145404892 Steel Galvanized

“What We Need to Know”

National Institute of Steel Detailinge of Steel Detailintailingtailing

Hot-DipGalvanizing

©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 verifi cation 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..............................................5Hot-Dip Galvanizing Process.................................6 Surface Preparation..................................6 Degreasing..................................6 Pickling........................................6 Fluxing.........................................6 Galvanizing.............................................7 Inspection................................................7Galvanized Coating Characteristics

Metallurgical bond...................................7 Coating Uniformity....................................7 Cathodic protection..................................8Galvanized Coating Performance

Time to First Maintenance.........................8 Exposure to High Temperature....................8 Additional Galvanizing Information

Galvanizing vs. Painting: By the Numbers.....9 Painting Hot-Dip Galvanized Steel.............9 Sheet Steel/Continuous Galvanizing...........9Design Considerations........................................10 Welding Procedure................................11 Flux & Slag Removal....................11 Stitch and Seal Welding .............12

Drilling and Cutting.................................................12

Venting and Drainage..............................................12 Handrail..........................................13 Cap and Base Plates.......................13 Cropping for Drainage.....................14 Repair of Vent Holes........................14 Masking.....................................................14 Marking.....................................................14 Barcode Tags..............................................15 Galvanized Bolts, Nuts, and Holes.................15

Temporary Bracing.......................................15 Lifting Aids..................................................16 Galvanizing Oversized Pieces......................16 Touchup and Repair.....................................16 Appearance................................................17

ASTM Standards.....................................................18Canadian Standards Association..............................18Frequently Asked Questions.....................................19Appendix of Detailed Sketches................................21Special Thanks........................................................27

Hot-DipGalvanizingHot-Dip

Table of Contents

44

55

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, galvanizing’s namesake, discovers the electrochemical process that takes place between metals during an experiment with frog legs.

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

1829 Michael Faraday discovers zinc’s sacrifi cial action, during an experiment involving zinc, salt water and nails.

1837 French engineer Stanislaus Tranquille Modeste Sorel took out a patent for the early 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 the zinc 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 engineer, fabricator, and detailer better understand the process of preparing steel for the highest quality corrosion resistant coating (galvanizing).

This book will assist you in your hot-dip galvanizing foundation by providing a look at the galvanizing history, galvanizing process, galvanized coating characteristics, performance, and design considerations. Following the information provided, the designer, fabricator, and detailer can ensure the highest quality galvanized coating.

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

• Surface preparation – a series of three cleaning processes to prepare the steel for immersion in the zinc bath, as zinc will not react with, nor adhere to unclean steel.

• Galvanizing – total immersion of the steel in the molten zinc bath.

• Inspection – visual inspection and coating thickness measurement to ensure conformance to appropriate specifi cations.

Small parts, such as fasteners, brackets, and clips less 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 PreparationDegreasing

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

PicklingDilute solution (between 8% to 15%) of either ambient hydrochloric or heated sulfuric acid removes surface rust and mill scale to provide a chemically clean metallic surface (Red Tank, Figure 1).

FluxingSteel is immersed in liquid fl ux (a zinc ammonium chloride solution) for two purposes. First, the fl ux will remove any remaining iron oxides. Additionally, the fl ux will create a protective fi lm to prevent oxidation prior to dipping into the molten zinc bath (Yellow Tank, Figure 1).

Hot-Dip Galvanizing Process

Figure 1: The Hot-Dip Galvanizing Process

Degreasing

PicklingRinsing

Rinsing

Fluxsolution

Causticcleaning

Drying Zincbath

Cooling andinspection

Surface Preparation

Pickling

77

Metallurgical BondDuring the galvanizing process, the zinc in the kettle and the iron in the steel metallurgically react to form the galvanized coating. This diffusion reaction creates a series of intermetallic zinc-iron alloy layers, which are harder than the base steel (see Figure 2). The metallurgical bond is much stronger than a mechanically bonded coating, as galvanized steel bond strength is around 3,600 psi compared to several hundred for most other coatings.

