Hot-dip galvanizing: process, coating thickness and threaded parts

Process steps

Galvanizing is a batch process applied after fabrication. Zinc only reacts with clean steel, so most of the process is surface preparation.

• Degreasing in a hot alkaline or acid cleaner to remove oil, dirt and marking paint.
• Pickling in dilute hydrochloric or sulfuric acid to remove mill scale and rust, or abrasive blast cleaning. Weld slag and paint do not pickle off and must be removed beforehand.
• Fluxing, usually in zinc ammonium chloride, to remove remaining oxide and protect the surface until dipping.
• Immersion in molten zinc at about 435–455 °C until the steel reaches bath temperature and the coating forms.
• Withdrawal and removal of excess zinc. Small parts and fasteners are spun in a centrifuge so threads stay clear.
• Cooling in air or a water quench, which may contain a passivating solution, then inspection of thickness and appearance.

The metallurgical bond

In the bath, iron and zinc diffuse into each other and form a series of zinc–iron intermetallic layers, normally finished with a layer of nearly pure zinc. The coating is part of the steel surface, not a film stuck to it.

The intermetallic layers are harder than the base steel, which gives the coating good resistance to abrasion in handling. Because the coating grows from the surface, edges, corners and the insides of hollow sections are coated where molten zinc can reach and drain.

Zinc protects steel in two ways. It forms a barrier that corrodes far more slowly than steel, and it corrodes preferentially at scratches and cut edges, protecting the exposed steel cathodically.

Coating thickness by steel thickness

Thicker steel holds heat longer and reacts longer with the zinc, so the standards set higher minimums for thicker sections.

ISO 1461, used in the UK and EU, sets a local minimum and a higher mean minimum for each steel thickness band. For articles that are not centrifuged, the mean minimum rises from 45 µm on steel under 1.5 mm to 85 µm on steel over 6 mm. Centrifuged threaded articles over 6 mm diameter have a lower requirement, 50 µm mean.

ASTM A123, used in the US, assigns a minimum average thickness grade by product category (structural shapes, strip and bar, plate, pipe, wire, reinforcing bar, forgings and castings) and steel thickness. Grades range from 35 to 100 µm. Hardware and fasteners that are spun are covered instead by ASTM A153, where fasteners over ⅜ in (9.5 mm) need 53 µm (2.1 mils) average, and by ASTM F2329 for fasteners.

JIS H 8641 (2021 revision) designates classes by minimum thickness, from HDZT 35 to HDZT 77 (35 to 77 µm). Earlier editions specified coating mass instead.

Steel chemistry also controls thickness. Reactive steels, particularly those with certain silicon and phosphorus levels, grow thicker and sometimes duller coatings than the minimums.

Service life factors

A galvanized coating is consumed at a fairly steady rate, so time to first maintenance is roughly the coating thickness divided by the zinc corrosion rate of the site.

ISO 9223 groups atmospheres into corrosivity categories with a zinc corrosion rate for each: below 0.1 µm a year in C1 (heated interiors), 0.7–2.1 µm in C3 (many urban and mild coastal sites), 4.2–8.4 µm in C5 and up to 25 µm in CX. On that basis a 100 µm coating in a C3 atmosphere gives an estimated 47 to 143 years to first maintenance.

Local conditions matter. Steel sheltered from rain but exposed to wind-blown salt corrodes faster than open steel nearby. Permanent wetness, soil, water immersion and contact with chemicals need separate assessment.

Threads, fasteners and over-tapped nuts

Bolts, studs and threaded rod are galvanized and then centrifuged to throw excess zinc off the threads. Thread areas are generally not held to the full thickness requirement.

A galvanized external thread is larger than an uncoated one, so it needs clearance. The usual method is to galvanize the nut blank and tap its thread oversize afterwards. The nut thread is then bare, but it is protected by the zinc on the mating bolt. ISO 10684 also allows the alternative of an undersize bolt thread with a standard nut.

ISO 10684 covers coarse threads from M8 to M64 and property classes up to 10.9 for bolts. Galvanizing threads smaller than M8, or with pitches below 1.25 mm, is not recommended. Oversize-tapped M8 and M10 nuts have reduced proof loads.

Very high-strength bolts are not galvanized. ASTM F3125 does not qualify hot-dip or mechanical galvanizing for Grade A490, and ISO 10684 does not extend to class 12.9, because of the risk of hydrogen embrittlement and the effect of the process on properties.