Electrochemical, Surface-Analytical, and Computational DFT Study of Alkaline Etched Aluminum Modified by Carboxylic Acids for Corrosion Protection and Hydrophobicity

Abstract

Carboxylic acids were investigated as a means to fabricate a superhydrophobic and corrosion resistant aluminum surface. Alkaline etching produced a hierarchically rough, superhydrophilic and hydroxylated Al surface which could then be modified by immersion in ethanol solution of carboxylic acids of different alkyl chain lengths, from hexanoic to octadecanoic. Acids with chain length longer than seven carbon atoms acted as corrosion inhibitors, but only those with long chains (e.g., octadecanoic acid) acted as a corrosion barrier and made the surface superhydrophobic with water contact angles over 150 degrees. The morphology, topography and chemical composition of unmodified Al and etched Al modified by carboxylic acids were studied using surface analytical tools (scanning electron microscopy with chemical analysis, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry). Modelling based on density functional theory was performed to help explain experimental observations and to provide a rationale of why only carboxylic acids with long enough chains were effective in reducing the rate of corrosion. The reason was attributed to their ability to form more stable and protective organic films. Aluminum surface prepared under appropriate conditions was superhydrophobic, corrosion resistant and durable and showed self-cleaning and delayed ice-melting properties.

Easy and Fast Fabrication of Self-Cleaning and Anti-Icing Perfluoroalkyl Silane Film on Aluminium

“…Although in recent years, numerous studies related to superhydrophobic surfaces have reported on their excellent performance [2,3,6], there is still a need to develop a convenient, environmentally acceptable and facile method for fabrication of superhydrophobic aluminium surfaces. The latter can trap air in the modified surface topography which prevents aggressive ions from reaching the aluminium surface, consequently offering an efficient mechanism for corrosion protection [2,3,[6][7][8][9]. Superhydrophobic surfaces have many applications because of their excellent properties such as on anti-icing [8,[10][11][12], anti-fouling [13] and anti-bacterial properties [14]; therefore, there is a high potential for applications in different fields [3].…”

“…The wettability of a solid surface is a function of two primary factors: surface roughness and surface chemistry [5]. There are several routes to construct superhydrophobic surfaces such as a sol-gel process [15], anodic oxidation [16,17], chemical vapour deposition [18] and chemical etching [9,19,20]. However, once the practical application of such a coating is considered, a method with simple operation, low-cost and short operation time are required.…”

“…However, once the practical application of such a coating is considered, a method with simple operation, low-cost and short operation time are required. From this point of view, chemical etching to produce a roughened surface followed by grafting a low-surface energy organic material, such as fatty acid [9,19] or alkyl and perfluoroalkyl silanes [8], are one of the easiest, economical and environmentally acceptable routes [2,20]. Chemical etching of aluminium has been reported in several studies (e.g., [21]); usually, it is performed in strong acid solutions (e.g., HCl, H 2 SO 4 ) in combination with other reagents such as HCl/H 2 O 2 [22], HCl/HF-Beck's solution [23][24][25] or alkaline solutions (e.g., NaOH) [9,26].…”

“…From this point of view, chemical etching to produce a roughened surface followed by grafting a low-surface energy organic material, such as fatty acid [9,19] or alkyl and perfluoroalkyl silanes [8], are one of the easiest, economical and environmentally acceptable routes [2,20]. Chemical etching of aluminium has been reported in several studies (e.g., [21]); usually, it is performed in strong acid solutions (e.g., HCl, H 2 SO 4 ) in combination with other reagents such as HCl/H 2 O 2 [22], HCl/HF-Beck's solution [23][24][25] or alkaline solutions (e.g., NaOH) [9,26]. The surface etching process can be performed also in solutions of metal chlorides such as NaCl [25], CuCl 2 [27,28], and FeCl 3 [21,27,29,30].…”

Contemporary Modes of Corrosion Protection and Functionalization of Materials

“…Superhydrophobicity of the surface is achieved by an appropriate combination of surface roughness, texture, and low surface energy. 97 To produce a superhydrophobic surface usually a two-step process is used. The first step is a mechanical modification, i.e.…”