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

Abstract

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.

Anodizing—Interplay between bath aging and oxide formation and properties

“…The spectra recorded from the front and the backsides of the sample are perfectly reproducible. The identification of the bands is based on comparison with the literature . Four bands are identified and discussed as follows: Band #1 with a maximum of between 940 and 960 cm −1 because of the Al‐O vibration mode and is commonly interpreted as typical for Al 2 O 3 fits well with earlier results of the authors …”

“…The authors explain this band by the overlapping of the AlOH deformation vibration due to AlOOH (cf. 1060‐1070 cm −1 , 1060‐1070 cm −1 , and 1150 cm −1 ), and this is frequently interpreted as γ‐AlOOH or (pseudo‐) boehmite, and SO stretching vibration (cf. 1140, 1126, 1152, and 1130‐1190 cm −1 ).…”

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

“…These are the same oxides that were formerly found to adsorb phenol molecules better than the other oxides, presumably due to the availability of weaker basic hydroxyl groups. Hence, the slight increase in peel strength after the transition from dry-to wet peel in here may be related to increased interfacial interactions in the presence of the weakly acidic phenols that are leached out of the resin in the presence of water to temporarily stabilize the interface 44 (since, as found earlier in Abrahami et al 24 , these reactions are reversible). This behavior has also been observed with other polymers including a study with poly(methyl methacrylate) (PMMA) polymer, Tannebaum et al 45 found that water helps to mediate bonding.…”

“…Also, no distinction is seen between the part in the panel that was tested under dry-and wet conditions. Considering the chemical composition of Redux 775, adsorption to all substrates via alcohol and phenol groups present on the phenolic resole product can be expected according to our previous studies using model molecules 24 . In this study, the highest peel strengths were achieved with the hydrothermal, alkaline and CAA oxides.…”

“…Epoxy adhesive is, for instance, often modeled using different types of amines that are used for epoxy curing [18][19][20] . Different studies on natural and (electro) chemically grown oxides have shown that these types of bonding involve relatively weak acid-base interactions [21][22][23][24] . On the other hand, a study by Marsh et al 25 showed no specific interaction with epoxypropylphenyl ether itself.…”