Aluminum alloys, prone to corrosion-induced degradation, are generally in need of protection by organic coatings 1À3 which also provide various surface properties such as color, wear resistance, and formability. Interfacial bonds at such polymer/(oxyhydr)oxide/aluminum joints have to withstand high mechanical forces and corrosive attack to protect the functional properties of the coated metals in its intended service application conditions. Therefore, it is crucial to control and to understand the adhesion mechanism originating at polymer/(oxyhydr)oxide/aluminum interfaces in order to achieve the long-term stability of these composites. 4 Adhesion at polymer/aluminum interfaces is mainly governed by the adsorption theory, taking into account covalent, polar, and acidÀbase interactions. 4 Therefore, the resistance of these interfaces is controlled not only by the adhesion-promoting functional groups in organic coating but also by the interrelated properties of passive layers formed on aluminum surfaces, e.g., hydroxyl content, 5 thickness, surface morphology 6,7 electronic structure, 8 and chemical composition. 9 Those properties of passive layers rely heavily on pretreatments of aluminum and also might be extremely sensitive to the small changes in the environmental conditions, e.g., humidity 10,11 and aging. 11,12 For instance, in the case of repeatability of coating application, humidity remains as almost a universal problem in various industrial areas such as packaging, coastal, marine, and aerospace, where coating must be applied within the humidity range at which moisture will not condense on the metal surface. 13 Scanning Kelvin probe (SKP) is a suitable technique to monitor Volta potential changes on bare metals. The chemical
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,N′-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.
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