The Study Of The Marine Atmospheric Corrosion Protection Mechanism Of A Superhydrophobic Surface

Marine atmospheric corrosion is a significant issue threatening the safety of marine facilities.Developing novel and efficient anti-corrosion technologies is a critical direction of the current research.From the perspective of the mechanism of atmospheric corrosion,the existence of water films/droplets on the metals provides the essential condition for the corrosion reactions.Accordingly,active removing the water film/droplets is a useful method to restrain corrosion.Superhydrophobic surfaces can inhibit the formation of surface water film/droplets.Our research group has proved that deliquesced Na Cl particles slip off an inclined superhydrophobic surface under gravity,thereby removing the corrosive media on the surfaces and inhibiting corrosion to some extent.However,the “lotus effect” of superhydrophobic surfaces only functions in external forces.Recent studies on some well-designed superhydrophobic surfaces have demonstrated that small droplets can undergo coalescence-induced droplet jumping behavior.Such coalescence-induced droplet jumping behavior on superhydrophobic surfaces,which is different from the “lotus effect”,is an autonomous motion that removes droplets without an external force and offers a new route for atmospheric corrosion protection by reducing the droplet residence duration and surface coverage.To date,the applications of coalescence-induced droplet jumping behavior have concentrated on condensation heat transfer,electrostatic energy harvesting,selfcleaning and anti-frosting applications.The possible applications of such functionalized behavior in atmospheric corrosion protection,however,have not been reported yet.In this study,we aim to reveal a novel atmospheric corrosion protection mechanism based on coalescence-induced droplet jumping behavior by studying the correlations of the surface structure(including the structure size,the structure feature,the complex structure,and the surface orientation),droplet jumping behavior,and atmospheric corrosion resistance of the superhydrophobic surface with a different structure.This study not only enriches the theory of atmospheric corrosion protection but also provides a theoretical basis for the development of atmospheric corrosion protection technologies based on coalescence-induced droplet jumping behavior,which has potential application value in the field of atmospheric corrosion protection.The main findings of this work are listed as follows.(1)Two kinds of superhydrophobic surfaces,namely,the nanostructured superhydrophobic surface and the microstructured superhydrophobic surface,were rationally fabricated over a zinc substrate with a hydrothermal method combined with the subsequent modification process.The effect of the microstructure size on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the two surfaces were studied.The results demonstrate that the nanostructured superhydrophobic surface realized coalescence-induced droplet jumping behavior due to the decreased solid-liquid contact area and interfacial adhesion,while the microstructured superhydrophobic surface realized droplet coalescence without droplet jumping behavior as a result of the increased solid-liquid contact area and interfacial adhesion.Electrochemical results demonstrate that the nanostructured superhydrophobic surface with coalescence-induced droplet jumping behavior presents a superior atmospheric corrosion resistance than the microstructured superhydrophobic surface without coalescence-induced droplet jumping behavior.This is because the coalescence-induced droplet jumping behavior of the nanostructured superhydrophobic surface offers a possible mechanism to switch the droplets from a partial wetting state to the mobile Cassie state,and this switch is critical for facilitating the recovery of the air film trapped in the microstructure of a surface.In particular,the recovered air film enhances the atmospheric corrosion resistance of a superhydrophobic surface due to its barrier-like character.(2)Two kinds of superhydrophobic surfaces,namely,the sheet-like structure superhydrophobic surface and the cluster-like structure superhydrophobic surface,were rationally fabricated over the copper substrate by the different solution-immersion process followed by the same low surface energy materials modification process.The effect of the microstructure feature on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the two surfaces were studied.The results demonstrate that the sheet-like structure superhydrophobic surface realized coalescence-induced droplet jumping behavior,while the cluster-like structure superhydrophobic surface realized droplet coalescence without droplet jumping behavior.Based on the study of the effect of water contact angle and surface roughness on the energy equations,a droplet jumping phase map that divided the jumping and non-jumping regions as a function of water contact angle and surface roughness is formulated from the perspective of energy.The exhibited phase map illustrates that a higher water contact angle and a lower surface roughness are more favorable to droplet jumping behavior due to a lower solid/liquid contact area and interfacial adhesion.And the critical water contact angle and surface roughness for coalescence-induced droplet jumping behavior are given.The results of the droplet jumping phase map are consistent with the experimental results and previous reports.Due to the droplet jumping-induced wetting transition,the sheet-like structure superhydrophobic surface with droplet jumping behavior exhibits a superior anti-corrosion performance than the cluster-like structure superhydrophobic surface without droplet jumping behavior after the simulated fog experiments.(3)Three kinds of superhydrophobic surfaces,namely,the microstructured superhydrophobic surface,the nanostructured superhydrophobic surface,and the complex superhydrophobic surface,were rationally fabricated over a zinc substrate with a respective acid etching method combined with the subsequent modification process,the hydrothermal method combined with the subsequent modification process,and the acid etching method and hydrothermal method combined with the subsequent modification process.The effect of the complex microstructure on the coalescenceinduced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the three superhydrophobic surfaces were studied.The results demonstrate that the nanostructured superhydrophobic surface and the complex superhydrophobic surface realized coalescence-induced droplet jumping behavior,while the microstructured superhydrophobic surface realized droplet coalescence without droplet jumping behavior.Due to the reduced solid-liquid contact area and interfacial adhesion,the existence of the nanostructure is an important factor affecting the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the surfaces.Due to the droplet jumping-induced wetting transition,the nanostructured superhydrophobic surface and the complex superhydrophobic surface with droplet jumping behavior exhibit a superior anti-corrosion performance than the microstructured superhydrophobic surface without droplet jumping behavior after the condensation experiments.(4)Based on the droplet jumping phase map,the knife-like structure Cu O superhydrophobic surface was rationally fabricated over a copper substrate by a facile chemical oxidation method combined with the subsequent low surface energy materials modification process.The effect of the surface orientation on the coalescence-induced droplet jumping behavior and the subsequent atmospheric corrosion resistance of the surface were studied.The results demonstrate that the horizontally oriented superhydrophobic surface exhibits a bigger droplet size distribution and surface coverage during the condensation process as a result of the droplets falling back to the surface.And compared with the vertically oriented superhydrophobic surface,the horizontally oriented superhydrophobic surface exhibits a better corrosion resistance after condensation.This is because the existence of the mobile Cassie state droplet on the top of the microstructure of the horizontally oriented superhydrophobic surface can occupy some surface sites and inhibit the condensation inside the microstructure,thereby leading to reduced water permeation and further enhancement of the atmospheric corrosion resistance.