Fluoride-Induced Interfacial Adhesion Loss of Nanoporous Anodic Aluminum 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 acids (PSA), with and without postanodizing immersion in NaF. Experimental results indicate that chemical surface modifications, as concluded from X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, 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-induced dissolution of surface phosphates in PSA compensates for fluoride adsorption by revealing new OH groups. As the net OH fraction remains similar there is no further adhesion loss under water-ingress. The surprising reduction of dry adhesion is contributed to an interplay between surface energy changes affecting the type of attractive forces across the interface, as well as the loss of fine surface features, as seen by transmission electron microscopy cross-section images.