Hydrogen permeation barrier coatings: preliminary results on the development of oxide coatings through chemical vapor deposition

In the framework of hydrogen exploitation for green energy production, it is pivotal for safe and effective H2 production, storage, and delivery, to develop hydrogen permeation barrier (HPB) coatings able to prevent the hydrogen embrittlement (HE) of metallic pipelines and tanks. Among various materials, alumina (Al2O3) is a promising barrier material because of its low hydrogen permeability, high thermal and chemical stability, and hardness. However, the coefficient of thermal expansion of alumina is lower than that of the metal constituting the material of the infrastructure for hydrogen storage and transport, typically steel. This can cause stress and/or defects in the barrier layer, consequently limiting its blockade properties. One strategy to alleviate the thermal mismatch is the insertion of an inter-layer of another material, having an intermediate thermal expansion coefficient between that of the metal and alumina. Compared to the single-layer counterparts, such materials often perform high hydrogen permeation reduction factor (PRF) because they form a barrier film that can simultaneously take advantage of the properties of the different materials and the interface can further increase the resistance to hydrogen permeation, acting as a hydrogen trap. Moreover, in the presence of nano-multilayers, the tendency for the formation of grains with a columnar structure, which are generally preferential routes for the diffusion of hydrogen, is strongly reduced. Titania (TiO2), having a thermal expansion coefficient intermediate between that of alumina and steel, can improve the adhesion of the Al2O3 film to the metal substrate. In this preliminary study, TiO2/Al2O3 composite films were deposited employing metal-organic chemical vapor deposition (MOCVD) technique. The MOCVD method can be used to prepare single layers, as well as for multilayer systems. Preliminary chemical, morphological, and structural characterizations of deposited composite materials were carried out. Moreover, the life cycle assessment (LCA) was carried out to highlight the environmental hot spots of the procedure.