steel galvanized

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What We Need to Know

National Insti tute of Steel Detailing

Hot-Dip

Galvanizing


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2009 National Institute of Steel Detailing and the American Galvanizers Association. The material provided herein has been developed to provide accurate

and authoritative information about after-fabrication hot-dip galvanized steel. This material provides information only and is not intended as a substitute for

competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation

or warranty on the part of the NISD or AGA. Anyone making use of this information assumes all liability arising from such use.

ContributorsFred Tinker

National Institute of Steel Detailing, Inc.

With Assistance From:Christine McCulloch - Education Committee

National Institute of Steel Detailing, Inc.Andrew Lesko - Calwest Galvanizing

Melissa Lindsley - American Galvanizers AssociationPaul Parks - Infosight Corporation

Photos contributed by the American Galvanizers AssociationFirst Printing: March 1, 2009


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Introduction..........................................................5Galvanizing History.............................................. 5

Hot-Dip Galvanizing Process.................................6

Surface Preparation..................................6Degreasing..................................6Pickling........................................6Fluxing.........................................6

Galvanizing.............................................7

Inspection................................................7Galvanized Coating Characteristics

Metallurgical bond...................................7Coating Uniformity....................................7

Cathodic protection..................................8Galvanized Coating Performance

Time to First Maintenance.........................8

Exposure to High Temperature....................8Additional Galvanizing Information

Galvanizing vs. Painting: By the Numbers.....9Painting Hot-Dip Galvanized Steel.............9

Sheet Steel/Continuous Galvanizing...........9Design Considerations........................................10

Welding Procedure................................11

Flux & Slag Removal....................11Stitch and Seal Welding .............12

Drilling and Cutting................................................

Venting and Drainage.............................................

Handrail........................................

Cap and Base Plates...................... Cropping for Drainage....................

Repair of Vent Holes.......................Masking....................................................

Marking....................................................

Barcode Tags.............................................

Galvanized Bolts, Nuts, and Holes................

Temporary Bracing......................................

Lifting Aids.................................................

Galvanizing Oversized Pieces.....................

Touchup and Repair....................................

Appearance...............................................

ASTM Standards....................................................

Canadian Standards Association.............................Frequently Asked Questions....................................

Appendix of Detailed Sketches..............................

Special Thanks.......................................................

Hot-DipGalvanizing

Table of Contents


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Galvanizing History79 AD Historical records show zinc usage in early construction.1742 P.J. Malouin, a French chemist, presents to the Royal Academy of Sciences several

experiments involving the coating of iron by molten zinc.

1772 Luigi Galvani, galvanizings namesake, discovers the electrochemical process that takes plabetween metals during an experiment with frog legs.

1801 Alessandro Volta discovers the electro-potential between two metals, creating acorrosion cell.

1829 Michael Faraday discovers zincs sacrificial action, during an experimentinvolving zinc, salt water and nails.

1837 French engineer Stanislaus Tranquille Modeste Sorel took out a patent for theearly galvanizing process.1850 British galvanizing industry is consuming 10,000 tons of zinc annually for the

production of galvanized steel.

1870 First galvanizing plant opened in the United States. Steel was hand-dipped in thezinc bath.

Today 600,000+ tons of zinc is consumed in North America to produce hot-dip galvanized steel.

IntroductionHot-dip galvanized steel has been effectively used for

more than 150 years. The value of hot-dip galvanizing

stems from the relative corrosion resistance of

zinc, which, under most service conditions, is

considerably better than iron and steel. In addition

to forming a physical barrier against corrosion, zinc,applied as a hot-dip galvanized coating, cathodically

protects exposed steel. Furthermore, galvanizing

for protection of iron and steel is favored because

of its low cost, the ease of application, and the

extended maintenance-free service it provides.

This book is to help the architect, design engin

fabricator, and detailer better understand the pro

of preparing steel for the highest quality corro

resistant coating (galvanizing).

This book will assist you in your hot-dip galvanifoundation by providing a look at the galvani

history, galvanizing process, galvanized coa

characteristics, performance, and design considerati

Following the information provided, the desig

fabricator, and detailer can ensure the highest qu

galvanized coating.


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The hot-dip galvanizing process (Figure 1) consistsof the following steps:

Surface preparation a series of three cleaningprocesses to prepare the steel for immersion inthe zinc bath, as zinc will not react with, noradhere to unclean steel.

Galvanizing total immersion of the steel in themolten zinc bath.

Inspection visual inspection and coatingthickness measurement to ensure conformance toappropriate specifications.

