Development of Superhydrophobic Coatings on Metallic Surfaces for Industrial Applications

Abstract

Recently, several superhydrophobic coatings on metallic surfaces have been developed, but they have not achieved great success in developing on commercial scale. Although these coatings have exceptional properties like self cleaning, anti-corrosive, anti-icing, etc., but these coatings cannot be used successfully in industry due to inability of complete mechanical, chemical and thermal stability data. Therefore, the main challenge is to create cheap, excellent adhesively, environmentally safe, durable, regenerative superhydrophobic coatings on various metallic surfaces. In the current work, various superhydrophobic coatings on metallic surfaces (aluminum, steel, copper, and steel mesh) for various industrial applications were synthesized by simple immersion technique.Superhydrophobic coatings on aluminium surfaces were synthesized using two-step and one-step processes. By using two-step process, two different superhydrophobic coatings were prepared using etchants such as KOH and HCl+HNO3 solutions independently and then by immersion in lauric acid solution for lowering surface energy. One-step process was used to synthesize superhydrophobic coating by immersing aluminium sample in a mixture of KOH etchant and lauric acid solution. Superhydrophobic coatings on steel and steel mesh surfaces were synthesized using two-step process. For the fabrication, first roughness was created by immersing in etchant (HCl+HNO3) and then by dipping the roughened surface in lauric acid low surface energy surface was created. Superhydrophobic coatings on copper surfaces were synthesized using two-step and one-step processes. In this case, after the creation of roughness and lowering the surface energy, the same chemicals were used. For each case, effects of process parameters like etching time and immersion time in lauric acid are studied. The result reveals that surface roughness and water contact angle increase with etching and immersion time. After fabrication, mechanical, thermal, and chemical stability tests were performed to identify the operating range. The SEM studies reveal the presence of a rough micropattern on the treated surfaces and the contact angle measurements confirm the superhydrophobicity. The water droplet dynamics study reveals that the water droplet with low impact velocity bounces from the surface. At critical impact velocity, pinning of water droplet is observed. Splashing of water droplet is observed beyond the critical impact velocity. Furthermore, the coated sample remains in floating condition on the water surface for several weeks, showing excellent water-repellent nature. Coated surface bounces off the high speed water jet stream and then still no change in superhydrophobicity is found. This behaviour confirms the excellent mechanical properties of the coatings. In addition to the above, it is observed that mechanical disturbances due to surface bending and repeated folding and de-folding do not have much effect on the superhydrophobicity. Mechanical durability of coatings was also evaluated by performing adhesive tape peeling and sand paper abrasion tests for the confirmation and quantification of the durability of coatings.
In chemical stability test, superhydrophobic nature remains unaffected after several days of immersion in NaCl solution and shows the excellent chemical stability of coating. Thermal stability of coatings was evaluated by annealing samples in temperature range 40 - 250 °C for one-hour period in hot air oven. It is found that superhydrophobicity of most of the coated samples remains unaffected after annealing in temperature range 40 - 140 °C, and this is considered as the excellent thermal stability of coatings. Coatings are found to be fully damaged after 24 hours annealing at 300 °C and superhydrophobicity of surface turns into superhydrophilicity. However, wettability of the surfaces can be restored by simple immersing the samples again in lauric acid solution. The analysis reveals that superhydrophobicity of the coatings is regained.
Self-cleaning property of the fabricated superhydrophobic aluminium, steel, and copper surfaces was studied. Coating shows the excellent self-cleaning property. No accumulation of moisture from air on the cooled superhydrophobic aluminium, steel, and copper surfaces is observed and this asserts the excellent anti-fogging property of coatings. By electrochemical tests, it is demonstrated anti-corrosion property of coating with low corrosion current density and high corrosion potential. For the replication of industrial applications in laboratory, petroleum ether-water and benzene-water mixtures which are oil-water mixtures were successfully separated from each other by a simple filtration method using the superhydrophobic steel mesh with the separation efficiency above 99%. Oil-water separation using the coated steel mesh is simple, fast, and repeatable for at least 10 times. The aforesaid durable and regenerable superhydrophobic metallic surfaces have potential industrial applications.