Imaging Local Surface Reactivity on Stainless Steels 304 and 316 in Acid Chloride Solution using Scanning Electrochemical Microscopy and the Scanning Vibrating Electrode Technique

•Direct screening of differences in the resistance against breakdown of the passive layers formed on steels by microelectrochemical methods.•Imaging domains of similar topography but different corrosion characteristics.•Distinguishing iron soluble species released from propagating pits.•Distinct effect of polarization on the onset of pit nucleation for different stainless steel compositions. Passive film breakdown and pit nucleation on 304 and 316 stainless steels in chloride-containing media were investigated using scanning microelectrochemical microscopy (SECM) and the scanning vibrating electrode technique (SVET). Experiments were performed for the alloys either at their corresponding open circuit potential, or under applied polarization, as to image domains of similar topography but different resistance against breakdown of the passive layers formed on these steels. Identification of the iron released species was accomplished, showing that pitting occurs with the formation of iron (II) species only. Detection of iron (III) species occurred when the steel sample was polarized at high positive overpotentials because it served as the reaction site to oxidize the iron (II) ions released from a propagating pit. The obtained results have revealed some difference of the reactivity of both specimens. Moreover the effect of the galvanic coupling has been investigated.