corrosion and protective coating
TRANSCRIPT
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McNeil Coatings Consultants, Inc.1132 Kay Drive
Greenwood, IN 46142
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Corrosion is defined as thedeterioration of a material, usuallya metal, because of a reaction with
its environment.
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Corrosion is:
A natural phenomenon that occursover time.
An electrochemical reaction (onmetals)
Happens at different rates withdifferent metals and in different
environments
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If we expose iron or steel to air andwater we can expect to see rust formin a short time, showing the familiar
color of red-brown iron oxide.
Depending on the environment therust may develop in minutes.
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With other metals such as copper,brass, zinc, aluminum, and stainless
steel we can expect corrosion totake place, but it might take longerto develop.
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One reason for the reduction of thecorrosion rate with these metals is
the potential formation of metallicoxides of copper, zinc, aluminum,and chromium.
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Unfortunately ordinary iron orsteel does not form this protective
layer, so must be separated fromthe environment by some othermeans. Generally protective
coatings are utilized for thispurpose.
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It is generally accepted that the
usual cause of corrosion of metalsinvolves electrochemistry. A flow
of electricity occurs from one area of
the metal to another area throughan electrolyte, i.e. any solution
capable of conducting electricitysuch as seawater, hard water, orother moisture on the substrate.
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We refer to the corrosion process asan electrochemical reaction,
sometimes referred to as a galvanicaction.
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For galvanic corrosion to occur wemust have four things:
AnodeCathode
Metallic PathwayElectrolyte
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The steel itself contains anodes and
cathodes (areas on the surface thathave different electrical potentials)and it is its own metallic pathway.
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Therefore, all we need for corrosion
cells to form is the electrolyte.
The electrolyte is provided in the
form of rain, dew, humidity, orchemicals.
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Factors that affect the rate ofcorrosion are:
Temperature Oxygen
Humidity Chemical Salts
Chemicals and airborne gases
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:There are several ways that we useto try to control corrosion:
Design Inhibitors Material Selection Protective Coatings
Cathodic Protection Control of the Environment Dehumidification
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Design
Generally speaking we want adesign that:
Avoids entrapment of water.
Allows access for surfacepreparation and coating operations.
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Inhibitors are materials that may beinjected into the system . They plateout on the surface and inhibit the
formation of corrosion cells. Theyare commonly used in pipelines andother vessels that will contain
materials that are corrosive.
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They are sometimes injected intothe water stream that may be usedfor the surface preparation of steel,as in the case of water jetting.
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In the formulation of some primers
inhibitive pigments are used .These inhibitive pigments inhibit, orinterfere, with the corrosion
process.
Examples of inhibitive pigments arered lead and barium meta borate.
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Some metals, such as gold andplatinum, corrode very slowly ornot at all. Choosing a corrosionresistant material can reduce therate of corrosion.
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Material Selection - Galvanic
Series [Seawater at 77 F.]Magnesium
Zinc
AluminumMild Steel
Cast Iron
Copper
Stainless Steel
Gold
Platinum
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Combining dissimilar metals canresult in corrosion. It may be veryrapid or it may be relatively slow,
depending on the metalscombined, the environment, andthe ratio of one to the other.
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We can also use this concept to
protect a surface, such as when wehot dip galvanize steel. The zinc weapply to the steel is the more active
metal and will sacrifice itself toprotect the steel. When we combinezinc and steel the zinc becomes theanode and the steel becomes thecathode.
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One of the most common ways ofpreventing corrosion to steelsurfaces is to apply protective
coatings.
The coatings we apply may be
inhibitive , barrier, or sacrificial.
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A clean steel surface should be free
of all: Oil Oxides Grease Corrosion Products
Dust Other foreign matter Dirt Mill scale Rust Coating
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Mill scale is the blue/black oxide
layer that forms on hot rolled steelduring the cooling process.
