Corrosion and surface: Superhydrophobic aluminum surfaces: Extended preparation routes and characterization

Superhydrophobicity (SH, earlier designated as ultrahydrophobicity) is a physicochemical phenomenon that has attracted a vast attention for more than one decade, with the exciting investigations of Barthlott and Neinhuis as an accelerating point. SH means an extreme waterrepelling property of natural as well as artificial surfaces. It is well substantiated that SH as a macroscopic surface property is based on the interplay of the roughness, mostly having a hierarchical character with micro- and sub-microscalic components, and the chemistry lowering the surface free energy. The way to produce SH depends on the type of the material to be treated. As for aluminum, which has clearly a hydrophilic character, a coating strategy must be necessarily adopted. The roughness may be produced by pure hot water treatment, alkaline or acid etching attack , anodic oxide formation or laser ablation. Other approaches, as straightforward variants of photolithography or micro-embossing at ambient temperature, failed in achieving two-tier roughness characteristics. Therefore, fluoro-organic compounds play an outstanding role for equipping a surface with SH. For Al substrates a great number of approaches were described. Forming very thin films of self-assembling molecules, simple alkanoic acids or even a perfluoroalkane were used. In order to covalently graft coating layers alkyl and fluoroalkyl silanes were applied. Fluorinated surfactants were anchored on aminosilane- pretreated substrates. Phosphonic and phosphoric acids with (fluoro)alkyl tails have a high affinity to be adsorbed onto several metal oxide surfaces. Suitable polymers are poly(dimethyl siloxanes), perfluoropolyether and the soluble fluorine-containing copolymer Teflon AF. It should be noticed that SH can be achieved on smooth substrates using fluoropolyelectrolytes or dispersed PTFE oligomers . In contrast to the 'wet-chemical' modifications mentioned above, 'dry' coating methods may be successfully applied to produce coatings in a very wide thickness range suitable for all substrate materials. PTFE and other fluorocarbon coatings could be generated by Hot Filament Chemical Vapor Deposition (HFCVD) and, derived from it, Initiated Chemical Vapor Deposition (iCVD), with deposition rates of over 1 mm/min. SH was produced