Hot Dip Galvanizing 101 Hot Dip Galvanizing Design Considerations

Protection against corrosion begins on the drawing board. No matter what corrosion

protection system is used, it must be factored into the design of the product.

Once the decision has been made to use hot dip galvanizing to provide corrosion

protection for the steel, the design engineer should ensure that the pieces can be

suitably fabricated for high quality galvanizing.

Certain rules must be followed to design components for galvanizing. These rules are

readily applied and, in most cases, are simply those which good practice would dictate

to ensure maximum corrosion protection. Adopting the following design practices will

ensure the safety of galvanizing personnel, reduces your coating cost, and produce

optimum quality galvanizing

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Liason Between Design Engineer, Fabricator, and Galvanizer

The most important rule is that the designer, fabricator, and galvanizer should work

together before the product is manufactured. This three-way communication can

eliminate most galvanizing problems.The designer can better appreciate hot dip

galvanizing design requirements if the basic steps of the galvanizing process are

understood. Though the process may vary from galvanizer to galvanizer, the

fundamental steps in the galvanizing problems are:

1. Soil and grease removal: A hot alkaline cleaner is usually used to remove oil, grease, shop oil, and

soluble paints. This will not, however, remove such things as epoxies, vinyls, asphalt, or welding slag.

These soils must be removed by grit blasting, or other mechanical cleaning which is normally not the

responsibility of the galvanizer.

2. Pickling: An acid bath is used to remove surface rust and mill scale to provide a chemically clean

metallic surface. Many galvanizers prefer the use of hydrochloric acid since it is more environmentally

friendly and will not effect the surface of the steel which may be possible with the use of sulfuric acid.

3. Prefluxing: A steel article is immersed in a liquid flux predip (usually zinc ammonium chloride

solution) to remove oxides and to prevent oxidation prior to dipping into molten zinc. By utilizing the dry

kettle process, a galvanizer can eliminate the potential of flux inclusion or entrapment on the galvanized

steel product. The wet kettle process, where the steel goes through a flux blanket on top of the

galvanizing bath, can result in these particles adhering to the steel surface.

4. Galvanizing: The article is immersed to molten zinc at approximately 850F (455C). This results in a

formation of a zinc and zinc-iron alloy coating which is metallurgically bonded to the steel.

5. Finishing: After the article is withdrawn from the galvanizing bath, excess zinc is removed by

draining, by vibrating, or, for small items, by centrifuging. The galvanized item is then cooled in air or

quenched in water. The air quenching process, which takes a bit longer than the water quenching

method, will result in a better surface finish which is an important consideration in architecturally

exposed steel.

6. Inspection: Thickness and surface condition inspections are the final steps in the galvanizing process.

Information on inspection procedures and quality control criteria is available.

Iron and steel articles hot dip galvanized after fabrication may range in size from

small pieces of hardware such as bolts and washers to large welded steel assemblies or

castings weighing several tons. The ability to galvanize these articles can be improved

by following the design practices recommended in this manual and by consulting with

the galvanizer during the design stage of a project.

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Materials Suitable for Hot Dip Galvanizing

Most ferrous materials are suitable for hot dip galvanizing. Cast iron, malleable iron,

cast steels, hot rolled steels and cold rolled steels all can be protected by hot dip

galvanizing. Structural steel shapes, including those of high strength low alloy

materials, are hot dip galvanized after fabrication to obtain the long lasting protection

afforded by the zinc coating.

Though most ferrous materials can be hot dip galvanized, the characteristics of the

galvanized coating will be primarily a function of the chemical composition of the

material.

The galvanized coating has as its basis a reaction between steel and molten zinc

resulting in the formation of a series of zinc-iron alloy layers, which are normally

covered by a layer of solidified zinc. For most hot rolled steels the zinc-iron alloy

portion of the coating will represent 50 to 70 percent of the total coating thickness.

Steel compositions vary depending upon strength and service requirements. Major

elements in the steel, such as carbon silicon, affect the necessary galvanizing

techniques as well as the structure and appearance of the galvanized coating. For

example, certain elements, when present in the steel, may result in coating that is

all, or nearly all, zinc-iron alloy.

While a description of the mechanism that causes this type of coating is beyond the

scope of this manual, a description of the characteristics of an all or nearly all zinc-

iron alloy coating is listed below:

Visual - Visually, the zinc-iron alloy coating may have a matte gray appearance due to

the absence of the free zinc layer. It is the free zinc layer which imparts the typical

bright finish to a galvanized coating.

Adherence - The coating which is all or nearly all zinc-iron alloy may have a lower

adherence when compared to the typical galvanized coating. This type of coating

tends to be thicker than the typical galvanized coating. As the thickness of this type

increases, a reduction of adherence may be experienced.

Corrosion Resistance - In general, galvanized coatings are specified more for their

corrosion resistance than for their appearance. Thus, designers primary interest is the

relative corrosion resistance of the two coating types. Fabricators and consumers

should be aware that while a gray or matte appearance may occur, this matte

appearance does not reduce the long term atmospheric corrosion protection of the

steel. For all practical purposes the corrosion resistance, mil for mil, of these coatings

is equal.

It is difficult to provide precise guidance to the designer in the area of steel erection

without qualifying all of the grades of steel commercially available. The guidelines

discussed below, however, will usually result in the selection of steels having good

galvanizing characteristics:

Plain carbon structural grade steel will, under most circumstances, galvanize with the production of a typical coating. However, it is known that levels of carbon less than 0.25%, phosphorous less than 0.05%

or manganese less than 1.35% are beneficial.

Silicon at levels less than 0.04% or between 0.15% and 0.25% is desirable.

Silicon may be present in many steels commonly galvanized even though it is not a

part of the controlled composition of the steels. This occurs primarily because silicon

is used in the de-oxidation process for the steel and is commonly found in continuous

cast steels. Steels containing the higher silicon levels may be exhibit bright, shiny

areas adjacent to gray matte areas, due to the silicon distribution. A recognized

method to combat the effects of high silicon steel is to add a trace amount of nickel,

usually between .05 - .09% to the zinc bath.

The galvanizer should always be advised of the grade of steel selected in order that he

might determine whether or not special galvanizing techniques will be required.

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Combining Different Materials and/or Surfaces

Optimum galvanizing quality is seldom obtained when different surface conditions,

different fabricating methods, or ferrous metals with different chemistries are

combined.

This is because different parameters for pickling (solution concentrations,

temperatures, immersion time) are required for:

excessive rusted surfaces pitted surfaces machined surfaces cast iron (especially with sand inclusions) cast steel malleable iron hot rolled steel cold rolled steel steels containing more than normal carbon, phosphorous, manganese and silicon. The use of old and new steel or castings and rolled steel in the same assembly should be avoided,

when possible. Where assemblies of cast iron, cast steel, malleable iron and rolled steel are unavoidable,

the entire assembly should be thoroughly shot or sand blasted prior to pickling in order to produce a

galvanized coating of acceptable quality.

