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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In this work, a new finite element modelling (FEM) approach is followed to model spatiotemporally resolved water uptake in organic coatings. To this aim, we start from a physical model, where not only Fickian diffusion of water is taken into account but also the adsorption/desorption reaction of water on the polymer matrix. Starting from a number of important coating properties and crucial model parameters, derived from gravimetric and Fourier transform infrared (FTIR) measurements as the model input, the local water concentration over the coating thickness as a function of time is modelled for a polyethylene glycol diacrylate (PEGDA) coating. The modelled water concentration is then used to calculate virtual capacitance values which are evaluated against experimental capacitance values extracted from impedance measurements. The constraints of the FEM model and ORP-EIS experiments and the discrepancies between them are critically discussed in order to carry out a meaningful model validation, eventually leading to model improvements.

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Electrodeposition from an environmentally friendly iron sulfate electrolyte with citric acid as carbon source has gained attention recently, because of excellent mechanical properties of the resulting Fe–C coatings with intentionally codeposited high-carbon concentrations. While being very attractive as protective coatings and sustainable alternatives for hard chrome coatings, comprehensive understanding of the coatings' chemical constitution including the type and location of carbon-containing phases is still lacking. The amount of codeposited carbon of up to about 0.8 wt.% significantly exceeds the solubility of carbon in ferrite, although carbon-free ferrite is the only unambiguously reported phase in as-deposited Fe–C coatings so far. In the present work, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and soft X-ray emission spectroscopy have been applied to identify the carbon-containing phases, which are present as minor secondary phases in the coatings but are known to have an important influence on the coatings' properties. Three carbon-containing phases could be distinguished, homogeneously distributed in the nanocrystalline ferrite base material. Iron acetates, amorphous carbon, and carbides were found in both as-deposited and annealed Fe–C coatings up to 300°C, but their fraction changes during postdeposition annealing.

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The role of prior austenite grain size (PAGS) on the passive layer properties of martensitic steels is studied. Electron backscatter diffraction analysis shows that PAGS between 5 and 66 µm were obtained after applying different heat treatments. The barrier properties of passive film deteriorate with grain refinement up to 28 µm, attributed to increased donor density and a decrease in the fraction of γ-Fe2O3 in passive films. However, the further refinement of PAGS to 5 µm leads to improvement in the barrier properties due to the changes within the martensite structure. This improvement stems from the increase in γ-Fe2O3 fraction. @en

The role of microstructural defects (dislocation density and grain boundary areas) on the passive film properties formed on cold- and hot-rolled interstitial free (IF) steels is investigated in 0.1 M NaOH solution. Electron backscattered diffraction (EBSD) shows higher microstructural defect density on cold-rolled samples. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) results exhibit the lower barrier properties of passive films with the increase in microstructural defects. This is attributed to the increase in donor density measured with Mott-Schottky analysis and the lower relative quantity of protective γ-Fe₂O₃ in passive films (composed of Fe₃O₄,γ-Fe₂O₃ and FeO(OH)) with the increase in microstructural defect density.

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The bonding properties of zirconium- and titanium-based conversion coatings were evaluated using model conversion solutions of H₂ZrF₆ and H₂TiF₆ with addition of various organic additives (PAA, PVA, PVP). Macroscopic testing techniques such as contact angle and pull-off adhesion measurements were performed on galvanized steel sheets. Complementary to this, molecular studies were performed on model zinc substrates using ATR-FTIR in the Kretschmann configuration. The macroscopic and molecular approaches showed a good correlation demonstrating ATR-FTIR in the Kretschmann configuration to be a valuable tool to gain fundamental insights in metal oxide-polymer interfacial phenomena. Zirconium-treated galvanized steel substrates were shown to have a higher bonding affinity for the polyester coil coat primer than titanium-treated galvanized steel substrates. The presence of organic additives did not further improve the bonding properties. Yet, organic additives initially improved the interfacial stability of titanium-treated substrates. However, on the long term, organic additives are shown to be detrimental for polyester coil coat adhesion. This adverse effect of organic additives on the long term was assigned to its selective dissolution during immersion and was most pronounced for titanium-treatments. The limited effect of organic additives in case of zirconium-treatments was attributed to the higher portion of chemical interfacial bonds, as well as its tendency for crosslinking reactions causing entanglement of polymeric compounds in the zirconium oxide structure.

