Integrated Scanning Electrochemical Probes Assisted by ECSTM for In Situ Imaging of the Pitting Corrosion Process of Carbon Steel in SCCP Solutions

Pitting corrosion of steel is a common phenomenon featuring a random and dynamic process. An understanding of the mechanism and origin of pitting initiation in the early stage remains ambiguous because it is difficult to sense the pitting activity, topographical variations, and local environment at or around pits of carbon steel in situ in a simulated carbonated concrete pore (SCCP) solution. In this work, integrated scanning electrochemical probes assisted by electrochemical scanning tunneling microscopy (ECSTM) were developed to concurrently image the local topography, local pH, and pitting activity in situ in the vicinity of the pits. Ex situ surface topographies and element contents were also determined by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). It was found that the pitting initiated at a high rate when carbon steel was immersed in SCCP solution, and then the pitting merged to form beam-like localized corrosion and uniform corrosion finally. The local pH became lower in the vicinity of the pits during the pitting initiation. The results demonstrated that the nuclei of pitting corrosion occurred close to the MnS inclusion or ferrite phase at the steel surface due to selective phase dissolution. Integrated scanning electrochemical probes enabled us to obtain correlated information about the pitting activity, topography, and local environment around pits, which is powerful for the in situ imaging of pitting corrosion with higher resolution and for further understanding the mechanism of localized corrosion of carbon steel in various service environments.