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 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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