Improved localized corrosion resistance of Alloy 600 by underwater laser peening: Microstructural and electrochemical evidence for the roles of oxide continuity and recast-layer integrity

The effect of underwater laser peening (ULP) on the localized corrosion resistance of Alloy 600 in deaerated 3.5 wt.% NaCl solution was investigated through a comparative study of four surface conditions: polished, heavy grinding (HG), single-pass ULP (ULP(x1)), and two-pass ULP (ULP(x2)). ULP was performed at a beam intensity of 4∼5 GW/cm 2 and a pulse density of 64 pulses/mm 2 without an ablative layer. While single-pass ULP provided initial improvements over the HG surface, the two-pass ULP treatment resulted in a superior electrochemical response, characterized by a significant noble shift in the breakdown potential ( : from −95 mV to +216 mV vs. SCE) and a marked increase in charge-transfer resistance ( > 10,000 kΩ·cm 2 ). Cross-sectional analyses revealed that the two-pass treatment facilitated the formation of a more continuous Cr-rich surface oxide and the reconstruction of a less porous recast layer. These features act as an effective barrier, thereby suppressing metastable pitting and stabilizing the passive state. It is found that Cr-rich oxide continuity and recast-layer physical integrity are the primary governing factors controlling the electrochemical corrosion behavior of ULP-treated nickel-based alloys in chloride environments.