Distortion & Warping

Protection against corrosion begins at the drawing board. No matter what corrosion protection system is used, it must be factored into the product's design. When the decision is made to hot-dip galvanize, the design engineer should ensure that the pieces can be suitably fabricated for highest-quality galvanizing.

Asymmetrical designs or structures containing sections of unequal thickness can be successfully galvanized, as can fabrications where cold-working techniques (bending, hole-punching, rolling, shearing) are employed.

Steel being galvanized progresses through a temperature cycle upon immersion into and withdrawal from the galvanizing bath. Because parts are immersed at an angle, uneven heating occurs, creating a temperature profile along the part being galvanized. This temperature profile allows the steel's internal stresses to be relieved at different times in the immersion cycle. These stresses may cause changes in shape and/or alignment (distortion and warping).

The following steps can be taken to minimize this risk:

• use assembly parts that are of equal or near-equal thickness, especially at joints,
• use standard rolled shapes,
• use symmetrically rolled sections rather than angle or channel frames; I-beams also are preferred to angles or channels,
• galvanize a channel frame with a plate separately and bolt or weld post-galvanizing,
• bend members to the largest acceptable radii to minimize local stress concentration,
• accurately pre-form members of an assembly so that it is not necessary to force, spring, or bend them into position during joining,
• avoid designs that, because of size, require more than one dip into the molten zinc bath ("progressive galvanizing," sometimes called "double-dipping"),
• relieve welding- or cold-working-induced stresses by heat-treating the part at 1100 F (590 C) for one hour per inch (2.5 cm) of section thickness,

Consult your galvanizer regarding the use of temporary bracing or reinforcing.

The guidelines for safeguarding against warping and distortion during hot-dip galvanizing of steel assemblies are outlined in ASTM A 384. 
 

Overlapping & Contacting Surfaces

To minimize the possibility of this occurrence, consider the following:

• Seal-welding ensures that cleaning solutions cannot become trapped in the weld area; if skip-welding is used, a gap of at least 3/32” (2.5 mm) must be provided.
• Cleaning solution salts can be retained in tight areas because they cannot be adequately rinsed; the galvanized coating may be of good quality in adjacent areas, but humidity encountered weeks or even months later may wet these acid salts, causing staining of the surrounding galvanized coating.
• When overlapping or contacting surfaces cannot be avoided and are 3/32” (2.5 mm) or less, all edges should be completely seal-welded; zinc’s viscosity prevents it from entering any space tighter than 3/32” (2.5 mm).
• There should be vent holes through one or both surfaces into the overlapped area (see tables).

 

Venting & Drainage

In the hot-dip galvanizing process, steel is completely coated with corrosion-inhibiting zinc, which forms a highly abrasion-resistant metallurgical bond with the base steel. In order to ensure that all interior and exterior surfaces are protected from corrosion, entire steel fabrications are lowered into and raised out of cleaning solutions, flux solutions, and molten zinc metal. In order to facilitate interior and exterior cleaning and coating, it is necessary to provide holes in fabrications to be galvanized.

The primary reason for vent and drain holes is to allow air to be evacuated from within and around the fabrication, allowing it to be completely immersed in the cleaning solutions and molten zinc and for the excess zinc and solutions to drain out and away from the part.

The secondary reason is that if fabrications to be galvanized are not properly vented, cleaning solutions or rinse waters trapped in overlapping or contacting surfaces flash to steam. The resulting pressure increase (up to 3600 psi [25MPa]) can rupture the fabrication. Additionally, trapped moisture that flashes to steam can result in localized uncoated surfaces.

Because items being galvanized are immersed in and withdrawn from all cleaning solutions and molten zinc at an angle, vent holes should be located at the highest point and drain holes at the lowest point as mounted during the galvanizing process. 


    1. Venting & Fabrication for Tubular Fabrications

• Galvanizers need to be able to visually confirm and inspect venting in complicated pipe assemblies, such as handrail.
• It is recommended that tubular structures be completely submerged in one dip in the galvanizing kettle to minimize potential internal coating problems, which, because of the size and shape of the item, may be difficult to discover during inspection.
• The diagrams above illustrate recommended designs for tubular fabrications and hollow structures. The vent dimensions are the minimum required.

   
 
    2.  Handrail

            Paragraph numbers correspond with number references on the following illustration:

1. External vent holes must be as close to the weld as possible and must be 25% the size of the internal diameter of the pipe, but not less than 3/8" (10 mm) in diameter.

2. Internal holes should be the full internal diameter of the pipe for the best quality, lowest cost galvanizing.

3. Vent holes in end sections or in similar sections must be 1/2 " (13 mm) in diameter.

4. & 5. Ends should be left completely open. Any device used for field-erection that prevents full openings on ends of horizontal rails and vertical legs should be galvanized separately and attached after galvanizing.