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In this work, in-situ ATR-FTIR in the Kretschmann configuration is proposed as an interfacial sensitive technique able to probe molecular processes at the buried interface of an industrial relevant polyester primer. Zinc, aluminium and magnesium oxide were used to represent oxides present at galvanized steel sheets used in coil coating. Two competing interactions with polyester resin and melamine-based crosslinker were shown to take place at metal hydroxide sites. This highlights the increased complexity of interfacial phenomena at metal–paint interfaces. Furthermore, in-situ ATR-FTIR was performed in deuterated water (D2O) to study the evolution of interfacial carboxylate bond degradation, without overlap of dominant water signals. For the first time, interfacial bond formation of paints and its degradation in an aqueous environment is studied in-situ. It is shown that the introduction of D2O at the interface initially increases the amount of interfacial carboxylate bonds, whereas upon longer exposure times bond degradation occurs. Significant delay of interfacial bond degradation on hexafluorozirconic acid treated oxides indicate successful stabilization of the metal-polymer interface by zirconium-based conversion coatings. Consequently, in-situ ATR-FTIR is able to demonstrate improved interfacial stability due to zirconium-based treatment in real-time and on a molecular level.@en

The effect of zirconium-based conversion of thermally vaporized zinc, aluminium and magnesium on the chemisorption of dimethylsuccinate was studied using attenuated total reflection – Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) analysis. Two competing chemisorption mechanisms contribute to interfacial bond formation. Hydrogen interactions on one hand were shown to occur between metal hydroxides and non-hydrolysed ester-groups of the molecule. On the other hand, both native and zirconium-treated substrates were shown to form interfacial carboxylate bonds with dimethylsuccinate, evidencing their capability of hydrolysing ester groups towards more reactive acid groups. Both interactions were shown to correlate to the metal oxide acid-base properties.@en

The overall performance of any organic coating system on a metal is largely dependent on the primer coating which has the crucial function to ensure adhesion to the metal substrate. Although performance in terms of adhesion can be empirically measured, the underlying chemical adhesion mechanism is difficult to unravel. A detailed molecular characterization of interfacial chemistry is required for this purpose, but brings up the challenge to reach the buried interface without inducing excessive damage to its molecular structure. In this work, argon gas cluster ions are being applied to sputter through an aminosilane coating on steel, in order to access the steel oxide-silane interface with time-of-flight secondary ion mass spectrometry (ToF-SIMS). In situ atomic force microscopy measurements during the sputter process demonstrate the importance of optimizing the Ar gas cluster ion beam in order to minimize sputter-induced roughness and molecular damage. ToF-SIMS spectra obtained at the buried steel oxide-aminosilane interface accessed by sputtering were compared to spectra from a steel oxide-aminosilane interface that was directly accessible without the need for sputtering. This comparison allowed us to identify contributions from sputter-induced damage in the buried interface spectra. Fragments characteristic for interfacial bonding interactions could be extracted, although there is a significant loss of molecular information because of sputtering. Nevertheless, insights into the role of steel surface hydroxyl groups in the adsorption mechanism of aminosilanes could be obtained through deuteration of the steel substrate.@en

Organofunctional silanes are applied as coupling agents between organic coatings and low carbon steel substrates to promote adhesion. Although the metal oxide-silane interface plays an important role in the performance of the entire overlying coating system, it remains challenging to obtain a clear understanding of the interfacial molecular bonding mechanism and its influence on adhesion. In this work, time-of-flight secondary ion mass spectrometry is used to study interfacial interactions between aminopropyl triethoxysilane (APS) and low carbon steel. APS is shown to bond to the steel substrate through silanol steel and amine-steel interactions, and coatings are cured at varying temperatures to evaluate the influence of curing on these different types of bonding interactions. Unambiguous evidence for hydrogen bond interactions between APS silanol groups and steel surface hydroxyl groups is provided for the first time in this work through deuteration of the steel substrate and allows to tackle long-lasting doubts about the most wide-spread bonding theory that has been postulated for silane adsorption on metals.

