Conversely,i nazinc-rich coating, the roleo ft he zinc pigmentp redominates. The high amount of zinc dust metal in the dried film determines
the coating’sb asic property,i .e., galvanicp rotection. For ac oating to be
considered zinc-rich, the commonruleofthumb is that there must be at least
75% by weight of zinc dust in the dried film. This may change because
conductive extenders (notably di-iron phosphide) have been added to
improve weldability and burn-through, with supposedly equivalent protection at lower zincloadings.
The primary advantage of zinc-rich coatings is their ability to galvanically
protect. The zinc coating preferentially sacrifices itselfinthe electrochemical
corrosion reaction to protect the underlying steel. This galvanic reaction,
together with the fillinga nd sealing effect of zincr eaction products
(primarily zinc carbonate, zinc hydroxide, and complex zinc salts), provides
more effectivec orrosionp rotectiont os teel substrates than does any
other typeo fc oating. Zinc-rich coatings can not be used outside the pH
rangeo f6–10.5.
7.13.1.2 Pigments
Pigments serve several functions as part of the coating: They reinforcet he
film structurally,p rovide color and opacity,a nd serve special purposes for
metal protection.
Typical materials used as color or hiding pigments and to provide
aestheticv alue, retention of gloss and color,a nd to help with the film
structure and impermeability are iron oxides, titanium dioxide, carbon or
lamp black, and others.
Pigments mustb es omewhat resistant to the environment and must be
compatible with the resin. For example, calcium carbonate, that is attacked
by acid, should not be used in an acid environment. Water-soluble saltsare
corrosion promoters, so special low salt containing pigments are used in
primers for steel.
Primers contain one of three kinds of pigments for special protective
properties as follows:
1. Inert or chemically resistant.These are used to form barriercoatings
in severe environments sucha sp Hl ess than 5o rg reater than 10.
Theya re also used as nonreactive extender, hiding,o rc olor
pigments in neutral environments.
2. Active.L eads, chromates, or other inhibitive pigments areu sed in
linseedo il/alkyd primers.
3. Galvanically sacrificial.Z inci se mployed at highc oncentrations to
obtain electrical contact for galvanic protection in environments
between pH 5a nd 10.
Table 7.15 shows the typea nd characteristics of these pigments.
Coatings
467
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
the coating’sb asic property,i .e., galvanicp rotection. For ac oating to be
considered zinc-rich, the commonruleofthumb is that there must be at least
75% by weight of zinc dust in the dried film. This may change because
conductive extenders (notably di-iron phosphide) have been added to
improve weldability and burn-through, with supposedly equivalent protection at lower zincloadings.
The primary advantage of zinc-rich coatings is their ability to galvanically
protect. The zinc coating preferentially sacrifices itselfinthe electrochemical
corrosion reaction to protect the underlying steel. This galvanic reaction,
together with the fillinga nd sealing effect of zincr eaction products
(primarily zinc carbonate, zinc hydroxide, and complex zinc salts), provides
more effectivec orrosionp rotectiont os teel substrates than does any
other typeo fc oating. Zinc-rich coatings can not be used outside the pH
rangeo f6–10.5.
7.13.1.2 Pigments
Pigments serve several functions as part of the coating: They reinforcet he
film structurally,p rovide color and opacity,a nd serve special purposes for
metal protection.
Typical materials used as color or hiding pigments and to provide
aestheticv alue, retention of gloss and color,a nd to help with the film
structure and impermeability are iron oxides, titanium dioxide, carbon or
lamp black, and others.
Pigments mustb es omewhat resistant to the environment and must be
compatible with the resin. For example, calcium carbonate, that is attacked
by acid, should not be used in an acid environment. Water-soluble saltsare
corrosion promoters, so special low salt containing pigments are used in
primers for steel.
Primers contain one of three kinds of pigments for special protective
properties as follows:
1. Inert or chemically resistant.These are used to form barriercoatings
in severe environments sucha sp Hl ess than 5o rg reater than 10.
Theya re also used as nonreactive extender, hiding,o rc olor
pigments in neutral environments.
2. Active.L eads, chromates, or other inhibitive pigments areu sed in
linseedo il/alkyd primers.
3. Galvanically sacrificial.Z inci se mployed at highc oncentrations to
obtain electrical contact for galvanic protection in environments
between pH 5a nd 10.
Table 7.15 shows the typea nd characteristics of these pigments.
Coatings
467
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485