Small parts, such as fasteners, brackets, and clipsess than 30 (76cm) in length, are galvanized with

the same process. However, these parts are spun or

centrifuged after galvanizing to remove excess zinc.

Surface Preparation

Degreasing

In the degreasing step, a hot, alkaline solution removesdirt, oil, grease, shop oil, some paints, and soluble markings(Green Tank, Figure 1). It will not remove some surfacecontaminants, such as epoxies, vinyls, asphalts,or welding slag. These contaminants must bemechanically cleaned by grinding or blasting prior toshipment to the galvanizing facility.

Pickling

Dilute solution (between 8% to 15%) of eitherambient hydrochloric or heated sulfuric acid removessurface rust and mill scale to provide a chemicallyclean metallic surface (Red Tank, Figure 1).

Fluxing

Steel is immersed in liquid flux (a zinc ammoniumchloride solution) for two purposes. First, the flux will

remove any remaining iron oxides. Additionally, the

flux will create a protective film to prevent oxidationprior to dipping into the molten zinc bath (YellowTank, Figure 1).

Hot-Dip Galvanizing Process

Figure 1: The Hot-Dip Galvanizing Process

Degreasing

PicklingRinsing

Rinsing

Flux

solution

Caustic

cleaning

DryingZinc

bath

Cooling and

inspection

Surface Preparation

Pickling


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Metallurgical BondDuring the galvanizing

process, the zinc in the kettle

and the iron in the steelmetallurgically react to formthe galvanized coating. Thisdiffusion reaction creates aseries of intermetallic zinc-iron alloy layers, which areharder than the base steel (seeFigure 2). The metallurgical

bond is much stronger thana mechanically bonded

coating, as galvanized steelbond strength is around3,600 psi compared toseveral hundred for mostother coatings.

Coating Uniformity

Galvanizing is a total immersion process, which ensuresall surfaces are coated, including the inside of hollowstructures. During the diffusion reaction in the galvanizing

kettle, the intermetallic layers grow perpendicular to the

surface, which means coating thickness at corners edges is at least as thick as flat surfaces. Paint tends tthinner at edges and corners, and painted hollow structhave no protection on the inside. These areas are wcorrosion often starts.

Galvanizing

The steel article is immersed in a bath of molten zincheated to between 815-850F (435-455C). Duringgalvanizing, the zinc metallurgically bonds to thesteel, creating a series of abrasion-resistant zinc-ironalloy layers, topped by a layer of pure zinc.

As the steel is withdrawn from the zinc bath, excess

zinc is removed by draining, vibrating, or for smallitems, centrifuging. It is important to remove allexcess to ensure the part is suitable for its intendeduse. The galvanized item is either cooled by air orwater, or dipped in a passivation solution to preventoxidation.

Inspection

The final step in the galvanizing process is the

inspection of the surface condition and coatingthickness. The inspection of galvanizing is relativelyeasy because zinc does not adhere to unclean steel.

So, if the steel has a continuous coating of zin

should meet the required specification. To conconformance, the coating thickness is measured ua magnetic thickness gauge.

Hot-Dip Galvanized Coating Characteristics

Figure 2: Photomicrograph of Galvanized CoatingDiamond Pyramid Number (DPN) = measure of hardness, the higher the number, the greater the hardness

Hot-Dip Galvanizing Process


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Cathodic Protection

Galvanized Coating Performance

Figure 3: Cathodic Protection

100

0

10

20

30

40

50

60

70

80

90

1.0 3.5 5.04.54.03.02.52.01.5

Rural

Suburban

Temperate Marine

Tropical Marine

Industrial

Key

Average Thickness of Zinc (mils)1 mil = 25.4m = 0.56oz/ft2

*Time to first maintenance is defined as the time to 5% rusting of the substrate steel surface.

TimetoFirstMaintenance*(years)

Figure 4: Time to First Maintenance Chart

Time to First MaintenanceThe Time to First Maintenance Chart (Figure 4) wasdeveloped from decades of real world corrosion dataollected from galvanized steel samples exposed tonvironments all over the world. This data was sortednto five characteristic environmental categories: rural,uburban, industrial, temperate marine and tropical

marine.

Time to first maintenance is defined as the period ofime until 5% of the substrate steel surface is showing

ron oxide (rust). At this point, it is unlikely theunderlying steel has been weakened or the integrity ofhe structure is compromised, but it is time to begin a

maintenance cycle on the structure to protect it fromurther corrosion.

