Development of nanostructured icephobic aluminium oxide surfaces for aeronautic applications

In-flight icing due to the impingement of supercooled water droplets modifies the shape of the aerodynamic surfaces of aircraft, resulting in lower stall speeds and higher fuel consumption. Icephobic coatings help reducing the adhesion strength of ice to a surface and represent a promising technology to support mechanical/thermal ice protection systems. Superhydrophobic surfaces are water-repellent and embody a straightforward solution to tackle icing: nanostructured porous aluminium oxide layers are generated with anodization and superhydrophobicity can be reached by tuning the pores size. However, not much research has been done to verify if such surfaces are generally icephobic in representative icing conditions, or if they need to have additional properties to effectively reduce ice adhesion. In this work, we investigate the ice adhesion strength on cladded Aluminium Alloy 2024 (AA2024), an alloy commonly used for aerospace components, anodized with different process parameters in sulphuric and oxalic acid and hydrophobized by a commercial fluorinated product. Upon the optimization of the two anodization processes, the micro-/nanostructures generated on the surface were effective in reducing the ice adhesion strength. From icing wind tunnel tests, the surfaces anodized in oxalic acid showed superior icephobic (i.e., lower ice adhesion) properties compared to the ones anodized in sulphuric acid because of their larger air-to-solid surface ratio. The proposed anodization process is fast, easy to perform and to implement in existing production lines, low-cost, and operator- and environmentally-friendly.