Characterization of pH Effect on Corrosion Resistance of Nuclear Steam Generator Tubing Alloy by In-situ Scanning Electrochemical Microscopy

The effect of pH on the corrosion resistance of Alloy 800, one of the preferred nuclear steam generator tubing materials, was investigated using in-situ scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). The experimental results show that positive feedback is observed in the probe approach curve (PAC) in acidic chloride solutions, indicating that Alloy 800 is active in acidic solutions; the EIS at the corrosion potential in acidic solutions exhibits an intact capacitance arc. However, negative feedbacks are observed in the PAC in either neutral or basic chloride solutions, showing that Alloy 800 is self-passivated in these two solutions. The EIS plots at different anodic potentials show incomplete capacitance arcs, and the arc radius decreases with increasing potential, indicating that the corrosion resistance of the passive film decreases. The SECM images show that the surface reactivity increases (or the dissolution rate of the passive film increases) as the polarization potential increases from the corrosion potential to the positive direction; this is verified by an increased tip current. Some "active spots" can be seen on the SECM images in neutral or basic chloride solutions, which are possibly related to grain boundaries, triple points, and/or inclusions.