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Threaded fasteners are galvanized in specialized galvanizing facilities that centrifuge the fasteners to remove excess zinc from the threads. Australian Standard AS1214 recommends the obligatory clearances on nuts for use with galvanized bolts to incorporate the extra coating applied to the threads.
Problems with Threads
The problems associated with the galvanizing of internal threads on nuts are overcome by galvanizing the nut blanks and later tapping them. A satisfactory level of corrosion protection is provided by the close contact of the uncoated steel on the thread of the nut with the galvanized thread on the bolt.
Galvanization of other threaded parts—such as bolt cages, threaded assemblies, tapped holes, and socket attachments—is a challenge for galvanizers, as well as their customers. These items are usually used in fabricated assemblies and may go unusable unless a provision is made to handle the cleaning of the threaded item.
Methods of Handling Threaded Components for Galvanizing
External Thread Galvanizing
While taking out items from the galvanizing bath, the excess molten zinc drains off the items. In the case of threaded items, a large quantity of zinc is trapped in the threads and forms a thick deposit on the bottom side of the thread.
These threads can be cleaned by the galvanizer using the following options:
- The threaded sections can be fettled when the zinc is still in a molten state, to discard the free zinc from the threads.
- The free zinc can be re-melted by heating the threaded section with a gas torch, and then, the thread can be wire-brushed cleanly. This has no impact on the durability of the coating since most of the coating is a zinc-iron alloy with a higher melting point (650 °C versus 420 °C).
- The threads can be re-tapped, which will take away the coating but consume plenty of time, and access difficulties might make it unworkable.
- The threads have to be protected before galvanizing to stop them from being galvanized. This can be carried out using proprietary stop-off materials such as Galvastop. High-temperature tapes from expert suppliers like 3M can be employed to cover threads. These processes will stop the threaded elements from being galvanized.
Internal Thread Galvanizing
Internal threads on tapped holes, nipples, and sockets will always be choked with zinc on the bottom side of the hole as it exits the galvanizing bath. The options for cleaning internal threads are limited to tapping out the threads after galvanization. The ideal method to prevent the build-up of zinc in internal threads is by preventing the zinc from making contact with the threads. This can be performed in the following ways:
- A best-suited, high-temperature silicone sealant can be used in small threaded holes to block off the hole. The sealant can then be mechanically removed after galvanizing.
- A proprietary stop-off product like Galvastop can be used.
- Prior to galvanizing, a stud or bolt can be inserted into the hole. This can be removed after galvanizing and may require heating with a gas torch to loosen the fastener to screw it out.
Special Threaded Items
Larger manufactured threaded items such as post-tensioning rods, roof bolts, threaded rods, and foundation bolts can be effectively galvanized if the volume is high enough to justify the installation cost for dedicated thread cleaning operations.
This is performed either by using special, advanced galvanizing equipment that removes the excess zinc from the threads during the galvanizing procedure, or by vibrating the items. It can also be performed with the help of robust post-cleaning systems that remove the excess zinc by heating the threaded sections and then brushing them.
To date, hot-dip galvanization is the most efficient technique to offer a heavy-duty anti-corrosion coating to threaded items. One more added benefit of this process is that zinc has self-lubrication properties, apart from having a hardness equivalent to that of the base steel.