The sol–gel process is a chemical surface preparation method based on hydrolysis and polycondensation reactions for enhanced adhesion for metallic substrates in adhesive bonding and coating applications. This paper describes an investigation into the effect of the microstructural complexity of two commonly used aerospace aluminium alloys (AAs) 2024-T3 and 7075-T6, on the response to different surface pre-treatments before deposition of the sol-gel coating and subsequent adhesive bonding. Different surface pre-treatments, including two abrasive treatments and three chemical surface pre-treatments were used, and their effect on surface chemistry, wettability and roughness was assessed. Surfaces were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, profilometry and static contact angles. A hybrid silane sol-gel film was deposited on the differently pre-treated aluminium alloys, an epoxy adhesive was applied and the adhesion properties were evaluated using pull-off testing. The role of the altered physicochemical properties of the pre-treated surfaces was related to the adhesion strength of the sol–gel reinforced epoxy/aluminium interfaces. The microstructural complexity of the aerospace alloys caused non-uniform responses to the pre-treatments, proving the importance of compatibility between material and treatment conditions. Statistical analysis revealed that, despite that overall higher adhesion values were obtained on rougher surfaces, only a strong correlation exists between the surface hydroxyl fraction and adhesion strength. The relation of roughness and water contact angle to interfacial adhesion was found to be non-significant. The findings of this study underscore the critical role of surface pre-treatments and their impact on adhesion strength in aerospace aluminium alloys, providing valuable insights for the effective utilization of sol-gel coatings in adhesive bonding and coating processes.

Three imidazole-based compounds (imidazole, 2-mercaptobenzimidazole, and 2-mercapto-1-methylbenzimidazole) were studied as corrosion inhibitors of Cu, Zn, and a family of Cu-xZn brass alloys (x = 10−40 wt%) in 3 wt% NaCl solution. The composition of inhibitor layers on the surface was studied using X-ray photoelectron and infrared spectroscopies. The molecular adsorption modes on Cu and Zn were modelled using density-functional theory (DFT) calculations. Mercapto-based inhibitors act as efficient inhibitors on all materials studied due to the formation of inhibitor layer through bonding with nitrogen and sulphur atoms. In contrast, imidazole is a moderate inhibitor for Zn, while it is much less efficient for Cu.

The often used simplistic correlations between molecular electronic parameters and experimentally determined corrosion inhibition efficiencies are critically evaluated for a set of 24 heterocyclic organic compounds, tested as corrosion inhibitors for copper in 3 wt.% NaCl aqueous solution. Twelve different molecular electronic descriptors—such as ionization potential, electron affinity, HOMO–LUMO gap, dipole moment—are tested and it is shown that none of them displays any noticeable correlation with the inhibition efficiency. Our results, therefore, cast serious doubt on reported correlations between such parameters and inhibition efficiency, obtained for only a few inhibitors, which are abundant in the literature. We also discuss some pros and cons of inhibition efficiency as a metric for evaluating the performance of corrosion inhibitors, and introduce a new metric termed inhibition power that uses the universal logarithmic scale and dimensionless decibel (dB) units.

The initiation of corrosion can be triggered by defects in the adsorbed layer of organic inhibitors. A detailed knowledge of the intermolecular forces between the inhibitor molecules and the interfacial bonding will be decisive to unravel the mechanisms driving the corrosion initiation. In this work, adsorbed organic layers of 2-mercapto-5-methoxybenzimidazole (SH-BimH-5OMe) and 5-amino-2-mercaptobenzimidazole (SH-BimH-5NH2) were compared regarding their performance mitigating copper corrosion. Atomic force microscopy was used to address the stability and intermolecular forces of the self-assembled monolayers, using imaging and force measurement modes. For a film formed by amino-derivative molecules, a gold-coated tip frequently picked up individual molecules (molecular fishing) in force-distance measurements. For layers of the methoxy-derivative, no fishing events were observed, pointing to a constant functional layer. X-ray photoelectron spectroscopy revealed that SH-BimH-5OMe molecules form a stronger bond with the surface and more stable SAM layers on Cu surfaces as compared to SH-BimH-5NH2 molecules. Results of computational density functional theory modeling and electrochemical corrosion tests are in line with the microscopy and spectroscopy results. In particular, with aid of computational modeling the less ordered structure of the SH-BimH-5NH2 monolayer is attributed to dual bonding ability of SH-BimH-5NH2 that can adsorb with either S or NH2 groups.

The sol-gel synthesis process is a versatile method used to produce a wide diversity of materials and is being increasingly used as a surface modification method to alter porosity, wettability, catalytic activity, biocompatibility and corrosion performance of underlying substrates. Silane sol–gel films deposited on aluminium and aluminium alloys have been widely studied as chemical conversion coatings and as coupling agent between the substrate and organic layers. This study set out to investigate the effect of the surface chemical treatment prior to sol-gel application on the interfacial adhesion properties of a hybrid sol-gel film. Different surface pre-treatments, including two abrasive treatments and three chemical surface pre-treatments were used and their effect on surface chemistry and surface roughness was assessed. Surfaces were characterized by scanning electron microscopy, x-ray photoelectron spectroscopy, roughness measurements and static contact angles. Cerium nitrate loaded hybrid sol-gel films were deposited and adhesion on commercially pure aluminium was evaluated using pull-off testing. Statistical analysis revealed that, although highest adhesion values were obtained on rougher surfaces, the strongest correlation exists between the surface hydroxyl fraction and adhesion strength.

The effect of the presence of an anodic film and hybrid sol-gel coating loaded with corrosion inhibitors was evaluated as a strategy for enhanced barrier and active corrosion protection of aluminium alloy 2024-T3. In this study, AA2024-T3 specimens were anodized in a modified sulphuric-citric acid bath (SCA) as the first layer of a corrosion protective multilayer system and subsequently protected by the application of silica-based hybrid sol-gel coatings. These coatings were doped with LiNO₃ and Ce(NO₃)₃ as corrosion inhibitors and studied in comparison with the inhibitor-free sol-gel coating in terms of morphology, composition and corrosion protection of intact and scribed specimens. The anodized AA2024-T3 with an overlaying inhibitor-free sol-gel coating showed the highest impedance modulus during long-term immersion in 0.1 mol·L⁻¹ NaCl aqueous solution. Active corrosion protection of scribed coated specimens was studied by exposure to a 0.5 mol·L⁻¹ NaCl solution and evaluated by surface analytical techniques. The addition of Li- and Ce-based salts into the hybrid sol-gel formulation showed active corrosion protection compared to the inhibitor-free scribed hybrid sol-gel coating. The Ce-doped sol-gel coating showed less visual corrosion and higher active corrosion protection than the Li-containing one during the long-term immersion test in 0.5 mol·L⁻¹ NaCl. Present findings reveal that the combination of the anodic/hybrid sol-gel layers on AA2024-T3 enhances the corrosion protective properties barrier properties of both stand-alone systems and the incorporation of Li- and Ce-based inhibitors provide active corrosion.

Efficiency of corrosion inhibitors in aqueous solutions depends on several interfacial parameters, which may vary over time. Therefore, reliable electrochemical techniques are demanded for screening the efficiency of corrosion inhibitors and monitoring their performance over time. Here, we evaluate corrosion inhibition efficiency of imidazole-based compounds on bare Cu surfaces and highlight the importance of electrochemical evaluation of the inhibitor over time, characterized by linear polarization resistance techniques as a reliable, instantaneous and non-invasive method for assessing intrinsic inhibitor performance in lab screening studies.