Coating UniformityGalvanizing is a total immersion process, which ensures all surfaces are coated, including the inside of hollow structures. During the diffusion reaction in the galvanizing kettle, the intermetallic layers grow perpendicular to the

surface, which means coating thickness at corners and edges is at least as thick as fl at surfaces. Paint tends to be thinner at edges and corners, and painted hollow structures have no protection on the inside. These areas are where corrosion often starts.

GalvanizingThe steel article is immersed in a bath of molten zinc heated to between 815-850ºF (435-455ºC). During galvanizing, the zinc metallurgically bonds to the steel, creating a series of abrasion-resistant zinc-iron alloy 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 small items, centrifuging. It is important to remove all excess to ensure the part is suitable for its intended use. The galvanized item is either cooled by air or water, or dipped in a passivation solution to prevent oxidation.

Inspection

The fi nal step in the galvanizing process is the inspection of the surface condition and coating thickness. The inspection of galvanizing is relatively easy because zinc does not adhere to unclean steel.

So, if the steel has a continuous coating of zinc, it should meet the required specifi cation. To confi rm conformance, the coating thickness is measured using a 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 ProtectionGalvanized coatings also offer cathodic protection, which simply means the zinc will sacrifi ce itself to protect the underlying base steel. Often steel pieces are roughly handled during shipment and/or erection, which can damage organic coatings. Galvanized steel can withstand this rough handling, and if damaged, the steel will still be cathodically protected by the surrounding zinc (see Figure 3). The same principle is used to protect outboard boat engines.

Galvanized Coating Performance

Figure 3: Cathodic Protection Figure 3: Cathodic Protection

100

010

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 = 25.4µm = 0.56oz/ft2*Time to first maintenance is defined as the time to 5% rusting of the substrate steel surface.

Tim

e to

Firs

t M

aint

enan

ce* (

year

s)

Figure 4: Time to First Maintenance Chart

Time to First MaintenanceThe Time to First Maintenance Chart (Figure 4) was developed from decades of real world corrosion data collected from galvanized steel samples exposed to environments all over the world. This data was sorted into fi ve characteristic environmental categories: rural, suburban, industrial, temperate marine and tropical marine.

Time to fi rst maintenance is defi ned as the period of time until 5% of the substrate steel surface is showing iron oxide (rust). At this point, it is unlikely the underlying steel has been weakened or the integrity of the structure is compromised, but it is time to begin a maintenance cycle on the structure to protect it from further corrosion.

As the chart illustrates, the zinc coating thickness is directly proportional to the time to fi rst maintenance. Other factors that infl uence the corrosion performance of the coating are: relative humidity, sulfur dioxide, airborne salinity, precipitation, and temperature.

For more information on the performance of hot-dip galvanized coatings, visit the American Galvanizers Association’s website at www.galvanizeit.org and download the publications Hot-Dip Galvanizing for Corrosion Protection: A Specifi er’s Guide and/or Service 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. The industry recommends the service temperature for galvanized coatings be less than 390ºF (200ºC) for long-term exposure. Possible concerns at continued exposure to temperatures above 390°F (200°C)

include peeling, some changes in mechanical properties, and obvious reduction in corrosion protection.

390 F 480 F

Temperature

No Peeling Some Peeling

Peeling

Figure 5: Galvanizing Performance at High

Temperatures

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Examples of duplex systems

Additional Galvanizing InformationGalvanizing vs. Painting: By the numbersAn economic analysis of galvanizing vs. painting on both an initial and life-cycle basis should be performed prior to the selection of either corrosion protection method. Galvanizing has long been known to be less expensive on a life-cycle basis, but many specifi ers do not realize galvanizing is also competitive on an initial cost basis. In order to facilitate the process of performing an economic analysis, an online Life-Cycle Cost Calculator was created at www.galvanizingcost.com. The interactive calculator allows the user to input information about any job and compare the initial and life-cycle cost of galvanizing to a number of paint systems.

