McNeil Coatings Consultants, Inc.
1132 Kay Drive
Greenwood, IN 46142
Corrosion is defined as the
deterioration of a material, usually
a metal, because of a reaction with
its environment.
Corrosion is:
•A natural phenomenon that occurs
over time.
•An electrochemical reaction (on
metals)
•Happens at different rates with
different metals and in different
environments
If we expose iron or steel to air and
water we can expect to see rust form
in a short time, showing the familiar
color of red-brown iron oxide.
Depending on the environment the
rust may develop in minutes.
With other metals such as copper,
brass, zinc, aluminum, and stainless
steel we can expect corrosion to
take place, but it might take longer
to develop.
One reason for the reduction of the
corrosion rate with these metals is
the potential formation of metallic
oxides of copper, zinc, aluminum,
and chromium.
Unfortunately ordinary iron or
steel does not form this protective
layer, so must be separated from
the environment by some other
means. Generally protective
coatings are utilized for this
purpose.
It is generally accepted that the
usual cause of corrosion of metals
involves electrochemistry. A flow
of electricity occurs from one area of
the metal to another area through
an electrolyte, i.e. any solution
capable of conducting electricity
such as seawater, hard water, or
other moisture on the substrate.
We refer to the corrosion process as
an electrochemical reaction,
sometimes referred to as a galvanic
action.
For galvanic corrosion to occur we
must have four things:
Anode
Cathode
Metallic Pathway
Electrolyte
The steel itself contains anodes and
cathodes (areas on the surface that
have different electrical potentials)
and it is its own metallic pathway.
Therefore, all we need for corrosion
cells to form is the electrolyte.
The electrolyte is provided in the
form of rain, dew, humidity, or
chemicals.
Factors that affect the rate of
corrosion are:
Temperature
Oxygen
Humidity
Chemical Salts
Chemicals and airborne gases
There are several ways that we use
to try to control corrosion:
:
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•
•
•
•
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Design
Inhibitors
Material Selection
Protective Coatings
Cathodic Protection
Control of the Environment
Dehumidification
Design
Generally speaking we want a
design that:
• Avoids entrapment of water.
• Allows access for surface
preparation and coating operations.
Inhibitors are materials that may be
injected into the system . They plate
out on the surface and inhibit the
formation of corrosion cells. They
are commonly used in pipelines and
other vessels that will contain
materials that are corrosive.
They are sometimes injected into
the water stream that may be used
for the surface preparation of steel,
as in the case of water jetting.
In the formulation of some primers
inhibitive pigments are used .
These inhibitive pigments inhibit, or
interfere, with the corrosion
process.
Examples of inhibitive pigments are
red lead and barium meta borate.
Some metals, such as gold and
platinum, corrode very slowly or
not at all. Choosing a corrosion
resistant material can reduce the
rate of corrosion.
Material Selection - Galvanic
Series [Seawater at 77⁰ F.]
Magnesium
Zinc
Aluminum
Mild Steel
Cast Iron
Copper
Stainless Steel
Gold
Platinum
Combining dissimilar metals can
result in corrosion. It may be very
rapid or it may be relatively slow,
depending on the metals
combined, the environment, and
the ratio of one to the other.
We can also use this concept to
protect a surface, such as when we
hot dip galvanize steel. The zinc we
apply to the steel is the more active
metal and will sacrifice itself to
protect the steel. When we combine
zinc and steel the zinc becomes the
anode and the steel becomes the
cathode.
One of the most common ways of
preventing corrosion to steel
surfaces is to apply protective
coatings.
The coatings we apply may be
inhibitive , barrier, or sacrificial.
The key to a successful coating
project is surface preparation. We
want a surface that is clean and
suitably roughened.
Adhesion is the most important
attribute of a coating. A clean
surface with the proper profile
provides a surface to which a
coating can adhere.
A clean steel surface should be free
of all:
• Oil
Oxides
• Grease
Corrosion Products
• Dust
Other foreign matter
• Dirt
• Mill scale
• Rust
• Coating
Mill scale is the blue/black oxide
layer that forms on hot rolled steel
during the cooling process.
