Regularities of the Formation of a Green Superhydrophobic Protective Coating on an Aluminum Alloy after Surface Modification with Stearic Acid Solutions

Aleksey Abrashov,Nelya Grigoryan,Tigran Vagramyan,Yaroslav Mezhuev,Yuri Korshak,Oleg Grafov

doi:10.3390/met11111718

Abstract

It has been shown that solutions of stearic acid in a dimethyl sulfoxide–water binary mixture allow superhydrophobic protective coatings to be created on an aluminum alloy surface with a minimum impact on the environment. The superhydrophobicity and self-cleaning ability of the coating that we developed have been confirmed by measurements of droplet wetting angles and roll-off angles. These properties appear due to the formation of a multimodal micro-rough surface that mainly consists of aluminum stearate. The coatings formed in this manner have been studied by ellipsometry, XPS, and scanning probe microscopy. Their protective ability has been estimated by the “droplet-express” method and in a salt fog chamber. The protective ability of the coating is determined by the DMSO/H2O ratio, the concentration of stearic acid, and the duration and temperature of modification of the aluminum alloy; it is controlled by a competition between the processes of aluminum stearate formation and hydrolysis. It has been shown that adsorption of stearic acid on an aluminum stearate coating increases its permeability and decreases its protective capability. The results presented in this article are useful for optimizing the conditions of applying green superhydrophobic stearate coatings on aluminum alloys in order to achieve a maximum protective effect.

Highlights

Materials with a water wetting angle above 150◦, with a droplet roll-off angle below 10◦, and a pronounced surface self-cleaning capability, are classified as superhydrophobic [1,2,3,4]
This article is aimed at the development of green superhydrophobic coatings and the determination of the regularities of variation in their protective ability depending on the conditions of layer formation on the surface of AMg6 aluminum alloy
It may be assumed that, on treatment of AMg6 alloy surface in air with a solution of stearic acid in the DMSO/H2O binary solvent system, aluminum stearate is predominantly formed on the surface

Summary

Introduction

Materials with a water wetting angle above 150◦, with a droplet roll-off angle below 10◦, and a pronounced surface self-cleaning capability, are classified as superhydrophobic [1,2,3,4]. Superhydrophobicity underlies the self-cleaning of surfaces in some plants and insects [3,4,5,6,7] and is often required in the formation of anti-corrosion, anti-fouling, and anti-icing coatings [8,9,10,11,12,13,14,15,16,17,18,19] as well as for the production of pollution-resistant textile materials [20]. The creation of continuous superhydrophobic self-cleaning films on the surfaces of aluminum and its alloys could be an in-demand alternative [6,8,21,22,23] to passivation with solutions of Cr(VI) compounds that manifest considerable toxicity. This article is aimed at the development of green superhydrophobic coatings and the determination of the regularities of variation in their protective ability depending on the conditions of layer formation on the surface of AMg6 aluminum alloy

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