Technical article · 5 min read
Fastener surface finishes compared
Zinc, zinc-flake, phosphate, nickel: each finish changes corrosion resistance, appearance — and the friction in the thread.
A finish protects the steel, sets the look of the part and changes the thread friction, which decides how much clamp load you get from a given tightening torque. Choose it with all three in mind.
Electroplated zinc with passivation
A thin zinc layer with a trivalent passivate (clear/blue, yellow or black). Economical and widely used. On high-strength parts (around 10.9 and above, or hardened parts) electroplating can cause hydrogen embrittlement, so parts are baked after plating to drive the hydrogen out.
Zinc-flake coatings
Zinc and aluminium flakes in a binder, dip-spun and cured (Geomet and Magni are well-known systems). No electrolytic hydrogen, high corrosion resistance at low thickness, and a controlled friction coefficient — common on wheel fasteners and chassis parts.
Phosphate and oil
Manganese or zinc phosphate with oil gives light corrosion protection and steady friction, and is a good base for paint. Typical on engine fasteners such as cylinder head bolts.
Nickel and nickel-chrome
Bright, hard and decorative — used where the part is seen, such as wheel nuts and furniture fittings.
Standards and values are summarised for general guidance — always work to your drawing and the current edition of the standard. Related REL page.
