Introduction of microcapillary techniques opened a new range of methods to study the behavior of microsystems. Although these measurements are widely employed and results obtained are generally recognized, certain aspects require careful consideration. We present the results of a parametric study performed by means of multi-ion modeling in order to clarify the influence of the microcapillary size and shape together with the permeability of the sealant for oxygen on the polarization curves obtained on metals with different corrosion activity. The obtained results show that when a corroding metal is (almost) non-active (e.g. aluminum), the oxygen limiting current increases with increase of the ratio between the radius of the capillary main part and the end radius γ = R/r but the corrosion potential remains the same. For more active metals and alloys (e.g. steel), a change of the aspect ratio γ leads not only to the proportional change of the limiting current, but also to a substantial shift of Ecorr. Analytical solutions derived for limiting current density in a microcapillary confirmed the simulated results. Permeability of the sealant for oxygen plays a significant role mostly for narrow capillaries with low ratio γ and more active metallic substrates. It is shown that the problem of comparison of polarization data obtained with capillaries of different size can be partially avoided by use of capillaries with the same aspect ratio γ but only if the quality of the sealant (low permeability for oxygen) is assured.

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.

In the present study, the effect of different types of iron oxides, which naturally exist on steel substrate, on the interfacial interaction between an epoxy coating and a carbon steel substrate was studied at the molecular/atomic level by employing molecular dynamics (MD) simulations and quantum mechanics (QM) calculations. Three types of iron oxide, that is, ferrous oxide (FeO), ferric oxide (Fe2O3, hematite), and ferrous ferric oxide (Fe3O4, magnetite), were considered for modeling, and their binding energies were calculated and compared by altering the concentration of hydroxide groups on the surface. To probe the effect of curing agent on interfacial interactions, computations were performed for either uncured or aminoamide-cured epoxy resins. The effect of the acid–base properties of the iron oxide on the molecular bonding was theoretically investigated by imposing diverse iron hydroxide/oxide termination groups. Noticeably, MD and QM calculations confirmed rather well earlier experimental evaluations on iron oxide acid–base properties obtained through X-ray photoelectron spectroscopy measurements in the view of chemisorption of different epoxy compounds. However, the interaction behavior of cured epoxy with hydroxylated iron oxide surfaces was quantified mechanistically in the current work from a closer view. For instance, it was found that aminoamide-cured epoxy was adsorbed on all oxide substrates through a mechanism of electrostatic and donor–acceptor interactions, with binding energies of −113.6, −1035.9, and −304.4 kcal/mol respectively assigned to FeO, Fe2O3, and Fe3O4 iron oxides. In the case of a hydroxylated surface, aminoamide-conjugated epoxy adhesive was found in the hydrogen bond interface as well, which was evidence of strengthened binding at a surface populated by hydroxide groups. Moreover, theoretical explorations showed that the type of covalent linkage between the curing molecule and epoxy resin governs the extent of cured epoxy adhesion to the surface of iron oxide.

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–, PO43–, and SO42– by curve-fitting the XPS data. Calculations showed that almost 40% of the surface species in PAA oxide are phosphates (PO43–), whereas about 15% are sulfates (SO42) 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.

•Cryogenic-rolling is used to obtain a microcrystalline copper, grain size 1µm.•ECSTM is used to study in situ the intergranular corrosion of microcrystalline Cu.•EBSD is used to link grain boundary type and susceptibility to corrosion.Electrochemical scanning tunneling microscopy is proved to be a powerful tool for providing valuable topographic information to study in situ the local corrosion properties of polycrystalline materials. It was applied to analyze the susceptibility to intergranular corrosion of different types of grain boundaries of microcrystalline copper in HCl and combined with electron backscatter diffraction to link the observed corrosion differences to a specific type of grain boundary. The superior resistance to intergranular corrosion of coherent twin boundaries over random grain boundaries is demonstrated.

Scanning electrochemical microscopy (SECM) with its high spatial resolution and chemical selectivity is a powerful technique for studying a wide range of corrosion processes. A common procedure to detect qualitatively a corrosion activity on a metal substrate consists of measuring the local oxygen concentration.The aim of this work is to clarify the local impact of the O2 measurement and the undesired effect on the corrosion process. By means of numerical simulation the variations of the anodic and cathodic currents on the aluminum substrate, as well as concentration profiles, are obtained. The multi-ion transport and reaction model (MITReM) is used considering the homogeneous reactions taking place in the solution, transport of species dominating the corrosion process and the electrochemical reactions. The model case study in this work is corrosion of pure aluminum in chloride solution. We concluded that amperometric O2 SECM measurements lead to a local increase of the solution pH and decrease of partial current densities for O2 reduction on the metal. This influence becomes significant when the distance to the substrate and when the size of the active surface is comparable with the size of the SECM probe.

