Wettability control of stainless steel surfaces via evolution of intrinsic grain structures

Wettability control of stainless steel surfaces has wide applicability due to the extensive use of stainless steels in industries that handle liquids. However, previous studies to achieve hydrophobicity on stainless steel surfaces rely on artificial roughness by added particles and residue formation that could compromise mechanical stability of the modified surface. In this study, water-repellent stainless steel surfaces were fabricated via the controlled topographical enhancement of intrinsic grain structures to achieve the required surface roughness. Electrochemical etching was used to control the relative etch rate between grains and grain boundaries by applying an anodic potential to stainless steel 316 (SS316). At low potential (1.2 V vs. Saturated Calomel Electrode), high selectivity between grain and grain boundary etching was observed, which resulted in significant etching at grain boundaries and smooth top grain surfaces. Morphologies with significant microscale roughness, and slight nanoscale roughness, were achieved with intermediate applied potentials (1.3–1.5 V). As the potential increased further (1.8 V), etch selectivity decreased and yielded microscopically smooth surfaces with sponge-like nanoscale roughness; at even higher potentials (2.4 V), electro-polishing was observed with surfaces that are smooth at the nanoscale. Based upon the relationship between anodic potential and topography, we designed a nano/microscale hierarchical SS316 surface by a two-step electrochemical etching combining low and high anodic potentials. The hierarchical SS316 surface resulted in a water contact angle of 137.5 ± 5.0°. Plasma-assisted deposition of a fluorocarbon film further reduced the wettability, with a contact angle of 163.9 ± 1.2° and a roll-off angle of 10.7 ± 1.8° for 4 μL water droplets, thereby rendering the modified SS316 surfaces superhydrophobic. Due to the inherently robust intrinsic grain structure of SS, we expect that this simple approach to generate water-repellent SS316 surfaces will display improved mechanical robustness relative to previous approaches.