Hot-dip galvanising of bolts to ISO 10684

In hot-dip galvanising, the parts are pickled and then dipped into molten zinc at around 450 °C. Zinc and steel combine to form an alloy layer with pure zinc on top; at 40 to 80 µm and more, the coating is many times thicker than with zinc electroplating and lasts for decades outdoors, in industrial atmospheres and in contact with soil. ISO 10684 covers the hot-dip galvanising of fasteners, designation tZn.

The thread needs room

A coating this thick does not fit into a standard 6g/6H thread. ISO 10684 therefore provides two options: either the external thread of the bolt is made undersize before galvanising, tolerance position 6az, or the nut is tapped oversize after galvanising, tolerance position 6AX; the oversize-tapped nut then remains uncoated in the thread and is protected by the zinc coating of the bolt. Both methods are common in practice, and the oversize-tapped nut is the standard for bolting assemblies. Bolt and nut should therefore be ordered together.

Suitable strength levels

Hot-dip galvanising is suitable without restriction up to property class 8.8, and for 10.9 in accordance with ISO 10684 with requirements for pickling and temperature control, because the zinc bath temperature must not affect the strength of tempered steels; 12.9 is not hot-dip galvanised. Typical parts are anchor bolts, foundation bolts, threaded rods, hex bolts and bolting assemblies to EN 15048 for masts, industrial buildings, bridges, railways and track construction.

How RS Randack goes about it

Hot-dip galvanising at partner companies, threads produced and parts marked beforehand, nuts from our own production tapped oversize, and after return, inspection of coating thickness, free running of the thread and appearance; galvanising certificate with the inspection certificate on request. RS Randack supplies bolting assemblies for load-bearing components with CE marking to EN 15048 and a Declaration of Performance.

What you should specify

Property class, oversize-tapped nut or undersize bolt, required minimum coating thickness, whether the assembly includes washers, Declaration of Performance to EN 15048, quantity and lengths.