Hot-dip galvanizing coats steel by cleaning it, fluxing it, and dipping it in molten zinc at about 450 °C for 3-5 minutes, which leaves a zinc coating 50-100 micrometres thick that lasts 20-50 years in most environments. The zinc protects even where it is scratched, because it corrodes in preference to the steel. Zinc fume causes metal fume fever and the flux fume is worse, so work outdoors or under strong extraction, and keep every part completely dry and vented before it touches the bath.
Part of Surface Treatments
Galvanizing is the coating of steel with zinc, usually by dipping in molten zinc, to protect it from rust by barrier and by sacrificial action.
Zinc Protects Two Ways
A zinc coating is a barrier that keeps water and air off the steel. And because zinc is more reactive than iron, in a damp scratch it corrodes first and the steel stays bright. The Advanced Materials article calls this sacrificial protection and gives a service life of 20-50 years in most environments. Rust Prevention explains that rust needs iron, water and oxygen together. The zinc steps in the way. During dipping, iron and zinc react at the surface and form layers of iron-zinc alloy bonded to the steel, with nearly pure zinc on top. That bonded alloy is why a hot-dip coating clings far better than paint.
What You Need
- Clean steel, free of grease, rust and scale.
- Hydrochloric acid for pickling, dilute.
- A flux bath of zinc ammonium chloride solution.
- A steel or cast-iron kettle of molten zinc, held near 450 °C. Zinc melts at 420 °C according to Temperature Reading, so 450 °C is only a little above melting and needs close control.
- Tongs or wire hangers, a skimmer, drying space, and ventilation.
Use clean zinc. Die-cast zinc scrap often carries lead, aluminium and other metals, and unknown scrap can bring cadmium, which is toxic as fume.
Method
The Advanced Materials article gives the sequence:
- Degrease and pickle. Clean thoroughly, then remove rust in dilute hydrochloric acid. Rinse.
- Flux. Dip in the zinc ammonium chloride solution and dry completely.
- Dip. Lower the part into molten zinc at 450 °C for 3-5 minutes. The bath bubbles as flux burns off, then settles.
- Withdraw and drain. Lift slowly, let excess drain, and wipe or shake drips away with care.
- Cool in air.
The result is a coating of 50-100 micrometres. Check it is complete and without bare spots, and touch up any bare area with paint or a zinc-rich coating.
Design The Part For The Dip
- Hollow parts must be vented at both ends, so that air and zinc can pass in and out. A sealed hollow part will burst.
- Avoid overlaps and tight seams. Pickling acid and moisture hide in them and spit out of the bath.
- Thick parts heat slowly, and thin parts overheat. Judge dip time by the part, using the Advanced Materials article’s 3-5 minutes as the starting range.
Worked Example: Zinc Needed
Coating area times thickness gives volume. A steel panel 0.5 m × 0.4 m dipped on both faces has 2 × 0.5 × 0.4 = 0.4 m² of surface (ignoring edges). At 75 micrometres (0.000075 m), the coating volume is 0.4 × 0.000075 = 0.00003 m³, or 30 cm³. Zinc has a density of about 7.1 g/cm³ (typical value), so that is 30 × 7.1 = 213 g of zinc retained, plus losses to dross and drips.
Zinc fume, flux fume, acid and steam
Overheated molten zinc gives off white zinc oxide fume that causes metal fume fever, with chills, fever and aching hours after exposure, and the flux of zinc ammonium chloride produces irritating fume that is worse. Work outdoors upwind, or under a strong extraction hood that pulls fume away from the face, and wear a suitable respirator. Keep the bath no hotter than needed and skim dross gently. Any water on the part or in a sealed cavity flashes to steam under molten metal and sprays it out, so dry and vent everything and keep the pit area dry. Hydrochloric acid fumes and splashes burn, and pickling steel releases hydrogen, which is flammable. Wear face shield, leather apron and gloves rated for molten metal splash.
Common Mistakes
- Damp flux or parts — spitting and explosions.
- Overheating the bath — more fume and a thick, brittle coating.
- Using galvanised parts as cooking surfaces or heating them — the coating gives off fume when heated, and acid food dissolves it.
What’s Next
- Learn next: Non-Destructive Testing — checking coated and uncoated parts for flaws
- Combine with: Electroplating — depositing zinc or other metals with current