Adhesive Bonding and Corrosion Performance Investigated as a Function of Aluminum Oxide Chemistry and Adhesives

The long-term strength and durability of an adhesive bond is dependent on the stability of the oxide/adhesive interface.As such, changes in the chemistries of the oxide and/or the adhesive are expected to modify its interfacial properties and so to affect the joint performance in practice.The upcoming transition to Cr(VI)-free surface pre-treatments makes it crucial to evaluate how the incorporation of electrolyte-derived sulfate and phosphate anions from, respectively, phosphoric acid-(PAA) and sulfuric acid-(SAA) anodizing are affecting the interfacial chemical properties.Hence, different types of featureless aluminum oxides with well-defined surface chemistries were prepared in this study.The relative amounts of O 2-, OH -, PO 4 3-and SO 4 2surface species were quantified using X-ray photoelectron spectroscopy (XPS).Next, bonding with two types of commercial aerospace adhesive films was assessed by peel and bondline corrosion tests.The presented results indicate that the durability of the oxide/adhesive interface depends on interplay between oxide and adhesive chemistries.Epoxy adhesion is highly affected by changes in the oxide surface chemistry, especially the amount of surface hydroxyls.However, the performance of anodic oxides with a lower hydroxyl fraction can be significantly enhanced by the presence of covalent bonds using a silane coupling-agent γ-amino propyl triethoxy (APS).On the contrary, results with Redux 775 adhesive exhibit very low sensitivity to variations in the surface chemistry.Bondline corrosion resistance of the joints is mainly determined by the nature of the adhesive, independent of the oxide varying chemistries.