Excessively rusted, pitted or forged steels should not be used in combination with new

or machined surfaces because difference in required pickling time can cause over

pickling of the machined surfaces. Where this combination is unavoidable, through

abrasive blast cleaning of the assembly (normally before any machining is done) will

remove rolled-in mill scale, impurities, and non-metallics prior to pickling. Products

containing different ferrous materials will then pickle in a more uniform matter,

providing an optimum galvanized coating.

Omission on blast cleaning of mixed material assemblies will result in combined under-

and over-pickling of the different surfaces. This omission may adversely affect the

quality of the galvanized coating.

Whenever possible, the materials described should be galvanized separately and

assembled after galvanizing. Whenever steels of different chemical composition or

different surface finishes of steel are joined in an assembly, the galvanized finish is

generally not uniform in appearance. The corrosion protection provided by the

galvanized coating, however, is not affected by variations in color and texture of

coating.

When abrasive blast cleaning is used to prepare a surface for galvanizing, a coating

thicker than the normal galvanized coating will be produced. Abrasive cleaning rough-

ens the surface and increases its surface area. The result is an increased reactivity

with the molten zinc. Greater zinc-iron alloy growth occurs during galvanizing of a

blast-cleaned steel, producing thicker coating at the expense of a moderately rougher

surface. These thicker coatings will sometimes have a dark gray appearance because

the alloy layers may extend to the outer surface.

Combinations of steels of different compositions may result in different compositions

may result in different coating thicknesses and surface appearances. This is not

necessarily detrimental to certain applications, but the designer and fabricator must

consider this an, in the planning stage, should consult with a galvanizer.

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Welding Procedures and Flux Removal

When welded items are galvanized, both the cleanliness of the weld area after

welding and the metallic composition of the weld itself affect galvanizing quality and

appearance at the weld.

The specifics of welding techniques can best be obtained from the American Welding

Society or your welding equipment supplier, but several welding processes and

techniques have been found to be most satisfactory for items to be galvanized. These

are:

1. 1. In welding, an uncoated electrode should be used wherever possible to prevent flux deposits.

2. If a coated electrode is used, all welding flux residues must be removed by wire brushing, flame

cleaning, chipping, grinding, pneumatic needle gun, or abrasive blast cleaning. Welding flux residues are

chemically inert in the normal pickling solutions used by galvanizers; their existence will produce rough

and incomplete zinc coverage. Flux residue removal is normally the fabricators responsibility unless other

arrangements have been made.

3. A welding process such as metal-inert gas (MIG), tungsten-inert gas (TIG) or C02 shielded arc is

recommended when possible since they produce essentially no slag.

4. In the case of heavy weldments, a submerged arc method is recommended.

5. If none of these are available, select a coated rod specifically designed for selfslagging, as

recommended by welding equipment suppliers.

6. Choose a welding rod providing a deposited weld composition as close as possible to the parent

metal. This will help prevent differential acid attack between the weld area and the parent metal during

acid cleaning.

7. Welding rods high in silicon may cause excessively thick and/or darkened coatings to form in the

welded area.

Materials which have been galvanized may be welded easily and satisfactorily by all

common welding techniques. Additional information can be found in Welding Zinc-

Coated Steel.*

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Mechanical Properties of Galvanized Steels

The hot dip galvanizing process produces no significant changes in the mechanical

properties of the structural steel commonly galvanized throughout the world.

The mechanical properties of 19 structural steels from major industrial countries of

the world were analyzed before and after galvanizing in a major 4-year research

project of the BNF Metals Technology Center, UK, under the sponsorship of

International Lead Zinc Research Organization. Included were steels to ASTM Standard

Specifications A36 and A572 Grade 60 and Canadian Standards Association (CSA)

Specifications G40.8 and G40.12.

The BNF report, Galvanizing of Structural Steels and Their Weldments (ILZRO, 1975),

concludes that the galvanizing process has no effect on the tensile, bend or impact

properties of any of the structural steels investigated when these are galvanized in the

as manufactured condition.

Many structures and parts are fabricated using cold rolled steel or cold working

techniques. In some instances, severe cold working may cause the steel to become

strain-age embrittled. While cold working increases the incidence of strain-age

embrittlement, the embrittlement may not be evident until after the work has been

galvanized. This occurs because aging is relatively slow at ambient temperatures but is

more rapid at the elevated temperature of the galvanizing bath.

Any forms of cold working reduces the ductility of steel. Operations such as punching

holes, notching, producing filets of small radii, shearing and sharp bending may lead

to strain-age embrittlement of susceptible steels.

Cold worked steels less than 1/8 inch (3.18 mm) thick which are subsequently

galvanized are unlikely to experience strain-age ebrittlement.

Since cold working is the strongest factor contributing to the embrittlement of

galvanized steel, the following precautions are recommended to reduce the incidence

of strain-age embrittlement when cold working is necessary:

a. Select steel with a carbon content below 0.25%.

b. Choose steel with low transition temperatures since cold work raises the ductile-brittle transition

temperatures and galvanizing (heating) may raise it even further.

c. Susceptibly to stain-age embrittlement is usually less of a potential problem with aluminum-killed

steels.

d. For steels having a carbon content between 0.1%-0.25%, a bending radius at least three times the

section thickness (3t) should be maintained. If less that 3t bending is unavoidable, the material should be

stress relieved at 1100F (595C) for one hour per inch (25.4mm) of section thickness.

e. Notches should be avoided since they are stress raisers. Notches may be caused during shearing or

punching operations. Flame cutting or sawing is preferred, particularly for heavy sections.

f. Drill, rather than punch, holes in material thicker than 3/4 inch (19.05 mm). If holes are punched,

they should be punched undersize then reamed an additional 1/8 inch (3.18 mm) overall or drilled to size.

Shapes between 1/4 and 3/4 inch thick are not seriously affected by cold punching if the punching is done

under good shop practice.

Small shapes up to 1/4 inch (6.5 mm) in thickness which have been cold worked by punching do not need

stress relieving operations before galvanizing.

g. Edges of steel sections greater than 5/8 inch (15.88 mm) thick subject to tensile loads should be

machined or machine cut. Edges of section up to 5/8 inch (15.88 mm) thick may be cut by shearing.

h. In critical applications, the steel should be hot worked above 1200F (650C) in accordance with the

steel makers recommendations. Where cold working cannot be avoided, stress relieve as recommended

in item d above.

