Publications

The galvanic compatibility of aerospace aluminum alloy AA7075 with cadmium (Cd), zinc (Zn), and zinc–cobalt–iron (Zn–Co–Fe, 32–37%Co and 1%Fe) alloys was investigated. A comparison of open circuit potential [OCP vs. saturated calomel electrode (SCE)] measurements in 0.6 mM NaCl showed that all coatings would act anodically to AA7075 with an exception of Zn–Co–Fe (37%Co + 1%Fe) alloy which was electropositive to AA7075. During the zero resistance ammetry (ZRA) measurement in 0.6 M NaCl electrolyte the coupled OCP and current density were measured during 7 days of immersion and both Zn and Cd acted anodic and thus sacrificial to AA7075. Galvanic coupling of AA7075 with (37%Co + 1%Fe) Zn–Co–Fe alloy resulted in the consequent dissolution of the AA7075 aluminum alloy. In contrast, Zn–Co–Fe (32%Co + 1%Fe) alloy was found to be anodic to AA7075 during the first 26 h of immersion but after dezincification and cobalt enrichment at the surface became cathodic to the AA7075 aluminum alloy. During coupling with Zn, some pitting was also observed on AA7075.

Atmospheric plasma technology is a promising next-generation alternative for replacing thermal chemical vapor deposition or wet chemical processes for the deposition of functional coatings. In this work, TiO2 films with various morphologies and thicknesses are synthesized in a controllable way by atmospheric Dielectric Barrier Discharges (DBD) under argon/oxygen ambient conditions. This method allows varying the density of the deposited coatings, from low density powders to dense compact anatase layers simple tuning of the parameters. The surface morphology and cross section of the as-synthesized films are observed by scanning electron microscopy (SEM). It is demonstrated that the total flow rate together with the absorbed power have a significant effect on the morphology of the film, which changes from granular to compact film by increasing the total flow rate or decreasing the power. In other words, changing the energy density (E d) in the plasma allows to control the morphology of the TiO2 films. A deposition mechanism is proposed to explain the effect of the synthesis parameters on the formed films. The chemical and phase composition of these films before and after annealing for 2 h in air at 673 K are comparatively studied. The photocatalytic activities and the reusability of the TiO2 films is investigated by degrading a methylene blue (MB) solution under Ultra-Violet (UV) light. It is compared to the activity of a P25 film, deposited by drop-casting. The photocatalytic rate of the annealed TiO2 film synthesized at a total flow rate of 2.5 slm turns out to be about 2 and 15 times higher than the one of P25 and as-synthesized TiO2 films, respectively. The annealed TiO2 film were shown to be reusable for at least three cycles, without significant loss of photocatalytic activity.

A gold–aluminium material combination is typically employed as an interconnection for microelectronic devices. One of the reliability risks of such devices is that of corrosion of aluminium bond pads resulting from the galvanic coupling between an aluminium bond pad and a gold wire. The research presented in this manuscript focuses on studying bond pad corrosion by selecting an appropriate model system and a dedicated set of electrochemical and analytical experimental tools. Taking into account the complex three-dimensional structure and the small dimensions of Au–Al interconnections (around 50–100 μm), a dedicated and novel experimental approach was developed. Au–Al covered silicon chips were developed under clean room conditions. Three-dimensional electrodes were mimicked as flat, two-dimensional bond pad model systems, allowing the use of microelectrochemical local probe techniques. Thin gold films were applied on Ti–Al covered silicon surfaces, and their morphology and electrochemical behaviour were analysed using the localised electrochemical cell, scanning vibrating electrode technique, scanning electron microscopy–energy dispersive X-ray spectroscopy, scanning electron microscopy–focused ion beam and Auger electron spectroscopy. The results revealed that the electrochemical behaviour of thin gold films was influenced by the underlying metal layers and the microstructural changes during heat treatment. The effect of the underlying layers on the electrochemical properties of the top gold layer was attributed to a mixed potential behaviour and microgalvanic interactions, which was more pronounced after heat treatment. Overall, the current experimental approach can be employed to study the mechanism of electrochemical events that are occurring on metallic interconnections in different electronic devices.

[This corrects the article DOI: 10.1039/C9NA00331B.].

