MODIFICATION OF STAINLESS STEEL SURFACES FOR ADVANCED FUNCTIONALITIES
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SUMMARY
In industry, business, and homes worldwide, metal alloys called stainless steels are used extensively. Stainless steel is a generic term for a large family of iron-based alloys containing a minimum of 10.5% chromium content by weight. The primary alloying element, chromium, forms a thin passive layer of chromium oxide/hydroxide when exposed to air and moisture, which is dense and impervious to water and air, thereby protecting the bulk metal beneath the surface layer. Stainless steel alloy composition, along with surface properties such as surface chemistry and topography, significantly impact the reliability and performance of stainless steels in their myriad applications. Typically, a smooth stainless steel surface with chromium enrichment is desired because of the enhanced corrosion resistance it displays. Recently, much attention has focused on controlling both surface structure and chemistry to achieve advanced functionalities such as liquid repellency, anti-bacterial, and anti-fouling properties. Most such studies have been conducted on soft materials like paper/cellulose and polymers. However, the extensive and varied applications of stainless steels often require exposure to complex and harsh environments such as corrosive liquids, seawater, food and beverages, and body fluids. As a result, considerable interest has developed in stainless steel surface modifications to achieve advanced functionalities beyond corrosion resistance alone. In this thesis, studies are described in which stainless steel surfaces are modified to control hydrophobicity, corrosion resistance, and anti-bacterial properties. Among the many different types of stainless steels, stainless steel 316L(SS316L) was chosen as the starting xvii material because of its extensive use in many industries, including maritime, bio-medical, and infrastructure applications. Metals and metal alloys are composed of grains with different orientations; grain boundaries represent interfaces between the grains. These grain boundaries have high interfacial energy and relatively weak atomic bonding, which renders them more susceptible to etching or dissolution under certain conditions than the grains themselves. The etching at different rates can therefore results in surface roughening. Here, electrochemical etching has been employed to modify SS316L surfaces. Variation in the applied anodic potential during electrochemical etching altered the etch selectivity between grains and grain boundaries, thereby producing various surface structures on SS316L. A relationship between the applied anodic potential and SS316L surface topography was established, which served as the basis to control roughness and thereby develop advanced functionalities. A super-hydrophobic stainless steel surface offers benefits in many applications including efficient fluid transport in pipes through drag reduction and effective drainage and cleaning of storage tanks. Generation of a hierarchical structure that combines nanoscale and microscale surface roughness is critical to achieve super-hydrophobicity. Based upon the established relationship, a two-step electrochemical etching process was developed for SS316L by consecutively using two different applied anodic potentials; this sequence accentuated both nanoscale and microscale roughness on SS316L surfaces. Deposition of a thin fluorocarbon layer on the hierarchically structured SS316L yielded superhydrophobicity with a static contact angle of 163.9 ° ± 1.2 and a roll-off angle of 10.7 ° ± 2.9 with 4 L water droplets. xviii Despite the corrosion resistance of stainless steel surfaces, they are not fully corrosionproof in corrosive environments such as seawater. Seawater is a solution with approximately 0.6 M sodium chloride and diverse maritime bio-organisms, which trigger local breakdown of passive layers and cause localized corrosion. In order to improve the localized corrosion resistance, electrochemical surface modification was conducted on SS316L with an anodic potential of 1.3 V vs. SCE, which resulted in a microscale grain boundary etched surface. Corrosion tests performed in 0.6 M sodium chloride solution displayed superior corrosion resistance with a narrow distribution of high breakdown potentials (0.96 to 1.05 V vs. SCE) compared to that of as-received SS316L (0.32 to 0.86 V vs. SCE). In addition, the grain boundary etched SS316L exhibited hydrophobicity with a static water contact angle of 135.7° ± 2.6. This unique combination of hydrophobicity and microscale topography on the SS316L surface offers the potential to reduce adhesion of marine organisms, which further deters localized corrosion and improves the reliability of this material in the maritime industry. Pathogenic bacterial adhesion on stainless steel surfaces is problematic because the existence of bacteria on implants, surgical tools, and biomedical devices can cause infectious diseases. Nanoscale surface topography can reduce bacterial adhesion by generating repulsive forces for bacteria and by inducing stress to the cell membrane where bacteria are attached. Nano-textures were achieved on SS316L surface by the electrochemical surface modification at an anodic potential of 2.2 V vs. SCE. E.coli, a gram-negative bacterium was used to investigate bacterial adhesion on SS316L, and a significant reduction in E.coli adhesion was observed on the nano-textured SS316L surface compared to an as-received SS316L surface. However, no difference in adhesion and xix metabolic activity of fibroblast (NIH-3T3) cells between nano-textured and as-received SS316L surfaces was observed. Therefore, the nano-textured SS316L surface can reduce bacterial adhesion while maintaining adhesion and biocompatibility with desired cells. A patterned surface with dissimilar wettability has many potential applications such as diagnostic sensors and microfluidic platforms. Copper is a relatively cheap engineering metal due to its abundance. A simple method to fabricate a hydrophobic pattern on superhydrophobic copper surface was devised as a proof of concept. A commercially available marker pen was utilized to directly draw patterns on copper surface, which served as masking layer, thereby growing nanowire selectively on background region in an aqueous solution. Fluorocarbon film deposition on the copper surface resulted in hydrophobic pattern/super-hydrophobic background copper surface. Diverse patterns including dot, line, and curve were attained on copper surface to demonstrate droplet manipulations.
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