Optimization of coating parameters for fabrication of robust superhydrophobic (SHP) aluminum and evaluation of corrosion performance in aggressive medium

The huge interest in superhydrophobic surfaces originates from their potential engineering applications in corrosion protection, antifouling, oil recovery, self-cleaning, anti-adhesion, anti-icing, medical dressing, drag reduction and radiative cooling. Despite the progress made in developing superhydrophobic (SHP) coatings, a key challenge is the ability to fabricate mechanically robust SHP surfaces and a proper understanding of the corrosion performance of SHP surfaces in aggressive conditions. Here, we report a simple, rapid, inexpensive, scalable and eco-friendly method for fabrication of mechanically durable superhydrophobic (SHP) aluminum surfaces with self-cleaning properties using a scalable dip-coating method. The coating parameters were optimized to obtain a robust coating with desirable properties such as water contact angle (WCA), sliding angle, self-cleaning, abrasion and corrosion resistance. To obtain good adhesion between the substrate and the coating, chemical etching and hydroxylation of the Al surface were used. At an optimal withdrawal speed of the substrate in the precursor solutions, a maximum WCA of 172.3° ± 1.1° (SA ~ 1° ± 1°) was achieved. The hierarchical micro-nano roughness and the presence of the Si-(CH3)3 group were responsible for the superhydrophobic behavior. The existence of a covalent bond between the Al surface and the middle connecting layer and –O-Si-CH3 groups on the surface of the SHP Al are confirmed by LRS and XPS, respectively. The electrochemical studies in 3.5 wt% NaCl solution show that the Rct value of SHP Al during the initial state was ~2 orders higher than that of the bare sample. The Cdl value of SHP Al was 6 orders less than that of bare after 1 h of exposure in chloride solution. On the SHP surface, a droplet with an impinging velocity of 0.36 m/s showed approximately 7 bounces. The superhydrophobicity of the SHP sample was preserved up to 150 cm abrasion distance. The hierarchical surface morphology and superhydrophobicity (WCA ~151°) were retained by the SHP samples even after 120 h of immersion in 3.5 wt% NaCl, which indicates its suitability in seawater applications such as ships, underwater instruments and pipelines and marine structures. Further, our work provides a framework to develop mechanically robust SHP coating for various applications.