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(c) barrier

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FIGURE 11.23 Categories of coating systems.

FIGURE 11.23 Categories of coating systems.

Sacrificial Coatings These coatings, as their name suggests are "sacrificed" to afford protection to the substrate, that is, the coating corrodes preferentially to the metal substrate (e.g., Zn, Cd). Noble Coatings These coatings are more corrosion resistant than the substrate being protected. In this respect they are required to be free from cracks and pores, otherwise preferential corrosion of the substrate will occur at defects in the coatings (e.g., Ni, Cr). Barrier Coatings These coatings are principally nonmetallic and nonconducting. Their primary mechanism of protection is one of separating the substrate from the corrosive environment (e.g., alkyd, epoxy).

The success of a coating system is largely dependent upon the surface preparation received by the substrate. Typically the following stages can be identified for a paint coating:

1. Chemical/physical cleaning with acids/alkalis, wire brushing, or blast cleaning

2. Chemical or physical pretreatment using chemical etchants, conversion coatings, or particle abrasion

3. Primary coat to form an adherent substrate/coating interface

4. Additional coats to build up the bulk properties of the coating

5. Top coat to provide additional resistance and aesthetic qualities

Metallic coatings applied via spraying or hot dipping may be restricted to stages 1 and 2. A brief description of some of the more common types of anticorrosion coatings will be given, although more extensive information may be found in references [53, 56, 57].

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