© 2006 by Taylor & Francis Group, LLC

Composition of the Anticorrosion Coating 45
2.3.8.2 Micaceous Iron Oxide
MIO is a naturally occurring iron oxide pigment that contains at least 85% Fe 2O 3.
The term ‘‘micaceous” refers to its particle shape, which is flake-like or lamellar:
particles are very thin compared to their area. This particle shape is extremely
important for MIO in protecting steel. MIO particles orient themselves within the
coating, so that the flakes are lying parallel to the substrate’s surface. Multiple layers
of flakes form an effective barrier against moisture and gases [40,109–116]. MIO
is fascinating in one respect: it is a form of rust that has been used as an effective
pigment in barrier coatings for decades to protect steel from … rusting.
For effective barrier properties, PVCs in the range of 25% to 45% are used, and
the purity must be at least 80% MIO (by weight). Because MIO is a naturally occurring
mineral, it can vary from source to source, both in chemical composition and in particle
size distribution. Smaller flakes mean more layers of pigment in the dried film, which
increases the pathway that water must travel to reach the metal. Schmid has noted
that, in a typical particle-size distribution, as much as 10% of the particles may be too
large to be effective in thin coatings, because there are not enough layers of flakes to
provide a barrier against water. To provide a good barrier in the vicinity of these large
particles, MIO is used in thick coatings or multicoats [88].
Historically, it has been believed that MIO coatings tend to fail at sharp edges
because the miox particles were randomly oriented in the vicinity of edges. Random
orientation would, of course, increase the capillary flow of water along the pigment’s
surface toward the metal substrate. However, Wiktorek and Bradley examined coverage
over sharp edges using scanning electron microscope images of cross-sections.
They found that lamellar miox particles always lie parallel to the substrate, even
over sharp edges. The authors suggested that when failure is seen at edges, the
problem is really thinner coatings in these areas [117].
In addition to providing a barrier against diffusion of aggressive species through
the coating, MIO confers other advantages:
• It provides mechanical reinforcement to the paint film.
• It can block ultraviolet light, thus shielding the binder from this destructive
form of radiation.
For the latter reason, MIO is sometimes used in topcoat formulations to improve
weatherability [40,109].
The chemical inertness of MIO means that it can be used in a variety of binders:
alkyd, chlorinated rubber, styrene-acrylic and vinyl copolymers, epoxy, and
polyurethane [40].
2.3.8.2.1 Interactions of MIO with Aluminum
It is not clear from the literature whether or not combining MIO and aluminum
pigments in a coating poses a problem. There are recommendations both for and
against mixing MIO with these pigments.
In full-scale trials of various paint systems on bridges in England, Bishop found
that topcoats with both MIO and aluminum pigments form a white deposit over
© 2006 by Taylor & Francis Group, LLC