Hot Dip Galvanizing
Hot dip galvanizing (HDG) is a corrosion-protection process where fabricated steel or iron is immersed in molten zinc at about 450°C (840°F). The zinc reacts with the base metal, forming metallurgically bonded zinc-iron alloy layers topped by an outer zinc layer. This produces a durable, low-maintenance coating used on structural steel, fasteners, brackets, and fabricated assemblies.
On the shop floor, hot dip galvanizing is as much a fabrication constraint as a finish. Parts that come off the CNC mill or laser are often nested, welded, or threaded before reaching the galvanizer, and that is where trouble starts. Closed sections need dedicated vent and drain holes in the drawings; without them, trapped acids and expanding air can blow out weld seams when the steel enters the molten zinc bath. Machinists must also think past bare steel dimensions. Every thread, bolt hole, and mating face grows by the zinc thickness, so tolerances that fit dry can lock up after coating. Galvanizing should be specified early because once the steel is dipped, there is no reliable way to machine off excess coating without exposing the substrate. Properly detailed parts come back coated inside and out, ready for assembly, while poorly planned ones are scrapped or reworked at significant cost.
Is hot dip galvanizing just a paint-like coating?
No. It is a metallurgical coating system, not a simple surface film; the zinc-iron alloy layers are integral with the steel substrate.
Why is cleaning so critical before galvanizing?
Zinc will not properly bond to oil, oxide, rust, or mill scale. Effective degreasing, acid pickling, and fluxing before immersion are necessary to produce a sound, fully adhering coating.
Why do fabricators add vent and drain holes to parts intended for galvanizing?
To allow trapped air and process liquids to escape and let molten zinc flow in and out freely. This improves safety during immersion and ensures complete interior and exterior coating coverage.