Fasteners, Bolts & Nails
Perhaps the most critical choice made in the design of connections is how to protect the fasteners from corrosion. Hot-dip galvanizing delivers long-term, maintenance-free corrosion protection in a variety of exposure conditions. Whether used in atmospheric, concrete, soil, or fresh- or salt-water applications, hot-dip galvanizing delivers maximum service life. There will be no difficult field repairs and no labor and material costs normally associated with replacing unprotected fasteners.
Left unprotected, steel will corrode and suffer loss of mechanical properties and integrity that is very dangerous when it happens to fasteners. Hot-dip galvanizing prevents corrosion by coating steel with zinc. The galvanized coating is metallurgically bonded to the underlying steel, forming an impervious barrier between the steel substrate and the corrosive environment. The hot-dip galvanized coating also preferentially corrodes to protect the fastener’s underlying steel and is able to protect small areas of the fastener’s steel that may become exposed when mechanically damaged. Hot-dip galvanizing is the most effective method for delivering both long-term barrier protection and cathodic protection.
Hot-dip galvanized fasteners are produced in the same galvanizing process, but are centrifuged in special equipment or spun when they are removed from the galvanizing bath to remove excess zinc. Items too long or too large to centrifuge, such as long threaded rods, may be brushed while hot to remove any excess zinc from the threads. Studs welded to assemblies may have to be cleaned after the assembly has cooled. This requires reheating with an acetylene torch and brushing to remove excess zinc. Alternatives to welded studs should be considered when possible.
The hot-dip galvanized process delivers an excellent fastener for a variety of reasons:
- Complete & consistent coverage – the complete immersion in molten zinc ensures excellent corrosion protection for 100% of the exposed surfaces, with a consistent zinc thickness.
- Coating thickness – hot-dip fasteners, depending upon diameter, will have from 1.7 to 3.4 (43 to 86 microns) mils of impervious zinc coating. Zinc-plated fasteners have a thin zinc coating, up to only 1 mil (25 microns).
- Cathodic protection – unlike other barrier coatings, such as paint, zinc is a sacrificial metal and preferentially corrodes to protect the underlying steel.
- Bond strength – hot-dip fasteners have a coating bond strength in the range of 3600 psi (24.82 MPa). The coating is extremely difficult to damage. Painted and zincplated coatings have a typical bond strength of only a few hundred PSI and are easily scratched and removed.
- Hardness – hot-dip fasteners have ironzinc alloy layers in the coating that are almost