The local electrochemical reactivity of high purity copper as a function of the grain orientation in the microstructure has been investigated. The different crystal orientations were determined by means of Electron backscatter diffraction (EBSD) followed by in-situ Scanning electrochemical microscopy (SECM) measurements operating in feedback mode across the very same copper surface. With this procedure it is possible to characterize in-situ the reactivity of selected grains in the polycrystalline microstructure of the copper. The study is focused on monitoring differences in the electrochemical response on <001>//ND and <111>//ND oriented grains. The behaviour of these selected orientations was studied under two surface conditions: passive state of copper after the formation of an anodic oxide layer, and corrosion or active state of the copper by immersion in chloride solution. In the case of the passivated copper it was observed that the grain with an orientation closer to <111>//ND shows a lower reactivity compared to the grain where the orientation is nearer to <001>//ND, which confirmed that grain orientation has an influence on the electrochemical reactivity of the passive layer formed on the metal.It was found that for the copper in the active state, the reactivity of a <111>//ND oriented grain is higher when compared to a <001>//ND oriented grain, which is contrary to the expected behaviour. This discrepancy could be explained by possible neighbouring effects between both grains.

The interfacial bonding properties of carboxylic polymers on a Zn substrate have been investigated. Poly(methyl vinyl ether-alt-maleic acid monobutyl ester) and cured propoxylated bisphenol A fumarate unsaturated polyester were applied on a set of differently treated Zn samples. The buried metal–polymer interface was studied by polymer removal and evaluation of the residue layers on Zn surfaces representing the metal–polymer interface region. Additionally, the interfacial bondings were mimicked by adsorption of the representative carboxylic monomers, i.e., succinic and myristic acids. The differently treated Zn surfaces were found to be capable of adsorption of the carboxyl functionality of the polymers, resulting in formation of carboxylates. A comparison of the interfacial bondings by the residue layers of the polymers with those formed due to the molecular adsorption showed comparable adsorption mechanisms. Additionally, it was found that the polymer–metal interfacial bonding density mainly depends on the Zn surface hydroxyl fraction, while Zn oxide semiconductor properties play an important role when a curing process occurs during the polymer interaction with Zn surfaces.

This study investigates the Volta potentials of the differently treated zinc surfaces and the interface dipole moments after adsorption of carboxylic acid and anhydride molecules on zinc surfaces by means of scanning Kelvin probe (SKP). The interfacial bonding properties of carbonaceous contamination as well as adsorbed succinic acid, myristic acid, and succinic anhydride molecules with zinc substrates have been investigated using Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The variation of the Volta potential due to the treatments applied on zinc was evaluated by means of SKP. Moreover, the dipole moments of adsorbed carboxylic acid and anhydride molecules were measured and correlated to surface hydroxyl fractions and oxide electronic properties. The results clearly showed that the zinc Volta potential varies with the oxide composition, resistance, and the configuration of the molecules adsorbed.

•A modified Zr-based conversion pre-treatment was studied on different metals.•Electrochemical analysis was used to study the deposition mechanism in-situ.•Surface analytical techniques were implemented to study the coating chemistry.•The thickness of Zr-based conversion layers depends on the underlying metal.•The elemental distribution of the Zr-based coatings depends on the substrate.In this work, a new surface pre-treatment based on a modified hexafluorozirconic acid solution with addition of copper was studied as a replacement of the phosphating process. The main purpose of this study is to compare the formation mechanism and kinetics of Zr-based conversion coatings on three different base substrates: Aluminium alloy (AA) 6014, cold rolled steel and hot dip galvanized steel. For that purpose, open circuit potential (OCP) measurements during the deposition of the conversion coatings are performed in combination with different ex-situ surface characterization techniques. The elemental distribution was analyzed by means of (Field Emission) Auger Electron Spectroscopy (FE-AES) depth profiles and mappings. Furthermore, the surface chemistry of the conversion layer was evaluated by X-ray Photoelectron Spectroscopy (XPS). The results showed that while the mechanism of formation is similar for the different substrates, the rate of formation strongly depends on the substrate type. Additionally, the thickness and lateral and in-depth elemental distribution of the Zr-based conversion layers also largely depend on the underlying metal.