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Faculty of Engineering,Faculty of Engineering,Cairo UniversityCairo University

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CORROSION PROCESSCORROSION PROCESS

Iron +Iron + Ox en + Water = RustOx en + Water = Rust

Iron will not rust unless both oxygen (fromIron will not rust unless both oxygen (froma r an water are present .a r an water are present .

The rate of rustin will var with theThe rate of rustin will var with thehumidity in the atmosphere.humidity in the atmosphere.

t e re at ve um ty s e owt e re at ve um ty s e ow , ron, ronwill not rust at all .will not rust at all .

Where liquid water is present the attack willWhere liquid water is present the attack will

. .

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RUSTING OF IRON IS ARUSTING OF IRON IS A

TWOTWO--FOLD PROCESSFOLD PROCESS1.1.A chemical change of iron to iron oxide andA chemical change of iron to iron oxide and

2.2.An e ectrica process invo ving current ow.An e ectrica process invo ving current ow.This rocess is called an electrochemical rocess and inThis rocess is called an electrochemical rocess and in

fundamental principals is identical to that occurring in carfundamental principals is identical to that occurring in car..

corrosion to occur we must have anode and cathode incorrosion to occur we must have anode and cathode ine ec r ca con ac n e presence o an e ec ro y e. ee ec r ca con ac n e presence o an e ec ro y e. e

essential point to remember is that it involves theessential point to remember is that it involves themovement of electrons through the metal and this ismovement of electrons through the metal and this is

another wa of sa in that durin corrosion there is aanother wa of sa in that durin corrosion there is apassage of electrical current.passage of electrical current.

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ANTICORROSIONANTICORROSION

Exclude oxygen.Exclude oxygen.

Exclude water (moisture).Exclude water (moisture).

..

Protective coatin s lacin a barrierProtective coatin s lacin a barrier

between the material and its environment.between the material and its environment.

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PaintPaint--films act as an electrical resistantfilms act as an electrical resistant. .

AntiAnti--corrosive paint containscorrosive paint containschemically reactive rust inhibitorschemically reactive rust inhibitors

hi h n r r rr i n.hi h n r r rr i n.

Paint films act as an active barrierPaint films act as an active barrieraga nst env ronmenta attac s.aga nst env ronmenta attac s.

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Oil and grease removal.Oil and grease removal.

Using solventUsing solvent problem of removing solvent.problem of removing solvent.

ropr e ary wa erropr e ary wa er--so u e remov ng u .so u e remov ng u .Treatment of surface defects.Treatment of surface defects.

Manual treatments,Manual treatments, hand scraping and wirehand scraping and wire

rus ng.rus ng.

Rotary wire brushing.Rotary wire brushing.

Mechanical deMechanical de--scaling.scaling. s ng.s ng.

Abrasive blastin .Abrasive blastin .

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Surface preparationSurface preparation

Film build and total thickness of systemFilm build and total thickness of system

Atmos heric conditions durin a licationAtmos heric conditions durin a lication

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METHODS OF PAINTMETHODS OF PAINT

APPLICATIONAPPLICATION Brush: simple, reaches difficult locationsBrush: simple, reaches difficult locations slow,slow,

m c ness con ro , over rus ng.m c ness con ro , over rus ng. Roller: fasterRoller: faster es eciall in o en areases eciall in o en areas cannotcannot

reach difficult places, overreach difficult places, over--rolling.rolling. Conventional Spray:Conventional Spray: rapid, simple equipmentrapid, simple equipment

. .

Airless brush:Airless brush: Heavy duty, very fast, does notHeavy duty, very fast, does not

require mixing paint with water, reducerequire mixing paint with water, reduce

. .

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Initial cost.Initial cost.

Coating system and color selection.Coating system and color selection.

. .

Sho v. field a lication.Sho v. field a lication.

Maintenance.Maintenance.

Exposure levels.Exposure levels.

