In this work, the correlation between electrolyte transport properties and the variation of pigment volume concentration (PVC) in a series of organic coatings is explored. Using an odd random phase electrochemical impedance spectroscopy (ORP-EIS) approach, the diffusion of ions independent from water take-up is analysed. A higher PVC resulted in a more homogeneous coating morphology, which could be associated with a faster diffusion of ions following a Fickian regime and enhanced water uptake. In the case of lower pigment loading, the obtained heterogenous morphology of the coating introduced new challenges to the physical interpretation of the proposed electrochemical equivalent circuit.

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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.

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The demand for lithium-ion batteries (LiBs) is rising, resulting in a growing need to recycle the critical raw materials (CRMs) which they contain. Typically, all spent LiBs from consumer electronics end up in a single waste stream that is processed to produce black mass (BM) for further recovery. It is desired to design a recycling process that can deal with a mixture of LiBs. Hence, this study investigates the structure and composition of battery modules in common appliances such as laptops, power banks, smart watches, wireless earphones and mobile phones. The battery cells in the module were disassembled into cell casing, cathode, anode and separator. Then, the cathode active materials (CAMs) were characterized in detail with XRD-, SEM-, EDX- and ICP-OES-analysis. No direct link was found between the chemistry of the active materials (NMC, LCO, LMO, LFP etc.) and the application. Various BM samples were submitted to a leaching procedure (2 M H2SO4, 50 °C, 2 h, 60 g BM/L) with varying concentration (0–4 vol%) of H2O2 to study the influence of their chemical composition on the dissolution of Li, Ni, Mn and Co. Only a part of the BMs dissolved completely at 4 vol% H2O2, which was attributed to the oxidation state of the transition metals (TMs). Exact determination of H2O2 consumption by redox titration confirmed this hypothesis.@en

An evaluation of neodymium‑iron‑boron permanent magnets (NdFeB PMs) from different end-of-life products, as a secondary resource of rare-earth elements (REEs), is presented. De-coating of PM was investigate as pre-treatment to facilitate efficient direct magnet recycling. Thus, critical aspects from disassembling to the de-coating of the magnets were addressed. A challenge for the de-coating process is that the magnets have different sizes, weights, and their coating compositions are not known beforehand. It was shown that ammonia-based solutions was thermodynamically suitable to dissolve nickel and zinc coatings selectively, while keeping the bulk magnet stable. Nevertheless, the dissolution of Zn was much faster than the Ni one, and more efficient.

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The adhesion capabilities of sulfuric acid and tartaric-sulfuric-acid anodic oxide films on cladded AA2024-T3 aluminium substrates have been investigated in a quantitative manner. A relatively simple, versatile and industrially applicable test methodology based on DIN EN ISO 29862 has been used. In addition, the effect of anodising process parameters on the oxide film morphology has been studied and correlated with the adhesion results. The process parameters considered are the chemistry of the acidic pickling pre-treatment step, the addition of tartaric acid to the sulfuric acid electrolyte, the formation voltage, and the electrolyte temperature. The suggested tape peel adhesion test is able to differentiate among pre-treatments and anodising conditions showing a good correlation with the morphological features at the ultimate surface. High peel adhesion strengths are measured when topographies such as pore mouth widening and dissolution-driven roughness take place. These morphologies are typically found in anodic layers formed at elevated electrolyte temperatures. The proposed method can be beneficial for a fast assessment of anodising parameters for good adhesion, which is of special interest for the industrial optimisation of anodising processes.

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The anodising process parameters (voltage, temperature, and electrolyte) control the morphology and the chemical composition of the resulting anodic oxide film by altering the balance between oxide growth and oxide dissolution reactions. The porosity of the oxide film is reduced by the addition of tartaric acid to a sulfuric acid electrolyte, while anodising at elevated temperatures enhances oxide dissolution, leading to wider pores and rougher surfaces. No significant changes in the oxide chemical composition as a function of anodising parameters was found; in particular, no tartrate incorporation took place. The resistance of uncoated anodic oxide films against aggressive media and galvanic stress as a function of anodising parameters has been studied by electrochemical methods. Anodising in a mixed tartaric and sulfuric acid electrolyte improves the resistance of the anodic oxide against galvanic stress and aggressive media in comparison to sulfuric acid ano-dising processes. However, the corrosion protection performance of the anodic oxide films in com-bination with a corrosion-inhibitor loaded organic coating is not governed by the blank oxide properties but by the adhesion-enhancing morphological features formed during anodising at elevated temperatures at the oxide/coating interface.