As the chart illustrates, the zinc coating thickness isdirectly proportional to the time to first maintenance.Other factors that influence the corrosion performanceof the coating are: relative humidity, sulfur dioxide,irborne salinity, precipitation, and temperature.

For more information on the performance of hot-dip

galvanized coatings, visit the American GalvanizersAssociations website at www.galvanizeit.org anddownload the publications Hot-Dip Galvanizing forCorrosion Protection: A Specifiers Guide and/orService Life Chart for Hot-Dip Galvanized Coatings.

Exposure to High TemperatureThere are some concerns with using hot-dip galvanized

steel in an elevated temperature environment. Theindustry recommends the service temperature for

galvanized coatings be less than 390F (200C) forlong-term exposure. Possible concerns at continuedexposure to temperatures above 390F (200C)

include peeling, somechanges in mechanicalproperties, and obviousreduction in corrosion

protection.

390 F 480 F

Temperature

No Peeling Some

PeelingPeeling

Figure 5: GalvanizingPerformance at High

Temperatures


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Examples of duplex systems

Additional Galvanizing Information

Galvanizing vs. Painting:By the numbers

An economic analysis of galvanizing vs. painting onboth an initial and life-cycle basis should be performedprior to the selection of either corrosion protectionmethod. Galvanizing has long been known to be lessexpensive on a life-cycle basis, but many specifiersdo not realize galvanizing is also competitive onan initial cost basis. In order to facilitate theprocess of performing an economic analysis, an

online Life-Cycle Cost Calculator was created atwww.galvanizingcost.com. The interactive calculatorallows the user to input information about any job and

compare the initial and life-cycle cost of galvanizing to anumber of paint systems.

Painting Hot-Dip Galvanized SteelPainting over hot-dip galvanized steel, called a duplexsystem, is a common practice for a number of reasons,including aesthetics, safety marking, and extended life.

Creating a successful duplex system requires propersurface preparation and communication with thegalvanizer about the intent to paint after galvanizing.

ASTM D 6386 has been developed to provide bestpractices for preparing a hot-dip galvanized surface forpainting.

Many products have been galvanized and paintedsuccessfully for decades, including automobilesand utility towers. For more information on duplexsystems, visit www.galvanizeit.org and downloadthe publicationsDuplex Systems: Painting Over HotDip Galvanized Steel and/or Practical Guide forPreparing Hot Dip Galvanized Steel for Painting.

Sheet Steel or ContinuousGalvanizing

Another series of hot-dip galvanized steel productsexists. Continuous galvanizing or sheet steel prodare still formed by dipping steel into molten zinc, buprocess is fully mechanized and done at very high speCoils of steel sheet metal are fed as ribbon througmolten zinc bath where it reacts to leave a protecsurface coating. Theoperation grew out

of traditional after-fabrication hot-dipgalvanizing into a

very sophisticatedprocess that can beused to apply thinand specific coatinggrades.

These coating grades are in the form of a letter Gand A followed by a coating weight in mass per a

For example, a G90 grade means the sheet has bgalvanized with 0.90 oz/ft2 (0.45 oz/ft2 per side) an A60 grade means the galvanized sheet was fur

annealed and has 0.60 oz/ft2 overall (0.30 oz/ft2 per s

This process is also called continuous galvanizingis specified in ASTM A 653/A 653 M, Steel ShZinc-Coated (Galvanized) or Zinc-Iron Alloy-Co

(Galvannealed) by the Hot-Dip Process. Comcoating weights specified for sheet products are: GG90 and G185. These also exist as metric counterpwith G90 being equivalent to a Z275 coating. more information about sheet steel products, con

the GalvInfo Center at www.galvinfo.com.

Galvanized Sheet


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Design Considerations

Now that we know the history, processes, and

performance characteristics of galvanizing, lets

examine characteristics for quality galvanizing.

Protection against corrosion begins at the drawing board.

No matter what corrosion protection system is specified,

t must be factored into the products design.

Once the decision has been made to hot-dip galvanize steel

for maximum corrosion protection, the design engineer

should ensure the pieces can be suitably fabricated for the

highest-quality galvanizing.

There are a few considerations when designing

components for galvanizing. These guidelines are relatively

simple and will help ensure maximum corrosion protection.

Things to consider while designing, detailing, and

fabricating steel to be galvanized:

The weight of fabricated items should be considered in the designof pieces for hot-dip galvanizing because the cranes/hoists usedin the handling processes required to move items though thegalvanizing facility have maximum limits.

Design a field splice at every other floor for heavy and longcolumns.

Increase the column size so doubler plates and cover plates are notrequired at the web and flange to satisfy the loads.