ASTM Recommended Practice Al 43, Safeguarding Against Embrittlement of Hot-Dip

Galvanized Structural Steel Products and Procedure for Detecting Embrittlement and

CSA Specification Gi 64, Galvanizing of Irregularly Shaped Articles, provide guidance

on cold working of susceptible steel is better avoided, if at all possible.

If there is concern with possible loss of ductility due to strain-age embrittlement, the

galvanizer should be alerted. A sample quantity of the cold-formed items should be

galvanized and tested before further commitment.

Hydrogen Embrittlement

Hydrogen embrittlement is a ductile-to-brittle change which occurs in certain high

strength steels. Hydrogen released during the pickling operation can cause this

embrittlement. This hydrogen can be absorbed into a steel during the acid pickling but

at galvanizing temperatures it is generally expelled from the steel.

Hydrogen embrittlement is not common, but precautions should be taken, particularly

if the steel involved has an ultimate tensile strength exceeding approximately 150,000

psi. If high strength steels are to be processed, grit blasting instead of acid pickling is

recommended to minimize the likelihood of hydrogen embrittlement.

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Size and Shape

With the increase in the size and capacities of galvanizing installations, facilities now

exist for galvanizing components covering a significant range of sizes and shapes.

Duncans galvanizing kettle measures 42 feet long by 7 feet deep by 5 feet 2 inches

wide. Almost any component can be galvanized by designing and fabricating in

modules suitable for available galvanizing facilities. However, it is wise to check

kettle size restrictions with your galvanizer at an early stage.

Large structures are galvanized by designing in modules or sub-units. These are then

assembled by shop welding or site bolting after galvanizing. Modular design techniques

often produce economies in manufacture and assembly because they simplify handling

and transport.

When an item is too large for total immersion in the molten zinc of the largest

galvanizing kettle available, but more than half of the item will fit into the kettle, one

end may be immersed and withdrawn, and then the other end may be galvanized. This

is referred to as the double dip process. ALWAYS CONSULT YOUR GALVANIZER BEFORE

PLANNING TO USE DOUBLE DIP GALVANIZING.

Large cylindrical objects may be galvanized by progressive dipping. This procedure can

be used when the width of the article exceeds that of the kettle. The item is

galvanized by using a series of dips or by rolling the article in the molten zinc of the

kettle.

The designer should consider the material handling techniques used in galvanizing

plants. The use of hoists and cranes is commonplace. Large assemblies are usually

supported by chain slings or by lifting fixtures, if provided. Special jigs and racks are

often used to galvanize large numbers of similar items simultaneously.

If aesthetics are important provide lifting fixtures for the galvanizer. Since all material

is immersed into the galvanizing kettle from overhead, chains, wire or other holding

devices will be used to support the material, unless special lifting fixtures are

provided. Chains and wire normally leave a mark on the galvanized article. This mark

is not necessarily detrimental to the coating but could affect the desired aesthetics.

Large pipe sections, open top tanks and similar structures may require the addition of

cross stays to maintain their shape during handling.

Although size normally brings large items to mind, the smaller items should also

receive attention. The galvanizing process can treat small items by racking. Pieces less

than about 15 inches (38.1cm) in length are frequently galvanized in perforated

baskets. The basket is then centrifuged to throw off excess zinc from the pieces and

provide smoother coatings. Fasteners, small brackets and clips typify work handled in

baskets.

The heavy weight of fabrications can be a factor in galvanizing-largely because of the

handling required to move items step to step. Thus, weight-handling capacity of your

galvanizer should be determined, if it appears this might be a factor in your design

considerations.

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Allowing for Proper Drainage

For effective galvanizing, cleaning solutions and molten zinc must flow into, over,

through and out of the fabricated article without undue resistance.

Failure to provide for this free, unimpeded flow is a frequent cause of problems for

both galvanizer and customer. Improper design for drainage results in galvanizing of

poor appearance and in excess buildups of zinc which are unnecessary and costly.

To ensure unimpeded flow of solutions, all stiffeners, gussets and bracing should be

cropped a minimum of 3/4 inch (19.05 mm).

Provide holes at least 1/2 inch (13 mm) in diameter in end plates on rolled steel

shapes, to allow access of molten zinc in the galvanizing bath and drainage during

withdrawal. Alternatively, holes at least 1/2 inch (13 mm) in diameter can be located

in the web within 1/4 inch (6 mm) of the end plate.

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Enclosed and Semi-enclosed Products

Tanks and enclosed vessels, should be designed to allow for acid cleaning solutions,

fluxes and molten zinc to enter and flow upwards through the enclosed space and out

through an opening at the highest point. This prevents air from being trapped as the

article is immersed. The design must also provide for complete drainage of both

interior and exterior details during withdrawal.

When both internal and external surfaces are to be galvanized, at least one filling and

draining hole and a vent hole must be provided. The filling hole should be as large as

the design will allow but at least 3 inches in diameter for each cubic yard (or 10 cm in

diameter for each 1.0 cubic meter) of volume with a minimum diameter of 2 inches

(50 mm). A vent hole of at least the same size should be provided diagonally opposite

the filling hole. This allows the air to escape and facilitates draining.

In tanks, internal baffles should be cropped on the bottom or provided with suitable

drainage holes to permit the free flow of molten zinc. Manholes, handholes, bosses

and openings should be finished flush inside to prevent trapping excess zinc.

Openings must be placed so that the flux on the vessel can float to the surface of the

bath. They will also prevent air pocket formations which would keep the acid bath

from completely cleaning the inside of the vessel.

The diameter of the opening should be at least 1 inch per foot (83.3 mm per meter) of

tank diameter. Minimum allowable diameter opening is 2 inches (50 mm). Tanks over

48 inches (1.22 meters) in diameter should have a manhole in one end and should have

all six holes.

Items such as vessels and air receivers which are to be galvanized on the outside only

must have snorkel tubes or extended vent pipes. These openings provide an air exit

from the vessel above the level of molten zinc in the galvanizing kettle. The

galvanizer should be consulted before using these temporary fittings.

It is always wise to have the galvanizer review the drawings of enclosed or partially-

enclosed vessels before they are fabricated. He can advise you of any changes that

would provide you a better product. If a change is needed to facilitate galvanizing, the

least expensive time to make the change is before fabrication.

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Tubular and Hollow Items

Tubular assemblies such as handrail, pipe columns, pipe girders, street light poles,

transmission poles, pipe trusses, and sign bridges are commonly galvanized.

Cleaning.

As with all steel to be galvanized, pipe and other hollow materials must be thoroughly

cleaned before the molten zinc will alloy with the steel to produce the galvanized

coating. Pipe commonly presents two special cleaning problems:

1. The mill coating (varnish, lacquer, and similar materials) applied by the manufacturer costs extra

to remove at the galvanizing plant. Further, some formulations, both foreign and domestic, are extremely

difficult to remove with common cleaning solutions; blasting may be required. Removing this mill coat at

the galvanizing plant can be avoided by ordering uncoated pipe from your supplier, for which there is

usually no extra charge.

