Gerald L. Witucki, The Dow Chemical Company Midland, MI
Introduction
Corrosion impacts nearly every facet of
our lives, from buildings, bridges and
utilities, to planes, trains and automobiles. Every metal bolt, screw, plate, pipe
or beam we put into service is susceptible
to corrosion. The cost is our time, money,
health and safety.
Many factors are outside of the con-
trol or the anticipation of the coatings
formulator, including:
•construction materials: sub-op-
timum metal alloys and surface
morphology;
•environmental contact: moisture,
chemicals, microbes, abrasive par-
ticles, etc.; and
•operating conditions: temperature,
flow rate, turbulence and phase
separation.
Coatings are a key approach to mitigating corrosion. In general, corrosion
occurs when oxygen, water and conductive ions (such as salts and acids) set up
an electrochemical cell on the metal surface. By slowing the penetration of these
corrosion elements, the formulator can
improve the durability of a paint film and
the service life of the coated object. This
article focuses on two coatings properties: permeability and adhesion, offering
recommendations for silicon-based technology remedies to coatings formulators.
Permeability
Aesthetics aside, the primary role of a
coating is to serve as a barrier between
the environment and the object to be
protected. The effectiveness of a coating’s
barrier performance is impacted by its
permeability which, in turn, is a function
of its hydrophobicity, porosity and occlu-
sive properties.
Epoxy resins are the industry standard
for corrosion control binders. The most
common are the Bis-Phenol A type epoxies, which are often crosslinked with
polyamines (Figure 1).
As a result of their high aromatic
content, epoxy resins possess excellent
barrier properties, but even these coatings can be permeable and susceptible to
hydrolytic attack. Though the secondary
amine can react with additional epoxide
rings, as a result of steric hindrance and
mixing constraints, residual hydrophilic
amines (both primary and secondary)
will be present in the film. In addition,
the resulting secondary hydroxyl group,
which is thought to provide polymer
adhesion through hydrogen bonding, is
hydrophilic and can also contribute to
water absorption.
Similarly, other traditional coating
resin binders (e.g., urethanes, polyesters,
etc.) are, to varying degrees, susceptible
to water attack. This deficiency can be
reduced with the use of polysiloxane
additives. The mobility and low rota-
tional energy of the siloxane backbone
allows the silicone polymer to orient at
the coating-air interface, creating a hy-
drophobic, low energy surface, which
results in water beading – prevent-
ing wetting and paint film penetration.
Functional groups (both organic and
inorganic) on a siloxane polymer can
form covalent bonds within the coating
matrix to enhance film integrity, thereby
further increasing hydrophobicity.
Coating films contain microscopic de-
fects, such as pin-holes, creating porosity
which can penetrate to the metal surface
leading to corrosion. Overcoating might
seal these defects, delaying the passage of
oxygen and water, but high film thickness
also has inherent disadvantages: (1) paint
sagging during thick film application can
ruin the appearance of the coating; (2)
solvent entrapment can lead to blistering
and craters; and ( 3) high film thickness on
complex structures (e.g., joints) can result
in cracking and ultimately corrosion.
Pigmentation can also have a signifi-
cant effect on film permeability. Increased
pigment loading up to (but not exceeding)
the critical pigment volume concentration
(CPVC) will result in reduced permeabil-
ity by physically blocking the passage of
liquids and gases (occlusion). Pigment
levels above the CPVC will result in voids
in the coating and permeability will in-
crease. For multi-coat systems, the pig-
mentation level of a primer, or basecoat,
can also impact topcoat permeability; a
flat (highly filled) primer can absorb resin
Coatings, Silicon-based
Technologies and Corrosion
Figure 1. Amine – BPA epoxy reaction.
