Nanoscale fluorescence microscopy maps cathodic pitting corrosion reactions on carbon steel

Fluorescent molecules that are sensitive to pH, metal ions, or electron transfer have recently been studied as real-time indicators of local metal corrosion. However, the fluorescence imaging systems used in previous studies were diffraction-limited and thus did not provide information below the single-micron scale size of a corrosion pit. We employ a state-of-the-art fluorescence microscope and single-molecule localization microscopy (SMLM) to access the nanoscale positions of the corrosion-sensitive fluorescent molecule resazurin within one corrosion pit. We expose A109 carbon steel to a chloride solution containing resazurin. Resazurin will accept electrons and become strongly fluorescent resorufin. We then map the positions of these fluorescence "turn-on" events relative to a corrosion pit. We complement our SMLM with reflection microscopy, scanning electron microscopy energy dispersive spectroscopy (SEM-EDS), and optical profilometry to correlate traditional methods of assessing pitting corrosion rates with our new technique. Our fluorescence microscopy quantifies the rate of electron transfer to the cathodic site within metastable pits that form in tens of minutes. The reaction rates observed by fluorescence correlates to sites with 30% increased oxygen content via SEM-EDS, depths of 0.5 um via profilometry, and 50% increases in pit diameter from reflection microscopy. Our work enables real-time observation of the cathodic pitting corrosion reaction rate on the nanoscale. In the future, this technique will allow us to examine how anode and cathode corrosion reactions are coupled on the nanoscale and test different proposed reaction pathways for pitting corrosion.