Surface coatings, discontinuous

Steady-State Permeation Through Discontinuous Surface Coatings... 

Figure 6. A, one symmetry unit (% lattice repeat) in a lattice consisting of a membrane with a discontinuous surface coating. B, the same unit with appropriate boundary conditions. (Reproduced with permission from Ref. 5. Copyright 1986 Elsevier.)...

Irregular surface coatings such as discontinuous oxide coatings, deposits of more noble metals (e g., copper on iron), and deposits of conducting materials, such as graphite, resulting from fabrication processes... 

Blech, 1. A. and Meieran, E. S. (1967), Enhanced x-ray diffraction from substrate crystals coating discontinuous surface films. Journal of Applied Physics 38, 2913-2919. 

Nonconsumable Anodes. Platinized anodes utilize a few micrometers thick surface coating of platinum on titanium, niobium, or tantalum substrates. Restricting the use of platinum to a thin surface film has important cost advantages. For extended life, the thickness of the platinum surface layer can be increased. The corrosion resistance of the substrate materials is important due to the presence of discontinuities in the platinum surface coating that invariably will occur in practice. It is important that the potential of unplatinized areas on these anodes does not exceed the critical depassivation value for a given substrate material. 

Proper thermal-spray and organic-coating surface prqiaration and sqpplication procedures must be followed to avoid premature failure in service. Figure 12 shows photographs illusuating corrosion evident on coated discontinuous silicon carbide/aluminum MMCs. 

Tin will protect copper from corrosion by neutral water. Pure tin is anodic to copper, and protects discontinuities by sacrificial corrosion. Both intermetallic phases are strongly cathodic to copper, and corrosion is stimulated at gaps in wholly alloyed coatings. An adequate thickness of tin is needed for long service, e.g. 25-50 xm. Another diffusion problem occurs with tin-plated brass. Zinc passes very quickly to the tin surface, where under conditions of damp storage zinc corrosion products produce a film... 

Two basic versions of the process plasma-based ion plating and vacuum-based ion plating. The coating material is vaporized in a manner similar to evaporation. Typically, the plasma is obtained by biasing the substrate to a high negative potential (5 kV) at low pressure. The constant ion bombardment of the substrate sputters off some of the surface atoms which results in improved adhesion and reduced impurities. Surface coverage of discontinuities is also improved. 

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