Background The galvanising of threaded fasteners is well established, and is done in specialised galvanising facilities that centrifuge the fasteners to remove excess zinc from the threads. Australian Standards (AS1214) nominates the required clearances on nuts for use with galvanised bolts to accommodate the additional coating applied to the threads. Problems With Threads The problems associated with the galvanising of internal threads on nuts are solved by galvanising the nut blanks and tapping them afterwards. The intimate contact between the galvanised thread on the bolt and the uncoated steel on the thread on the nut provides an acceptable level of protection from corrosion.
The galvanising of other threaded components such as bolt cages and threaded assemblies, tapped holes and socket attachments, is an issue of concern to both galvanisers and their clients. These items are frequently included in fabricated assemblies and may be rendered unserviceable unless provision to deal with the cleaning of threaded item is dealt with.
Methods Of Dealing With Threaded Components For GalvanizingExternal Thread GalvanisingWhen items are withdrawn from the galvanising bath, the excess molten zinc drains off the work. On threaded items, much of this zinc is trapped in the threads and forms a thick build up on the bottom side of the thread.
There are a number of options available to the galvaniser for dealing with the cleaning of these threads. These are:
· Fettling the threaded sections while the zinc is still molten to bump or brush the threaded section to shake the free zinc of the threads.
· Heating the threaded section with a gas torch to re-melt the free zinc and wire brush the thread clean. This does not affect coating durability as most of the coating is a zinc-iron alloy with a higher melting point (650oC versus 420oC) and provided the area is not overheated, only the free zinc will be removed.
· Re-tapping the threads. This will remove the coating and is time consuming and access issues may make it impractical.
· Protecting the threads prior to galvanising to prevent them being galvanised. This can be done with proprietary stop-off materials such as Galvastop. High temperature tapes from specialist suppliers like 3M can be used to mask threads. These procedures will leave the threaded elements ungalvanised.
Internal Thread GalvanisingInternal threads on sockets, nipples and tapped holes will always fill up with zinc on the down side of the hole as it exits the galvanising bath. The options for cleaning internal threads are limited to tapping out the threads after galvanising. The best methods of preventing zinc build-up in internal threads, is by preventing the zinc coming in contact with the threads. This can be done in a number of ways. These include:
· Inserting a stud or bolt in the hole prior to galvanising. This can be removed after galvanising and may require heating with a gas torch to free the fastener and allow it to be screwed out.
· Using a suitable high temperature silicone sealant in small threaded holes that can be used to block off the hole. This can be mechanically removed after galvanising.
· Using a proprietary stop-off product such as Galvastop.
Special Threaded ItemsGalvanising of larger manufactured threaded items such as roof bolts, threaded rod, foundation bolts and post-tensioning rods can be done successfully if the volume justifies the set-up cost for specialised thread cleaning operations.
This is done by either installing special progressive galvanising equipment that removes the excess zinc from the threads as part of the galvanising process, efficient post cleaning systems that remove the excess zinc by heating the threaded sections and brushing, or vibrating the items.
ConclusionHot dip galvanising remains the best way to provide a heavy duty anti-corrosion coating to threaded items, with the additional advantage that the zinc has self-lubrication properties along with a hardness that equals or exceeds that of the base steel.