Coating
There are many finishes or coating applied to fasteners; some corrosion protective, some decorative, or there may be no added coating at all.
(Black - Self Colour)
- An 'as produced' finish on carbon steel products having an oil residue which provides some shelf life but no real corrosion protection when in use. Today, less than 20% of carbon steel fasteners would be purchased plain finish.
- Stainless steel, brass and other non-ferrous materials protect themselves through a reaction of the surface to oxygen, creating a protective chromium oxide film.
Zinc Plated
- The most economic and common fastener finish, comprising a thin coating of zinc applied either by electroplating or mechanically. A shiny silver grey appearance, it will normally be enhanced by a chemical chromate passivation conversion which applies a harder surface film. This can be clear (bluish tinge), or iridescent yellow which is thicker and gives marginally better protection.
Clear is referred to as zinc, zinc clear, blue zinc.
Yellow is referred to as zinc plate gold (ZPG), zinc yellow chromate (ZYC), zinc di-chromate, zinc yellow pass.
Cadmium Plated
- Formerly a popular electroplated or mechanically applied finish, looking like but giving slightly better protection than zinc and providing increased lubricity; also chromate converted. Very seldom used today due to its toxicity and environmental non-acceptability. If specified, it is usually through habit, error or ignorance and possible confusion with zinc.
Galvanised
- A very heavy coating of zinc applied by hot dipping in a bath of molten zinc, then centrifuge spinning for even distribution and removal of the excess, or mechanically cold welding a zinc powder in a barrel rumbling process. The hot dip finish is rougher and duller than electroplated finishes but because of the thickness achieved, gives considerably enhanced protection. Often it is wax coated to provide assembly lubrication.
Phosphate
- A thin, dull grey phosphate coating obtained by insertion in a solution containing phosphoric acid. Gives a lower level of protection than zinc in mild environments, but gives an excellent base for painting or organic lubrication. Often used in automotive industry.