What Hot-Dip Galvanizing Does
Hot-dip galvanizing is a diffusion coating process: a chemically clean steel workpiece is immersed in molten zinc, and iron from the steel reacts with the zinc to form a series of zinc-iron intermetallic layers that are metallurgically bonded to the substrate. Because the bond is formed by reaction rather than by adhesion, the coating cannot peel or flake in the way a paint system can. The process is used for structural sections, lattice towers, transmission poles, guardrail, cable trays, fasteners and any fabricated steel item that will be exposed outdoors for decades.
Process Flow from Black Steel to Coated Part
| Step | Purpose | Key control |
|---|---|---|
| Degreasing | Remove oil, grease and drawing compounds | Alkaline bath concentration and temperature |
| Pickling | Remove mill scale and rust | Acid concentration, temperature, immersion time |
| Rinsing | Remove iron salts from the surface | Water quality and rinse stage count |
| Fluxing | Activate the surface and prevent re-oxidation | Flux composition, iron content, pH |
| Drying | Remove water to avoid zinc explosion and skips | Workpiece temperature before immersion |
| Zinc immersion | Form the zinc-iron coating | Bath temperature, immersion time, withdrawal rate |
| Cooling | Solidify the coating without damage | Air or water quench practice |
| Passivation | Suppress white rust in storage | Chromate-free or trivalent passivation control |
Pre-treatment: Degreasing, Pickling, Fluxing and Drying
Pre-treatment decides coating quality more than any other stage, because zinc only reacts with clean steel. Degreasing removes oils applied during machining or drawing. Pickling in hydrochloric acid removes mill scale and rust; within normal operating ranges a lower acid concentration requires a longer immersion time, and bath temperature has an even stronger effect on the pickling rate than concentration. Rinsing then removes iron salts, which if carried forward increase zinc ash and dross formation. Fluxing, normally in a zinc ammonium chloride solution, activates the surface and deposits a protective film that prevents re-oxidation before the part reaches the zinc. Drying is the last essential step: any residual moisture flashes to steam in the bath and causes zinc explosions, bare patches and excessive fumes.
Immersion in Molten Zinc
Zinc melts at about 419.5 degrees Celsius, and general galvanizing baths are operated a little above that, commonly in the range of roughly 445 to 465 degrees Celsius for structural work. Immersion time depends on the mass and wall thickness of the part, because the coating grows by diffusion and needs time to reach the specified thickness; heavy sections are held longer than thin ones. Withdrawal rate and angle matter as well, since they govern drainage, coating uniformity and the amount of zinc that returns to the bath as dross. Bath chemistry is controlled continuously, with aluminium additions used to suppress excessive alloy growth and small additions of nickel or other elements used where reactive steels would otherwise produce an over-thick, brittle coating.
What Forms Inside the Coating
The coating is not a single layer of zinc. It is a sequence of intermetallic phases, each with a different iron content, topped by a layer of nearly pure zinc:
| Layer | Phase | Iron content | Character |
|---|---|---|---|
| Eta | Pure zinc | Less than 0.03% | Soft, ductile outer layer that provides the sacrificial protection |
| Zeta | FeZn13 | Roughly 6% | Columnar crystals, the thickest alloy layer in most coatings |
| Delta | FeZn7 | Roughly 7 to 12% | Compact barrier layer that controls diffusion |
| Gamma | Fe3Zn10 | Roughly 20 to 28% | Thin layer adjacent to the steel, hard and brittle |
Thick coatings are therefore harder and more brittle than thin ones, because alloy layers make up a larger share of the total. This is why bend radius, not coating mass alone, is the limiting factor for galvanized parts that must be formed after coating.
Acceptance Requirements to ISO 1461
ISO 1461 is the reference standard for hot-dip galvanized coatings on fabricated iron and steel articles. It specifies minimum mean and local coating thicknesses that increase with the thickness of the steel being coated:
| Steel thickness | Minimum mean coating thickness | Minimum local thickness |
|---|---|---|
| 6 mm and above | 85 micrometres | 70 micrometres |
| 3 mm to under 6 mm | 70 micrometres | 55 micrometres |
| 1.5 mm to under 3 mm | 55 micrometres | 45 micrometres |
| Under 1.5 mm | 45 micrometres | 35 micrometres |
Thickness is verified with a magnetic or electromagnetic gauge, and the standard also covers appearance, adhesion and the treatment of small areas such as drilled holes and cut ends. Because the coating is consumed at a predictable rate, the measured thickness is also the basis for estimating service life in a given corrosivity category.
Limits of the Process
Hot-dip galvanizing is unsuitable where dimensional accuracy is critical, because the coating adds to every surface and can bridge fine threads unless they are cut after coating. Parts containing enclosed spaces must be vented and drained so that air and moisture cannot be trapped. High-strength fasteners above a defined strength class, and steels with sulphur or phosphorus contents outside the ranges the standard considers galvanizable, require discussion before processing because of the risk of hydrogen embrittlement or an excessively thick, poorly adherent coating.
Frequently Asked Questions
Q: Why must the workpiece be completely dry before it enters the zinc bath?
A: Residual moisture flashes to steam on contact with the melt, which throws zinc out of the bath and leaves bare patches or pinholes on the workpiece.
Q: How thick is a hot-dip galvanized coating?
A: ISO 1461 requires a minimum mean thickness from 45 micrometres for steel under 1.5 mm thick to 85 micrometres for steel 6 mm and thicker, with corresponding local minima of 35 and 70 micrometres.
Q: Does a thicker coating always last longer?
A: Thicker coatings have a longer life in a given environment because more zinc is available, but past a certain point the coating becomes brittle and cannot survive bending, so thickness must be balanced against forming.
Q: Can galvanized parts be welded?
A: Yes, but the zinc coating volatilises in the arc, so fume extraction is required, the weld quality is affected, and the affected zone must be repaired with a zinc-rich coating after welding.
Q: What is the purpose of passivation after galvanizing?
A: It suppresses the white rust that forms when freshly coated parts are stacked and stored in humid conditions, keeping the surface acceptable for handling and subsequent painting.