Corrosion and Protective Coating

8/10/2019 Corrosion and Protective Coating

1/54

McNeil Coatings Consultants, Inc.1132 Kay Drive

Greenwood, IN 46142


2/54

Corrosion is defined as thedeterioration of a material, usuallya metal, because of a reaction with

its environment.


3/54

Corrosion is:

A natural phenomenon that occursover time.

An electrochemical reaction (onmetals)

Happens at different rates withdifferent metals and in different

environments


4/54

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.


5/54


6/54


7/54


8/54

With other metals such as copper,brass, zinc, aluminum, and stainless

steel we can expect corrosion totake place, but it might take longerto develop.


9/54

One reason for the reduction of thecorrosion rate with these metals is

the potential formation of metallicoxides of copper, zinc, aluminum,and chromium.


10/54


11/54

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.


12/54

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.


13/54

We refer to the corrosion process asan electrochemical reaction,

sometimes referred to as a galvanicaction.


14/54

For galvanic corrosion to occur wemust have four things:

AnodeCathode

Metallic PathwayElectrolyte


15/54

The steel itself contains anodes and

cathodes (areas on the surface thathave different electrical potentials)and it is its own metallic pathway.


16/54

Therefore, all we need for corrosion

cells to form is the electrolyte.

The electrolyte is provided in the

form of rain, dew, humidity, orchemicals.


17/54

Factors that affect the rate ofcorrosion are:

Temperature Oxygen

Humidity Chemical Salts

Chemicals and airborne gases


18/54

:There are several ways that we useto try to control corrosion:

Design Inhibitors Material Selection Protective Coatings

Cathodic Protection Control of the Environment Dehumidification


19/54

Design

Generally speaking we want adesign that:

Avoids entrapment of water.

Allows access for surfacepreparation and coating operations.


20/54


21/54


22/54

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.


23/54

They are sometimes injected intothe water stream that may be usedfor the surface preparation of steel,as in the case of water jetting.


24/54

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.


25/54

Some metals, such as gold andplatinum, corrode very slowly ornot at all. Choosing a corrosionresistant material can reduce therate of corrosion.


26/54

Material Selection - Galvanic

Series [Seawater at 77 F.]Magnesium

Zinc

AluminumMild Steel

Cast Iron

Copper

Stainless Steel

Gold

Platinum


27/54

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.


28/54

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.


29/54

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.


30/54


31/54

A clean steel surface should be free

of all: Oil Oxides Grease Corrosion Products

Dust Other foreign matter Dirt Mill scale Rust Coating


32/54

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.


33/54


34/54

So lets look at the common coating

systems that are used to protect andbeautify steel and concretestructures.


35/54

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.


36/54

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.


37/54

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


38/54

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.


39/54

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.


40/54

Some general classifications of

coatings are:

OrganicInorganicNon-Convertible

Convertible


41/54

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.


42/54

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


43/54

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


44/54

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.


45/54

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


46/54


47/54

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.


48/54

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.


49/54

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.


50/54

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.


51/54

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.


52/54

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.


53/54

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.


54/54

QUESTIONS?

Documents

Corrosion and Protective Coating