Use W & WT members for bracing in place of back to backstitched angle.

Use connections that can be welded all around.

Provide shear plate connections in place of clip angles (Figure 6).

Incorporate one-sided clip connections in place of clip angles(Figure 7).

Figure 6: Shear Connection

Figure 7: One-sided Clip Angle


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Use seated connections in place of clip angles(Figure 8).

Design end plate connections in lieu of clip angles(Figure 9).

Attach curb plates after galvanizing.

Avoid combining different materials & finishesbecause pickling time and immersion time in thezinc bath may affect the coating appearance and/or cause slight warpage and/or distortion due to

varying temperature gradients (Figure 10).

Asymmetrical steel sections

Use W shapes for fill beams to avoid the

distortion of asymmetrical pieces.

Weld stair stringer and steps into frames to add

symmetry and support during galvanizing.

Steel section of unequal thickness and size

There are ways to fabricate steel weldments to

guard against warping. Typically, bracing or usingstructural steel of symmetrical shape and similar

thickness provides quality finished product with little

or no distortion or warpage. See ASTM A 384 for

best practices, and then contact your local galvanizer

for more information.

Welding ProcedureIt is common practice to weld steel prior to

galvanizing, which ensures the entire structure iscoated with zinc. There are a few things to considerwhen welding before galvanizing, including the

removal of contaminants and the viscosity of zinc.

Flux & Slag Removal

As with any fabrication to be galvanized, the steelssurface needs to be completely free of any residuesincluding weld flux and weld slag. Welding flux isthe material used to prevent the formation of, or to

dissolve and facilitate removal of, oxides and otherundesirable substances. Weld slag is the materialresulting from the combination of weld material andweld flux and both will inhibit localized formation ofthe galvanizing coating.

Neither can be removed by the chemicals used in thegalvanizing process, and thus they will need to beremoved by mechanical means before shipping to the

galvanizers facility.

Figure 10: Design Guidelines to Avoid

Overlapping Surfaces

Figure 8: Seated Connection

Figure 9: End Plate Connection

Ductile iron pipe with

machined flange

Forged bolt with

machined threads

Steel with different

surface conditions

Old&

Pitte

d

New&Clean

Castings with

mild carbon steel

Machine surfaces

on pitted steel


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For highest quality galvanizing and final appearance,smooth clean welds free of flux and slag are

required.

Stitch- and Seal-welding

Stitch-welding and seal-welding are both commonlyused in fabrications for galvanizing. However, there arebest practices for using one or the other. Consider thefollowing:

The viscosity of molten zinc is low and thus

prevents it from entering gaps of 3/32 and

smaller, but cleaning solutions used in the process

can penetrate such openings.

Overlapping and contacting surfaces, like stitch

welds, allow the cleaning solutions used in the

galvanizing process to penetrate between the steel.

If cleaning solutions penetrate a gap, and zinc cannot,

pressure and steam can build up along the weld. Thisnot only may result in flash steaming that prevents the

galvanized coating from forming around the weld but

also creates steam pressure that may compromise the

integrity of the weld. Also, the trapped solutions may

eventually react with the uncoated steel hidden by the

weld or overlapping surfaces. This manifests as iron

oxide that weeps out to form an unsightly brown stain

on the galvanized surface..

Best welding practice for galvanizing is to stitch weld witha gap greater than 3/32 or seal-weld when this gapdistance is not possible. If the areas to be enclosed byseal welding are greater than 16 in2, vent holes must besupplied in the design to allow the expanding gas in theenclosed area to be vented during galvanizing. ASTMA 385 gives guidance on hole sizes and quantitiesbased on the area to be enclosed.

Seal-welding a weld used primarily to obtaintightness and prevent the flow of cleaning solutionsand zinc into otherwise enclosed areas, to preventflash steaming causing localized ungalvanized areas.

Stitch-welding a weld with at least 3/32 gap whichwill allow cleaning solutions and zinc to flow into andout of the weld area.

Drilling and CuttingDrill holes in place of punching in thicker material

and gas cut in place of shearing to avoid cracks atedges. Punching and shearing are cold-working forcesthat put internal stress on steel. The punched holeor shear location may result in an accelerated rate ofembrittlement of the steel.

Sheared Edge

If these edges are exposed during the hot-dip

galvanizing process, the microcracks that formedon the sheared edges may propagate into the steel.These edges may need to be ground to remove anymicrocracks formed during shearing.