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Public facilities:Public facilities: High degree of corrosion protection.High degree of corrosion protection.

ttract ve appearance.ttract ve appearance.

exposure.exposure. Interior steel may also be subject to extendedInterior steel may also be subject to extended

..

This may affect color and gloss of paint, but isThis may affect color and gloss of paint, but isusually not long enough to affect corrosion.usually not long enough to affect corrosion.

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COATING SYSTEMSCOATING SYSTEMSExterior exposure:

--acrylic topcoats.acrylic topcoats.

. .

22 coats high performancecoats high performance

industrial acrylic emulsion.industrial acrylic emulsion.--

Polyurethane topcoatPolyurethane topcoat

. .

11 coat high build aliphatic acryliccoat high build aliphatic acrylicpo yuret ane.po yuret ane.

--

Polyurethane topcoatPolyurethane topcoat

. .

11 coat high build polyamid epoxycoat high build polyamid epoxy

11 coat high build aliphatic acryliccoat high build aliphatic acrylic

ol urethane.ol urethane.

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.

wowo--coat system w tcoat system w tPolysiloxane topcoat.Polysiloxane topcoat.

11 coat norgan c z nc pr mer.coat norgan c z nc pr mer.11 coat ro rietar ol siloxane.coat ro rietar ol siloxane.

Interior exposure

TwoTwo--coat system withcoat system withPolyamide EpoxyPolyamide Epoxy 11 coat organic zinc rich primer.coat organic zinc rich primer.11 coat hi h build ol amidcoat hi h build ol amidtopcoat.topcoat. epoxy.epoxy.

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COATING SYSTEMSCOATING SYSTEMS

-- .. Manual surface re aration is not recommendedManual surface re aration is not recommended..

Increasing the quality of surface preparation to SPIncreasing the quality of surface preparation to SP--1010 w su stant a y ncrease cost w t out mucw su stant a y ncrease cost w t out muc

im rovement in life ex ectanc .im rovement in life ex ectanc .

Most paints are based on organic zinc primer withMost paints are based on organic zinc primer withepoxy or polyurethane binder.epoxy or polyurethane binder.

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COATING SYSTEMSCOATING SYSTEMS

inorganic zinc as they have a well definedinorganic zinc as they have a well definedcure period based on temperature andcure period based on temperature and

..

Inorganic zinc is sensitive to excessive filmInorganic zinc is sensitive to excessive film

build. Only subjective tests (such as nickelbuild. Only subjective tests (such as nickel--

the degree of curethe degree of cure..

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EPOXY SYSTEMSEPOXY SYSTEMS

. . E ox lasts a lon time on interior surfaces.E ox lasts a lon time on interior surfaces.

Recoating will be required for aesthetic orRecoating will be required for aesthetic or

yy ..

Unless there is h sical dama e to the surfaceUnless there is h sical dama e to the surface

in a potentially wet area, corrosion is rarely anin a potentially wet area, corrosion is rarely anssue.ssue.

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Three coat system, where the intermediateThree coat system, where the intermediate

erosion protection.erosion protection.

Intermediate coat also provides betterIntermediate coat also provides better

..

Ensure primer is dry before application.Ensure primer is dry before application.

Primed surfaces must be masked forPrimed surfaces must be masked forapp cat on o nterme ate coats n case oapp cat on o nterme ate coats n case o

slisli --critical connections.critical connections.

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OneOne--component topcoat.component topcoat.

.. Relativel eas to touchRelativel eas to touch--u as it is available asu as it is available as

a singlea single--component product.component product.

Appearance of the touchedAppearance of the touched--up area may be ofup area may be of

concern.concern.

Suitable for application where the Client shallSuitable for application where the Client shalluse his own maintenance personnel.use his own maintenance personnel.

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--

Provides excellent color and gloss retensionProvides excellent color and gloss retension..

resistance.resistance.

Spot touchSpot touch--up is not easy due to its hardness.up is not easy due to its hardness.

May require repainting the entire surface toMay require repainting the entire surface to

achieve a uniform a earance.achieve a uniform a earance.

