Evolution of wettability in Ni based metallic hydrophobic coatings

Hydrophobic metallic coatings have garnered significant scientific and industrial interest due to their exceptional mechanical durability and extensive application potential. A critical yet not fully understood phenomenon associated with certain metallic surfaces is the spontaneous and reversible transition from an initial hydrophilic state to a hydrophobic state over time. This study aims to elucidate the physicochemical mechanisms underlying the time-dependent wettability transition. The Ni based alloy coating, characterized by an intricate, rough hierarchical structure, was successfully fabricated on 304 stainless steel (SS) substrates through electrodeposition. The temporal evolution of the coating's wetting behavior was systematically monitored, confirming a transition from hydrophilicity to hydrophobicity following aging in ambient air. To disentangle the underlying factors contributing to this phenomenon, plasma cleaning was employed to systematically remove adsorbed airborne hydrocarbons from the coating surface. Comparative X-ray photoelectron spectroscopy (XPS) analysis of the surfaces before and after plasma treatment revealed a substantial reduction in the atomic ratio of airborne hydrocarbon groups (from 79.2 % to 62.7 %), accompanied by a notable increase in lattice oxygen content. These findings demonstrate that the observed wettability transition is a multifaceted process arising from synergistic effects involving both physical adsorption of environmental hydrocarbons and concurrent chemical changes occurring on the coating surface. The interaction between adsorbed hydrocarbon molecules and lattice oxygen has been identified as a key driver for the gradual evolution of surface wettability. Collectively, these insights enhance the understanding of dynamic interfacial behaviors exhibited by metallic coatings and establish a fundamental theoretical framework for rational design and optimization of stable hydrophobic metallic surfaces.