Venting & DrainageProper venting is required on tubular assemblies suchas handrails, pipe columns and pipe trusses. Thisallows trapped air to escape the part and preventsthe air from becoming superheated steam in the

Punched Hole EmbrittlementSheared Edge Embrittlement

Seal WeldGood Weld


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emptied or freed of molten zinc.

Venting providing holes in fabrications to begalvanized to allow entrapped, heated liquids andgases to escape as temperature and pressure increase.

Handrail Preferred Venting & Drainage

In the picture below, the numbers correspond to the

following items:1. External vent holes2. Internal vent holes3. Open end drains

Venting and Drainage: Cap & Base Plates

There is a reason for base and cap plates to have venand drainage holes as shown here. When they enter

galvanizing bath air can escape and allow zinc to comcontact with the entire inside surface of the pipe or t

Additionally, when they are removed from the galvanibath, zinc is not trapped inside.

In the picture above, the end plate design is such thaholes are used for drainage but only in the orienta

shown. If turned 90 degrees the base plates will trap upon removal from the galvanizing bath. Contact your galvanizer for the proper way to vent pipes and tubes.

If steel is not adequately or properly vented, it mbecome a danger to galvanizer personnel, as

as allow explosive pressure to build, resultingirreparable damage to the steel.

molten zinc that could build up pressure. This builtup pressure may not only damage the coating, but

can also physically explode and endanger galvanizingpersonnel. Structures may be internally or externallyvented (see Figure 11).

Drainage the act, process, or mode of becoming

Proper Baseplate Drainage

(See detail sketch, page 21, for more information)Common baseplate venting

(See detail sketch, page 22, for more informati

Figure 11: Internal and External Venting


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Cropping For Drainage

To achieve effective galvanizing, the cleaningsolutions and molten zinc must flow completely into,over, through and out of the fabricated steel. Below

(Figures 12-15) are recommended types of drainagedesign to avoid improper drainage resulting in poorappearance, bare spots, and/or excessive buildup ofzinc. This buildup may make the part heavier than

anticipated in the design. Proper communicationthroughout the project will help attain good designfor drainage.

All stiffeners and gusset plates should be cropped(See Figure 12&14) to provide an opening with aminimum of 0.3 in2 or 13/16 in. hole at the corners ofall stiffeners. (See Figure 13&15).

Repair of Venting HolesIf vent holes need to be closed after galvanizing, as

they often are in handrail pieces, aluminum or zincplugs can be used.

MaskingIt is possible to masksections of a part toavoid the development ofthe galvanized coating.Examples where maskingis commonly used:

1. Field welded shear studs2. Slip critical bolt surfaces3. Field welded splice areas

There are 4 categories of masking material:

Acid-resistant, high temperature tapes

Water-based pastes and paint-on formulations Resin-based, high temperature paints High temperature greases

Masking using a material to produce intentionallyungalvanized areas, typically used on surfaces to be welded,

on faying surfaces, or areas where the galvanized steel

coating is not necessary for uniform corrosion protection.

MarkingPermanent identification practices include:

Stamping the surface of the material using die-cut deep stencils or a series of punch-markstoward the center of the pieces.

A series of weld beads to mark letters or numbersdirectly onto the material. It is essential that allweld flux be removed in order to achieve thehighest-quality galvanized coatings.

Deep stenciling a steel tag (minimum #12 gauge)and firmly affixing it to the material with aminimum #9 gauge steel wire. If desired, tagsmay be seal-welded directly onto the material.

Before After

Masking

Common identification practices

Figure 12: Cropped

Corners (Preferred)

Figure 13: Hole

close to corner

Figure 15: Holes

at Corner (Alternative)Figure 14: Cropped

Corners (Preferred)

(See detail sketch, page 23-24,for more information)


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Barcode TagsMetal barcode tags can also be used to identify materials.These tags are resistant to caustic wash and acid pickling.The tags will survive the molten zinc bath with minimaldamage, as they are durable in a wide temperature range(-22F to 1400F (-30C to 760C)).

Additional information can be stored in the bar code

besides the piece mark, including job name andnumber, grade of steel, weight of piece, name of

customer, etc.

Galvanized Bolts, Nuts, and HolesNuts and threaded holes fabricated in steel to be hot-dip galvanized should be retapped or rethreaded aftergalvanizing to remove the zinc coating and provideclearance for the coated bolt. When the fastener system

is assembled, the coating from the bolt will provide

protection for the uncoated threads on the nut or holesince zinc coatings cathodically protect uncoatedsteel. Retapping is done to the nut so no uncoatedthreads (Figure 16) on the bolts (outside the nut) areexposed to weather without galvanized protection.Standard practice for structural connections is togalvanize the nuts as blanks and then tap the threads

after galvanizing.

