Fluorocarbon lubricant impregnated nanoporous oxide for omnicorrosion-resistant stainless steel
Graphical abstract
Introduction
Corrosion causes many serious problems precluding satisfactory lifetime of metallic materials. Therefore, corrosion-resistant surfaces of metallic materials are of great importance in a broad range of engineering systems and applications. For long decades, various strategies have been employed to prevent the corrosion of metallic materials regarding the service environments. For example, thin organic (or polymer) coatings, such as simple painting, spraying and shrink-wrapping [1], [2], [3], [4], [5], [6], have been convenient and widely used ways to form passive layers on metals. Although these organic coatings inhibit the mass transfer of corrosion reactant (e.g., water, oxygen and chloride) they can be easily degraded by UV exposure and temperature change, so that the corrosion reactants are allowed to be transported to the interface of metal/polymer resulting in crevice corrosion and the delamination of coating from a metal surface [7]. Hard ceramic protective layers by thermal spraying and passive oxide layers by sol–gel coating have also been employed for anti-corrosion of metals [8], [9], [10]. However inhomogeneous coatings with surface cracks by poor adhesion are still hurdles to be overcome [11], [12].
Recently it has been studied that hydrophobic or superhydrophobic surfaces have a potential to enhance the corrosion resistance on metallic surfaces [13], [14], [15], [16]. Especially, the superhydrophobic passivation has shown the prevention of corrosion in wet environments accompanied with corrosive water by harbouring air layer within the surface structures [16], [17], [18], [19]. However, it should be noted that the corrosive media can be airborne (e.g., in sea coast and chemical plants) and transported via a vapor phase [20], [21], so that the superhydrophobic passivation fails to inhibit the penetration of corrosive gas into the underlying metallic substrate. Therefore, an alternative strategy to realize omnicorrosion-resistant passivation, which protects the metallic substrate from corrosive media in both liquid and gas phases, is required.
A lubricant-impregnated surface, which is one of representative liquid repellent surfaces, has also been studied for corrosion-resistant coating of metals. Due to the mobile and low surface tension lubricant layer, most liquids including water and aqueous corrosive media can effectively be repelled from the surface resulting in the protection of the metal surface against corrosive liquids. Although the lubricant-impregnated surface shows good corrosion resistance, the protection efficacy against atmospheric corrosion in a vapor phase has not yet been investigated. In addition, a meniscus (or wetting ridge) of lubricant surrounding a liquid drop can be rinsed off when a liquid drop slides on the lubricant-impregnated surface, which is a serious issue on durability of conventional lubricant impregnated surfaces [22], [23], [24].
In this study, we design a method to fabricate a highly stable fluorocarbon lubricant impregnated nanoporous oxide (FLINO) surface on metals as an omnicorrosion resistant coating (i.e., corrosion protection against corrosive media in both liquid and vapor phases). The effect of FLINO is investigated especially on stainless steel. Stainless steel, employed in many applications such as chemical plants, heat exchangers, seawater desalination and marine systems, is exposed to severe corrosive environments where the corrosive media can be transported via both liquid and vapor phases. The anodic oxidation (or anodizing) process is adapted to realize disconnected high-aspect-ratio and dead-end nanopore structures [25], [26], which have not yet been examined for stainless steel to realize omniphobic de-wetting property. Such nanoporous geometry has advantages in immobilizing a lubricant liquid within the structures and allows superior durability along with a unique self-healing capability [27]. The surface featured with a nanoporous oxide layer composited with polytetrafluoroethylene (PTFE) coating and water-immiscible lubricant liquid physically precludes the penetration of aqueous corrosive media into the oxide and inhibits the corrosion of metallic substrates [27], [28], [29]. In addition, the lubricant perfectly covering the surface also blocks vaporized media in corrosive atmospheres. This combination of nanoporous oxide and lubricant exhibits highly durable and effective anti-corrosivity against both aqueous and vaporized media, which opens a new door to develop an omnicorrosion coating on metals.
Section snippets
Pretreatment of stainless steel
For stainless steel substrate, AISI 304 (Fe-18Cr-8Ni, Alfa Aesar) sheet with 0.1 mm in thickness was employed. It was cut into 45 mm × 20 mm in size and used for a specimen. The specimen was degreased in acetone with ultrasonication for 3 min. Before anodizing, the surface of stainless steel was electropolished in a mixture of percholoric acid and ethylene glycol (1:19 volumetric ratio) under voltage of 40 V for 5 min at 10 °C.
Anodization of stainless steel
Ethylene glycol based electrolyte was employed for anodizing to
Nanoporous anodic oxide of stainless steel
For the fluorocarbon lubricant impregnated nanoporous oxide (FLINO) coating, a stable nanoporous oxide should be first prepared on the surface of stainless steel followed by the surface modification with low surface energy polytetrafluoroethylene and the impregnation of fluorocarbon lubricant (Fig. 1a). The stainless steel plate can be anodized in ethylene glycol-based solution (7.5 g L−1 NH4F + 2.0 g L−1 water) resulting in a yellowish film (Fig. 1b). Since the as-anodized structure of
Conclusions
Various technologies creating porous surface structures have been applied to realize the omniphobic lubricant impregnated surfaces on commercial materials [43], [47], [48]. Such lubricant impregnated porous surfaces show multifunctional properties, such as anti-bacterial adhesion [49], [50], [51], anti-wetting transparency [43], [52], anti-corrosion [53], enhanced condensation [54] and anti-icing [55]. Among the porous structures, the disconnected high-aspect-ratio dead-end nanopore structures
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the US Office of Naval Research (ONR) Award N00014-14-1-0502. J. Lee acknowledges the support from the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT, Ministry of Science and ICT) (No. 2018R1C1B6006156) and the local industry promotion business linked with public institutions (Gyeongnam) (P0004798) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This research was also supported by a grant (19CTAP-C151876-01)
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