either H 2 SO 4 or HClO 4 electrolyte.The current variations with respect to time provided an unambiguous characterization of the single pit evolution. Different pit shapes were observed depending on both the nature of the electrolyte and potential applied to the electrode. Substituting SO 4 2-for ClO 4 -gave smaller (in diameter) but deeper pits at the early stage of pitting. However, when using a different setup that allows the sustaining of the pit propagation with a continuous supply of Cl -, the deeper pits were observed in HClO 4 rather than H 2 SO 4 . The formation of an iron sulphate salt film at the bottom of the pit by precipitation of dissolution products in H 2 SO 4 slowed down the corrosion rate. At high potentials, the repassivation mechanism outweighed the metal dissolution in the ClO 4 -solution containing solution.

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