Film/coating formation pinholes

Corrosion Initiation Defects in coatings are always preferential sites for corrosion initiation. Apart from the cases mentioned above - soluble salts inclusions, volatile components - the accidental formation of defects during its service life is common, that is, in the form of pinholes or scratches. The electrochemical description of a defect next to an intact coating area is shown in Fig. 4. When a small defect is exposed to a corrosive environment - which may be either a bulk liquid phase or only a thin film of condensed water - the part of the substrate that is directly exposed will start to corrode, forming metal oxides and hydroxides that block the defect. These corrosion products are permeable to water but impermeable to oxygen. Therefore, a separation between the cathodic and the anodic areas occurs. Underneath the oxides, that is, at the center of the defect, the anodic reaction takes place, whereas the cathodic reaction occurs further away from the defect [80] (Fig. 6). 

Polymers can be evaporated, deposited as a thin film, and cured in a vacuum system to provide a basecoat. For example, acrylate coatings can be deposited and cured with an e-beam. The deposited liquid flows over the surface and covers surface flaws, reducing pinhole formation. This technique can be used in vacuum web coating and has been found to improve the barrier properties of transparent barrier coatings. 

Moreover, thin (less than 2 pm thickness) and pinhole-free Pd-Cu alloy composite membranes with a diffusion barrier have been fabricated on mesoporous stainless steel supports (MSSS) by vacuum electro-deposition (Nam and Lee, 2001). The deposition film was fabricated by multilayer coating and diffusion treatment and the formation of Pd-Cu alloys was achieved by annealing the as-deposited membranes at 450°C in nitrogen atmosphere. To improve the structural stability of Pd alloy/Ni-MSSS composite membranes, a thin intermediate layer of silica by sol-gel method was introduced as a diffusion barrier between Pd-Cu active layer and a modified MSSS substrate. The composition and phase structures of the alloy film were studied by energy dispersive analysis (EDS) and XRD the typical Pd-Cu plating had a composition of 63% Pd and 37%Cu and the atomic inter-diffusion of Pd and Cu resulted in Pd-Cu alloys in an fee structure. The electron probe microanalyser (EPMA) profiling analysis indicated that the improved membranes were structurally stable. The Pd-Cu alloy composite membrane obtained in this study yielded excellent separation performance with H2 permeance of 2.5 x 10 cm /(cm cmHg s) and Hj/Nj selectivity above 70000 at 450°C. 

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