Used in many outdoor applications.Used in many outdoor applications.

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Proprietary topcoatProprietary topcoat..

in accelerated weathering tests.in accelerated weathering tests.

Advertised to have the advantages of theAdvertised to have the advantages of the 33--

coa o yure ane opcoa w on ycoa o yure ane opcoa w on y --coa s,coa s,

i,.e., less expensive.i,.e., less expensive.

Resists stains, graffiti and dirt accumulationResists stains, graffiti and dirt accumulationue to ts ar smoot sur ace.ue to ts ar smoot sur ace.

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There are three basic methods of applicationThere are three basic methods of application

Shop applied primer & shop applied topcoat, withShop applied primer & shop applied topcoat, withe touc up.e touc up.

Sho a lied rimer intermediate coat & fieldSho a lied rimer intermediate coat & fieldapplied topcoat.applied topcoat.

om na on o o ase on access y anom na on o o ase on access y ansubsequent construction activities.subsequent construction activities.

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Field application limited to touchup ofField application limited to touchup of

bolts and surfaces of connections.bolts and surfaces of connections.

Special primer is applied to slipSpecial primer is applied to slip--criticalcritical

topcoat.topcoat.

Touchup in field ranges fromTouchup in field ranges from 2020 toto 4040% of% of. .

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Disadvantages:Disadvantages:

ay requ re a comp e e recoa o prov eay requ re a comp e e recoa o prov euniformit .uniformit .

Multiple coats in the shop present timingMultiple coats in the shop present timingconstra nts w t t e r s t at t e a r catorconstra nts w t t e r s t at t e a r cator

ma not allow the rimer full cure beforema not allow the rimer full cure before

topcoating is applied.topcoating is applied.

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Best coatin from an aesthetic oint of view.Best coatin from an aesthetic oint of view.Touchup of the primer/intermediate coat isTouchup of the primer/intermediate coat is

prov e a er o ng s comp e e.prov e a er o ng s comp e e. Field a lied finish coat will rovide aField a lied finish coat will rovide a

complete and uniform finish coat over all thecomplete and uniform finish coat over all the

.. Spray application is preferred. However,Spray application is preferred. However,

overspraying may damage adjacent surfaces.overspraying may damage adjacent surfaces. ,,

achieve the required film thickness.achieve the required film thickness.

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Areas that are considered to be inaccessibleAreas that are considered to be inaccessible

after erection are coated based on Method I.after erection are coated based on Method I.or areas a are access e an or requ re aor areas a are access e an or requ re a

higher degree of aesthetics, Method II is used.higher degree of aesthetics, Method II is used. Requires high level of coordination betweenRequires high level of coordination between

a r cator an e team.a r cator an e team.

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protection?protection?

Many owners repaint large areas of theirMany owners repaint large areas of their. .

Color usually fades, especially due toColor usually fades, especially due toprolonged exposure to UV.prolonged exposure to UV.

atc es w appear t e structure wasatc es w appear t e structure wastouchedtouched--u usin a different to coat than theu usin a different to coat than the

original.original.

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on:on:

Quality of surface preparation.Quality of surface preparation.

Application techniques.Application techniques.

Environmental conditions.Environmental conditions.

. .

Regular repair and touchup.Regular repair and touchup.

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Ins ection re uires s ecific focus on criticalIns ection re uires s ecific focus on criticalareas, such as bolt heads, edges of members,areas, such as bolt heads, edges of members,

-- -- ..

Service life is measured to the time ofService life is measured to the time of 100100%%repainting, when there is rust that covers asrepainting, when there is rust that covers as

much asmuch as 1010% of the surface area.% of the surface area. Before rust occurs, the surfaces of coatedBefore rust occurs, the surfaces of coated

stee eg n to rea own.stee eg n to rea own.

Aesthetics is not taken into consideration inAesthetics is not taken into consideration inevaluating service life.evaluating service life.

