High contrast corrosion mapping of dissimilar metal weld joints using alternating current scanning electrochemical microscopy: A case study with Zr-4–Ti–304L SS weld

Intermittent contact alternating current scanning electrochemical microscopy (IC-AC–SECM) has been used in the present study to image the local electrochemical activity of Zr-4–Ti–304L SS dissimilar weld joint without a redox mediator at an open circuit potential. The perturbation frequency and bias potential applied to the Pt ultramicroelectrode (UME) are found to affect the contrast of IC-AC-SECM imaging. The optimum parameters for high contrast imaging with high sensitivity were determined by performing potentiodynamic polarization and approach curves of Pt UME. The potentiodynamic polarization behaviour of Pt UME carried out in tap water revealed that the Pt UME biased with potential at −0.5 V was ideal for obtaining better sensitivity due to the maximum reduction of dissolved oxygen in the electrolyte. The effect of applied perturbation frequencies on Pt UME examined by the approach curves using the AC-SECM technique showed a transition from negative to positive feedback over 304L SS at about 10 kHz whereas for Zr-4 and Ti the transition was observed at 50 kHz but with only a minor difference in their feedback. Better contrast in mapping local current and impedance magnitude across the weld joint has been achieved by applying an optimum dc bias (–0.5 V) to the Pt tip and perturbation frequency (10 kHz). The IC-AC-SECM results have been compared with the conventional potentiodynamic polarization and electrochemical impedance spectroscopy results of Zr-4, Ti, 304L SS and the weldment in tap water, where the weld sample undergoes preferential corrosion of 304L SS region due to the galvanic coupling. The present work provides new insight into the optimal IC-AC-SECM parameters for best lateral imaging to map electrochemical activity with good contrast across a multilayer weld joint in tap water.