202
Fundamentals of Corrosion
preferentially, while at the original metal surface only the cathodic reaction
takes place.
Passivating pigments reconstruct and stabilize the oxide film on the
exposed metal substrate. Chromates (e.g., zinc chromate, strontium chromate) with limited water solubility are used for this purpose. In aqueous
systems, they may cause anodic passivation of a metal surface with a very
stable chromium and oxygen containing layer.
Color or hiding pigments are selected to provide aesthetic value, retention
of gloss and color, as well as help with film structure and impermeability.
Examples include iron oxides, titanium dioxide, carbon or lampblack, and
others.
Pigments must be compatible with the resin and should also be somewhat resistant to the environment; for example, calcium carbonate, which is
attacked by acid, should not be used in an acidic environment. Water-soluble
salts are corrosion promoters, so that special low-salt-containing pigments
are used as primers for steel.
For special protective properties, primers contain one of three kinds of
pigments:
1. Inert or chemically resistant pigment. These are for use in barrier coatings in severe environments, such as conditions below an acidity of
pH 5 or above an alkalinity of pH 10, or as a nonreactive extender,
hiding or color pigments in neutral environments.
2. Active pigment. Leads, chromates, or other inhibitive pigments are
used in linseed oil/alkyd primers.
3. Galvanically sacrificial pigment. Zinc is employed at high concentrations to obtain electrical contact for galvanic protection in environments between pH 5 and 10.
Types and characteristics of these pigments are presented in Table 7.3.
7.2.3 Solvents
The purpose of a solvent is to reduce the viscosity of the binder and other
components so as to enable their homogeneous mixing. In addition, the
reduced viscosity makes it possible to apply the coating as a thin, smooth,
continuous film on a specific surface. The roles of a solvent in a coating prior
to application and after application are contradictory. In the liquid state,
before application, paint should form a solution or a stable dispersion or
emulsion of binder, pigments, and additives in the solvent. All solid components should remain more or less homogeneously distributed in the liquid
phase. This requires high compatibility between solvents and components
and the presence of repulsive forces between solvents and components to
avoid clustering. In contrast, after applying the paint, a major attractive force
Fundamentals of Corrosion
preferentially, while at the original metal surface only the cathodic reaction
takes place.
Passivating pigments reconstruct and stabilize the oxide film on the
exposed metal substrate. Chromates (e.g., zinc chromate, strontium chromate) with limited water solubility are used for this purpose. In aqueous
systems, they may cause anodic passivation of a metal surface with a very
stable chromium and oxygen containing layer.
Color or hiding pigments are selected to provide aesthetic value, retention
of gloss and color, as well as help with film structure and impermeability.
Examples include iron oxides, titanium dioxide, carbon or lampblack, and
others.
Pigments must be compatible with the resin and should also be somewhat resistant to the environment; for example, calcium carbonate, which is
attacked by acid, should not be used in an acidic environment. Water-soluble
salts are corrosion promoters, so that special low-salt-containing pigments
are used as primers for steel.
For special protective properties, primers contain one of three kinds of
pigments:
1. Inert or chemically resistant pigment. These are for use in barrier coatings in severe environments, such as conditions below an acidity of
pH 5 or above an alkalinity of pH 10, or as a nonreactive extender,
hiding or color pigments in neutral environments.
2. Active pigment. Leads, chromates, or other inhibitive pigments are
used in linseed oil/alkyd primers.
3. Galvanically sacrificial pigment. Zinc is employed at high concentrations to obtain electrical contact for galvanic protection in environments between pH 5 and 10.
Types and characteristics of these pigments are presented in Table 7.3.
7.2.3 Solvents
The purpose of a solvent is to reduce the viscosity of the binder and other
components so as to enable their homogeneous mixing. In addition, the
reduced viscosity makes it possible to apply the coating as a thin, smooth,
continuous film on a specific surface. The roles of a solvent in a coating prior
to application and after application are contradictory. In the liquid state,
before application, paint should form a solution or a stable dispersion or
emulsion of binder, pigments, and additives in the solvent. All solid components should remain more or less homogeneously distributed in the liquid
phase. This requires high compatibility between solvents and components
and the presence of repulsive forces between solvents and components to
avoid clustering. In contrast, after applying the paint, a major attractive force