Electrochemically-assisted low power density laser writing on stainless steel via enrichment of chromium oxides

Laser color marking produces nearly permanent, environmentally friendly, vibrant colors on surfaces. However, previous work has used high power density pulsed lasers to induce the physicochemical reactions for marking. Here, we perform laser color marking on stainless steel 304 (SS304) with a less expensive continuous wave (CW) laser and a power density five orders of magnitude below previously reports by combining an electrochemical cell with a fluorescence microscope. Using a combination of optical microscopy, x-ray photoelectron spectroscopy, and bulk electrochemistry, we demonstrate that the laser-induced luminescence and colors are due to enrichment (32±9% increase) of Cr2O3 in the SS304 passive film. We show that the enrichment proceeds by a different physicochemical mechanism than the oxygen pyrolysis that occurs in typical laser color marking. Our technique provides a new pathway for laser color marking of metals in industrial settings with applications as diverse as solar absorbers or corrosion prevention.