Hot Dip Galvanizing 101
Protection against corrosion from Maine to New York (or throughout the Northeast) 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
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:
- 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.
- 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.
- 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.
- Galvanizing: The article is immersed to molten zinc at approximately 850°F (455°C). This results in a formation of a zinc and zinc-iron alloy coating which is metallurgically bonded to the steel.
- 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.
- 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.
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, designer’s 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.
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.
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. In welding, an uncoated electrode should be used wherever possible to prevent flux deposits.
- 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 fabricator’s responsibility unless other arrangements have been made.
- 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.
- In the case of heavy weldments, a submerged arc method is recommended.
- If none of these are available, select a coated rod specifically designed for “selfslagging”, as recommended by welding equipment suppliers.
- 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.
- 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.*
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:
- Select steel with a carbon content below 0.25%.
- Choose steel with low transition temperatures since cold work raises the ductile-brittle transition temperatures and galvanizing (heating) may raise it even further.
- Susceptibly to stain-age embrittlement is usually less of a potential problem with aluminum-killed steels.
- 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 1100°F (595°C) for one hour per inch (25.4mm) of section thickness.
- 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.
- 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.
- 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.
- In critical applications, the steel should be hot worked above 1200°F (650°C) in accordance with the steel maker’s 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 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.
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.
Duncan’s 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.
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.
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.
Tubular assemblies such as handrail, pipe columns, pipe girders, street light poles, transmission poles, pipe trusses, and sign bridges are commonly galvanized.
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:
- 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.
- 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.
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.
- Vent holes must be as close to the weld as possible and not less than 3/8” in diameter.
- Internal holes should be the full l.D. of the pipe for best galvanizing quality and lowest galvanizing cost.
- Vent holes in end sections on similar sections must be 1/2” diameter.
- 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:
- 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.
- Vent holes in end sections or in similar sections must be 1/2” diameter.
- 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
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
- Most desirable - completely open.
- 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
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
- Most desirable - completely open ‘same diameter.’
- Equal substitutes - openings should be at least 30% of the area of the inside diameter.
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.
H + W
Tapered - Signal Arm
A. Small end open completely.
Pole Plate End
- Most desirable - completely open.
- 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
- Most desirable - completely open “same diameter” as section top and bottom.
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.
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:
- Where possible, use symmetrical rolled sections in preference to angle or channel frames. I-beams are preferred to angles or channels.
- Use parts in an assembly that are of equal or near equal thickness, especially at joints.
- Bend members to the largest acceptable radii to minimize local stress concentration.
- Accurately perform members of an assembly so that it is not necessary to force, spring or bend them into position during joining.
- 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).
- 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.
- 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.”
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:
- 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.
- 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.
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:
- Avoid sharp corners and deep recesses.
- Use large pattern numerals and generous radii to facilitate abrasive cleaning.
- 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.
- 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:
- Heat at a temperature of 1250°F (677°C) until all sections have reached that temperature (no soak required).
- Perform finish machining and/or heat treating after abrasive blasting.
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|
|* 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:
- Low carbon bars are recommended since high carbon or high silicon causes a heavier, rougher galvanized coating on the threads.
- 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.
- 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.
- Standard sized threads are cut on the bolt while standard sized nuts are retapped oversize after galvanizing.
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.
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:
- 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.
- 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.
- 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.
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.
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 galvanizer’s facility.
See AGA publication “Wet Storage Stain” for more details.
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.
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.