Protective coatings typical layered structure

Typical layered structure of a conventional protective coating. 

FIGURE 16.15 (a) Typical cross-sectional structure of an optical disk and (b) the intensity profile of the read-out system in binary code as a function of time (t). ffere, T is the transparent polymer support, R is the reflective metal layer, PC is the protective coating, and P represents a depression created for information storage. (Reproduced with permission from HUthig and Wepf Verlag.)... 

Excellent bonding can be achieved with zinc phosphate and mixed metal phosphates but the particle size and quantity of phosphate applied are very important. There are two main physical forms of phosphate that are used in commerce. One is an amorphous structure applied at a level typically below 4 g/m and the other an acicular (needle shaped) structure typically applied at a level of 15 g/m. The acicular form is used as an absorbent substrate for coatings and oils used to enhance the phosphate layer as protective coating. The acicular form is unsuitable for bonding as the crystal structure can fracture under the bonding agent primer. 

Contemporary protective coatings have, as a rule, a quite complex structure, which is, in turn, closely related to the underlying substrate and to the specific demands on the corrosion resistance. However, it is possible to point out some common elements present in the vast majority of protective coatings. A typical protective coating is a layered system consisting of (i) a pre-treatment layer or conversion layer (ii) a primer layer and (iii) a top-coat layer (Fig. 10.1). Quite often, a layer of electrochemically more active metal (Zn, Cd, etc) is deposited on the top of a metal or alloy substrate to provide additional cathodic protection (Brock etal, 2009). 

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

Typical PlacemeTd Specifucaiions for Reinforcement. The minimum clear distance between parallel bars should be 2 times the side dimensions for square bars and 1 4 times the diameter for round bars. Reinforcement of footings and columns should be sealed with at least 3 in. of plain concrete on the ground cont"ct surface. Surfaces exposed to weathering should have at least a 2-in. protective layer of plain concrete. Structures subject to fire hazards should have a fire-resistant coating of concrete 1 in. thick for slabs and 2 to 4 in. thick for structural members. 

Coatings such as a zinc silicate primer covered with a layer of an epoxy-based polymer are routinely applied to steel structures to protect them against corrosion. However, cracks or flaws in the coating expose Fe which then undergoes oxidation in an anodic process. To prevent this, a second protection system is put in place cathodic protection. By placing a block of a more electropositive metal on the surface, this second metal is preferentially oxidized. This is the same principle as the use of zinc in galvanized steel (see Section 6.7). From Table 8.1, you can see why Zn, A1 and Mg (or alloys of these metals) are typically chosen as sacrificial anodes. The most electropositive metals (Li, Na, K and Ca) are unsuitable because they react with cold water. The relevant half-equations (at pH 7) are now ... 

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