Lesson 11.4CoatChapter 11 · Coatings and surface finishing

Hot-dip galvanizing

Steel dipped in molten zinc forms a bonded zinc layer

Hot-dip galvanizing protects steel from rust by dipping it in a bath of molten zinc. The dull gray, slightly spangled finish on highway guardrails, electricity pylons and farm gates comes from this process. A galvanized part can often last several decades outdoors with no maintenance.

Photo: Wertuose, CC BY-SA 4.0, via Wikimedia Commons

Hot-dip galvanizing · the tank line, the coating layers and sacrificial protectionCircled numbers match the steps below. The insets show the coating up close and why a scratch does not rust.

How it works

Degrease. The steel hangs from a crane and is dipped in a hot alkaline bath (a strong soapy cleaner) that removes oil, grease and paint marks.
Pickle. A dilute acid bath, usually hydrochloric acid, dissolves rust and mill scale (the dark oxide skin left by rolling). Zinc only bonds to perfectly clean steel.
Flux and dry. A dip in a flux (a zinc ammonium chloride salt solution) leaves a thin film that stops new rust and helps the zinc wet the steel. The part is then dried.
Dip in molten zinc. The part goes into a kettle of zinc at about 450 °C for a few minutes. Iron and zinc react and grow layers of zinc-iron alloy that are bonded into the steel surface.
Lift out and drain. The part is withdrawn slowly so excess zinc runs off. A layer of almost pure zinc stays on top of the alloy layers.
Cool and inspect. The part is quenched in water or cooled in air, then checked for thickness and bare spots, and drips are removed.

The same idea runs continuously in steel mills: coils of sheet unroll through a zinc bath, often at more than 100 m per minute. That sheet becomes car bodies, roofing and appliance panels.

Key facts

Materials: Carbon and low-alloy steel, and cast iron; zinc bath with small additions such as aluminum
Typical parts: Structural beams, guardrails, pylons, poles, fences, fasteners, trailers, farm equipment
Part size: From bolts to beams many meters long; the largest kettles are over 15 m long
Precision: Coating usually 50–150 µm and uneven; threads and close-fitting holes need cleaning or masking
Surface finish: Dull gray or shiny with a crystal pattern (spangle), turning matte gray with age; drips and rough spots are common
Batch size it suits: One-off structures to large series; galvanizers price it by weight of steel
Tooling and machine cost: No part-specific tooling; a galvanizing plant costs millions
Speed: Minutes in the zinc; a full batch cycle with cleaning takes hours; continuous sheet lines run at high speed

Strengths and limits

Strengths

Very long life outdoors, often several decades with no upkeep
Covers every surface, inside tubes and corners, in one dip
Tough, metallurgically bonded coat that resists knocks
Zinc protects scratches and cut edges by corroding first

Limits

Only for steel and iron; looks industrial unless painted
At about 450 °C thin parts can warp, and closed hollows must be vented to avoid bursting
Thick, uneven coat, so it is poor for precise fits and threads
Welding or cutting afterward burns off zinc and gives off fumes

Where it's used

ConstructionStructural steel, stairs, balconies, rebar
Transport infrastructureHighway guardrails, sign gantries, lighting columns, bridges
EnergyElectricity pylons, substation frames, solar panel mounting structures
AgricultureGates, fences, barn structures, animal pens
AutomotiveGalvanized steel sheet for car bodies (continuous line), trailers
HardwareBolts, nuts, nails, chains, wire

How to recognize it

A dull silver-gray or shiny surface, sometimes with a frost-like crystal pattern called spangle.
Small drips, runs or lumps of zinc at edges and lower ends, where the zinc drained off.
Vent and drain holes drilled in the corners of tubes and closed shapes.
On older outdoor steel, an even matte gray with no rust, even at cut edges and scratches.

Compare with

Electroplating11.5also puts zinc on steel, but with electricity at room temperature; much thinner (typically 5–25 µm) and more precise, for bolts and small parts.Powder coating11.2adds color; it is often applied on top of galvanizing in a “duplex” system.Thermal spraying11.10sprays molten zinc onto structures too large to dip, such as bridge sections.

Watch it

Galvanizers Association · The process steps, explained by the UK industry body

American Galvanizers Association · What the finished coating looks like, and why

Joseph Ash Galvanizing · A real dip into molten zinc and the withdrawal

Brooke Alden · A plant tour past every tank

See real photos

Wikipedia: Hot-dip galvanization — process steps, batch and continuous lines, history, with photos
Wikimedia Commons photo search

Remember

Clean steel dipped in zinc at about 450 °C grows a coat that is bonded into its surface.
Zinc protects twice: as a barrier, and by corroding first wherever the steel is exposed.
It is the long-life choice for outdoor steel, but the coat is thick and not precise.