2. Welding of mill-coated pipe burns and carbonizes the varnish in the surrounding heated areas. This

soot must be removed by blasting or other mechanical means. The burned coating could be removed

when blasting to remove weld flux, but if welding has been done with an uncoated rod, any blasting or

other hand cleaning is expensive and highly impractical.

Venting.

It is mandatory that tubular fabrications and hollow structurals be properly vented.

Any pickling acid or rinse waters that might be trapped in a blind or closed joint

connection will be converted to superheated steam and can develop a pressure of up

to 3800 psi when immersed in molten zinc at 850~F (455~C). This is a serious potential

hazard to galvanizing equipment and to personnel.

Since proper galvanizing demands that the inside, as well as the outside, be

completely cleaned and coated with zinc, air and frothy fluxes must be allowed to

flow upward and completely out; cleaning solutions and molten zinc must be allowed

to flow in and completely wet the surfaces.

Simply stated, the structure must be lowered into the solution without trapping any

air. It must be raised from the solution without trapping any solution. Consequently,

ample passageways which allow flow in and out must be designed into the assemblies.

Since items, to be galvanized are immersed and withdrawn at an angle, the vent holes

should be located at the highest point and drainage holes at the lowest point each

member.

All sections of fabricated pipework should be interconnected with full open tee or

with miter joints. Each enclosed section must be provided with a vent hole at each

end.

Most galvanizers prefer to visually identify the venting from outside when the

assembly is received. This is necessary to check the adequacy of the venting as well as

to determine that it has not been omitted by mistake. Some galvanizers may hesitate

to process complicated pipe assemblies (such as hand railing) unless all venting is

visible on the outside and readily accessible for inspection.

Base plates and end plates must be designed to facilitate venting and draining. Fully

cutting the plate provides minimum obstruction to a full, free flow into and out of the

pipe. Since this is not always possible, the use of vent holes in the plate often provides

a solution.

Vent holes can be closed with drive caps or plugs installed after galvanizing. To do

this, pear shaped-shaped lead weights are often used. These can easily be hammered

in and filed off flush with surrounding surfaces.

It is recommended that tubular structures be completely submerged in one dip in the

galvanizing kettle. This may be difficult to discover during inspection because of the

size and shape of the item.

Various methods of providing vent holes are acceptable but the subsequent plugging of

these holes should be kept in mind when necessary.

Internal gusset plates and end flanges should also be provided with vent and drainage

holes. In circular hollow shapes these should be located diametrically opposite to each

other at opposite ends of the member.

In rectangular hollow shapes, the four corners of the internal gusset plates should be

cropped. Internal gusset plates in all large hollow sections should be provided with an

additional opening at the center. Where there are flanges or end plates, it is more

economical to locate holes in the flanges or plates rather than in the section.

Handrail.

1. Vent holes must be as close to the weld as possible and not less than 3/8 in diameter.

2. Internal holes should be the full l.D. of the pipe for best galvanizing quality and lowest galvanizing

cost.

3. Vent holes in end sections on similar sections must be 1/2 diameter.

4. Any device used for erection in the field that prevents full openings on ends of horizontal rails and

vertical legs should be galvanized separately and attached after galvanizing.

Vent holes should be visible on the outside of any pipe assembly.

If full internal holes (the full I.D. of the pipe) are not incorporated in the design of the

handrail, the following should occur:

1. Each vent hole must be as close to the welds as possible and must be 25% of the l.D. of the pipe, but

not less than 3/8 diameter.

2. Vent holes in end sections or in similar sections must be 1/2 diameter.

3. Any device used for erection in the field that prevents full openings on ends of horizontal rails and

vertical legs should be galvanized separately and attached after galvanizing.

Vent holes should be visible on the outside of any pipe assembly.

Rectangular Tube Truss

Vertical Sections

Each vertical member should have two (2) holes at the top and bottom, l8Oj apart in

line with the horizontal members. The size of the holes preferably should be equal and

the combined area of the 2 holes at either end of the verticals should be at least 30%

of the cross sectional area.

End PLates Horizontal

1. Most desirable - completely open.

2. If H+W=24 or larger, area of holes. Clips should equal 25% of the area of the tube (H+W).

If H+W less than 24 to and including 16 - 30%.

If H+W less than 16 to and including 8 - use 40%.

If H+W under 8 - leave open.

Pipe Truss 3 & Larger

Vertical Sections

Each vertical member should have two (2) holes at the top and bottom, 180 apart in

line with the horizontal members. The size of the holes preferably should be equal and

the combined area of the 2 holes at either end of the verticals should be at least 30%

of the cross sectional area.

End PLates Horizontal

1. Most desirable - completely open same diameter.

2. Equal substitutes - openings should be at least 30% of the area of the inside diameter.

Box Sections

INTERNAL GUSSETS should be spaced a minimum of 36.

Box Sections - H+W-24 or larger - the area of hole plus clips should equal 25% of the

cross sectional area of the box (H+W).

Box Sections - H+W less than 24 but greater than or equal to 16 - use 30%.

Box Sections - H+W less than 16 but greater than or equal to 8 - use 40%.

Box Sections - H+W under 8 leave completely open; no end plate or internal gussets.

The following table is for square box sections only. For rectangular sections, calculate

required area and check with your galvanizer for positioning of openings.

Box Size

H + W

Holes

A-DIM

Clipped Corneres

B-DIM

48 8 6

36 6 5

32 6 4

28 6 3

24 5 3

20 4 3

16 4 2

12 3 2

Tapered - Signal Arm

A. Small end open completely.

Pole Plate End

1. Most desirable - completely open.

2. Acceptable alternates - half circles or slot and round hole must equal 30% of the area of the I.D. of

the pole end of the tapered arm for 3 and larger I.D. The opening must equal 45% of the area of the pole

end of the tapered arm if the I.D. is under 3.

Pipe Columns, Pipe Girders, Street Light Poles and Transmission Poles With Base Plates

and With or Without Cap Plates.

Location of Openings

1. Most desirable - completely open same diameter as section top and bottom.

Dimensions

Openings at each end must be at least 30% of I.D. area of pipe for pipe 3 and over

and 45% of the I.D. area for 3 pipe or smaller.

The following is an example of sizes for 6 diameter section. Allow 30% of the area of

the l.D. for hole sizes at each end.

#2 Half Circle A - 1-3/4 R.

#3 Slot B = 3/4 - Center Hole C = 3 Diameter.

#4 Half Circle D = 1-5/8 R.

