Studies on localized electrochemical activity of 304L SS-Zr-4 dissimilar joints using alternating current scanning electrochemical microscopy

Abstract Alternating current-intermittent contact-scanning electrochemical microscopy (AC-IC-SECM) technique is used for the first time for simultaneous mapping of local electrochemical activity and topography of as-polished, passivated and pickled 304L SS-Zr-4 dissimilar weld joint (explosive welded) surfaces in fresh water at open circuit potential (OCP) without redox mediator. The local surface activity showed good contrast and uniformity, even when the surfaces are in tilted condition and topographic resolution is in good agreement with contact and non-contact profilometry. The difference in current and impedance values on SS and Zr-4 surfaces were ∼100 nA and 200 kΩ, respectively. The images acquired on pickled 304L SS and Zr-4 surfaces after immersion in HNO3+HF exhibited a relatively higher current and lower impedance compared to as-polished and passivated surfaces. XPS analysis of Zr-4 pickled surface revealed the presence of ZrOF2, ZrO2 and Zr, while pickled 304L SS showed a higher metallic to oxide ratio, indicating less protective thinner passive films, resulting in a relatively higher electrochemical activity. Though 304L SS in passivated condition revealed Cr-oxide enrichment, the higher metallic concentration on 304L SS, resulted in similar electrochemical activity as in as-polished condition. The quantitative XPS analysis corroborated the local electrochemical activity results obtained from AC-SECM technique.