Mill scale is cathodic to the steel. Ifmill scale is left on the surface, and
painted over, the steel will rust andsacrifice itself trying to protect themill scale.
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So lets look at the common coating
systems that are used to protect andbeautify steel and concretestructures.
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First of all lets look at whats in thecan.Coatings consist of pigment, vehicle,solvent, and additives.
Pigment is a discrete particulatesolid.Vehicle is the binder (resin) and
other liquid components.
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Most coatings are named after thebinder (resin).Examples:
Alkyds
EpoxiesUrethanesAcrylics
Other coatings may be named afterthe pigment such as zinc rich primer
or red lead primer.
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There are hundreds of pigments
available to the formulators. Theyare selected based on what wewant the coating to do for us.
Pigments come in three basicshapes. These are:
NodularAcicular
Lamellar
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Nodular pigments are lump shaped.
Examples are titanium and zincphosphate.
Acicular pigments are needleshaped. Examples are asbestine andmagnesium silicate.
Lamellar pigments are plate like.Examples are glass flake and MIO.
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Nodular pigments generally add
color. Most pigments are nodular.
Acicular pigments give the coating
mechanical or cohesive strength.
Lamellar pigments improve thefilms impermeability to moisture.
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Some general classifications of
coatings are:
OrganicInorganicNon-Convertible
Convertible
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Organic resins are made ofsomething that was once alive suchas vegetable oil or fish oil.
A lot of our coatings are made frompetroleum products.Most resins in use in todays
industrial coatings are synthetic orman made.
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Non-convertible coatings are thosecoatings that dry and cure by solventevaporation. When they are dry theyare cured. Examples are:
VinylChlorinated Rubber
AsphaltBitumen
Convertible coatings are those that
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Convertible coatings are those thatcure principally by some type of
polymerization. The solvent mustevaporate before the polymerizationtakes place. After polymerization it
is a different chemical compound.Examples are:
Epoxies2-Pack UrethanesPolyesters
Vinyl Esters
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Coatings are generally applied as
systems. These systems consist of oneor more coating layers.Coating systems may be:
Single layerMultiple layers of the same generic
type.Multiple layers of different generictypes.
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A typical coating system consists of
a primer, an intermediate coat, andone, or more, topcoats.An example of a coating system for a
bridge might be:
Zinc Rich Primer
Epoxy Intermediate CoatUrethane Topcoat
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Sacrificial primers protect the steel
substrate by containing metallicpigments that are anodic to thesteel. The most common sacrificial
primer is a zinc rich primer. Zinc isa more active metal than steel andwill become an anode when in
direct contact with the less activesteel substrate.
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Inhibitive primers contain pigmentsthat passivate the steel and thereforemitigate corrosion. Red lead primer
is one of the best of the inhibitiveprimers. Unfortunately we do notsee much red lead used any more
because of health concerns.
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There are many bridges out there
that have red lead primer on themand they have been protected fromcorrosion for years.
Pigments that have replaced red leadfor inhibitive qualities are barium
meta borate, iron oxide, and zincoxide to name a few.
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Barrier type primers create a barrier
to the passage of moisture throughthe film to the substrate, thuspreventing the electrolyte from
getting to the steel. Withoutelectrolyte we cannot havecorrosion. Barrier type primersoften contain lamellar typepigments.
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Intermediate coats also serve asbarriers in the coating system, aswell as adding film thickness.
Intermediate coats must becompatible with the primer as wellas the topcoat.
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Topcoats in the system must protectthe substrate, and previous coats,from the environment in which the
coating will be exposed. This maybe simply atmospheric exposure, ora more severe environment such as
immersion or chemical.
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Protective coatings have been used
to mitigate corrosion for years andnew technology is making coatingsbetter and better every year.
This has been a very basic andsimple explanation of how coatingshelp to protect steel and concretefrom corrosion. The subject ismuch more complex that what I
have presented here.
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QUESTIONS?