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Minimizing Distortion

Some fabricated assemblies may distort at the galvanizing temperature as a result of

the stresses induced during manufacturing of the steel and in subsequent fabricating

operations.

To minimize distortion, design engineers should observe the following

recommendations:

1. Where possible, use symmetrical rolled sections in preference to angle or channel frames. I-beams

are preferred to angles or channels.

2. Use parts in an assembly that are of equal or near equal thickness, especially at joints.

3. Bend members to the largest acceptable radii to minimize local stress concentration.

4. Accurately perform members of an assembly so that it is not necessary to force, spring or bend them

into position during joining.

5. Continuously weld joints using balanced welding techniques to reduce uneven thermal stresses.

Staggered welding techniques to produce a continuous weld are acceptable. For staggered welding of 1/8

inch (3.18 mm) or lighter material weld centers should be closer than 4 inches (10.16 cm).

6. Avoid designs which require double dip galvanizing or progressive galvanizing. It is preferable to build

assemblies and sub-assemblies in suitable modules so that they can be immersed quickly and fully in a

single dip. In this way, the entire fabrication can expand and contract uniformly. Where double dip or

progressive galvanizing is required, consult with your galvanizer if you anticipate a wide variance of

section size.

7. Consult with your galvanizer regarding the use of temporary bracing and/or reinforcing to minimize or

prevent warpage and distortion during galvanizing.

Guidelines for minimizing distortion warpage are provided in ASTM Recommended

Practice A384, Safeguarding Against Warpage and Distortion During Hot-Dip

Galvanizing of Steel Assemblies and CSA Specification G164, Hot Dip Galvanizing of

Irregularly Shaped Articles.

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Overlapping and Contacting Surfaces

When designing articles to be galvanized after fabrication, it is best to avoid narrow

gaps between plates, overlapping surfaces, and back-to-back angles and channels.

When overlapping or contacting surfaces cannot be avoided, all edges should be

completely sealed by welding. This is because the viscosity of the zinc keeps it from

entering any space tighter than 3/32 inch (2.38 mm). Less viscous pickling acids will

enter, but zinc will not

Two further problems encountered with tightly overlapping surfaces are:

1. Pickling acids that may be trapped will flash to steam when the part is immersed in the galvanizing

bath. The blowing out of this steam can prevent zinc from adhering to the area adjacent to the lap

joint.

2. Pickling acid salts can be retained in these tight areas due to impossibility of adequate rinsing. The

galvanized coating may be of good quality in the adjacent area, but humidity encountered months, or

even weeks, later may wet these acid salts. This will cause an unsightly rust staining to run out on top of

the galvanized coating.

Cleaning solutions will not effectively remove oils and greases trapped between

surfaces in close contact. Any residual oil and grease will partially volatilize at the

galvanizing temperature. This will prevent a satisfactory zinc coating in the immediate

area of the lap joint. It is important to specify that contacting joint surfaces be

thoroughly cleaned before fabrication and then completely sealed by welding.

If the area of seal-welded overlap is large, there should be vent holes through one or

both sides into the lapped area. This is to prevent any moisture which gets in through

a pin hole in the weld from building up explosive pressure while in the galvanizing

bath. This venting becomes more important when the area is large or steel is thin.

Consult Recommended Details for Galvanized Structures for vent size and numbers.

Vent holes can be sealed after galvanizing. Seal welding is not mandatory, but does

not prevent trapping moisture, internal rustling and seepage, all of which are possible

in any unsealed connection regardless of the protective coating used.

Where two bars come together at an angle, a gap of at least 3/32 inch (2.38 mm) after

welding must be provided to ensure the area is wetted by the molten zinc. An

intermittent fillet weld may be used. This can be on one side of the bar only or, where

necessary, an intermittent staggered fillet weld may be employed on both sides so

that no pocket can be formed. This type of welding, however, is not suitable for load

bearing members.

ASTM Recommended Practice A385 Providing High Quality Zinc Coatings (HotDip)

provides guidance for galvanizing overlapping surfaces.

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Castings

Cleanliness is very important to achieve proper and complete galvanizing of castings.

Thorough abrasive cleaning is the most effective method of treatment for the removal

of foundry sand and impurities. Conventionally, this is accomplished by grit shot, or

sand blasting. Grit blasting or a combination of grit and shot is generally preferred.

Usually, castings are cleaned at the foundry since most galvanizers do not have

abrasive blasting facilities.

Conventional acid cleaning process employed by most galvanizers does not clean

castings well because sand and other surface inclusions are not removed by

hydrochloric or sulfuric acid. After castings have been abrasively cleaned, they may

then be flash pickled to prepare them for galvanizing.

Galvanizing sound, stress-free castings with good surface finish will produce high

quality galvanized coatings. The following design and preparation rules should be

applied for castings to be galvanized:

1. Avoid sharp corners and deep recesses.

2. Use large pattern numerals and generous radii to facilitate abrasive cleaning.

3. Specify uniform wall sections. Non-uniform wall thicknesses in certain casting designs may lead to

distortion and/or cracking. These results from stresses developed as the temperature of the casting is

increased during the galvanizing process. Uniform wall sections and a balanced design will reduce this.

4. Heat treat castings before galvanizing. Under certain conditions of composition or thermal history,

the impact resistance of malleable iron castings may be significantly reduced as a result of galvanizing.

This can be avoided if the castings are heat treated prior to galvanizing as follows:

a. Heat at a temperature of 1250F (677C) until all sections have reached that temperature

(no soak required).

b. Perform finish machining and/or heat treating after abrasive blasting.

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Threaded Parts

Galvanized fasteners are recommended for use with hot dip galvanized subassemblies

and assemblies. Galvanized nuts, bolts and screws in common sizes are readily

available from commercial suppliers.

Bolted assemblies should be sent to the galvanizer while in the disassembled

condition. Nuts and bolts or studs to be galvanized should also be supplied

disassembled.

When the item to be galvanized incorporates threaded assemblies, the pitch diameter

of the female threads must be increased to permit hand assembly after the addition of

zinc to the male threads of the mating part.

Bolts are thus completely galvanized, but internal threads of nuts must be tapped

oversize after galvanizing to accommodate the increased diameter of the bolts. While

tapping or retapping of the nuts after galvanizing results in an uncoated female

thread, the zinc coating on the engaged male thread will protect both components

from corrosion. For economy, nuts are usually galvanized as blanks and the threads

tapped oversize after galvanizing.

To remove excess zinc and produce smoother coatings, small parts including fasteners

are centrifuged in special equipment when they are removed from the galvanizing

bath.

Items too long or too large to be centrifuged, such as long threaded rods, may be wire

brushed while hot to remove any excess zinc from threads.

