Superhydrophobic Coatings on Aluminium Based on Carboxylic Acids
Authors: Milosev Ingrid , Bakaric Tina , Rodic Peter , Zanna Sandrine , Seyeux Antoine , Poberznik Matic , Cornette Pauline , Costa Dominique , Kokalj Anton , Marcus Philippe 
Published: February 2020

Development of new coatings for corrosion protection should be directed to environmentally friendly yet efficient technologies. Industrial applications additionally demand energy sustainability and cost effectiveness. Among varieties of new coatings being developed superhydrophobic coatings are also in the focus of current research in both academic and industrial sectors. The background of their use, inspired by effects in nature, is the prevention of ingress of electrolyte to underlying substrate and properties of self-cleaning and anti-sticking. Superhydrophic surfaces can be applied to different substrates, from metal, textile, plastics, etc. Superhydrophobicity can be related to surface texture or surface chemistry, e.g., use of silane-, fluoro-based or other compounds [1].
The basic idea of the current research study is to design multifunctional compounds by integrating corrosion inhibitors and hydrophobic agents with lipophilic or fluorous properties into hybrid matrices so as to produce a corrosion resistant hydrophobic coating that display good adhesion to Al substrates. The first step is to test potential candidates for (i) hydrophobic compounds and (ii) corrosion inhibitor compounds. Among hydrophobic compounds, carboxylic acids were tested since they offer environmentally friendly and cheap option. In order to adsorb carboxylic acids on metal surface an appropriate pre-treatment is required. In the present work we investigated the possibility to fabricate superhydrophobic surfaces on aluminium metal pre-treated by alkaline etching at 90 °C [2] which represents a green solution-phase method to produces hierarchical growth of the aluminium hydroxide array. After immersion in ethanol solution of carboxylic acid the surface turns superhydrophobic.
In the present study we have investigated the effect of several parameters on the formation of superhydrophobic surface on aluminium pre-treated by etching: type of etching as a pre-treatment, length of carboxylic chain, time of immersion in carboxylic acid and concentration of carboxylic acid. The effect of these parameters on the water contact angle and corrosion properties in 0.5 M NaCl solution were determined. Depending on the type of carboxylic acid a remarkable decrease in both cathodic and anodic current density can be achieved. Optimal parameters were denoted as those achieving high water contact angle, and low corrosion current density combined with a broad passive range. XPS showed that after alkaline etching the aluminium surface was fully covered by an hydroxide layer. ToF-SIMS provided direct evidence of adsorption of octanoic and stearic acids. Coatings prepared on Al by self-assembling in carboxylic acids were tested also by long-term immersion in NaCl solution. Surface durability in terms of resistance in acid and alkaline solution, as well as sliding angle were determined.
Morphology of self-assembled carboxylic layers on alkaline etched aluminium analysed by FE-SEM shows nano-rough surface with numerous micro-cones with steps or terraces giving the surface a dual roughness. Composition of the surface was investigated using various surface analytical methods, i.e. FTIR, XPS and SIMS.
Adsorption bonding of carboxylic acids to hydroxylated Al surfaces is also being investigated by DFT calculations. We utilized periodic slab model of passivated surface with hydroxylated oxide-layer supported on Al metal. These calculations help us to scrutinize details of how carboxylic head-groups attach to hydroxylated surface and how lateral intermolecular interactions and their self-assembly depends on the length of the aklyl tail. The effect of the organic layer composition on the resulting electronic properties of the passive layer (band gap, electronic workfunction) is also investigated.
Fig. 1. FE-SEM image of aluminium surface etched in NaOH and immersed in 5 mM ethanol solution of stearic acid. Detail: contact angle of water drop 156°.
Acknowledgments: This work is a part of M.ERA-NET project entitled “Design of corrosion resistant coatings targeted for versatile applications”, acronym COR_ID. The financial support of the project by Ministry of Education, Science and Sport of Republic of Slovenia (Programmes of International Scientific Cooperation) and Agence Nationale de la Recherche, France, is acknowledged.
References:
[1] N. J. Shirtcliffe et al., Adv. Colloid Interfac 161 (2010) 124–138
[2] L. Liu, X. Feng, M. Guo, J. Phys. Chem. C 117 (2013) 25519–25525

Figure 1