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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.

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Self-organized anodization of copper in 0.1 M Na2CO3 electrolyte was studied in order to obtain nanostructured oxide surface on the metal substrate. Linear sweep voltammetry (LSV) revealed that the most suitable voltage range for anodic film formation is from 3 to 31 V. In this range (except between 3 and 7 V), the oxide is formed as nanorods, with the diameter of the anodically grown nanostructures increasing with the applied voltage. The smallest diameter of the nanorods was found to be 28 ± 9 nm (15 V), while the greatest diameter was 109 ± 15 nm (30 V). X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman spectroscopy pointed out that the nanorods consist of crystalline CuO (tenorite) and Cu2O (cuprite), and amorphous Cu(OH)2. Moreover, the greater the anodizing voltage, the greater the CuO content versus Cu2O. The formed nanostructured materials may find applications in photocatalysis and catalytic electrochemical reduction of carbon dioxide into light hydrocarbons.@en

This work focuses on the effect of surface roughness and surface chemistry on the initial adhesion strength and corrosive de-adhesion properties of adhesive bonds. The adherend used in this study is a S690 low-alloy steel whereas the adhesive is a 2-component epoxy-amine adhesive (Araldite 2015). The steel surface is subjected to different surface pre-treatment methods such as mechanical abrasion, grit blasting, zirconium conversion treatment and silane treatment. The effect of these different pre-treatments on the surface morphology, roughness and chemistry is addressed. Single-lap joint tests were performed at ambient conditions to assess the initial bond strength of the joint. Static wedge tests were performed in saltwater immersions to study the environmental ageing of the adhesive joints. Unloaded delamination of adhesive films from the steel surface was studied by means of scanning Kelvin probe (SKP) at high relative humidity. This unique combination of different techniques allows thorough evaluations of the bond performance under different environmental and loading conditions. Experimental results indicate that surface roughening plays an important role in the initial adhesion in the single-lap joint test but a minor role in the durability of the bonded steel surfaces. The improved initial adhesion is mainly attributed to the increased interfacial bond area at higher surface roughness. The presence of complex texture or morphology shows a more profound effect than the average roughness on both the initial adhesion and the durability of the interfacial adhesion. The results from the static wedge test show the large contribution of mechanical interlocking, caused by texturing of the surface, on the durability of the interfacial adhesion. In the absence of complex texture, surfaces with altered chemistry by zirconium- or silane treatment exhibit a significant increase of the initial bonding strength due to enhanced physicochemical interactions across the interface. Assessment of the interfacial delamination kinetics by SKP show that despite the absence of any surface topography, chemically altered surfaces prove to have higher resistance to delamination.@en

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

Passivated metal surfaces are commonly coated by organic coatings to protect them against various types of corrosion such as pitting, intergranular, and exfoliation. Achieving durable coating protection in hostile conditions is a ubiquitous problem of interface engineering, which requires a local understanding on chemical interactions at the hybrid polymer/metal oxide interface. However, it is very challenging to get useful information directly from this solid/solid interface. This article explains different synthesis and analysis approaches that allow the (in situ) investigation of the buried interface, leading to the extraction of information regarding the molecular interfacial chemistry by surface analysis techniques. Different pretreatments of the metal oxide substrate prior to coating can improve interfacial bonding strength and durability. Therefore, it will be shown that certain surface oxide properties are directly correlated with the quality of interfacial interactions. Next, different hybrid structure synthesis approaches are discussed, which allow to characterize chemical interactions locally at the interface by conventional surface analysis techniques such as X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ions mass spectrometry (ToF-SIMS), and other spectroscopic techniques. A new generation of techniques such as ambient pressure XPS (APXPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) in the Kretschmann geometry, and sum-frequency generation vibrational spectroscopy are nowadays utilized to investigate the influence of external conditions on the hybrid interface such as changing ambient conditions in situ.@en

