steel galvanized
TRANSCRIPT
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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