Studs welded to assemblies may have to be cleaned after the assembly has cooled.

This requires reheating with an acetylene torch and wire brushing to remove excess

zinc. Alternatives to welded studs should be considered when possible.

Masking to prevent galvanizing threads on pipe or fittings is very difficult. The

recommended practice is to clean or tap after galvanizing.

Anchoring devices (such as threaded rods and anchor bolts) are sometimes specified to

be galvanized in the threaded areas only or in the areas to be exposed above ground.

This can be more expensive than galvanizing the complete unit because of the

additional handling required. Complete galvanizing can be specified for items to be

anchored in concrete. Research has proved the high bond strength and performance of

galvanized steel in concrete.

Tapped-through holes must be retapped oversize after galvanizing if they are to

contain a galvanized bolt after assembly. Tapping of all holes after galvanizing is

recommended to eliminate double tapping costs and the possibility of cross threading.

The recommended over tapping for nuts and interior threads is as follows:

Bolt or Stud Size Diameter, inches

Minimum Overtapping of Female Threads, inches*

7/16 and smaller 0.016

Over 7/16 to 1 0.021

Over 1 0.031

* Applies to both pitch and minor diameters, minimum and

maximum limits.

On threads over 1 & 1/2 inches it is often more practical, if design strength allows, to

have the male thread cut 0.031 inches (0.79 mm) undersize before galvanizing so a

standard tap can be used on the nut. ASTM Specification A563 (A563M) Carbon and

Alloy Steel Nuts discuss the required minimum diametral amount of overtapping of

nuts used with hot dip galvanized bolts. (Note: overtapping allowances contained in

A563 as of this printing are undergoing review and revision.)

Manufacturers of threaded parts recognize that special procedures must be followed in

their plants where certain items are to be galvanized. Following are some examples:

1. Low carbon bars are recommended since high carbon or high silicon causes a heavier, rougher

galvanized coating on the threads.

2. Hot formed threading or bending requires cleaning at the manufacturing plant to remove scale before

threading. Otherwise, over-pickling of threads will result during scale removal.

3. Sharp manufacturing tools are mandatory. Ragged and torn threads open up in the pickling and

galvanizing processes. Worn tools also increase bolt diameters. Frequent checking is necessary on long

runs.

4. Standard sized threads are cut on the bolt while standard sized nuts are retapped oversize after

galvanizing.

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Moving Parts

When a galvanized assembly incorporates moving parts (such as drop-handles, shackles

and shafts), a radial clearance of not less than 1/16 inch (1.59 mm) must be allowed

to ensure full freedom of movement after the addition of zinc during galvanizing.

It is recommended that, whenever possible, work be designed so that hinges can be

bolted to frames, covers, bodies and the like after galvanizing.

Hinges should be galvanized separately and assembled after galvanizing. All hinges to

be galvanized should be of the loose pin type. Before galvanizing, any adjacent edges

should be ground to give at least 1/32 inch (0.8 mm) clearance. The pin holes can be

cleared of excess zinc at time of assembly. After hinges are galvanized, it is

recommended that an undersized pin be used to compensate for the zinc picked up

during the galvanizing process. If desired, the pin holes in the hinges may be reamed

1/32 inch (0.8 mm) after galvanizing to permit the use of regular size pins.

At times, it is necessary to reheat moving parts in order to make them work freely.

Heating may cause discoloration of the galvanized coating near the reheated area.

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Marking for Identification

Identification markings on fabricated items should be carefully prepared before

galvanizing, so they will be legible after galvanizing.

Do not use paint to apply addresses, shipping instructions, and job numbers on items

to be galvanized. Oil based paints and crayon marks are not removed by the pickling

acids. This results in extra work and extra charges by the galvanizer to properly

prepare the steel for galvanizing.

For temporary identification, detachable metal tags or a water soluble marker should

be specified.

Where permanent identification is needed, there are three suitable alternatives for

marking steel fabrications to be hot dip galvanized. Each will enable items to be

rapidly identified after galvanizing and at the assembly site. The three marking

alternatives are:

1. Deep stenciling a steel tag (minimum #12 gauge) and firmly affixing it to the fabrication with a

minimum #9 gauge steel wire. The tag should be wired loosely to the work so that the area beneath the

wire can be galvanized and the wire will not freeze to the work when the molten zinc solidifies. It is

also recommended that more than one tag be used on each piece of work requiring identification.

Handling in transportation can result in loss of an occasional tag. If desired, the tags may be seal-welded

directly to the materials.

2. Stamping the surface of the item using die cut deep stencils or series of center punch marks. These

marks should be placed in a standard position on each of the members. They should be a minimum of 1/2

inch (12.7 mm) high and 1/32 inch (0.08 mm) deep to ensure readability after galvanizing. This method

should not be used to mark fracture critical members.

3. Using a series of weld beads to mark letters or numbers directly on the fabrication. However, it is

essential that all weld flux be removed.

Do not use aluminum, plastic, paper, or paint to mark an item before galvanizing.

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Galvanized Surface Repair

Sometimes hot dip galvanized coatings are damaged by excessively rough handling

during shipping or erection. Damage may also be the result of welding or flame

cutting.

Where limited areas are damaged, the use of low melting point zinc alloy repair rods

or powders, the use of organic zinc rich paint or the use of sprayed zinc (metallizing)

is recommended to protect the area.

ASTM Recommended Practice A780 Repair of Damaged Hot Dip Galvanizing Coatings

covers acceptable methods of reconditioning the damaged areas.

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After-Galvanizing Considerations

Wet Storage Stain Prevention

When galvanizers know items will be stacked or stored in humid environments after

they have been galvanized, they will often suggest application of an after-galvanized

treatment which will inhibit wet storage stain (white rust).

Wet storage stain is an attack on the galvanized coating producing a white corrosion

product. It is caused by retention of condensation or runoff water between contacting

surfaces when air circulation is poor. While the attack is frequently superficial,

despite the relative bulkiness of the corrosion product, its appearance may be

objectionable. Your galvanizer can discuss simple treatments which can be applied at

the galvanizers facility.

See AGA publication Wet Storage Stain for more details.

Painting

In general, galvanized coatings used alone provide the most economic corrosion

protection for steel. When galvanized coatings are painted it is usually for aesthetic

reasons, for identification or warning, for camouflage, or added corrosion resistance

under severe service or exposure conditions.

In many applications duplex systems of galvanizing-plus-paint are an ideal

combination. The galvanized coating provides a stable base which greatly increases

paint life, while the paint film protects the zinc coating to give a synergistic effect in

which the combination lasts considerably longer than the total of each coating alone.