Diamond is known as a promising electrode material in the fields of cell stimulation, energy storage (e.g., supercapacitors), (bio)sensing, catalysis, etc. However, engineering its surface and electrochemical properties often requires costly and complex procedures with addition of foreign material (e.g., carbon nanotube or polymer) scaffolds or cleanroom processing. In this work, we demonstrate a novel approach using laser-induced periodic surface structuring (LIPSS) as a scalable, versatile, and cost-effective technique to nanostructure the surface and tune the electrochemical properties of boron-doped diamond (BDD). We study the effect of LIPSS on heavily doped BDD and investigate its application as electrodes for cell stimulation and energy storage. We show that quasi-periodic ripple structures formed on diamond electrodes laser-textured with a laser accumulated fluence of 0.325 kJ/cm² (800 nm wavelength) displayed a much higher double-layer capacitance of 660 μF/cm² than the as-grown BDD (20 μF/cm²) and that an increased charge-storage capacity of 1.6 mC/cm² (>6-fold increase after laser texturing) and a low impedance of 2.74 ω cm² turn out to be appreciable properties for cell stimulation. Additional morphological and structural characterization revealed that ripple formation on heavily boron-doped diamond (2.8 atom % [B]) occurs at much lower accumulated fluences than the 2 kJ/cm² typically reported for lower doping levels and that the process involves stronger graphitization of the BDD surface. Finally, we show that the exposed interface between sp² and sp³ carbon layers (i.e. the laser-ablated diamond surface) revealed faster kinetics than the untreated BDD in both ferrocyanide and RuHex mediators, which can be used for electrochemical (bio)sensing. Overall, our work demonstrates that LIPSS is a powerful single-step tool for the fabrication of surface-engineered diamond electrodes with tunable material, electrochemical, and charge-storage properties.

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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

Copper foil was anodized in 1 M KOH at potentials ranging from −400 to −100 mV vs. Ag|AgCl electrode. Cyclic voltammetry showed a distinct peak with a maximum at around −150 mV. For anodizing at −100 and −200 mV, nanowires with diameters of ca. 19 and 24 nm respectively were found to be grown. Moreover, photoluminescence and X-ray diffraction show that the dominant phase is the CuO phase. At lower potentials, cuboidal micron sized crystals were formed and Cu₂O was found to be the major phase.

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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.@en

Zirconium-based conversion treatment of zinc, aluminium and magnesium oxides have been studied in-situ using ATR-FTIR in a Kretschmann geometry. This set-up was coupled to an electrochemical cell, which allowed to obtain chemical and electrochemical information simultaneously as a function of conversion time. This elucidated the strong relation between physico-chemical surface properties and zirconium-based conversion kinetics. Whereas the surface hydroxyl density of zinc and aluminium increased during conversion, magnesium (hydr)oxide was shown to dissolve in the acid solution. Due to this dissolution, strong surface alkalization can be expected, explaining the rapid conversion kinetics. AES depth profiling was used to determine the final oxide thickness and elemental composition. This confirmed that magnesium is most active and forms a zirconium oxide layer approximately 10 times thicker than zinc. On the other hand, the presence of zirconium oxide on aluminium is very low and can be considered as not fully covering the metal oxide. Additionally, the converted oxide chemistry was related to the bonding mechanisms of amide functionalized molecules using ATR-FTIR and XPS. It was shown that inclusion of zirconium altered the acid-base properties, increasing the substrate proton donating capabilities in case of magnesium oxide and increasing hydrogen bonding and Bronsted interactions due to increased surface hydroxide fractions on zinc and aluminium substrates.

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For more than six decades, chromic acid anodizing (CAA) has been the central process in the surface pre-treatment of aluminum for adhesively bonded aircraft structures. Unfortunately, this electrolyte contains hexavalent chromium (Cr(VI)), a compound known for its toxicity and carcinogenic properties. To comply with the new strict international regulations, the Cr(VI)-era will soon have to come to an end. Anodizing aluminum in acid electrolytes produces a self-ordered porous oxide layer. Although different acids can be used to create this type of structure, the excellent adhesion and corrosion resistance that is currently achieved by the complete Cr(VI)-based process is not easily matched. This paper provides a critical overview and appraisal of proposed alternatives to CAA, including combinations of multiple anodizing steps, pre- and post anodizing treatments. The work is presented in terms of the modifications to the oxide properties, such as morphological features (e.g., pore size, barrier layer thickness) and surface chemistry, in order to evaluate the link between fundamental principles of adhesion and bond performance.[Figure not available: see fulltext.]

