The Hot-Dip Galvanizing Line, Step by Step
Hot-dip galvanizing forms a metallurgical zinc alloy bond, not just a paint-like layer. The line sequence is: degreasing (alkaline bath removes oils) → rinsing → pickling (hydrochloric or sulfuric acid removes mill scale) → fluxing (zinc-ammonium chloride promotes wetting) → drying → immersion in molten zinc at roughly 445–455°C → controlled withdrawal → water quench → passivation or chromating on request.
What Happens in the Kettle
In the molten zinc, iron and zinc react to form layered intermetallic phases — gamma, delta and zeta alloy layers topped by a nearly pure eta zinc layer. The alloy layers grow with time in the kettle; silicon content in the steel accelerates growth (the Sandelin effect), which is why reactive steels can come out with thicker, darker-grey coatings. The final thickness is controlled by steel chemistry, immersion time and withdrawal speed — not by paint-like deposition.
| Phase | Layer | Character |
|---|---|---|
| Gamma | Innermost, iron-rich | Thin, brittle |
| Delta | δ — FeZn₇ | Ductile, adherent |
| Zeta | ζ — FeZn₁₃ | Hard, grows with dwell time |
| Eta | Outer free zinc | Ductile, gives shine |
Required Coating Thickness
ISO 1461 requires a minimum average of 85 µm for steel sections ≥ 6 mm thick (70 µm local minimum); thinner sections have proportionally lower minima in the standard’s table. ASTM A123 works with coating grades — for structural shapes at least 1/4 in (6.4 mm) thick, the minimum average is coating grade 100 (100 µm). Specify the governing standard on the PO and ask for coating-thickness readings (magnetic gauge) in the inspection report.
Coating life tracks thickness: in a moderate (C3) atmosphere, 85 µm of zinc commonly delivers decades of maintenance-free service; check your environment’s corrosivity category before fixing a spec.
Quality Checks That Matter
Acceptance is visual plus measured: the coating must be continuous (no uncoated spots, no acid traps in double dips), reasonably smooth, and free of flux inclusions or ash. Blistering, flaking and bare patches are grounds for rejection and re-galvanizing. For pipe specifically, also check the ID — small bores can leave heavy zinc runs or uncoated interiors if the line is not set up for tube.
Frequently Asked Questions
Why do some galvanized surfaces look grey instead of shiny?
Fully alloyed coatings (all zeta/delta, little free zinc) appear matte grey. It is thicker and often longer-lasting in abrasive service — appearance is not a quality criterion in ISO 1461.
Can welded fabrications be galvanized after welding?
Yes, and it is common — but welds must be continuous and free of slag, and vent/drill holes are needed for hollow sections so trapped gas cannot blow out in the kettle.
Does galvanizing affect steel strength?
No measurable loss at normal kettle temperatures; the steel is briefly held below its transformation temperature. Bent or cold-worked parts should be stress-relieved before galvanizing to avoid distortion.
CREATEEL supplies hot-dip galvanized pipe and sections to ISO 1461 / ASTM A123 with coating-thickness reports per lot. Contact us for galvanized line pipe, structural tube and hollow sections.
Standards & references: ISO 1461 · ASTM A123 · International Zinc Association
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