Hot-Dip Galvanizing
Also: galvanizing, HDG, hot dip galvanizing, hot-dip galvanized, galvanising, batch galvanizing, ASTM A123, ASTM A153, spin galvanizing, galv, ISO 1461
Hot-dip galvanizing dips fabricated steel in molten zinc at about 450 °C, bonding a 50-150 µm zinc coating that protects outdoor steel for decades.
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Hot-dip galvanizing coats finished steel fabrications by immersing them in molten zinc. The steel is degreased in a hot caustic, pickled in acid to remove rust and mill scale, dipped in a zinc ammonium chloride flux, and then lowered into a kettle of zinc at about 445-455 °C (830-850 °F). The zinc reacts with the iron to form layers of zinc-iron alloy, topped by a layer of nearly pure zinc as the part is withdrawn. The coating is metallurgically bonded, it covers edges, corners and the inside of vented tubes, and it protects in two ways: as a barrier, and sacrificially, since zinc corrodes in preference to exposed steel at a scratch or a cut edge.
ASTM A123 covers fabricated products (structural shapes, plate, bar, pipe and tube, weldments) and sets a minimum average coating thickness by material category and steel thickness. ASTM A153 covers hardware: fasteners, castings and small parts, which are galvanized in baskets and centrifuged (spun) to fling off excess zinc so threads stay usable. ASTM A385 gives design practice, A384 covers warpage and distortion, A143 covers embrittlement, and A780 covers repair of damaged areas. Coating life scales with thickness and with the environment: several decades in rural and suburban air, less in industrial and marine exposure.
The steel chemistry controls the coating. Silicon and phosphorus speed the zinc-iron reaction: steels with silicon between roughly 0.04% and 0.15% (the Sandelin range), or above about 0.25%, grow thick, dull grey and sometimes brittle coatings that can flake on impact. For a bright, even coating, order steel with silicon below about 0.04% or between about 0.15% and 0.25%, and low phosphorus. The 450 °C dip also releases residual stress and can warp thin plate, long asymmetric sections and weldments that mix thin and thick members, and it tempers any part that was hardened and tempered below that temperature.
At a glance
| Typical tolerances | ASTM A123 minimum average thickness runs from 45 µm (1.8 mils) on steel under 1.6 mm (1/16") thick to 100 µm (3.9 mils) on structural shapes and plate 6.4 mm (1/4") and thicker; actual coatings are commonly 85-150 µm (3.3-5.9 mils), and 200 µm or more on reactive steels. ASTM A153 fastener coatings are thinner, about 43-53 µm (1.7-2.1 mils) minimum. The coating builds unevenly, with drips and runs at drain points, so holes shrink, threads fill and fits bind: nuts are tapped oversize after galvanizing, and hinges and moving parts need about 1.5 mm (1/16") radial clearance. Flatness and straightness are not guaranteed on thin or asymmetric weldments. |
|---|---|
| Size limits | Kettle-limited. U.S. galvanizers commonly run kettles 9-18 m (30-60 ft) long, about 1.5-2 m (5-7 ft) wide and 2-3.5 m (7-12 ft) deep. A part longer or deeper than the kettle can often be progressive-dipped, one end and then the other, if more than half of it fits. Crane capacity limits weight, commonly several tonnes per lift. Fasteners and small parts go through spin lines in baskets. Galvanizers are regional, so large fabrications also carry trucking both ways. |
| Surface finish | Metallic zinc, bright and spangled to matte grey depending on steel chemistry and cooling; it weathers to a uniform dull grey patina within a year or so. The surface is rougher than paint, with drips, runs and occasional dross pimples: protective, not cosmetic. The zinc-iron alloy layers are harder than mild steel (roughly 180-250 HV against about 160 HV), which gives good abrasion resistance; the outer zinc layer is soft (about 70 HV). Freshly galvanized parts stacked wet develop white rust (wet storage stain). Paint or powder over galvanizing, a duplex system, needs its own surface preparation (ASTM D6386 for paint, D7803 for powder). |
| Lead time | Typically 3-7 business days at the galvanizer plus freight both ways; 1-2 weeks door to door is normal for structural fabrications. Spin-galvanized fasteners and hardware run 1-2 weeks. A duplex paint or powder system adds about a week. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
Carbon and low-alloy steels: structural shapes and plate (A36, A572), hollow structural sections and pipe, rebar, sheet, and fabrications made from them; gray, malleable and ductile iron castings (abrasive blasted rather than pickled, since acid does not remove foundry sand); and fasteners. Steels at or above about 1,000 MPa (150 ksi) tensile risk hydrogen embrittlement from pickling, and quenched and tempered parts tempered below 450 °C lose hardness in the kettle. Stainless, aluminum, copper alloys and other non-ferrous metals are not galvanized.