Press-hardened steels are commonly protected with hot-dip Al-Si coating. Due to the electrochemical complexity of this system, either before or after hot-stamping process, SKPFM was used to investigate the influence of the thermo-mechanical process on the electrochemical behaviour of the galvanic coupling. The hot-stamping process changed significantly the anodic/cathodic coupling of the coating/steel due to iron enrichment in the coating layer. Hence, a concurrent mechanism was thoroughly established, i.e., while the press hardening enhances the corrosion properties of the steel system (steel and metallic coating) through diffusion, at the same time it diminishes the cathodic protection of the Al-Si layer.

No abstract is provided for this article.

The influence of Al2O3 inclusions on the localized corrosion behaviour of Q460NH steel was investigated in a simulated marine environment. According to the current sensing atomic force microscopy (CS-AFM) result, there was no galvanic couple between Al2O3 inclusions and the adjacent steel. Therefore, a different mechanism from that traditionally used (considering the galvanic coupling between inclusions and matrix) to explain the selective dissolution of the matrix around inclusions, dominated the pit initiation and propagation process in this system. Electron backscattered diffraction (EBSD) analysis showed a region of high dislocation density around the inclusions. Both microcrevices and the high dislocation density region around inclusions can easily induce localized corrosion. The catalytic-occluded cells and oxygen-concentration cell could accelerate the propagation of the pits. Furthermore, inclusion clusters had a larger negative effect than single inclusions on the corrosion resistance of the steel. Scanning vibrating electrode technique (SVET) measurements confirmed that inclusions play a key role in the corrosion process.

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 chemistries of the oxide and/or the adhesive are expected to modify its interfacial properties and so to affect the joint performance in practice.The upcoming transition to Cr(VI)-free surface pre-treatments makes it crucial to evaluate how the incorporation of electrolyte-derived sulfate and phosphate anions from, respectively, phosphoric acid-(PAA) and sulfuric acid-(SAA) anodizing are affecting 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 O 2-, OH -, PO 4 3-and SO 4 2surface species were quantified using X-ray photoelectron spectroscopy (XPS).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 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 (APS).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 oxide varying chemistries.

The performance of columns packed with 1.7 µm particles for aqueous size-exclusion chromatography was assessed at high-pressure conditions and linked to particle- and column-bed integrity. Decreasing the particle size from 3.5 µm to 1.7 µm increases the resolution due to the improved mass-transfer characteristics, allowing to significantly speed-up analysis without compromising the selectivity. A sub-minute separation of intact proteins was realized on a 4.6 mm i.d × 75 mm long column packed with 1.7 µm SEC particles applying a flow rate of 1.8 mL/min, corresponding to a column pressure of 530 bar. Ultra-high pressure operation (exceeding manufacturer's recommendations) resulted in peak deformation, a shift towards earlier retention times, and an alteration in selectivity. To gain insights in the mechanisms of column deterioration, short 30 mm long columns were operated at UHPLC conditions, maximizing the pressure drop over individual particles. This resulted in the presence of fractured particles situated at the column outlet, as verified by scanning electron micrographs. Mercury-intrusion porosimetry and argon-adsorption measurements did not reveal significant differences in intraparticle volume between particle batches sampled before and after pressure stress testing. As particles at the column outlet fracture (but not collapse) at high pressure operation, a void was formed at the column inlet. The degradation of the separation performance appeared to be the result of a decrease in interparticle pore volume.

The electrodeposition operating conditions for Zn–Co and Zn–Co–Fe alloys from chloride baths were studied. The electrodeposition was performed on a high strength steel substrate, under galvanostatic conditions, for a range of current densities at varying Co2+ and Fe2+ bath concentrations and at different temperatures. A transition current density was noticed above which a transition from normal to anomalous deposition took place. Below the transition current density electrodeposition of alloys with a higher amount of Co was obtained. Above the transition current density (i.e. in the anomalous range), both Zn–Co and Zn–Co–Fe alloys were deposited with a Co content lower than the composition reference line. This transition current density that resulted in normal to anomalous deposition was attributed to the shift in cathodic potential or polarization. However, it was found that under certain conditions the transition occurred from very high wt.% Co to almost equal to the amount of Co in the electrolyte. The increase of Fe2+ ions in the electrolyte assisted in increasing the Co content and decreasing the Zn content in the deposits. In addition, the increase of Fe2+ resulted in shifting the transition current density to a lower value. However, the cathodic current efficiency decreased with the addition of Fe2+ ions in the electrolyte.