Organic pigments metallic pigment surfaces

In some cases, a pigment s thermal and chemical resistance can be improved by the encapsulation of the pigment particles by an iasoluble, colorless layer of metal oxide or oxide—hydroxide, eg, siUca, Si02. The function of such a shell is to prevent direct contact and reaction between the pigment surface and the organic matrix ia which the pigment is dispersed (11). 

If a paint film is to prevent this reaction, it must be impervious to electrons, otherwise the cathodic reaction is merely transferred from the surface of the metal to the surface of the film. Organic polymer films do not contain free electrons, except in the special case of pigmentation with metallic pigments consequently it will be assumed that the conductivity of paint films is entirely ionic. In addition, the films must be impervious to either water or oxygen, so that they prevent either from reaching the surface of the metal. 

Organic pigments are used for colored paints. Carbon blacks are classed as organic and come in a variety of properties which depend on the method of production. Metallic powders and flakes are often used especially for hot surfaces. Aluminum, copper, and bronze powders also serve as decorative coatings. 

Active anticorrosive pigments inhibit one or both of the two electrochemical partial reactions. The protective action is located at the interface between the substrate and the primer. Water that has diffused into the binder dissolves soluble anticorrosive components (e.g., phosphate, borate, or organic anions) out of the pigments and transports them to the metal surface where they react and stop corrosion. The oxide film already present on the iron is thereby strengthened and sometimes chemically modified. Any damaged areas are repaired with the aid of the active substance. Inhibition by formation of a protective film is the most important mode of action of the commoner anticorrosive pigments. 

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