Organic primer coatings loaded with environmentally harmful Cr(VI) corrosion inhibitive pigments still play an important role in corrosion protection of aluminium alloys for the aerospace industry. A potential “green” alternative coating system has recently been developed, loaded with lithium salt corrosion inhibitors. Under exposure to neutral salt spray, lithium salts leach from the organic coating into coating defects to induce the formation of a corrosion protective layer. In this work the composition and growth of this protective layer is investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS imaging is successfully applied to monitor the lateral spread of leaching lithium salts in artificial 1-mm-wide scribes. The chemical composition of the protective layer is revealed by comparing the mass spectra of salt spray exposed scribe areas to the mass spectra of pseudoboehmite and aluminium-lithium layered double hydroxide reference samples. The insights obtained in this work have led to a thorough understanding of the formation mechanism of the protective layer and provide local chemical and structural information which can be linked to corrosion protection behavior.

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Nanoporous anodic aluminum oxides (AAOs) are used as templates in various technological applications, including load-bearing aircraft structures. But in spite of their popularity, the important aspects that control their (dis-)bonding to an organic coating are not fully understood. To study the mechanisms behind the negative effect of fluorides on AAOs adhesion we employed both porous and barrier AAO specimens. These were prepared by anodizing in sulfuric acid (SAA) or a mixture of phosphoric and sulfuric acids (PSA), with and without postanodizing immersion in NaF. Experimental results indicate that chemical surface modifications, as concluded from X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, are dependent on the initial oxide composition. A partial replacement of surface hydroxyls (OH) by fluorine on SAA leads to adhesion loss due to removal of these stable sites for oxide-to-adhesive interfacial bonding. Conversely, fluoride-induced dissolution of surface phosphates in PSA compensates for fluoride adsorption by revealing new OH groups. As the net OH fraction remains similar there is no further adhesion loss under water-ingress. The surprising reduction of dry adhesion is contributed to an interplay between surface energy changes affecting the type of attractive forces across the interface, as well as the loss of fine surface features, as seen by transmission electron microscopy cross-section images.@en

Probing initial interactions at the interface of hybrid systems under humid conditions has the potential to reveal the local chemical environment at solid/solid interfaces under real-world, technologically relevant conditions. Here, we show that ambient pressure X-ray photoelectron spectroscopy (APXPS) with a conventional X-ray source can be used to study the effects of water exposure on the interaction of a nanometer-thin polyacrylic acid (PAA) layer with a native aluminum oxide surface. The formation of a carboxylate ionic bond at the interface is characterized both with APXPS and in situ attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann). When water is dosed in the APXPS chamber up to 5 Torr (∼28% relative humidity), an increase in the amount of ionic bonds at the interface is observed. To confirm our APXPS interpretation, complementary ATR-FTIR Kretschmann experiments on a similar model system, which is exposed to an aqueous electrolyte, are conducted. These spectra demonstrate that water leads to an increased wet adhesion through increased ionic bond formation.

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Understanding the stability of chemical interactions at the polymer/metal oxide interface under humid conditions is vital to understand the long-term durability of hybrid systems. Therefore, the interface of ultrathin PMMA films on native aluminum oxide, deposited by reactive adsorption, was studied. The characterization of the interface of the coated substrates was performed using ambient pressure X-ray photoelectron spectroscopy (APXPS), Fourier transform infrared spectroscopy in the Kretschmann geometry (ATR-FTIR Kretschmann) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The formation of hydrogen bonds and carboxylate ionic bonds at the interface are observed. The formed ionic bond is stable up to 5 Torr water vapour pressure as shown by APXPS. However, when the coated samples are exposed to an excess of aqueous electrolyte, an increase in the amount of carboxylate bonds at the interface, as a result of hydrolysis of the methoxy group, is observed by ATR-FTIR Kretschmann. These observations, supported by ToF-SIMS spectra, lead to the proposal of an adsorption mechanism of PMMA on aluminum oxide, which shows the formation of methanol at the interface and the effect of water molecules on the different interfacial interactions.@en

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 chemistry of the oxide and/or the adhesive are expected to modify the interfacial properties and affect the joint performance in practice. The upcoming transition to Cr(VI)-free surface pretreatments makes it crucial to evaluate how the incorporation of electrolyte-derived sulfate and phosphate anions from, respectively, phosphoric acid anodizing and sulfuric acid anodizing affect 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 O2−, OH−, , and surface species were quantified using x-ray photoelectron spectroscopy. 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 the 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. 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 varying oxide chemistries.@en

The transition to Cr(VI)-free production is a great challenge in the global aerospace industry that currently still relays on it for the preparation of aluminum for bonding. Proper surface pretreatment is a prerequisite for strong and durable adhesive joint. Despite decades of experience, the nature and contribution of the different adhesion forces between the aluminum and organic adhesive remain under discussion. Herein we studied the adhesion of epoxy resin as a function of the surface chemistry of barrier-type anodic oxides prepared in sulfuric acid (SAA), phosphoric acid (PAA), and mixtures of phosphoric–sulfuric acids (PSA) and chromic acid (CAA) at different anodizing temperatures. X-ray photoelectron spectroscopy (XPS) data measured on model specimens were curve-fitted to calculate the relative amounts of O2–, OH–, PO43–, and SO42– species at the surface. The amounts of these species were then related to the mechanical performance of the joint measured by the floating roller peel test. Results show that significant initial adhesion is achieved without mechanical interlocking and independent of the type of electrolytes used for the pretreatment. Conversely, bonding stability under wet conditions is highly influenced by the surface chemistry. The wet adhesion strength increases with the hydroxyl concentration at the aluminum (oxide) surface, indicating that interfacial bonding is established through these surface hydroxyls. Phosphates and sulfates anions were not found to contribute to bonding with this type of adhesive.@en

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