Galvanized fasteners are recommended for use with hot dip galvanized subassemblies and assemblies. Galvanized nuts, bolts and screws in common sizes are readily available from commercial suppliers.
Bolted assemblies should be sent to the galvanizer while in the disassembled condition. Nuts and bolts or studs to be galvanized should also be supplied disassembled.
When the item to be galvanized incorporates threaded assemblies, the pitch diameter of the female threads must be increased to permit hand assembly after the addition of zinc to the male threads of the mating part.
Bolts are thus completely galvanized, but internal threads of nuts must be tapped oversize after galvanizing to accommodate the increased diameter of the bolts. While tapping or retapping of the nuts after galvanizing results in an uncoated female thread, the zinc coating on the engaged male thread will protect both components from corrosion. For economy, nuts are usually galvanized as blanks and the threads tapped oversize after galvanizing.
To remove excess zinc and produce smoother coatings, small parts including fasteners are centrifuged in special equipment when they are removed from the galvanizing bath.
Items too long or too large to be centrifuged, such as long threaded rods, may be wire brushed while hot to remove any excess zinc from threads.
Studs welded to assemblies may have to be cleaned after the assembly has cooled. This requires reheating with an acetylene torch and wire brushing to remove excess zinc. Alternatives to welded studs should be considered when possible.
Masking to prevent galvanizing threads on pipe or fittings is very difficult. The recommended practice is to clean or tap after galvanizing.
Anchoring devices (such as threaded rods and anchor bolts) are sometimes specified to be galvanized in the threaded areas only or in the areas to be exposed above ground. This can be more expensive than galvanizing the complete unit because of the additional handling required. Complete galvanizing can be specified for items to be anchored in concrete. Research has proved the high bond strength and performance of galvanized steel in concrete.
Tapped-through holes must be retapped oversize after galvanizing if they are to contain a galvanized bolt after assembly. Tapping of all holes after galvanizing is recommended to eliminate double tapping costs and the possibility of cross threading.
The recommended over tapping for nuts and interior threads is as follows:
|Bolt or Stud Size Diameter, inches||Minimum Overtapping of Female Threads, inches*|
|7/16 and smaller||0.016|
|Over 7/16 to 1||0.021|
|* Applies to both pitch and minor diameters, minimum and maximum limits.|
On threads over 1 & 1/2 inches it is often more practical, if design strength allows, to have the male thread cut 0.031 inches (0.79 mm) undersize before galvanizing so a standard tap can be used on the nut. ASTM Specification A563 (A563M) “Carbon and Alloy Steel Nuts” discuss the required minimum diametral amount of overtapping of nuts used with hot dip galvanized bolts. (Note: overtapping allowances contained in A563 as of this printing are undergoing review and revision.)
Manufacturers of threaded parts recognize that special procedures must be followed in their plants where certain items are to be galvanized. Following are some examples: