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 -, PO 4 3-and SO 4 2-by curve-fitting the XPS data.Calculations showed that almost 40% of the surface species in PAA oxide are phosphates (PO 4 3-), while about 15% are sulfates (SO 4 2 ) 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.

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–, PO4 3–, and SO4 2– 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.

For more than six decades, chromic acid anodizing has been the main step in the surface treatment of aluminum for adhesively bonded aircraft structures. Soon this process, known for producing a readily adherent oxide with an excellent corrosion resistance, will be banned by strict international environmental and health regulations. Replacing this traditional process in a high-demanding and high-risk industry such as aircraft construction requires an in-depth understanding of the underlying adhesion and degradation mechanisms at the oxide/resin interface resulting from alternative processes. The relationship between the anodizing conditions in sulfuric and mixtures of sulfuric and phosphoric acid electrolytes and the formation and durability of bonding under various environmental conditions was investigated. Scanning electron microscopy was used to characterize the oxide features. Selected specimens were studied with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy to measure resin concentration within structurally different porous anodic oxide layers as a function of depth. Results show that there are two critical morphological aspects for strong and durable bonding. First, a minimum pore size is pivotal for the formation of a stable interface, as reflected by the initial peel strengths. Second, the increased surface roughness of the oxide/resin interface caused by extended chemical dissolution at higher temperature and higher phosphoric acid concentration is crucial to assure bond durability under water ingress. There is, however, an upper limit to the beneficial amount of anodic dissolution above which bonds are prone for corrosive degradation. Morphology is, however, not the only prerequisite for good bonding and bond performance also depends on the oxides' chemical composition.

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Using adhesives for connection technology has many benefits. It is cost-efficient, fast, and allows homogeneous stress distribution between the bonded surfaces. This paper gives an overview on the current state of knowledge regarding the technologically important area of adhesive materials, as well as on emergent related technologies. It is expected to fill some of the technological gaps between the existing literature and industrial reality, by focusing at opportunities and challenges in the adhesives sector, on sustainable and eco-friendly chemistries that enable bio-derived adhesives, recycling and debonding, as well as giving a brief overview on the surface treatment approaches involved in the adhesive application process, with major focus on metal and polymer matrix composites. Finally, some thoughts on the connection between research and development (R&D) efforts, industry standards and regulatory aspects are given. It contributes to bridge the gap between industry and research institutes/academy. Examples from the aeronautics industry are often used since many technological advances in this industry are innovation precursors for other industries. This paper is mainly addressed to chemists, materials scientists, materials engineers, and decision-makers.

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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.