What drives the cost
- Weight: galvanizers price per pound or kilogram of steel, with a minimum lot charge
- Steel chemistry: reactive, high-silicon steels pick up far more zinc
- Contaminants pickling cannot remove (paint, marker, weld slag, anti-spatter, casting sand), which must be blasted or ground off first
- Geometry: parts that need extra venting, special handling, double dipping or straightening after the dip
- Spin (centrifuged) processing of threaded parts and small hardware
- Freight to and from the plant for large or heavy fabrications
- Duplex topcoats, masking, and thickness reports or certs to ASTM A123 or A153
When to use it
- Outdoor structural steel: platforms, stairs, handrails, poles, towers, bridges, highway and utility hardware
- Trailer frames, agricultural equipment, marine docks and parts that live wet and get scratched
- Welded tube and pipe fabrications that need protection inside as well as out (vented)
- Long service life with no maintenance, where a painted part would need recoating in a few years
- Anchor rods, structural bolts and fasteners (ASTM A153 or F2329) for exposed structures
- A base for a duplex paint or powder system in the harshest environments
When not to
- Precision parts, fine threads and close fits: the coating is thick and uneven
- Thin sheet-metal assemblies and long asymmetric weldments that will warp at 450 °C
- Steels at or above about 1,000 MPa (150 ksi) tensile, or hardened parts tempered below 450 °C
- Cosmetic parts that need a smooth, uniform appearance: use powder or e-coat, or a duplex system
- Closed sections that cannot be vented: trapped air keeps the zinc out and trapped moisture can burst the part
- Continuous service above about 200 °C (390 °F), or strong acids and alkalis, which attack zinc
Design tips
- Call out the spec: "Hot-dip galvanize per ASTM A123" for fabrications, "per ASTM A153" or "F2329" for hardware, and ask for a coating thickness report.
- Vent and drain every closed section at diagonally opposite corners, with holes sized per ASTM A385 and the galvanizer's guidance; a sealed tube can burst in the kettle.
- Seal-weld overlapping plates all the way around or leave a gap; skip-welded laps trap pickling acid that weeps rust stains later.
- Keep member thicknesses similar and designs symmetric to limit distortion; review large weldments against ASTM A384 and with the galvanizer before building.
- Order steel with silicon below about 0.04% or between 0.15% and 0.25% if appearance matters, and do not mix reactive and non-reactive steels in one visible assembly.
- Remove paint, marker, weld slag and anti-spatter before shipping; acid does not remove them and the zinc will not wet those spots.
- Tap nuts and internal threads oversize after galvanizing, and allow about 1.5 mm (1/16") clearance on hinges and moving parts.
- If the part will be painted or powder coated, tell the galvanizer so they skip the post-dip chromate quench, which spoils paint adhesion.
Frequently asked questions
- How long does hot-dip galvanizing last?
- Decades in most atmospheres. Life is roughly proportional to coating thickness and depends on the environment: a 100 µm coating commonly lasts 50 years or more in rural and suburban air, and less in industrial or coastal exposure. The American Galvanizers Association publishes time-to-first-maintenance estimates.
- How thick is hot-dip galvanizing?
- ASTM A123 minimums run from 45 µm on thin steel to 100 µm (3.9 mils) on structural steel 6.4 mm (1/4") and thicker. Real coatings are usually 85-150 µm, and reactive high-silicon steels grow 200 µm or more. Fasteners per ASTM A153 carry about 43-53 µm.
- What is the difference between hot-dip galvanizing and zinc plating?
- Galvanizing dips the part in molten zinc and bonds a 50-150 µm alloyed coating that lasts decades outdoors. Zinc plating electrodeposits 5-25 µm of zinc with a chromate topcoat for indoor or mild service; it is smoother, holds threads and fits, and costs less on small parts.
- Why is galvanized steel dull grey instead of shiny?
- Usually the steel chemistry. Silicon in the Sandelin range (about 0.04-0.15%) or above about 0.25%, or high phosphorus, lets the zinc-iron alloy grow through to the surface, giving a thicker, matte grey coating. It protects as well or better, but it looks different and can be more brittle.
- Does hot-dip galvanizing warp steel?
- It can. The 450 °C dip releases residual stress from rolling and welding, and uneven heating of thin and thick members distorts the part. Symmetric designs, similar section thicknesses, balanced welds and a single dip keep distortion down; ASTM A384 is the guide.