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For more than six decades, chromic acid anodizing (CAA) has been the central process in the surface pre-treatment of aluminium for adhesively bonded aircraft structures in Europe. Unfortunately, this electrolyte contains hexavalent chromium (Cr(VI)), a compound known for its toxicity and carcinogenic properties. The approaching ban on the use of hexavalent chromium (Cr(VI)) makes its elimination a high-priority R&D topic within the aerospace industry and the Cr(VI)-era will soon have to come to an end. Anodizing aluminium in acid electrolytes produces a self-ordered porous oxide layer with a thin barrier layer underneath. This special type of oxide readily adheres to the organic resin and provides protection against corrosion. Although Cr(VI)-free candidates such as sulphuric acid- (SAA), phosphoric acid- (PAA) and mixtures of phosphoric-sulphuric acid anodizing (PSA) can be used to create this type of structure, the excellent adhesion and corrosion resistance that is currently achieved by the Cr(VI)-based process is not easily matched. To gain a better understanding of the underlying physical and chemical mechanisms that contribute to the adhesion and durability in these structures, this study investigates the correlation between the oxide’s chemical and morphological characteristics, as influenced by the anodizing electrolyte, and bond performance. The major challenge in the mechanistic understanding of the adhesion in bonded components is to differentiate between the different forces acting at the oxide/resin interface. In the first part of this PhD thesis, studies focus on the role of surface chemistry. To exclude the contribution of mechanical interlocking between the oxide and the resin, featureless oxides were prepared by stopping the anodizing during the formation of the barrier layer. Surface characterization of the different anodic oxides by means of Fourier transform infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) revealed no significant net change in the acid-base properties of the different anodic oxides. It was found that local chemical changes were introduced due to the incorporation of electrolyte-driven anions. Therefore, a model was developed to quantify the relative amounts of O2-, OH−, PO4 3−, and SO4 2−, showing significant changes in the type and amount of surface species. Consequently, measurements showed that the pretreatments and the molecule type affected oxide/molecule interfacial interactions. To evaluate the contribution of adsorptive interaction in practice, peel tests were performed on featureless oxides bonded with commercial aerospace adhesives. Results showed that significant initial dry adhesion is achieved with FM 73 epoxy without mechanical interlocking, and independent of the type of pretreatment. However, the formed bonding was not water resistant, with the amount of applied stress needed for peeling linearly increasing with the amount of surface hydroxyls. Moreover, the application of a thin γ-APS silane layer before bonding with epoxy has confirmed that the stability of the interface is also determined by the nature of the bond, showing much more stable interfaces in the presence of covalent interactions. When peel tests were performed with a phenolic-based adhesive (Redux 775), no correlation to the surface chemistry was found. Nevertheless, the bonded joints on the basis of the weakly acidic character of the phenolic adhesive showed better resistance to corrosion in salt spray tests, compared to those on the basis of the epoxy adhesive. Therefore, we conclude that both oxide surface- and adhesive chemistries play a role in the formation and long-term stability of the oxide/resin interface. In the second part of this thesis industrial porous oxides were applied. Fundamental investigations show that changing the voltage during anodizing can produce morphological variations across the oxide thickness. The effect of the initial voltage sweeps, however, was limited by the oxide dissolution action of phosphoric acid in PSA, since prolonged anodizing in this electrolyte not only leads to an increase of the pore diameter, but also completely dissolves the upper most part of the oxide. Morphological changes were distinguished between geometrical modifications that affect the pore size and changes in the surface roughness that was caused by extended chemical dissolution at higher anodizing temperatures and/or phosphoric acid concentration. Measured carbon concentration profiles within the pores using high-resolution transmission electron microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) indicated that resin penetration is affected by both aspects. Moreover, mechanical performance in peel tests indicates that these parameters, rather than the oxide layer thickness are critical for moisture-resistant adhesion. Both adhesion mechanisms: adsorption and mechanical interlocking seem to contribute to the adhesion in these structural bonds. A higher degree of dissolution during anodizing is beneficial for the adhesion, facilitating a composite-like interphase. Too much dissolution, however, reduces the resistance to bondline corrosion. Overall, the presented results illustrate the need to consider both chemical and morphological changes in the selection of Cr(VI)-free alternatives for structural adhesive bonding. @en