Where steel is exposed to highly corrosive environments or where access is difficult

and the longest possible systems of hot dip galvanized coating-plus-paint provide the

best available protection for steel. Very long service life is achieved even under

adverse exposure conditions, resulting in outstanding economics compared to other

coating systems.

The longer life of correctly chosen and applied paint coatings on zinc coated steel

surfaces results from the stable zinc substrate which prevents initiation of rust at

pores and scratches, and resulting creep corrosion beneath the paint film.

Test results show that suitable paint coatings on galvanized steel achieve a synergistic

effect in which the duplex coating lasts up to three times as long before maintenance

as equivalent paint coatings on black steel.

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Pertinent Specifications

American Society for Testing and Material (ASTM)

A90 Test Methods for Weight of Coatings on Zinc Coated (Galvanized) Iron or Steel

Articles

A123 Zinc (Hot Dip Galvanized) Coatings on Iron and Steel Products

Al 43 Recommended Practice for Safeguarding Against Embrittlement of Hot Dip

Galvanized Structural Steel Products and Procedure for Detecting Embrittlement

Al 53 Zinc Coating (Hot Dip) on Iron and Steel Hardware

A325 High-Strength Bolts for Structural Steel joints, including Suitable Nuts and Plain

Hardened Washers

A384 Recommended Practice for Safeguarding Against Warpage and Distortion During

Hot Dip Galvanizing of Steel Assemblies

A385 Recommended Practice for Providing High Quality Zinc Coatings (Hot Dip) on

Assembled Products

A394 Galvanized Steel Transmission Tower Bolts and Nuts

A780 Practice for Repair of Damaged Hot-Dip Galvanized Coatings

B6 Zinc Metals (Slab Zinc)

E376 Recommended Practice for Measuring Coating Thickness by Magnetic-Field or

Eddy-Current (Electromagnetic) Test Methods

Canadian Standards Association

C164-M Hot Dip Galvanizing of Irregularly Shaped Articles

Duncan Galvanizing Specifications

Section 05030-Galvanizing and Metal Coatings-available on disk in Mac or PC format or

CD-ROM.

FAQ

The following is a list of frequently asked questions about hot-dip galvanizing.

1. 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 sulfides 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. What are the steps in the galvanizing process?

There are four steps:

Pre-inspection where the fabricated structural steel is viewed to ensure it has, if

necessary, the proper venting and draining holes, bracing, and overall design characteristics

necessary to yield a quality galvanized coating

Cleaning steel is immersed in a caustic solution to remove organic material such as

grease and dirt, followed by dipping in an acid bath (hydrochloric or sulfuric) to remove mill

scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and

retards further oxidation of the steel (steel will not react with zinc unless it is perfectly

clean)

Galvanizing the clean steel is lowered into a kettle containing 850 F molten zinc

where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and

one pure zinc layer

Final inspection the newly galvanized steel is sight-inspected (if it looks good, it is),

followed up by measurement of coating thickness with a magnetic thickness gauge

View animation

3. How does the cost of hot-dip galvanizing

compare to other corrosion protection systems,

such as paints?

When compared with paint systems, hot-dip galvanizing after fabrication has

comparable initial application costs and, almost always, lower life-cycle costs . In

fact, the lower life-cycle costs of a hot-dip galvanized project make galvanizing the

smart choice for today and tomorrow.

4. How long can I expect my galvanized steel

project 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. To get a good idea of how long your project will last, see the service-life

chart.

5. 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.

6. What causes wet storage stain and how can it

be prevented?

Zinc on newly galvanized steel is very reactive and wants to form zinc oxide and zinc

hydroxide corrosion products that eventually become the stable zinc carbonate.

When galvanized steel is tightly stacked or stored in wet boxes that dont allow for

free flowing air, the zinc forms excessive layers of zinc hydroxide, otherwise known

as wet storage stain. Most wet storage stain can be easily removed with a cleaner or

nylon brush. To prevent wet storage stain, store galvanized steel indoors or block it

so that there is ample free flowing air between each galvanized article.

7. 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 steel chemistry is the primary determinant of galvanized coating thickness and

appearance. Continuously cast steel produced by the steel companies has a wide

variety of chemistries, thus the different coating appearances. There are several

different additives galvanizers may put in their zinc kettle to enhance the coating

appearance by making it shiny, spangled or matte gray. The appearance of the

coating (matte gray, shiny, spangled) does nothing to change the corrosion

protection of the zinc coating.

8. Can galvanized steel in service withstand high

temperatures for long periods of time?

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

an intermittent basis.

9. Why would you want to paint over galvanized

steel?

Called duplex coatings, zinc and paint in combination (synergistic effect) produce a

corrosion protection approximately 2X the sum of the corrosion protection that 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.

10. What are the specifications governing hot-dip

galvanized steel?

Structural steel (plate, wide-flange beams, angles, channels, pipe, tubing) are

galvanized to ASTM A 123/A 123M. Fasteners and small parts that fit into a

centrifuging basket are galvanized to ASTM A 153/A 153M. Reinforcing steel is

galvanized to ASTM A 767/A 767M.

11. Isnt 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 lifetime costs should be considered when making a project

decision on the corrosion prevention 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.

12. What if the article to be galvanized is larger

than the dimensions of the galvanizers 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 often erroneously referred to as double dipping.

13. What is the difference between hot-dip

galvanized fasteners and zinc-plated fasteners?

Hot-dip fasteners generally have about 10 times as much zinc on the surface and

are suitable for use in all exterior and interior applications. Zinc-plated fasteners will

provide a disappointing performance if used outside, especially when used to

connect hot-dip galvanized structural steel members.

14. How long will hot-dip galvanizing protect my

steel from corrosion?

The corrosion rate of zinc and how long it will provide protection is a function of the

coating thickness and the amount of corrosive elements in the atmosphere. For

example, in rural settings where there is less automotive/truck exhaust and plant

emissions, galvanized steel can easily last for 100 150 years without maintenance.

Industrial and marine locations contain significantly more aggressive corrosion

elements such as chlorides and sulfides and galvanized steel may last for 50 100

years in those cases. The relationship between coating thickness and atmospheric

conditions is contained in a popular graph developed by the AGA. Please see the

publication Hot-Dip Galvanizing for Corrosion Protection: A Specifiers Guide.

15. Are there any special design and fabrication

considerations required to make steel ready for

hot-dip galvanizing?

Yes. Specifically, fabricated steel must allow for easy flow 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

configurations, weld flux removed, overlapping surfaces must be seal-welded, and

light gauge material temporarily braced. The details of design and fabrication are

contained in the AGA publication The Design of Products to be Hot-dip Galvanized

After Fabrication.