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Chemical throwing power, being the distance over which an inhibitor is able to protect a defect effectively, is an important parameter for active protective coatings. This study investigates the chemical throwing power of lithium-based leachable corrosion inhibitors exhibiting different leaching kinetics, from coatings at different inhibitor loading concentrations. The results demonstrate that Li-salt loaded coatings provide corrosion protection of defect areas up to a width of 6 mm. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used to detect the lateral spread of Li in the defect areas and provide the chemical speciation of corrosion protective layers in the defect areas.

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A spectro-electrochemical setup of Fourier transform infrared spectroscopy (FTIR) in the Kretschmann geometry and odd random phase multisine electrochemical impedance spectroscopy (ORP EIS) is applied to in situ monitor the effect of an aqueous electrolyte on the polymer/metal oxide interface. The interfacial interactions of an ultrathin polyacrylic acid (PAA) film on an aluminum oxide surface are identified as carboxylate ionic bonds and changes induced by the effect of water diffusion at the interface are monitored in situ. Initially after electrolyte exposure, an increase in ionic bonding is observed. However, eventually the interfacial interactions are replaced by water molecules, leading to macroscopic delamination. By comparing a variation of oxide types, the stability of the interfacial bonds is linked to the amount of free hydroxyl groups on the aluminum oxide surface. An electric equivalent circuit is proposed to model the ORP EIS response of the PAA/aluminum oxide system and the fitted resistance values could be interpreted in a physically meaningful way. Finally, a poly(methyl methacrylate) (PMMA) deposition on aluminum oxide is investigated to explore the effect of a variation in functional groups present at the polymer/metal oxide interface. It is shown that PMMA forms a more stable interface than PAA on native aluminum oxide. This work demonstrates that IR spectroscopy in the Kretschmann geometry and ORP EIS are suited techniques to in situ probe interfacial bonds at polymer/metal oxide systems exposed to aqueous conditions. Moreover, a variation of the surface properties of the metal oxide as well as the functional groups of the polymer alter the stability of their mutual interface when exposed to aqueous conditions.

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The mechanisms governing coating/metal oxide delamination are not fully understood, although adhesive interactions at the interface are considered to be an important prerequisite for excellent durability. This review aims to better understand the formation and degradation of these interactions. Developments in adhesion science made it clear that physical and chemical interfacial interactions are key factors in hybrid structure durability. However, it is very challenging to get information directly from the hidden solid/solid interface. This review highlights approaches that allow the (in situ) investigation of the formation and degradation of molecular interactions at the interface under (near-)realistic conditions. Over time, hybrid interfaces tend to degrade when exposed to environmental conditions. The culprits are predominantly water, oxygen, and ion diffusion resulting in bond breakage due to changing acid–base properties or leading to the onset of corrosive de-adhesion processes. Therefore, a thorough understanding on local bond interactions is required, which will lead to a prolonged durability of hybrid systems under realistic environments.@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

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

A common approach to investigate chemical interactions at the polymer/metal oxide interface is by monitoring ultrathin polymer films onto a metal oxide substrate by a variety of surface analysis techniques. The deposition of this nanometer-thin overlayer is frequently carried out by reactive adsorption from dilute polymer solutions. However, the influence of the solvent on the metal oxide chemistry is seldom taken into account in interface studies. The overall amount of available adsorption sites on the metal oxide surface might decrease due to competing adsorption of the solvent and the polymer adsorbate. Therefore, in this work, the adsorption of a common organic solvent (methanol) onto a physical vapor-deposited aluminum oxide surface is monitored in situ by an integrated attenuated total reflectance Fourier transform infrared spectroscopy in the Kretschmann geometry and odd random phase multisine electrochemical impedance spectroscopy system. It is shown that methanol immediately physisorbs onto the aluminum oxide surface and replaces the initial adventitious carbon layer. This process is followed by methanol chemisorbing onto the oxide surface to form methoxide species at the liquid/solid interface. Additionally, chemisorption is validated ex situ by X-ray photoelectron spectroscopy.

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