The role of acid‐base properties in the interactions across the oxide‐primer interface in aerospace applications

In the transition to environmentally friendly production, chromic acid anodizing is being replaced by sulfuric acid, phosphoric acid or phosphoric‐sulfuric acid anodizing. While it is known that using these acids leads to the incorporation of anions into the oxide, little is known on how they affect the nature of the oxide and how this, in turn, affects the interactions across the oxide‐primer interface. In the current work, we investigate these changes in the interactions, especially the acid‐base bonding, across the interface as a function of the pretreatment. To exclude contribution of surface roughness, oxides with no surface features were prepared in the aforementioned anodizing solutions by stopping the oxide growth during the formation of the barrier layer. Treatments in alkaline solution and boiling water were also included. XPS and Fourier transform infrared were first used to probe the acid‐base properties of the oxide before its interactions with 4‐ethylphenol (4‐EP), and 4‐hydroxybenzyl alcohol (4‐HbA) was measured. These two molecules represent functional groups of phenolic‐based thermoset resins with different acidities. Results show that both the pretreatments and the choice of molecule play a role in the interactions across the interface. While 4‐HbA showed no preference, thin films of 4‐EP were not detected on oxides that were prepared in phosphoric acid. Copyright © 2015 John Wiley & Sons, Ltd.