Fastener Surface Finishes Explained: Zinc, Dacromet, Galvanizing & More
The finish decides how long your fasteners survive. This guide compares zinc plating, hot-dip galvanizing, Dacromet, black oxide, and other common fastener finishes.
Why finish matters
Bare steel rusts. The surface finish is what protects the fastener — and determines corrosion resistance, appearance, friction behavior, and cost. Selecting the wrong finish is one of the most common specification mistakes we see in RFQs.
Zinc plating — the everyday choice
Electroplated zinc is the most economical protective finish, offering 72-120 hours of salt spray resistance at typical thicknesses (5-8 µm). It works for indoor and moderate outdoor use. Clear, blue, and yellow (trivalent passivation) versions are available. For 10.9 and 12.9 high-strength bolts, zinc plating must be controlled to avoid hydrogen embrittlement.
Hot-dip galvanizing — outdoor heavy duty
HDG applies a thick zinc layer (45-85 µm) by dipping the fastener in molten zinc. It delivers 500+ hours of salt spray resistance and is the standard for structural steel, transmission towers, and marine-adjacent applications. The trade-off: thread clearance must be accounted for (parts are commonly over-tapped), and the matte grey finish is not cosmetic.
Dacromet — thin and corrosion-proof
Dacromet is a zinc-aluminum flake coating applied by dipping and baking. A thin 8-10 µm layer delivers 500-1000+ hours of salt spray resistance with no hydrogen embrittlement risk — making it ideal for high-strength bolts (10.9/12.9) and automotive fasteners. It is the premium choice when salt spray hours matter.
Black oxide — appearance, not protection
Black oxide (blackening) gives a clean black appearance with minimal dimensional change and almost no added corrosion resistance. It suits indoor precision parts where look matters and lubrication is helpful. Treat it as decorative — for outdoor use, combine with oil or wax.
Other common finishes
Nickel plating: bright, corrosion-resistant, common for decorative hardware and electronics. Passivation: applied to stainless steel to enhance its natural oxide layer — essential for food and medical applications. Zinc-nickel (Zn-Ni): a newer alloy plating offering 1000+ hours for automotive underhood parts.
How to specify the finish in an RFQ
State the finish AND the standard: for example, "zinc plated, clear passivation, 8 µm, per ISO 4042" or "Dacromet, salt spray 720h". Specifying only "zinc plated" leaves the thickness and passivation to the factory's discretion. At Taily Metal we run salt spray tests per batch and provide reports with every shipment.
Unsure which finish you need?
Tell us the environment, expected service life, and any plating standards. Email [email protected] or use the contact form — we will recommend the most cost-effective finish that meets your salt spray requirement.
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