"To accommodate the thickness of zinc when galvanizing threaded components, extra clearance must be provided on the female threads."
Galvanized nuts and bolts
As a general rule, nuts, bolts and washers down to 8 mm diameter can be galvanized and a wide range of threaded components can now be processed using special equipment. For ISO metric fasteners, the galvanizing of one thread either internal or external requires an extra clearance of four times the coating thickness.
In practice, it is normal for standard bolts from stock to be fully galvanized, but for nuts to be galvanized as blanks and then tapped up to 0.4 mm oversize with the threads then lightly oiled. When assembled, the nut thread is protected by contact with the coating on the bolt.
Even after many years of service, galvanized nuts can readily be unfastened even though the threads have never been galvanized.
Nuts and bolts coating uniformity
Nuts and Bolts Surface finish and appearance
Galvanized fasteners usually have a bright light grey appearance, but with certain grades of high yield and high tensile bolts, the coating may be matt grey because the higher silicon content of the steel makes them more reactive towards the molten zinc.
Fasteners hot dip galvanized at high temperature (around 550°C) tend to take on a uniform, matt grey appearance due to the structure of the coating formed when the component is cooled.
"Galvanized fasteners should be stored under dry, well-ventilated conditions to minimise the occurrence of wet storage stain."
Specification for hot dip galvanized fasteners
| Type | Coating Thickness |
|---|---|
| Zinc electroplated | 5-12 µm |
| Sherardized | 15-30 µm |
| Hot dip galvanized EN ISO 10684 | 50 µm (min) |
Costs of Nuts and Bolts
Relative costs
The initial cost of hot dip galvanizing threaded fasteners is generally a little higher than zinc plating. However, in terms of cost per year of rust-free life, hot dip galvanizing is by far the most economical coating.
Can high strength bolts be galvanised?
Friction grip connections
Most recent testing of galvanized products has shown that a value for the coefficient of friction of circa 0.3 can normally be achieved and therefore additional treatment is not usually required. As slip commences, however, friction rapidly builds up and ‘lock-up’ occurs due to cold welding between the coated surfaces.
If a small amount of slip can be tolerated it is therefore unnecessary to treat the surfaces, but if all slip must be avoided, the coefficient of friction can be raised by roughening the surface of the galvanized coating.
The coefficient of friction can be raised by wire brushing (up to 0.5), while sweep blasting or use of a pneumatic chisel hammer or needle gun may achieve a value in the range 0.5-0.6.
In the United States, galvanizing is one of the few coatings permitted on the contact surfaces in the specification for friction grip joints. This is approved by the Research Council for Riveted and Bolted Structural Joints of the Engineering Foundation.
The ‘lock-up’ effect described above can cause galling in the threads of galvanized fasteners used in friction grip connections and lubrication may be required in order that the required clamping force is developed. Beeswax has been found to be a most effective lubricant and molybdenum disulphide, or tallow, have also been specified for this purpose.
Related reading
How long will galvanizing last in your area?
Galvanized Steel: Solution for the Circular Economy?