A similar process is suggested for oversizing oholes. The hot-dip galvanizing process addcoating of zinc to steel in the range of 2-8 mils. Wdesigning open holes, it is necessary to plan forincreased thickness on both the fastener and the (see Table 1). If after galvanizing, the hole is stillarge enough, it can be reamed. A small amoureaming will not affect the corrosion protection.

The numbers in the parenthesis are equal tonumber outside of the parenthesis and can be useeasier calculations.

Note: When over-sizing holes, check with the deengineer for bearing surface area of the bolt head

Bolts used in a bridge structureFigure 16: Bolt Micrograph

Galvanized Table for Oversized HolesNot certified by AISC or AGA

Table 1: Standard Clearance Hole Diamet

Barcode Tags

Nominal boltDiameter (db)(in)

StandardClearanceHole Diameter (in.)

OversizedClearanceHole Diameter

db < 1/2 db + 1/16 db + 2/16

1/4 (4/16) 5/16 3/8 (6/16)

1/2 (8/16) 9/16 5/8 (10/16)

1/2 < db < 1 db + 1/16 db + 3/16

5/8 (10/16) 11/16 13/163/4 (12/16) 13/16 15/16

7/8 (14/16) 15/16 1 1/16 (17/16)

1 < db < 1 1/8 db + 1/16 db + 4/16

1 (16/16) 1 1/16 (17/16) 1 1/4 (20/16)

db > 1 1/8 db + 1/16 db + 5/16

1 1/8 (18/16) 1 3/16 (19/16) 1 7/16 (23/16)


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Temporary Bracing

Large diameter, thin-walled pipe and many long orcomplex fabrications may require temporary bracingo prevent possible distortion. The slow (3 ft/min)mmersion of steel items into the zinc bath creates an

uneven heating and cooling gradient.

Temporary bracing metal attached to a fabricationprior to galvanizing in order to provide added supporto the steel does not change shape during heating

and cooling. Temporary bracing is removed aftergalvanizing.

Lifting Aids

With respect to providing lifting points, consider thefollowing:

Where possible, lifting points (see illustrationbelow) should be provided at the quarter points forsymmetrical parts; this avoids chain or wire markson the sides of the parts.

Holes for hooks may be included in the designto allow the galvanizer to hang the material fromoverhead fixtures.

Lifting points connectors (sometimes temporary)

directly on the steel article that aid the galvanizer inhandling the article throughout the galvanizing process,

especially if the piece to be galvanized is oversized

Galvanizing Oversized PiecesProgressive dipping, sometimes erroneously referred toas double dipping, is used when pieces are too large to fitin the galvanizing kettle in one pass. Progressive dipping

increases the potential for warpage and distortion since asection of the steel fabrication will be outside the moltenzinc, and therefore, cold and stiff while the immersedsection of the steel is hot and ductile.

This uneven temperature gradient may cause distortionof the steel fabrication. Other issues associated

with progressive dipping include additional handlingcosts and an overlap line (albeit having no effect on thecorrosion protection provided). When possible, designfor a splice to allow pieces to be dipped in one pass.

Touchup and Repair

ASTM A780 describes three acceptable methods of repairinghot-dip galvanized steel (zinc solder, metallizing, and zinc rich

paint). The touch-up and repair method chosen should considerthe specific use of the galvanized steel and the performancecharacteristics of each method. Corrosion protection shouldalways be the primary consideration, but certain uses and

conditions may warrant selection on the basis of otherperformance characteristics.

Progressive Dipping

Zinc Rich Paint

1/4 points

(See detail sketch, page 25, for more information)

Temporary bracing


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Shiny surface Dull surface

AppearanceWhen steel parts are removed from the molten zinc bath,the hot-dip galvanized coating can appear bright andshiny, spangled, matte gray, or a combination of these.Regardless of the appearance, the corrosion protectionafforded is the same. After a few months of exposureto the atmosphere, hot-dip galvanizing forms a protectivelayer of zinc corrosion byproducts that will give all pieces

a uniform, matte gray appearance.

To learn more about design guidelines for galvanized svisit www.galvanizeit.org and download the publicatThe Design of Products to be Hot-Dip Galvan

After Fabrication and/or Recommended DetailsGalvanizing Structures.

