Protective Paint-and-Varnish Coatings

coatings applied to the surface of metal articles and structures to protect them from corrosion and to provide them with a decorative finish. Protective paint-and-varnish coatings make no essential changes in the electrochemical nature of the processes that take place on the surface of corroding metals, merely reducing their speed. The coatings act as a diffusion barrier that effectively slows down the penetration of aggressive outside agents to the surface of the metal, and in most cases they also alter the metal’s electrical potential.

Protective paint-and-varnish coatings are multilayered systems that consist of an undercoat in direct contact with the metal and of upper covering layers. The undercoat must ad-here dependably to the metal and possess good anticorrosive properties. It contains film-forming materials and pigments. The film-forming substances are alkyd resins, epoxy resins, vegetable oils, and pigments (these pigments include red lead and iron oxide for undercoats on ferrous metals and zinc chromate for undercoats on nonferrous metals.) Moreover, so-called protective undercoats are used. They consist of a binder (about 5 percent) and zinc dust (up to 95 percent), and like the zinc coating they protect the metal electrochemically.

The upper paint-and-varnish coatings should be impervious to moisture, steam, gases, and ions of electrolytes and must not swell or crack in the working medium. The most common film-forming substances used in protective coatings are the alkyd resins and their combinations with melamineformaldehyde resins and urea-formaldehyde resins. Coatings with a phenol aldehyde resin, epoxide resin, and polyvinylchloride base have stable chemical qualities. The upper layers of heat-resistant protective paint-and-varnish coatings use silicon-organic polymers and polyimides. The introduction of pigments raises the heat resistance of protective paintand-varnish coatings and slows down their rate of aging.

Prior to applying protective paint-and-varnish coatings, the surface of the metals is specially prepared: scale, oxides, fatty substances, and moisture are removed. Sometimes the surface is sandblasted, phosphated, or anodized. Protective paint-and-varnish coatings are applied to surfaces being painted by pneumatic spraying, power spraying, and other methods. The coatings may be dried at room temperature or raised temperature, depending on the properties of the film-forming material and the size of the object or the structure.