Using superhydrophobic surfaces to reduce marine corrosion

Marine atmospheric corrosion – the effect of seawater and sea air in coastal areas – is costly to our societies in terms of safety, economics, functionality, and cultural heritage. To deal with marine atmospheric corrosion, the current general protection measures mainly aim to improve the corrosion resistance of the material itself, or to isolate the base from the corrosive medium, such as alloying, organic coating, metal coating, etc. As science and technology develops faster, people have increasingly higher standards for what they will accept in terms of corrosion protection technology. The limitations of traditional protection methods are becoming increasingly prominent; such as the toxicity of organic coating and the pollution of metal coating. Therefore, it is of great practical significance to develop novel and efficient marine atmospheric corrosion protection materials.

From the perspective of the mechanism of marine atmospheric corrosion, marine atmospheric corrosion is a kind of electrochemical corrosion formed by rain, fog, dew, and salt deliquescence (the process of becoming liquid by absorbing moisture from the atmosphere). The existence of liquid film/droplets is an important prerequisite for the occurrence of the corrosion reactions. Therefore, actively stopping the formation of liquid film/droplet on the material’s surface is an effective means to solve atmospheric corrosion problem.

Superhydrophobic surfaces (SS) can effectively prevent the formation of liquid film/droplets on the surface of the materials. Our research group has confirmed the marine atmospheric corrosion protection mechanism is based on the ‘lotus effect’ – named after the lotus flower whose leaves have a self-cleaning mechanism whereby the dirt particles are picked up by water and roll off. Studies have confirmed that raindrops and deliquesced salt (NaCl) particles can easily roll off an inclined SS, thus avoiding the corrosion effect of the strong electrolyte solution formed by raindrops and deliquesced NaCl particles. However, the ‘lotus effect’ of SS only applies to SS under the action of external forces such as wind and precipitation, which inevitably limits their application in practical environment.

Recent studies on some well-designed SS have demonstrated that small droplets with a size <100 µm can undergo coalescence-induced droplet jumping behaviour. The so-called coalescence-induced droplet jumping behaviour refers to when two or more droplets coalesce on a SS, they can spontaneously jump away from the surface as a result of the release of excess surface energy. Such coalescence-induced droplet jumping behaviour on SS, which is different from the ‘lotus effect’, is an autonomous motion that removes droplets without an external force and offers a new route for an innovative method of atmospheric corrosion protection. To date, the applications of coalescence-induced droplet jumping behaviour have concentrated on condensation heat transfer, anti-frosting, self-cleaning, and electrostatic energy harvesting applications. The possible applications of such functionalised behaviour in atmospheric corrosion protection, however, have not been reported yet.

What kind of microstructure is best?

Coalescence-induced droplet jumping behaviour on SS is highly dependent on the material’s microstructures. As part of the study, a series of SS with different microstructures were formed, and the effect of microstructure on coalescence-induced droplet jumping behaviour of these SS was studied. We discovered that the nanostructured SS, the complex SS and the sheet-like structure SS lend themselves well to coalescence-induced droplet jumping behaviour in the condensation/fogging environment, and the droplets formed on the surface were relatively sparse.

The microstructured SS and the cluster-like structure SS, however, were not well suited to coalescence-induced droplet jumping behaviour in the condensation/fogging environment, and the droplets formed on the surface were denser. Based on the effects of roughness and contact angle on droplet jumping energy equations of the above four SS, a droplet jumping phase map based on the roughness and contact angle to distinguish the droplet jumping zone from the droplet non-jumping zone was constructed from the energy perspective. The phase map illustrates that a higher water contact angle and a lower roughness are more favourable to droplet jumping behaviour due to a lower solid/liquid contact area and interfacial adhesion. Therefore, to realise the droplet jumping behaviour of an SS, a larger water contact angle together with a smaller roughness is preferred. Additionally, 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 behaviour and the subsequent atmospheric corrosion resistance of the SS (Xiaotong Chen et al 2019, Xiaotong Chen et al 2022a).

How does it work?

Electrochemical experiments show that the nanostructured SS, the complex SS, and the sheet-like structure SS with coalescence-induced droplet jumping behaviour have better atmospheric corrosion protection performance than the microstructured SS and the cluster-like structure SS without coalescence-induced droplet jumping behaviour after condensation/fogging. This is because coalescence-induced droplet jumping behaviour of the nanostructured SS, the complex SS, and the sheet-like structure SS 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 SS due to its barrier-like character (Xiaotong Chen et al 2019, Xiaotong Chen et al 2022 a).

The angle matters

Based on the droplet jumping phase map, the knife-like structure CuO SS was rationally fabricated. The effect of the surface orientation on the coalescence-induced droplet jumping behaviour and the subsequent atmospheric corrosion resistance of the surface were studied. The results demonstrate that the horizontally oriented SS 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 SS, the horizontally oriented SS 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 SS can occupy some surface sites and inhibit the condensation inside the microstructure, thereby leading to reduced water permeation.

Xiaotong Chen^a, Peng Wang^{a, *}, Dun Zhang^{a, *}

^a Key Laboratory of Marine Environmental Corrosion and Bio-fouling, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

^* Corresponding authors.

E-mail: wangpeng@qdio.ac.cn (P. Wang); zhangdun@qdio.ac.cn (D. Zhang).