Direct Answer
Hot-dip galvanizing (HDG) coats structural steel with a zinc layer by immersing cleaned steel in a bath of molten zinc at approximately 830°F. The zinc bonds metallurgically with the steel, providing corrosion protection that lasts 50+ years in many environments. Fabrication requirements include: cleaned surfaces (no paint, scale, or oil), specific hole sizes for drainage, and venting holes on closed sections to prevent steam explosion.
Full Explanation
Galvanizing process for fabricated steel: (1) Steel is degreased, pickled in acid, fluxed, and then immersed in the zinc bath. (2) Zinc thickness is 3–4 mils on structural steel. (3) All closed hollow sections must have vent holes at prescribed locations to allow steam to escape and zinc to flow through. (4) Members must be designed to drain zinc after withdrawal — no trapped corners without drainage holes. (5) Bolt holes should be 1/16" oversize (before galvanizing) to account for zinc coating in the holes. (6) Weld spatter must be removed completely — it falls off in the acid bath and creates bare spots.
What This Means for Your Shop
Galvanizing changes the fabrication sequence: all welding and mechanical finishing must be complete before galvanizing. Any touch-up (grinding, re-welding) after galvanizing requires either cold galvanizing compound or a zinc-rich weld overlay — both of which are less effective than the original galvanized coating.
Common Mistakes
Not oversizing holes before galvanizing. If bolt holes are drilled to nominal diameter before galvanizing, the zinc coating in the hole reduces the effective hole diameter and bolts won't fit. Holes must be 1/16" oversize.
Recommended Process
Before releasing galvanized steel to fabrication, review the requirements: vent hole sizes and locations (per ASTM A385 and your galvanizer's requirements), drain hole requirements for hollow sections, and hole size. Coordinate with your galvanizer early — turnaround time and kettle size limits can affect your schedule.