Fastener Surface Treatments Explained
Plain and Black Oxide
Plain fasteners are untreated and are only suitable for dry indoor use or for parts that will be painted after assembly.
Black oxide produces a thin dark film that holds oil well. It is common on socket head screws and machine tools where dimensional accuracy matters more than corrosion resistance.
Zinc Plating and Passivation
Electroplated zinc with a clear or yellow passivation is the most common finish for indoor hardware and general machinery.
A yellow or blue passivation slows the formation of white corrosion products. A black passivation gives the same protection with a dark appearance.
Hot-Dip Galvanizing
Hot-dip galvanizing is the standard for outdoor structural work, giving a thick, hard coating with a long service life.
Because the coating thickens the thread, galvanized nuts are tapped oversize and must be supplied with their matching bolts.
Specialist Coatings
Dacromet is a zinc-aluminium flake coating applied in a dip-spin process. It avoids hydrogen embrittlement, so it suits high grade bolts such as 10.9 and 12.9.
PTFE and Xylan coatings reduce friction and resist chemicals, useful for valves, pumps and food equipment.
For the most aggressive environments, stainless steel or a duplex fastener removes the need for a coating altogether.
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