Painting Hot-Dip Galvanized SteelPainting over hot-dip galvanized steel, called a duplex system, is a common practice for a number of reasons, including aesthetics, safety marking, and extended life. Creating a successful duplex system requires proper surface preparation and communication with the galvanizer about the intent to paint after galvanizing. ASTM D 6386 has been developed to provide best practices for preparing a hot-dip galvanized surface for painting.

Many products have been galvanized and painted successfully for decades, including automobiles and utility towers. For more information on duplex systems, visit www.galvanizeit.org and download the publications Duplex Systems: Painting Over Hot Dip Galvanized Steel and/or Practical Guide for Preparing Hot Dip Galvanized Steel for Painting.

Sheet Steel or Continuous GalvanizingAnother series of hot-dip galvanized steel products also exists. Continuous galvanizing or sheet steel products are still formed by dipping steel into molten zinc, but the process is fully mechanized and done at very high speeds. Coils of steel sheet metal are fed as ribbon through a molten zinc bath where it reacts to leave a protective surface coating. The operation grew out of traditional after-fabrication hot-dip galvanizing into a very sophisticated process that can be used to apply thin and specifi c coating grades.

These coating grades are in the form of a letter G, Z, and A followed by a coating weight in mass per area. For example, a G90 grade means the sheet has been galvanized with 0.90 oz/ft2 (0.45 oz/ft2 per side) and an A60 grade means the galvanized sheet was further annealed and has 0.60 oz/ft2 overall (0.30 oz/ft2 per side).

This process is also called continuous galvanizing and is specifi ed in ASTM A 653/A 653 M, Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process. Common coating weights specifi ed for sheet products are: G60, G90 and G185. These also exist as metric counterparts with G90 being equivalent to a Z275 coating. For more information about sheet steel products, contact 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, let’s examine characteristics for quality galvanizing.Protection against corrosion begins at the drawing board. No matter what corrosion protection system is specifi ed, it must be factored into the product’s 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 design of pieces for hot-dip galvanizing because the cranes/hoists used in the handling processes required to move items though the galvanizing facility have maximum limits.

• Design a fi eld splice at every other fl oor for heavy and long columns.

• Increase the column size so doubler plates and cover plates are not required at the web and fl ange to satisfy the loads.

• Use “W” & “WT” members for bracing in place of back to back stitched 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 & fi nishes because pickling time and immersion time in the zinc 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 fi ll 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 sizeThere are ways to fabricate steel weldments to guard against warping. Typically, bracing or using structural steel of symmetrical shape and similar thickness provides quality fi nished 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 is coated with zinc. There are a few things to consider when welding before galvanizing, including the removal of contaminants and the viscosity of zinc.

Flux & Slag Removal

As with any fabrication to be galvanized, the steel’s surface needs to be completely free of any residues including weld fl ux and weld slag. Welding fl ux is the material used to prevent the formation of, or to dissolve and facilitate removal of, oxides and other undesirable substances. Weld slag is the material resulting from the combination of weld material and weld fl ux and both will inhibit localized formation of the galvanizing coating.

Neither can be removed by the chemicals used in the galvanizing process, and thus they will need to be removed by mechanical means before shipping to the galvanizer’s 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 differentsurface conditions

Old & Pitted

New & Clean

Castings with mild carbon steel

Machine surfaces on pitted steel

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For highest quality galvanizing and fi nal appearance, smooth clean welds free of fl ux and slag are required.

Stitch- and Seal-welding

Stitch-welding and seal-welding are both commonly used in fabrications for galvanizing. However, there are best practices for using one or the other. Consider the following: • 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. This not only may result in fl ash 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 with a gap greater than 3/32” or seal-weld when this gap distance is not possible. If the areas to be enclosed by seal welding are greater than 16 in2, vent holes must be supplied in the design to allow the expanding gas in the enclosed area to be vented during galvanizing. ASTM A 385 gives guidance on hole sizes and quantities based on the area to be enclosed.

Seal-welding – a weld used primarily to obtain tightness and prevent the fl ow of cleaning solutions and zinc into otherwise enclosed areas, to prevent fl ash steaming causing localized ungalvanized areas.

