© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 47
There are two types of aluminum pigment: leafing and nonleafing. Leafing pigment
orients itself parallel to the substrate at the top of the coating; this positioning enables
the pigment to protect the binder against UV damage but may not be the best location
for maximizing barrier properties. Leafing properties depend on the presence of a
thin fatty acid layer, commonly stearic acid, on the flakes. Nonleafing aluminum
pigments have a more random orientation in the coating and are very effective in
barrier coatings [109]. De and colleagues, for example, have obtained favorable
results with aluminum in a chlorinated rubber vehicle in seawater trials in India [126].
2.3.8.4.2 Zinc Flakes
Zinc flakes should not to be confused with the zinc dust used in zinc-rich coatings:
the size is of a different magnitude altogether. Some research suggests that zinc
flakes could give both the cathodic protection typical of zinc dust and the barrier
protection characteristic of lamellar pigments [109]. However, in practice, this could
be very difficult to achieve because the zinc dust particles in zinc-rich paints have
to be in electrical contact to obtain cathodic protection. Designing a coating in which
the zinc particles are in intimate contact with each other and with the steel, and yet
completely free of gaps between pigment and binder or between pigment particles,
is difficult. The lack of any gaps is critical for a barrier pigment, because it is
precisely these gaps that provide the easy route for water and oxygen to reach the
metal surface. In fact, Hare and Wright’s [127] research shows that zinc flakes
undergo rapid dissolution in corrosive environments when they are used as the sole
pigment in paints; their coatings are prone to blistering.
2.3.8.4.3 Other Metallic Pigments
Other metallic pigments, such as stainless steel, nickel, and copper, have also been
used in recent years. Their use in coatings of metals with more noble electrochemical
potential than carbon steel entails a certain risk of galvanic corrosion between the
coating and the substrate. The pigment volume concentrations in such paints must
be kept well below the levels at which the metallic pigment particles are in electrical
contact with each other and the carbon steel. If this condition is not met, pitting
follows. Bieganska recommends using a nonconducting primer as an insulating layer
between the steel substrate and the barrier coating, if it is necessary to use a strong
electropositive pigment in the barrier layer [109]. The same author also warns that,
although the mechanical durability and high-temperature resistance of stainless steel
flake makes this type of pigment desirable, it is not suited to applications where
chlorides are present [109].
Nickel flake-filled coatings can be useful for strongly alkaline environments.
Cupronickel flakes (Cu – 10% Ni – 2% Sn) are used in ship protection because of
their outstanding antifouling properties. The alloy pigment is of interest in this
application because its resistance to leaching is better than that of copper itself [109].
2.3.9 CHOOSING A PIGMENT
Before choosing a pigment and formulating paint, one question must be answered:
will an active or a passive role be required of the pigment? The role of the pigment
— active or passive — must be decided at the start for the fairly straightforward
© 2006 by Taylor & Francis Group, LLC