16. Where are galvanized steel products used?

First of all, the variety of things galvanized is broad. Structural steel (angles,

channels, wide-flange beams, I-beams, H-beams), grating, expanded metal,

corrugated sheets, wire, cables, plate, castings, tubing, pipe, bolts & nuts. The

industries that utilized hot-dip galvanized steel range from bridge &

highway (reinforcing steel for decks and column concrete, girders, stringers, light and

signposts, guardrail, fencing), water & wastewater treatment plants (walkway

grating/expanded metal, handrails) architectural (facades, exposed structural steel,

lentils), parking garages (reinforcing steel for concrete decks, exposed structural

steel columns and barriers), pulp & paper plants (structural steel, walkways,

handrail), OEMs (motor housings, electrical cabinets, frames, heat exchanger

coils), electrical utilities (transmission towers, distribution poles, substations, wind

turbine poles), communication (cell towers), rail transportation (poles, switchgear,

miscellaneous hardware), chemical/petro-chemical (pipeline hardware,

manufacturing buildings, storage tanks, walkways), recreation (boat trailers,

stadiums, arenas, racetracks), and many more.

17. What are the size limitations of steel that is to

be galvanized?

The hot-dip galvanizing process can accommodate various different shapes and

sizes of steel. Kettle sizes vary in dimensions from one galvanizer to the next. You

can view the online listing of all the galvanizers in North America and their kettle

sizes.

18. What types of products can be galvanized?

Numerous different fabrications for a variety of applications are galvanized each

year. To view a list of the different types of products that have been hot-dip

galvanized click here.

19. 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

coatings intermetallic layers being exposed. Performance is not affected. Coating

appearance depends on the amount of zinc in the coating.

View animation

20. Is the zinc coatings 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. Members of the American Galvanizers

Association galvanize to ASTM standards, which define minimum average coating

thickness grades for various material categories.

View animation

21. What can I do to minimize possible warping &

distortion? Is it possible to determine prior to

galvanizing which pieces might be prone to this

occurrence?

Minimizing potential warpage and distortion is easily done in the projects design

stages by selecting steel of equal thicknesses for use in every separate

subassembly that is to be hot-dip galvanized, using symmetrical designs whenever

possible, and by avoiding the use of light-gage steel (

23. 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 figure can vary greatly based on

numerous factors. The fabrications shape, size, and steel chemistry all play a major

role, and other factors like the black weight, the different types of steel that get

welded together, and the galvanizing bath chemistry can also have an effect.

24. Are slip-critical connections a concern when

the steel is to be galvanized?

When galvanized parts are used for slip-critical connections, they must either be

brushed, abrasive blasted, or painted with zinc-silicate paint to increase the surface

roughness and, thus, the slip factor.

View animation

25. Im interested in specifying hot-dip galvanizing

for reinforcing steel. Are there any concerns with

fabricating rebar after galvanizing?

Rebar is commonly fabricated after galvanizing. In order to minimize the possibility

for coating damage, avoid bending the rebar at a radius of more than 8 times its

radius. ASTM A 767, Specification for Zinc-Coated (Galvanized) Steel Bars for

Concrete Reinforcement, has a table that provides maximum bend diameters for

various-sized rebar.

View animation

26. 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) it may significantly increase the coating thickness and cause a brittle

coating.

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

Double-dipping is the progressive dipping of steel that is too large to fit into the

kettle in a single dip. Double-dipping cannot be used to produce a thicker hot-dip

galvanized coating.

View animation

28. What is the reason for incorporating venting &

drainage holes into a projects 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 flow entirely into, over, and throughout the part, and then back into the

tank or kettle.

View animation

29. If I stitch-weld, will there be uncoated areas

after galvanizing?

When stitch-welding is used, there is a possibility of gas release between gaps,

which will prevent the galvanized coating from forming in these areas. By leaving at

least a 3/32 (2.4 mm) gap between the contacting surfaces, gases are allowed to

escape and cleaning solutions and molten zinc are allowed to flow in between the

surfaces for a complete and uniform coating.

View animation

30. What is white rust and how can it be

avoided?

White rust is the term mistakenly applied to wet storage stain, which actually is a

milder corrosion product than white rust. Wet storage stain can be avoided by

properly stacking freshly galvanized articles, avoiding unprotected exposure to wet

or humid climates, or by using a surface passivation treatment after galvanizing. Wet

storage stain typically weathers away once the part is in service. (True white rust is

most commonly associated with galvanized cooling towers.)

View animation

31. 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.

View animation

32. Is there any environmental impact when the

zinc coating sacrificially 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 (25th most abundant element

in the earth), 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.

33. Should I be concerned when galvanized steel

comes 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 recommended to insulate

galvanized steel so that it doesnt come in direct contact with dissimilar metals.

Rubber or plastic, both non-conductive, are often used to provide this insulation.

34. What is the difference between hot-dip

galvanizing after fabrication and continuous hot-

dip galvanized sheet?

The process steps are similar but the production equipment is very different. After

fabrication galvanizing is a more manual process where structural steel (fabricated

plate, wide-flange beams, angles, channels, tube, pipe, fasteners) is suspended by

wire, chain or hook from crane hoists and immersed in the cleaning solutions and

zinc. Continuous sheet galvanizing involves uncoiling sheet, passing it through the

cleaning steps and molten zinc bath at speeds up to 500 feet per minute, drying and

recoiling. The uses of after-fabrication galvanized steel are usually exterior in nature

because the zinc coating is relatively thick (3.0 6 mils, 75 150 microns, 1.7 to 3.6

oz/sq. ft.) and will protect steel from corrosion in most atmospheric conditions for 50

to 100 years. Galvanized sheet is suitable for interior applications because of the

relatively thin coating (0.45 oz on each side), unless it is painted after galvanizing.

35. What is a G90 or A60 coating?

G90 is a grade of galvanized sheet produced to ASTM A653. It has 0.90 oz/sq. ft. of

zinc overall or 0.45 oz/sq. ft. per side. A60 is also a grade, has 0.30 oz/sq. ft. per

side, and has been annealed after galvanizing to produce a surface that promotes

good adhesion of paint.

36. Is a salt spray test in a laboratory appropriate

to estimate the corrosion rate of zinc coated steel?

In order for zinc to develop its protective patina of zinc carbonate that is very stable

and non-reactive, it requires a wetting and drying cycle like that produced by nature.

Salt spray tests keep the zinc wet and essentially wash the zinc corrosion products

off as they develop, inflating the corrosion rate of zinc. This lab test is not reflective

of real-world performance of zinc coatings.

37. Can galvanized steel in service withstand high

temperatures for long periods of time?

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

an intermittent basis.

38. 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) it may significantly increase the coating thickness and cause a brittle

coating.

39. What is cold galvanizing?

There is no such thing as cold galvanizing. The term is often used in reference to

painting with zinc-rich paint. Galvanizing by definition 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.