Stitch-welding – a weld with at least 3/32” gap which will allow cleaning solutions and zinc to fl ow into and out 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 at edges. Punching and shearing are cold-working forces that put internal stress on steel. The punched hole or shear location may result in an accelerated rate of embrittlement of the steel.

Sheared EdgeIf these edges are exposed during the hot-dip galvanizing process, the microcracks that formed on the sheared edges may propagate into the steel. These edges may need to be ground to remove any microcracks formed during shearing.

Venting & DrainageProper venting is required on tubular assemblies such as handrails, pipe columns and pipe trusses. This allows trapped air to escape the part and prevents the air from becoming superheated steam in the

Punched Hole EmbrittlementSheared Edge Embrittlement

Seal WeldGood Weld

1313

emptied or freed of molten zinc.

Venting – providing holes in fabrications to be galvanized to allow entrapped, heated liquids and gases 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 venting and drainage holes as shown here. When they enter the galvanizing bath air can escape and allow zinc to come in contact with the entire inside surface of the pipe or tube. Additionally, when they are removed from the galvanizing bath, zinc is not trapped inside.

In the picture above, the end plate design is such that the holes are used for drainage but only in the orientation shown. If turned 90 degrees the base plates will trap zinc upon removal from the galvanizing bath. Contact your local galvanizer for the proper way to vent pipes and tubes.

If steel is not adequately or properly vented, it may become a danger to galvanizer personnel, as well as allow explosive pressure to build, resulting in irreparable damage to the steel.

molten zinc that could build up pressure. This built up pressure may not only damage the coating, but can also physically explode and endanger galvanizing personnel. Structures may be internally or externally vented (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 information)

Figure 11: Internal and External Venting

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Cropping For Drainage

To achieve effective galvanizing, the cleaning solutions and molten zinc must fl ow completely into, over, through and out of the fabricated steel. Below (Figures 12-15) are recommended types of drainage design to avoid improper drainage resulting in poor appearance, bare spots, and/or excessive buildup of zinc. This buildup may make the part heavier than anticipated in the design. Proper communication throughout the project will help attain good design for drainage.

All stiffeners and gusset plates should be cropped (See Figure 12&14) to provide an opening with a minimum of 0.3 in2 or 13/16 in. hole at the corners of all 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 zinc plugs can be used.

MaskingIt is possible to mask sections of a part to avoid the development of the galvanized coating. Examples where masking is commonly used:

1. Field welded shear studs 2. Slip critical bolt surfaces 3. 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 intentionally ungalvanized 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 identifi cation practices include:

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

• A series of weld beads to mark letters or numbers directly onto the material. It is essential that all weld fl ux be removed in order to achieve the highest-quality galvanized coatings.

• Deep stenciling a steel tag (minimum #12 gauge) and fi rmly affi xing it to the material with a minimum #9 gauge steel wire. If desired, tags may 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: Holesat Corner (Alternative)

Figure 14: Cropped Corners (Preferred)

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

1515

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 minimal damage, as they are durable in a wide temperature range (-22ºF to 1400ºF (-30ºC to 760ºC)).

Additional information can be stored in the bar code besides the piece mark, including job name and number, 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 after galvanizing to remove the zinc coating and provide clearance 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 hole since zinc coatings cathodically protect uncoated steel. Retapping is done to the nut so no uncoated threads (Figure 16) on the bolts (outside the nut) are exposed to weather without galvanized protection. Standard practice for structural connections is to galvanize the nuts as blanks and then tap the threads after galvanizing.

A similar process is suggested for oversizing open holes. The hot-dip galvanizing process adds a coating of zinc to steel in the range of 2-8 mils. When designing open holes, it is necessary to plan for the increased thickness on both the fastener and the hole (see Table 1). If after galvanizing, the hole is still not large enough, it can be reamed. A small amount of reaming will not affect the corrosion protection.

The numbers in the parenthesis are equal to the number outside of the parenthesis and can be used for easier calculations.

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

Bolts used in a bridge structureFigure 16: Bolt MicrographFigure 16: Bolt Micrograph

Galvanized Table for Oversized HolesNot certified by AISC or AGA

Table 1: Standard Clearance Hole Diameter

Barcode Tags

Nominal boltDiameter (db) (in)

Standard ClearanceHole Diameter (in.)