MAINTENANCEMAINTENANCE

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MAINTENANCEMAINTENANCE

Periodic inspection should identify physicalPeriodic inspection should identify physicalamage e ore s tuat on ecomes ser ous.amage e ore s tuat on ecomes ser ous.

Re ular re air of dama ed areas will rolonRe ular re air of dama ed areas will rolon

life expectancy, while waiting till active rustinglife expectancy, while waiting till active rusting. .

Quality touchup includes surface preparationQuality touchup includes surface preparationand accurate application of matching topcoat.and accurate application of matching topcoat.

relatively easy as maintenance personnel mayrelatively easy as maintenance personnel mayprepare t e amage sur ace any app y t eprepare t e amage sur ace any app y t esingle component product by brush or roller.single component product by brush or roller.

REPAINTINGREPAINTING

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REPAINTINGREPAINTING

At the end of the effective service life of theAt the end of the effective service life of thecoat ng, a comp ete repa nt ng s requ re .coat ng, a comp ete repa nt ng s requ re .

A com lete surve of the facilit is necessar toA com lete surve of the facilit is necessar to

determine the actual conditions of each element.determine the actual conditions of each element. oun , g y a ere ex s ng pa n s ou nooun , g y a ere ex s ng pa n s ou no

be removed unless the coating is found brittle.be removed unless the coating is found brittle.The new paint system should be fully compatibleThe new paint system should be fully compatible

. .

A spot touchup of the existing surface shall takeA spot touchup of the existing surface shall takecare of the effects of physical damage andcare of the effects of physical damage and

corrosion.corrosion.

REPAINTINGREPAINTING

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REPAINTINGREPAINTING

A full topcoat will restore the aesthetics andA full topcoat will restore the aesthetics andcorrosion protection of the entire system.corrosion protection of the entire system.

with an alkaline cleanser and highwith an alkaline cleanser and high--pressurepressure

potable water washing.potable water washing.

w w vw w vor otherwise damaged coatings.or otherwise damaged coatings.

Waste from potable water washing should not beWaste from potable water washing should not beazar ous an may e a owe o ow n o eazar ous an may e a owe o ow n o e

astewater s stem.astewater s stem.

RECOMMENDATIONSRECOMMENDATIONS

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RECOMMENDATIONSRECOMMENDATIONS

In a new project, there are three main zones:In a new project, there are three main zones: Roof and lighting structure.Roof and lighting structure.

. .

Building frame in public areas.Building frame in public areas.

For roof and lighting structures (least accessible),For roof and lighting structures (least accessible),

..monitored whereas aesthetics is not critical. Amonitored whereas aesthetics is not critical. A

paint that repels dirt and allows highpaint that repels dirt and allows high--pressurepressure. .

RECOMMENDATIONSRECOMMENDATIONS

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RECOMMENDATIONSRECOMMENDATIONS

For more easily accessed structures, aestheticsFor more easily accessed structures, aestheticsand corrosion are important, and frequentand corrosion are important, and frequent

or staging slung from the roof.or staging slung from the roof.

For areas easily accessed by the public, a systemFor areas easily accessed by the public, a system

,,owner may opt to repaint on a more regularowner may opt to repaint on a more regular

basis.basis.

FINAL COMMENTFINAL COMMENT

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FINAL COMMENTFINAL COMMENT

In dealing with paint manufacturers, in manyIn dealing with paint manufacturers, in manycases you are ea ng w t a sa esman. s or t ecases you are ea ng w t a sa esman. s or t etechnical expert or request the technical catalog.technical expert or request the technical catalog.

Ask for compliance with reference codes, suchAsk for compliance with reference codes, such

ASTM DASTM D638638 Tensile Properties of plastics.Tensile Properties of plastics.ASTM DASTM D790790 Flexural Properties of unreinforcedFlexural Properties of unreinforced

and reinforced plastics.and reinforced plastics.

ASTM DASTM D543543 Resistance of plastics to chemicalResistance of plastics to chemicalrea ents.rea ents.

ASTM DASTM D638638 Durometer hardness, type D.Durometer hardness, type D.