XPS Analysis of the Surface Chemistry and Interfacial Bonding of Barrier-Type Cr(VI)-Free Anodic Oxides

In the transition to environmental friendly pretreatment of aerospace aluminum alloys, chromic acid anodizing (CAA) is being replaced by sulfuric acid (SAA), phosphoric acid (PAA), or phosphoric-sulfuric acid (PSA) anodizing. While generally the main concern is controlling the film morphology, such as the pore diameter, oxide-, and barrier layer thickness, little is known on how the anodic oxide chemistry affects the interactions at the interface upon adhesive bonding. To study the link between surface chemistry and interfacial bonding, featureless oxides were prepared by stopping the anodizing during the formation of the barrier layer. A model was developed to quantify the relative amounts of OH–, PO43–, and SO42– by curve-fitting the XPS data. Calculations showed that almost 40% of the surface species in PAA oxide are phosphates (PO43–), whereas about 15% are sulfates (SO42) in SAA. When both anions were present in the electrolyte, phosphate incorporation was inhibited. Studies of the interaction between this set of Cr(VI)-free oxides and diethylenetriamine (DETA)—an amine curing-agent for epoxy resin—showed that all oxides interact with the nitrogen of DETA. However, larger ratios of Lewis-like acid–base bonding between the amine electron pair and the acidic hydroxyl on phosphate surface sites were observed.