Oversized ClearanceHole Diameter (in.)

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 13/16

3/4 (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)

1616

Temporary BracingLarge diameter, thin-walled pipe and many long or complex fabrications may require temporary bracing to prevent possible distortion. The slow (3 ft/min) immersion of steel items into the zinc bath creates an uneven heating and cooling gradient.

Temporary bracing – metal attached to a fabrication prior to galvanizing in order to provide added support so the steel does not change shape during heating and cooling. Temporary bracing is removed after galvanizing.

Lifting AidsWith respect to providing lifting points, consider the following:

• Where possible, lifting points (see illustration below) should be provided at the quarter points for symmetrical parts; this avoids chain or wire marks on the sides of the parts.

• Holes for hooks may be included in the design to allow the galvanizer to hang the material from overhead fi xtures.

Lifting points – connectors (sometimes temporary) directly on the steel article that aid the galvanizer in handling the article throughout the galvanizing process, especially if the piece to be galvanized is oversized

Galvanizing Oversized PiecesProgressive dipping, sometimes erroneously referred to as double dipping, is used when pieces are too large to fi t in the galvanizing kettle in one pass. Progressive dipping increases the potential for warpage and distortion since a section of the steel fabrication will be outside the molten zinc, and therefore, cold and stiff while the immersed section of the steel is hot and ductile.

This uneven temperature gradient may cause distortion of the steel fabrication. Other issues associated with progressive dipping include additional handling costs and an overlap line (albeit having no effect on the corrosion protection provided). When possible, design for a splice to allow pieces to be dipped in one pass.

Touchup and RepairASTM A780 describes three acceptable methods of repairing hot-dip galvanized steel (zinc solder, metallizing, and zinc rich paint). The touch-up and repair method chosen should consider the specifi c use of the galvanized steel and the performance characteristics of each method. Corrosion protection should always be the primary consideration, but certain uses and conditions may warrant selection on the basis of other performance 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 and shiny, spangled, matte gray, or a combination of these. Regardless of the appearance, the corrosion protection afforded is the same. After a few months of exposure to the atmosphere, hot-dip galvanizing forms a protective layer of zinc corrosion byproducts that will give all pieces a uniform, matte gray appearance.

To learn more about design guidelines for galvanized steel, visit www.galvanizeit.org and download the publications The Design of Products to be Hot-Dip Galvanized After Fabrication and/or Recommended Details for Galvanizing 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 Quality Steel

ASTM STANDARDS RELATING TO HOT-DIP GALVANIZING 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 And Steel Products

A 143 Practice For Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural 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 Steel Structural Tubing in Rounds and Shapes

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

A 563 Standard Specification for Carbon and Alloy Steel Nuts

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

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

A 780 Practice For Repair Of Damaged And Uncoated Areas Of Hot-Dip Galvanized 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 And Steel 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 and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfi des to attack the steel. Zinc is more importantly anodic to steel – meaning it will corrode before the steel, until the zinc is entirely consumed.

2. How long can I expect my galvanized steel projects to last in service?Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions.

3. Does the galvanized steel coating of zinc resist abrasion?The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and 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 function of the amount of zinc in the coating, more zinc equals longer life.

5. Can galvanized steel in service withstand high temperatures for long periods of time?Constant exposure to temperatures below 390F (200C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390F (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 2.5 times the sum of the corrosion protection each alone would provide. Additionally, duplex coatings make for easy repainting, excellent safety marking systems, and good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.

7. Isn’t galvanizing more expensive than paint?Depending on the product mix, square feet per ton, and condition of the steel surface, galvanizing is often less expensive on an initial cost basis. However, as with any purchase, the life-cycle costs should be considered when making a project decision on the corrosion protection system to utilize. And, with galvanizing, the life-cycle cost, i.e. the cost per year to maintain, is almost always less than a paint system. Paint systems require maintenance, partial repainting and full repainting several times over a 30-year project life. The costs can be staggering, making the decision to paint a costly one in the long run. To run the comparison yourself, visit www.galvanizingcost.com.

