Morphology-driven white coloration on stainless steel via underwater nanosecond laser processing

Conventional laser color marking of stainless steel predominantly relies on laser-induced oxide formation, resulting in wavelength-selective, interference-based coloration that inherently limits the achievable color space. In particular, the generation of white surfaces remains challenging due to the intrinsic dependence of oxide films on selective optical interference and oxide-related chromatic contributions. In this work, we demonstrate an underwater nanosecond laser processing route that overcomes this limitation by altering laser–matter interactions. The liquid environment suppresses oxidation while promoting controlled surface modification. Colorimetric measurements reveal a pronounced increase in surface lightness and indicate a diminished specular contribution and a transition toward a more matte surface under optimized processing conditions. Reflectance spectra show enhanced reflectance levels with a flat, broadband response across the visible range. Raman spectroscopy, together with elemental analysis, indicates that oxidation is significantly suppressed relative to processing in ambient air, while scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical imaging reveal the development of rough, porous surface morphologies induced by underwater laser processing. Collectively, these results show that the observed white coloration originates from morphology-driven diffuse reflection without reliance on oxide-based interference mechanisms. This study establishes underwater laser processing as an effective route for tailoring surface optical response and surface appearance in metallic materials, offering new opportunities for industrial laser surface engineering.