Corrosion of Paint 201
the lm-forming agent of the paint. Its density and composition are primar-
ily responsible for determining the permeability, corrosion resistance, and
ultraviolet (UV) resistance of the coating.
A continuous lm is formed either by physical curing, chemical curing,
or a combination of the two. A typical physical curing process is the sinter-
ing of thermoplastic powder coatings. Prior to application, this type of paint
consists of a large number of small binder particles. After depositing these
particles on a metal surface, they are baked in an oven to form a continuous
lm by sintering.
Chemical curing involves lm formation through chemical reaction. These
reactants can either be reactive curing or oxidative curing. In reactive cur-
ing, a polymer network is formed through polycondensation or polyaddition
reactions. This may be the case with multicomponent coatings where the
binder reacts with crosslinkers. In oxidative curing, oxygen from the atmo-
sphere reacts with the binder monomers, causing polymerization.
It is not uncommon for both physical and chemical curing to take place, as
in the case with the lm formation of thermosetting powders. At elevated
temperatures, physical sintering of the particles takes place, followed by
chemical reaction between different components in the powder. Another
example is lm formation of solvent-based reactive coatings, such as com-
mon house paints. With these paints, the solvent physically evaporates from
the curing lm, causing the binder molecules to coalesce and start chemical
polymerization reactions.
7.2.2 Pigments
The addition of pigments serves two purposes. First, pigments provide color
to the coating system to improve its aesthetic appeal, and second, they can
be added to improve the corrosion-protection properties of the coating. This
latter improvement can be obtained, for example, by incorporating ake-
shaped pigments parallel to the substrate surface. When a large volume con-
centration is used, the akes will hinder the permeation of corrosive media
into the coating by elongating their diffusion pathways.
Alternatively, anti-corrosion pigments can be added that will provide
active protection against corrosive attack. These pigments tend to dissolve
slowly in the coating and provide protection by covering corrosion-sensitive
sites under the coating by sacricially corroding themselves, thereby pro-
tecting the substrate metal, or by passivating the surface.
Blocking pigments can adsorb at the metal surface, thereby reducing the
active area for corrosion and forming a transport barrier for ionic species to
and from the substrate. Typical of this type is a group of alkaline pigments
such as lead carbonate, lead sulfate, and zinc oxide. These can form soaps via
interaction with organic oils.
Galvanic pigments are metal particles that are not noble related to the
metal substrate. On exposure, these particles (zinc dust on steel) corrode