Mill scale is cathodic to the steel. If
mill scale is left on the surface, and
painted over, the steel will rust and
sacrifice itself trying to protect the
mill scale.
So let’s look at the common coating
systems that are used to protect and
beautify steel and concrete
structures.
First of all let’s look at what’s in the
can.
Coatings consist of pigment, vehicle,
solvent, and additives.
Pigment is a discrete particulate
solid.
Vehicle is the binder (resin) and
other liquid components.
Most coatings are named after the
binder (resin).
Examples:
Alkyds
Epoxies
Urethanes
Acrylics
Other coatings may be named after
the pigment such as zinc rich primer
or red lead primer.
There are hundreds of pigments
available to the formulators. They
are selected based on what we
want the coating to do for us.
Pigments come in three basic
shapes. These are:
Nodular
Acicular
Lamellar
Nodular pigments are lump shaped.
Examples are titanium and zinc
phosphate.
Acicular pigments are needle
shaped. Examples are asbestine and
magnesium silicate.
Lamellar pigments are plate like.
Examples are glass flake and MIO.
Nodular pigments generally add
color. Most pigments are nodular.
Acicular pigments give the coating
mechanical or cohesive strength.
Lamellar pigments improve the
films impermeability to moisture.
Some general classifications of
coatings are:
Organic
Inorganic
Non-Convertible
Convertible
Organic resins are made of
something that was once alive such
as vegetable oil or fish oil.
A lot of our coatings are made from
petroleum products.
Most resins in use in today’s
industrial coatings are synthetic or
man made.
Non-convertible coatings are those
coatings that dry and cure by solvent
evaporation. When they are dry they
are cured. Examples are:
Vinyl
Chlorinated Rubber
Asphalt
Bitumen
Convertible coatings are those that
cure principally by some type of
polymerization. The solvent must
evaporate before the polymerization
takes place. After polymerization it
is a different chemical compound.
Examples are:
Epoxies
2-Pack Urethanes
Polyesters
Vinyl Esters
Coatings are generally applied as
systems. These systems consist of one
or more coating layers.
Coating systems may be:
Single layer
Multiple layers of the same generic
type.
Multiple layers of different generic
types.
A typical coating system consists of
a primer, an intermediate coat, and
one, or more, topcoats.
An example of a coating system for a
bridge might be:
Zinc Rich Primer
Epoxy Intermediate Coat
Urethane Topcoat
There are three basic types of
primers. They are:
Sacrificial
Inhibitive
Barrier
Sacrificial primers protect the steel
substrate by containing metallic
pigments that are anodic to the
steel. The most common sacrificial
primer is a zinc rich primer. Zinc is
a more active metal than steel and
will become an anode when in
direct contact with the less active
steel substrate.
Inhibitive primers contain pigments
that passivate the steel and therefore
mitigate corrosion. Red lead primer
is one of the best of the inhibitive
primers. Unfortunately we do not
see much red lead used any more
because of health concerns.
There are many bridges out there
that have red lead primer on them
and they have been protected from
corrosion for years.
Pigments that have replaced red lead
for inhibitive qualities are barium
meta borate, iron oxide, and zinc
oxide to name a few.
Barrier type primers create a barrier
to the passage of moisture through
the film to the substrate, thus
preventing the electrolyte from
getting to the steel. Without
electrolyte we cannot have
corrosion. Barrier type primers
often contain lamellar type
pigments.
Intermediate coats also serve as
barriers in the coating system, as
well as adding film thickness.
Intermediate coats must be
compatible with the primer as well
as the topcoat.
Topcoats in the system must protect
the substrate, and previous coats,
from the environment in which the
coating will be exposed. This may
be simply atmospheric exposure, or
a more severe environment such as
immersion or chemical.
Protective coatings have been used
to mitigate corrosion for years and
new technology is making coatings
better and better every year.
This has been a very basic and
simple explanation of how coatings
help to protect steel and concrete
from corrosion. The subject is
much more complex that what I
have presented here.
QUESTIONS?