Role of Surface Oxide Properties on the Aluminum/Epoxy Interfacial Bonding

Organic coatings are extensively used in various industries (e.g., automotive, aerospace) to protect the aluminum surface against corrosion. A crucial parameter determining the lifetime of coated metals is the durability of the metal/polymer bonding, in which a clear understanding of molecular-level interfacial chemistry lacks due to the challenges in probing the buried metal/polymer interfaces. In this study, the scanning Kelvin probe (SKP) technique was used to monitor the interfacial bonding-induced potential changes on the aluminum oxide surfaces (electropolished, acid-pretreated, alkaline-pretreated, and pseudoboehmite) after the application of an epoxy coating or, alternatively, model compounds (N,Ni'-dimethylsuccinamide and N-methyldiethanolamine) representing interfacial functional groups of epoxy/aluminum interface. The observed potential changes are then discussed in terms of hydroxyl fraction, adsorption amount, and bonding mechanisms, as characterized by X-ray photoelectron spectroscopy (XPS). Furthermore, the potential of the pseudoboehmite oxide, exhibiting the highest chemical interaction after the application of an epoxy coating or model compound, was investigated as a function of the pretreatment duration. It is shown that the N-methyldiethanolamine molecules are the major potential-decreasing component of the epoxy coating. Although the magnitude of potential drop is mainly controlled by the hydroxyl fraction of the oxides in the case of model compound deposition, such a correlation is not observed after the application of epoxy coating, supposedly due to the segregation of the model compounds creating an excess amount of molecules at the interface region with respect to the bulk polymer.