Nanosecond Pulsed-Laser Treatment Couples Chloride Removal with Oxide Transformation in Salt-Corroded Carbon Steel

Abstract: Maintaining carbon steel in marine environments requires surface treatments capable of simultaneously removing corrosion products and chloride contaminants whilst modifying the residual oxide layer. In this study, salt-contaminated SS400 carbon steel was treated using a Q-switched pulsed fibre laser characterized by a full width at half maximum (FWHM) of approximately 150 ns and a decaying tail extending to about 600 ns. The pulse energy was fixed at 1 mJ, and the repetition frequency was varied between 50 and 200 kHz to investigate the effects of cumulative thermal accumulation. Under 100 W (100 kHz) conditions, WDS-EPMA analysis confirmed that Na and Cl levels dropped to near-background values, demonstrating the comprehensive removal of sea-salt-derived contaminants. SEM observations revealed the transition of the porous rust layer into a dense, laser-modified layer, whilst XPS analysis confirmed the suppression of the $\text{Fe}^{3+}$ satellite feature in the $\text{Fe}$ $2p$ spectrum, establishing a distinct phase transformation from a haematite-dominant rust to a protective magnetite-rich scale. These results elucidate a single-step, two-stage mechanism: the high-peak-power leading edge of the pulse drives the ablation of the corrosion layer, whilst the 600 ns trailing tail delivers continuous thermal energy that promotes oxide resolidification and phase transformation. This approach offers a promising, non-contact methodology capable of concurrent decontamination and surface functionalisation in a single processing step.