ZnO/Stearic Acid Composite Coating on 316L Stainless Steel: A Multifunctional Surface with Enhanced Corrosion Resistance and Antibacterial Activity in Ringer Solution

This study presents the development of a multifunctional ZnO/SA composite coating on 316L SS aimed at simultaneously enhancing corrosion resistance and antibacterial performance in Ringer solutions. The coating was fabricated through a two-step process involving electrochemical deposition of ZnO nanolamellae followed by immersion in an ethanolic SA solution. Structural and surface analyses (SEM, EDS, XPS, and contact angle measurements) confirmed the formation of a uniform bilayer consisting of an inner ZnO layer and an outer hydrophobic SA film, with contact angles exceeding 85°. Electrochemical tests in Ringer solution at 37 °C demonstrated that the ZnO/SA coating significantly improved corrosion resistance compared to both bare and single ZnO-coated 316L SS, as evidenced by positive shifts in OCP, higher charge transfer resistance in EIS, and a two-order-of-magnitude decrease in corrosion current density in Tafel experiments. Furthermore, the ZnO/SA film maintained surface integrity and hydrophobicity after four days of immersion, preventing corrosion attack. Antibacterial assays against Staphylococcus aureus revealed that both ZnO and ZnO/SA coatings inhibited bacterial growth, with dynamic tests under stirring conditions confirming effective bactericidal activity driven by Zn ion release. The results demonstrate the synergistic effect of ZnO nanostructures and stearic acid, providing a durable, hydrophobic, and biocompatible coating suitable for advanced biomedical implant applications.