Dull and Shiny surfaceSpangled surface


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CANADIAN STANDARDS ASSOCIATIONG40.8* Structural Steel with Improved Resistance to Brittle Fracture

G40.12* General Purpose Structural Steel

G164 Galvanizing of Irregularly Shaped Articles

* Superseded by G40.20/G40.21 General Requirements for Rolled or Welded Structural QualitySteel

ASTM STANDARDS RELATING TO HOT-DIPGALVANIZING AND HOT-DIP GALVANIZED MATERIALS

A 36 Specification for Structural Steel

A 123/ A 123 M Specification For Zinc (Hot-Dip Galvanized) Coatings On Iron AndSteel Products

A 143 Practice For Safeguarding Against Embrittlement of Hot-Dip GalvanizedStructural Steel Products and Procedure for Detecting Embrittlement

A 153/ A 153 M Specification For Zinc Coating (Hot-Dip) On Iron And Steel Hardware

A 384/ A 384 M Practice For Safeguarding Against Warpage And Distortion During Hot-Dip Galvanizing Of Steel Assemblies

A 385 Practice For Providing High-Quality Zinc Coatings (Hot-Dip)

A 500 Specification for Cold-Formed Welded and Seamless Carbon SteelStructural Tubing in Rounds and Shapes

A 501 Specification for Hot-Formed Welded and Seamless Carbon Steel

Structural TubingA 563 Standard Specification for Carbon and Alloy Steel Nuts

A 572 Specification for High-Strength Low-Alloy Columbium-Vanadium Steels ofStructural Quality

A 767/ A 767 M Specification For Zinc-Coated (Galvanized) Steel Bars For ConcreteReinforcement

A 780 Practice For Repair Of Damaged And Uncoated Areas Of Hot-DipGalvanized Coatings

A 992 Specifications for Steel Structural Shapes For Use in Building Framing

B 6 Specification For Zinc

D 6386 Practice For Preparation Of Zinc (Hot-Dip Galvanized) Coated Iron AndSteel Products And Hardware Surfaces For Painting

E 376 Practice For Measuring Coating Thickness By Magnetic-Field Or Eddy-Current (Electromagnetic) Test Methods


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Frequently Asked Questions1. How does galvanizing protect steel from corrosion?

Zinc metal used in the galvanizing process provides

an impervious barrier between the steel substrate andcorrosive elements in the atmosphere. It does not allowmoisture and corrosive chlorides and sulfides to attack thesteel. Zinc is more importantly anodic to steel meaning

it will corrode before the steel, until the zinc is entirelyconsumed.

2. How long can I expect my galvanized steel projectsto last in service?

Hot-dip galvanized steel resists corrosion in numerousenvironments extremely well. It is not uncommon

for galvanized steel to last more than 70 years undercertain conditions.

3. Does the galvanized steel coating of zinc resistabrasion?

The three intermetallic layers that form during thegalvanizing process are all harder than the substrate steeland have excellent abrasion resistance.

4. Why do galvanized steel appearances differ from

project to project and galvanizer to galvanizer, and

is there any difference in the corrosion protection

offered by the different appearing coatings?

The appearance of the coating (matte gray, shiny,spangled) does nothing to change the corrosion protection

of the zinc coating. The corrosion protection is a functionof the amount of zinc in the coating, more zinc equalslonger life.

5. Can galvanized steel in service withstand hightemperatures for long periods of time?

Constant exposure to temperatures below 390F (200C) is

a perfectly acceptable environment for hot-dip galvanizedsteel. Good performance can also be obtained whenhot-dip galvanized steel is exposed to temperatures above390F (200C) on an intermittent basis.

6. Why would you want to paint over galvanized steel?

Called duplex coatings, zinc and paint in combination(synergistic effect) will protect a structure 1.5 to 2.5times the sum of the corrosion protection each alonewould provide. Additionally, duplex coatings make foreasy repainting, excellent safety marking systems, and

good color-coding. Painting over galvanized steel thathas been in service for many years also extends the lifeof the zinc coating.

7. Isnt galvanizing more expensive than pa

Depending on the product mix, square feet per ton

condition of the steel surface, galvanizing is oftenexpensive on an initial cost basis. However, as withpurchase, the life-cycle costs should be considered wmaking a project decision on the corrosion protec

system to utilize. And, with galvanizing, the cycle cost, i.e. the cost per year to maintain, is almalways less than a paint system. Paint systems reqmaintenance, partial repainting and full repain

several times over a 30-year project life. The costsbe staggering, making the decision to paint a costlyin the long run. To run the comparison yourself,

www.galvanizingcost.com.

8. What if the article to be galvanized is larger tha

the dimensions of the galvanizers kettle? Can it

still be galvanized?

