Layer-by-Layer Assembly of End-Functional Zirconium Phosphonate Multilayers for Multifunctional Pretreatments of Aluminum Surfaces

Aluminum with its unique combination of strength, lightness, and recyclability is popular choice for a wide range of applications in the packaging, automotive and construction sector. Even though its corrosion properties are more favorable compared to other structural metals such as steel, the economic burden of corrosion-related damages remains significant. This study introduces an approach to corrosion protection of aluminum surfaces through the creation of ultra-thin yet highly protective zirconium phosphonate multilayer coatings. The process involves the formation of a α,ω-bisphosphonic acid self-assembled monolayer (SAM) directly on aluminum, followed by a layer-by-layer (LbL) deposition approach using simple alternating dip-coating into α,ω-bisphosphonic acid and Zr(IV) salt solutions. While the hydrophobic alkyl sublayers contributed to inhibiting water and ion penetration, a functionalization of the surface was achieved by using different end-functional phosphonic acids as the terminating layer, In this way, the resulting multilayer structures gave rise to simultaneously excellent corrosion performance, particularly also against pitting corrosion and in near-application settings, improved surface wetting as required for forming, as well as a remarkable bonding performance under corrosive conditions due to the synergistic interplay of covalent bond formation to the adhesive and corrosion protection due to the Zr(IV)-linked multilayer assembly.