Detailing hollow steel sections for hot-dip galvanizing

Why venting matters

Hot-dip galvanizing is a total immersion process. Cleaning solutions must reach and wet every surface of a hollow fabrication, and no solution should remain trapped inside when it is withdrawn.²

The American Galvanizers Association gives two reasons for vent and drain holes. The primary reason is to let air escape so the piece can be completely immersed in the cleaning solutions and the molten zinc. The secondary reason is to prevent damage: pickling solution or rinse water trapped in a blind or closed connection can turn to superheated steam or gas and develop very high pressure when immersed, which can damage the fabrication and put galvanizing personnel and equipment at risk.¹

Where the openings go

Pieces are dipped and withdrawn at an angle, so vents are located at the highest point and drains at the lowest. Fabricated pipework should be interconnected with full open tee or mitre joints, each enclosed section needs a vent at each end, and tubular structures are best submerged completely in one dip.¹

Many galvanizers prefer venting that is visible from the outside so they can confirm it is adequate and has not been omitted, and some may hesitate to process complex pipe assemblies without it.¹

Base plates and end plates should be designed for venting and draining. Cutting the plate fully open gives the least obstruction; where that is not possible, vent holes in the plate are the usual solution. Holes can be left open or plugged after galvanizing. The guide notes aluminium plugs for mild and moderate environments and zinc plugs for more corrosive ones, to avoid galvanic corrosion.¹

The AGA guide illustrates recommended arrangements for handrail, tube and pipe trusses, pipe columns, box sections and similar items, and states minimum vent dimensions for those arrangements.³

Enclosed, overlapping and sealed details

For tanks and enclosed items galvanized inside and out, solutions and zinc should enter at the bottom and air should leave through an opening at the highest point, with at least one fill or drain hole and one vent. The guide recommends reviewing drawings of enclosed or partly enclosed items with the galvanizer before fabrication.⁴

Narrow gaps, overlapping plates and back-to-back angles or channels are best avoided. Where a tight overlap cannot be avoided, the guide recommends sealing all edges by welding, because zinc will not enter the gap but cleaning solutions will and can later weep out as rust staining. A sealed weld must be free of pinholes, and larger sealed overlaps need vent holes; the guide tabulates vent sizes by overlap area.⁵

Distortion, welds and material

Steel is heated to about 443 °C during galvanizing, and every heating and cooling cycle adds stress to a fabrication. Kettle size limits what can be dipped in one go, so dimensions should be checked with the galvanizer early and large items designed in modules where needed.⁶

To limit distortion the guide recommends parts of similar thickness at joints, symmetrical sections where possible, a balanced welding sequence, temporary bracing on thin or asymmetric work, and optimised venting, drainage and lifting points on long, slender products.⁷

There is a small potential for cracking at flame-cut beam copes and at the corners of rectangular hollow sections. The guide recommends visual inspection after galvanizing and repair and touch-up before delivery.⁸

Welding flux residues are not removed by the galvanizer’s chemical cleaning and must be removed beforehand, and high-silicon weld metal can produce a thicker, darker coating over the weld.⁹ Steel chemistry, particularly silicon and phosphorus, affects coating thickness and appearance, so the galvanizer should know the steel grade.¹⁰

Scope and limits

The AGA guide is US industry guidance and refers to ASTM practices. It presents itself as general information, not a substitute for competent professional examination of a specific item.¹¹