Materials Performance

SEP 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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The Fundamentals of a Coating 1 9 4 3 – 2 0 1 8 NACE INTERNATIONAL 75 The fundamentals of a coating could refer to several things, depending on the purpose or the use of the coating. For example, an antifouling paint would have the fundamental prop- erty of inhibiting the growth of animal or vegetable organisms on the coating. A fire-resistant coating must fundamentally resist burning, or at least retard the burning of the substrate. A coating to be applied over concrete must have a fundamental property of resistance to strong alkali. All corrosion-resistant coatings, however, must fundamentally resist the corrosive atmosphere and prevent it from reaching the basic structure. Thus, there are as many variations in the types of coatings as there are in the forms of corrosion. The design of an effective anticorrosive coating is a complex task, which requires an extensive knowledge of not only corrosion principles but of the science and chemistry of coating formation as well. Without such inclusive information, the development of effective corrosion-resistant coatings would be impossible. A coating is not a self-supporting structure. It is part of an overall system, which includes the basic structure that supports the coating. Although it is always on a substrate of one kind or another, a coating can be thought of in the same light as a building. In order to be strong, a build- ing must have a heavy, carefully constructed foundation; in order to be durable, a coating must also have a carefully designed (formulated) and constructed (applied) foundation (substrate and primer). A building also consists of a number of interlocking parts—the foundation, the superstruc- ture, and the roof—and each one has a different function. The corresponding parts of a coating are the primer, intermediate coats, and topcoat. In the case of a small building with a relatively short, useful life, the foundation and superstruc- ture may be minimal. The same is true of a coating applied only for decorative purposes where surface preparation, application, and long life may be easily overlooked. In the instance of a substantial industrial structure, however, durability, reliability, and long life are required. Again, the same holds true for an industrial corrosion-resistant coating, which likewise must be engineered with a properly prepared substrate, a sound foundation coat or primer, a strongly reinforced inter- mediate coat, and long-lasting weather and corrosion-resistant topcoats. In constructing a build- ing, the substrate (the soil or ground) indicates the type and extent of the foundation since sand, clay, or rock all have different foundation requirements. The same, of course, is true of coatings. The primer must be designed specifically for the substrate, whether it is steel, aluminum, concrete, plastic, or wood. In fact, the surface over which a coating is applied may be more important from the standpoint of long life and durability than the design of the coating itself. The fundamental concepts involved in corrosion-resistant coatings, then, include those of coating protection, component design, component function, and coating formulation. Many coat- ings contain as many as 15 to 20 ingredients, each of which has its own function in the overall performance of the coating. A coating system may employ one or more of the basic coating concepts of impermeability, inhibition, and anodic or cathodic pigments. While many coating systems employ only one of these concepts, some of the most successful anticorrosive systems combine two of the concepts into one coating system. This article is adapted from Corrosion Prevention by Protective Coatings, Third Edition, Charles G. Munger, Louis D. Vincent, assoc. ed. (Houston, TX: NACE International, 2014), p. 63. Corrosion Basics A19 SEPTEMBER 2018 MATERIALS PERFORMANCE: VOL. 57, NO. 9

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