Scalable surface structuring of stainless steel plate heat exchangers using chloride-based chemical etching for enhanced two-phase heat transfer

Heat exchangers play a key role in the functioning of industry, and improving their thermal-hydraulic effectiveness is essential for optimizing energy efficiency. This study develops a scalable passive surface modification method for stainless steel plate heat exchangers using static chloride-based chemical etching. The etching process chemically modifies the surface via Cl⁻ and H + -induced passive film breakdown, Fe 3+ -driven oxidation, and Cu removal, producing hierarchical micro/nanoscale surface structures. Thorough analysis of etching reaction mechanisms and the resultant microstructures are obtained by quantifying micro and nanostructured surface morphologies using scanning electron microscopy, and focused ion beam sectioning. Additionally, X-ray diffraction, X-ray photoelectron spectroscopy, and energy-dispersive X-ray spectroscopy were performed to evaluate the surface chemistry prior to and after etching. Evaporation tests were performed on unetched and etched Cu-brazed stainless steel 304 plate heat exchangers with surface roughness values of 0.8 μm and 5.2 μm, respectively. Flow boiling experiments with R-134a refrigerant revealed that the etched heat exchangers achieved a 54% enhancement in flow boiling heat transfer coefficient compared to their unetched counterparts, particularly at low vapor qualities. The enhancement is suggested to arise from the combined effects of increased roughness, enlarged effective surface area, modified wettability, and micro/nanoscale cavities that can serve as candidate vapor-entrapment and bubble-nucleation sites. Qualitative bubble visualization on etched coupons extracted from the BPHX plate material further supports enhanced surface-assisted bubble generation. Because chloride-based etching intentionally modifies the passive film of stainless steel, long-term durability, corrosion resistance, and fouling behavior require further evaluation. These findings highlight chemical etching as a rapid and scalable route for improving two-phase heat transfer in compact stainless steel heat exchangers.