 
 
    3. Rectangular Tube Truss

End-plates – Horizontal

1. The most desirable fabrication is completely open.

2. If H + W = 24” (61 cm) or larger, the area of the hole, plus clips, should equal 25% of the area of the tube (Area = H x W).

If H + W is between 16” and 24” (41-61 cm), the area of the hole, plus clips, should equal 30% of the area of the tube.

If H + W is between 8” and 16” (20-41cm), the area of the hole, plus clips, should equal 40% of the area of the tube.

If H + W is less than 8” (20 cm), the tube should be left open.

  

     4. Pipe Truss 3" (8 cm) & Larger  

        Vertical Sections

• Hole locations for the vertical members should be as shown in Examples A and B, by the arrows on the figure.
• Each vertical member should have two holes at each end and 180° apart, in line with the horizontal members as indicated by the arrows. The size of the holes preferably should be equal and the combined area of the two holes at either end of the verticals (Areas C and D or Areas E and F) should be at least 30% of the cross-sectional area.

         End-plates Ð Horizontal

1. The most desirable fabrication is completely open with the same hole diameter as the tube's internal diameter.

2. & 3. & 4. Equal substitutes would have openings as shown above and would be at least 30% of the area of the internal diameter.

 
 
        5. Pipe Columns, Pipe Girders, Street Light Poles, & Transmission Poles
            (With base-plates and with or without cap-plates)

                Location of Opening

1. The most desirable fabrication is to have the end completely open, with the same diameter as the section top and bottom.

2. & 3. & 4. This is an equal substitute if the full opening is not allowed.

5. This must be used when no holes are allowed in the cap or base-plate: two half-circles 180° apart and at opposite ends of the pole.

                Dimensions

• Openings at each end must be at least 30% of the cross-sectional area of the pipe, for pipe 3" (8 cm) and greater and 45% of the cross-sectional area for pipe smaller than 3" (8 cm).

       
 
         6. Box Sections

• The figure shows the location of holes and clipped corners, which must be flush.
  Using the following formulas, the table shows typical sizes of holes.
• If H + W = 24" (61 cm) or larger, the area of the hole, plus clips, should equal 25% of the cross-sectional area of the box (Area = H x W). If H + W is between 16" and 24" (41-61 cm), the area of the hole, plus clips, should equal 30% of the cross-sectional area of the box. If H + W is between 8" and 16" (20-41 cm), the area of the hole, plus clips, should equal 40% of the cross-sectional area of the box. If H + W is less than 8" (20 cm), leave completely open, with no end-plate or internal gusset.
• The table is for square box-sections only. For rectangular sections, calculate the required area and check with the galvanizer for positioning of openings.

• The small end should be completely open.

Pole Plate End

1. The most desirable fabrication is to have the end completely left open.

2. & 3. & 4. For acceptable alternatives, the half-circles, slots, and round holes must equal 30% of the area of the internal diameter of the pole end of the tapered arm for 3" (8 cm) and larger internal diameters. The opening must equal 45% of the area of the pole end of the tapered arm if the internal diameter is less than 3" (8 cm).
 

• When both internal and external surfaces are to be galvanized, at least one fill/drain hole and one vent hole must be provided.
• The fill/drain hole should be as large as the design will allow, but at least 3” (8 cm) in diameter for each cubic yard (10 cm in diameter for each cubic meter) of volume. The minimum diameter is 2” (5 cm).
• Provide vent holes of the same size diagonally opposite the fill/drain hole. This allows the air to escape.
• In tanks, internal baffles should be cropped on the top and bottom or provided with suitable drainage holes to permit the free flow of molten zinc.
•  Manholes, hand holes, and openings should be finished flush inside to prevent trapping excess zinc.
• Items such as vessels or heat exchangers that are galvanized on the outside only must have snorkel tubes or extended vent pipes. The galvanizer should be consulted before using these temporary fittings because special equipment is needed.
• The galvanizer should always review the drawings of enclosed or partially enclosed vessels before fabrication. Galvanizers may recommend changes that would provide a better-galvanized product. If a change is needed to facilitate galvanizing, the least expensive time to make the change is before fabrication.

        9. Drainage

• Where gusset plates are used, generously cropped corners provide for free drainage. When cropping gusset plates is not possible, holes at least 1/2" (13 mm) in diameter must be placed in the plates as close to the corners as possible.
• To ensure unimpeded flow of solutions, all stiffeners, gussets and bracing should be cropped a minimum of 3/4" (19 mm).
• Provide holes at least 1/2" (13 mm) in diameter in end-plates on rolled steel shapes to allow molten zinc access during immersion in the galvanizing bath and drainage during withdrawal.
• Alternatively, holes at least 1/2" (13 mm) in diameter can be placed in the web within 1/4" (6 mm) of the end-plate. To facilitate drainage, end-plates should have holes placed as close to interior corners as possible.

 
 
Source: American Galvanizers Association
www.galvanizeit.org