11871 Properties and Types of Paint Systems

The properties of a paint coating will primarily be dependent upon the bulk properties of the system, and in this respect paint coatings generally consist of a primary coat(s), undercoat(s), and a top coat. From a corrosion protection viewpoint, these systems have two essential roles: (a) to provide an adherent bond between primer and substrate, and (b) to restrict the access of air, moisture, and aggressive ions to the substrate. It should be recognized at this stage that all paint coatings are permeable to a greater or lesser extent. It is the pigment that limits the rate of diffusion through the paint film. The performance of a paint is, however, often limited because of poor substrate surface preparation and it is crucial that the structure being painted is free from dust, grease, or corrosion products. This can be achieved chemically or mechanically as discussed

TABLE 11.9 Typical Paint Types and Applications

Type of Coating Uses and Comments

Alkyd General metal finishing. Mild corrosive atmospheres. Not recommended for immersion conditions. Fast drying, economic. Poor resistance to alkalis.

Phenolic Can and tank linings. Immersed structural steels or high-humidity atmospheres. Good chemical resistance.

Acrylic Automotive top coats, coil coats. Protection of steel in mild corrosive environments.

Epoxy resins Chemical processing. Good chemical resistance especially to alkalis. Good adherence properties. Surface deterioration (chalking) occurs in sunlight.

Urethane Aircraft finishes. Low-temperature applications. Excellent abrasion and impact resistance. Variable corrosion resistance depending upon formulation. Good gloss retention.

Epoxy coatings Air conditioners, tanks, heat exchangers. 'White goods.'

Excellent corrosion resistance. Versatile, flexible, and good hardness.

Vinyl coatings Used widely in the chemical industry for severe corrosive environments. Not recommended above 50-60°C. Can be used as thin or high-build film thickness deposits. Poor resistance to solvents.

Metal-rich primers Act sacrificially to protect the underlying steel substrate. Good (Zn) abrasion and temperature resistance. Used in conjunction with other top-coat systems.

previously. Paints should thereby be highly impermeably and resistant to abrasion, be suitable for the desired temperature range, and be flexible, thereby resisting cracking on movement of the substrate.

Selection of the most suitable coating system is not trivial and, as previously noted, depends upon a number of considerations. One of the more important steps in coating selection will be a thorough evaluation of the environmental conditions under which the coating will operate. Table 11.9 provides a brief summary of some of the more common types of paint and their typical applications.

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