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Hydrogen barrier coatings are protective layers consisting of materials with a low intrinsic hydrogen diffusivity and solubility, showing the potential to delay, reduce or hinder hydrogen permeation. Hydrogen barrier coatings are expected to enable steels, which are susceptible to hydrogen embrittlement, specifically cost-effective low alloy-steels or light-weight high-strength steels, for applications in a hydrogen economy. Predominantly, ceramic coating materials have been investigated for this purpose, including oxides, nitrides and carbides. In this review, the state of the art with respect to hydrogen permeation is discussed for a variety of coatings. Al2O3, TiAlN and TiC appear to be the most promising candidates from a large pool of ceramic materials. Coating methods are compared with respect to their ability to produce layers with suitable quality and their potential for scaling up for industrial use. Different setups for the characterisation of hydrogen permeability are discussed, using both gaseous hydrogen and hydrogen originating from an electrochemical reaction. Finally, possible pathways for improvement and optimisation of hydrogen barrier coatings are outlined.

Amorphous alumina coatings, intended as hydrogen barriers, were successfully deposited on Fe-8 wt% Cr substrates by plasma ion-assisted deposition technique. The amorphous structure of the coatings was confirmed by transmission electron microscopy and X-ray diffraction. The interfacial and mechanical properties of the coating-substrate system were evaluated using an in-house custom-designed backside electrochemical hydrogen charging method. In this approach, the substrate side faces the electrolyte (hydrogen entry side) and the mechanical behavior was tested on the coating side (hydrogen exit side). A Kelvin-probe-based measurement was performed to determine the hydrogen diffusivity in these amorphous alumina coatings at room temperature using a similar backside charging approach. Chemical and microstructural characterizations, in combination with scratch and hardness testing, show that interfacial hydrogen accumulation is strongly responsible for drastic changes in the scratch morphology of the coating and its adhesion to the substrate. Scratch testing promises to be a quick and easy technique to fingerprint changes at the coating/substrate interface upon hydrogen exposure. Graphical abstract