8. What if the article to be galvanized is larger than the dimensions of the galvanizer’s kettle? Can it still be galvanized?Galvanizers can progressively dip such a fabrication or article of steel. They dip one half in the molten zinc bath, remove it, turn it around or over and immerse the other half in the zinc. This method is sometimes erroneously referred to as “double dipping”.

9. Are there any special design and fabrication considerations required to make steel ready for hot-dip galvanizing?Yes. Specifi cally, fabricated steel must allow for easy fl ow of the cleaning chemicals and molten zinc metal over and through it. This means that gussets must be cropped, holes put in the proper location for draining and venting of zinc from tubular confi gurations, weld fl ux removed, overlapping surfaces must be seal-welded, and light gauge material temporarily braced.

10. Sometimes, the galvanized coating is shinier in some places than others. Why is that?The galvanized coating appearance may either be bright and shiny resulting from the presence of an outer layer of pure zinc, or duller, matte gray as the result of the coating’s intermetallic layers being exposed. The appearance has no affect on the corrosion performance of the coating. Over time and exposure to the environment, all galvanized coatings become a uniform, matte gray.

11. Is the zinc coating’s thickness consistent over the entire piece?Coating thickness depends on the thickness, roughness, chemistry, and design of the steel being galvanized. Any or all of these factors could produce galvanized coatings of non-uniform thickness.

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12. How much weight will my material gain from galvanizing?As an average, the weight of the article will increase by about 3.5% due to zinc picked up in the galvanizing process. However, that fi gure can vary greatly based on numerous factors. The fabrication’s shape, size, and steel chemistry all play a major role in the fi nal weight.

13. I’m interested in specifying hot-dip galvanizing for reinforcing steel. Are there any concerns with fabricating rebar after galvanizing?Rebar can be fabricated after galvanizing, but the fabrication process may induce damage into the protective coating and reduce the life of the material.

14. Can I specify how much zinc to put on the steel?No, the steel chemistry and surface condition are the primary determinants of zinc coating thickness. Leaving the steel in the molten zinc a little longer than optimal may have one of two effects: 1) it may increase the coating thickness, but only marginally; 2) or it may signifi cantly increase the coating thickness and cause a brittle coating.

15. What does it mean to “double-dip” steel?“Double-dipping” is the progressive dipping of steel too large to fi t into the kettle in a single dip. Double-dipping cannot be used to produce a thicker hot-dip galvanized coating.

16. What is the reason for incorporating venting & drainage holes into a project’s design?The primary reason for vent holes is to allow otherwise trapped air and gases to escape; the primary reason for drain holes is to allow cleaning solutions and molten zinc metal to fl ow entirely into, over, and throughout the part, and then back into the tank or kettle.

17. Is there a way to provide for intentionally ungalvanized 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 zinc coating sacrifi cially corrodes? Is zinc a safe metal?There are no known studies to suggest zinc corrosion products cause any harm to the environment. Zinc is a naturally occurring element (27th most abundant element in the earth’s crust), and necessary for all organisms 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 steel comes in contact with other metals?Zinc is a noble metal and will sacrifi ce itself (i.e. corrode, give up its electrons and create a bi-metallic couple) to protect most metals. So, it is recommended to insulate galvanized steel so it doesn’t come in direct contact with dissimilar metals. Rubber or plastic, 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 by defi nition means a metallurgical reaction between zinc and iron to create a bond between the zinc and the steel of approximately 3600 psi. There is no such reaction when zinc-rich paints are applied and the bond strength is only several hundred psi.

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

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

Special Thanks To:Michael Tinker – Pacific Drafting Inc.Rodelio Carpio – Pacific Drafting Inc.

Bernardo Duran – American Galvanizers AssociationJenny Clawson - American Galvanizers Association

Cecile Elliott – American Galvanizers Association

Kevin Hobson – Calwest Galvanizing

National Institute of Steel Detailing7700 Edgewater Dr. Ste. 670

Oakland, CA 94621-3022510.568.3741www.nisd.org