© 2006 by Taylor & Francis Group, LLC

42 Corrosion Control Through Organic Coatings
and calcium [93]. The major benefit of this inorganic film seems to be that it prevents
Cl − and other corrosion-initiating species from reaching the metal surface.
The dual action of entrapment of aggressive cations and release of inhibitor gives
calcium-exchange silica two advantages over traditional anticorrosion pigments:
1. The “inhibitor” ion is only released in the presence of aggressive cations,
which means that no excess of the pigment to allow for solubility is
necessary.
2. No voids are created in the film by the ion-exchange; the coating has
fairly constant permeability characteristics [38,93–95].
2.3.7.2 Barium Metaborate
Barium metaborate is a pigment to avoid. It contains a high level of soluble barium,
an acute toxicant. Disposal of any waste containing this pigment is likely to be
expensive, whether that waste is produced in the manufacture or application of the
coatings or much later when preparing to repaint structures originally coated with
barium metaborate.
Barium metaborate creates an alkaline environment, inhibiting the steel; the
metaborate ion also provides anodic passivation [23]. The pigment requires high
loading levels, up to 40% of coating weight, according to Beland. It is highly soluble
and fairly reactive with several kinds of binders; this leads to stability problems
when formulated with such products as acidic resins, high-acid number resins, and
acid-catalyzed baking systems. A modified silica coating is often used to reduce and
control solubility. One way to decrease its reactivity and, therefore, increase the
number of binders with which it can be used, is to modify it with zinc oxide or a
combination of zinc oxide and calcium sulphate [23]. The high loading level required
for heavy-duty applications implies that careful attention must be paid to the
PVC/CPVC ratio when formulating with this pigment.
Information regarding actual service performance of barium metaborate coatings
is scarce, and what does exist does not seem to justify the use of this pigment. In
the early 1980s, the state of Massachusetts repair-painted a bridge with barium
metaborate pigment in a conventional oil/alkyd vehicle. The result was not satisfactory:
after six years, considerable corrosion had occurred at the beam ends and on
the railings above the road [22]. It should perhaps be noted that an alkyd vehicle is
not the ideal choice for a pigment that generates an alkaline environment; better
results may perhaps have been obtained with a higher-performance binder. However,
because of the toxicity problems associated with soluble barium, further work with
barium metaborate does not seem to be warranted.
2.3.7.3 Molybdates
Molybdate pigments are calcium or zinc salts precipitated onto an inert core such
as calcium carbonate [47,96–98]. They prevent corrosion by inhibiting the anodic
corrosion reaction [47]. The protective layer of ferric molybdate, which these pigments
form on the surface of the steel, is insoluble in neutral and basic solutions.
© 2006 by Taylor & Francis Group, LLC