Galvanizers can progressively dip such a fabricatioarticle of steel. They dip one half in the molten zinbath, remove it, turn it around or over and immerse

the other half in the zinc. This method is sometimeerroneously referred to as double dipping.

9. Are there any special design and fabricationconsiderations required to make steel ready for

dip galvanizing?

Yes. Specifically, fabricated steel must allow for e

flow of the cleaning chemicals and molten zinc meover and through it. This means that gussets must cropped, holes put in the proper location for draininand venting of zinc from tubular configurations, we

flux removed, overlapping surfaces must be seal-welded, and light gauge material temporarily brace

10. Sometimes, the galvanized coating is shinier insome places than others. Why is that?

The galvanized coating appearance may either bebright and shiny resulting from the presence of

an outer layer of pure zinc, or duller, matte grayas the result of the coatings intermetallic layers

being exposed. The appearance has no affect on thcorrosion performance of the coating. Over time

exposure to the environment, all galvanized coatinbecome a uniform, matte gray.

11. Is the zinc coatings thickness consistent over tentire piece?

Coating thickness depends on the thickness,roughness, chemistry, and design of the steel bein

galvanized. Any or all of these factors could prodgalvanized coatings of non-uniform thickness.


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2. How much weight will my material gain fromgalvanizing?

As an average, the weight of the article will

increase by about 3.5% due to zinc picked up in thegalvanizing process. However, that figure can varygreatly based on numerous factors. The fabricationsshape, size, and steel chemistry all play a major role

in the final weight.

3. Im interested in specifying hot-dip galvanizingfor reinforcing steel. Are there any concerns with

fabricating rebar after galvanizing?

Rebar can be fabricated after galvanizing, but thefabrication process may induce damage into the

protective coating and reduce the life of the material.

4. Can I specify how much zinc to put on the steel?

No, the steel chemistry and surface condition arethe primary determinants of zinc coating thickness.Leaving the steel in the molten zinc a little longerthan optimal may have one of two effects:

1) it may increase the coating thickness, but onlymarginally; 2) or it may significantly increase thecoating thickness and cause a brittle coating.

5. What does it mean to double-dip steel?

Double-dipping is the progressive dipping of steel

too large to fit into the kettle in a single dip. Double-dipping cannot be used to produce a thicker hot-dipgalvanized coating.

6. What is the reason for incorporating venting &

drainage holes into a projects design?The primary reason for vent holes is to allowotherwise trapped air and gases to escape; the

primary reason for drain holes is to allow cleaningsolutions and molten zinc metal to flow entirely into,over, and throughout the part, and then back into the

tank or kettle.

17. Is there a way to provide for intentionallyungalvanized areas?

Yes, but because masking or stop-off materials may

not be 100% effective, contact your galvanizer for

suggestions.

18. Is there any environmental impact when the zinccoating sacrificially corrodes? Is zinc a safe metal?

There are no known studies to suggest zinc corrosion

products cause any harm to the environment. Zinc isa naturally occurring element (27th most abundant

element in the earths crust), and necessary for allorganisms to live. It is a recommended part of our diet(RDA 15 mg) and necessary for reproduction. It is

used in baby ointments, vitamins, surgical instruments,sunscreens and cold lozenges.

19. Should I be concerned when galvanized steelcomes in contact with other metals?

Zinc is a noble metal and will sacrifice itself (i.e.corrode, give up its electrons and create a bi-metallic

couple) to protect most metals. So, it is recommendedto insulate galvanized steel so it doesnt come indirect contact with dissimilar metals. Rubber orplastic, both non-conductive, are often used to

provide this insulation.

20. What is cold galvanizing?

There is no such thing as cold galvanizing. The term is

often used in reference to zinc-rich paint. Galvanizing bydefinition means a metallurgical reaction between zincand iron to create a bond between the zinc and the steel ofapproximately 3600 psi. There is no such reaction when

zinc-rich paints are applied and the bond strength is onlyseveral hundred psi.

For additional information please visit

the American Galvanizers Associations website www.galvanizeit.org


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For additional information please visitthe American Galvanizers Associations website www.galvanizeit.org

Special Thanks To:Michael Tinker Pacific Drafting Inc.

Rodelio Carpio Pacific Drafting Inc.

Bernardo Duran American Galvanizers Association

Jenny Clawson - American Galvanizers AssociationCecile Elliott American Galvanizers Association

Kevin Hobson Calwest Galvanizing


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National Institute of Steel Detailing7700 Edgewater Dr. Ste. 670

Oakland, CA 94621-3022510.568.3741www nisd org

steel galvanized

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