Fluoride-Induced Interfacial Adhesion Loss of Nanoporous Anodic Aluminium Oxide Templates in Aerospace Structures

Nanoporous anodic aluminum oxides (AAOs) are used as templates in various technological applications, including load-bearing aircraft structures. But in spite of their popularity, the important aspects that control their (dis-)bonding to an organic coating are not fully understood. To study the mechanisms behind the negative effect of fluorides on AAOs adhesion we employed both porous- and barrier AAO specimens. These were prepared by anodizing in sulfuric acid (SAA) or a mixture of phosphoric- and sulfuric acid (PSA), with- and without post-anodizing immersion in NaF. Experimental results indicate that chemical surface modifications, as concluded from X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), are dependent on the initial oxide composition. A partial replacement of surface hydroxyls (OH) by fluorine on SAA leads to adhesion loss due to removal of these stable sites for oxide-to-adhesive interfacial bonding. Conversely, fluoride-assisted dissoluti...