Zinc Plating for Fasteners - How Much Do You Know

Fasteners and gaskets share one job in a bolted joint: hold pressure and keep the environment out of the metal. Zinc plating is the most common protection for bolts, nuts and studs, but the process chosen changes coating thickness, hardness, salt-spray life and cost. Below are the five zinc coating methods used on fasteners — electroplating, hot-dip galvanizing, sherardizing, mechanical plating and Dacromet — and how each one behaves next to flange gaskets in a flanged joint.

Five Zinc Coating Processes at a Glance

Every process builds the zinc layer in a different way, so thickness, appearance and protection life vary widely. The table collects the figures that matter when a drawing calls for a zinc finish.

Thickness, hardness and salt-spray figures are typical process values; confirm the coating specification on the fastener drawing before ordering.

Electroplating and Hot-Dip Galvanizing

Electroplating, also called cold galvanizing, deposits zinc on the steel surface by an electrochemical reaction. The layer is thin, uniform and bright, so dimensional accuracy, appearance and conductivity are preserved — but corrosion resistance is weaker than with thicker coatings. Common baths are cyanide, zincate, chloride and sulfate zinc plating; each has its own balance of cost, brightness and coverage.

Because the coating is thin, electrogalvanized parts are used where fit and looks matter. Chromate passivation produces clear, blue, blue-white or yellow iridescent finishes. The main risk is hydrogen embrittlement, so high-strength bolts must be dehydrogenated promptly after plating.

Hot-dip galvanizing immerses pre-treated fasteners in molten zinc and forms a zinc or zinc-iron alloy layer with a metallurgical bond to the steel. The coating runs 45–55 μm and is hard to control precisely, but it gives the best atmospheric corrosion resistance of the five methods. Because the layer builds up, hot-dip galvanized stud bolts are common on outdoor steel towers, pole structures, photovoltaic racking and exposed pipe flanges.

• Electroplated zinc: thin, uniform and bright, up to 72 hours of neutral salt spray; special sealants push this past 200 hours at about 5–8 times the cost of standard zinc plating.
• Hot-dip galvanizing: 45–55 μm, metallurgical bond, full coverage of recesses and corners, lower processing cost than paint systems, and simple visual or gauge inspection.
• Hydrogen control: electroplating requires prompt dehydrogenation, while the hot-dip process itself creates no such embrittlement risk.
• Trade-offs: electroplated zinc has a short protection life and poor acid and alkali resistance; hot-dip galvanizing is the less environmentally friendly process.

Sherardizing, Mechanical Plating and Dacromet

Sherardizing, or zinc diffusion coating, forms a zinc-iron layer through a diffusion reaction in a drum of heated zinc powder mixed with fillers such as alumina, fireclay and ammonium chloride. The layer is mainly the δ phase (FeZn17) with 7–11 % iron by mass, dense and tough, with corrosion resistance not below hot-dip galvanizing and a hardness around 250 HV.

The coating is uniform and causes neither hydrogen embrittlement nor part distortion, which makes it suitable for nuts, bolts, small hardware and complex shapes. Working conditions are poor, pollution is significant and automation is difficult.

Mechanical zinc plating builds the layer at room temperature by tumbling parts with glass beads, zinc powder, dispersants and accelerators in water. The impact deposits zinc with the physical properties of metallic zinc. There is no hydrogen embrittlement risk for high-strength steels, energy use is low, and small or complex parts are easy to process — but thickness can be uneven and the finish is less smooth and bright than electroplating.

Dacromet is a zinc flake coating made from zinc flakes, aluminium flakes, chromic acid and deionized water. It is applied as a liquid and cured, so it reaches deep holes, narrow gaps and tube interiors that electroplating cannot reach because of electrostatic shielding.

• Dacromet is only 4–8 μm thick, yet its rust protection is 7–10 times better than electroplating, hot-dip galvanizing or paint; treated parts pass over 1200 hours of salt spray without red rust.
• Heat resistance up to 300 °C, while conventional zinc plating starts to peel around 100 °C.
• No hydrogen embrittlement in the process, so it suits stressed, high-strength fasteners.
• Good adhesion to metal and to added paint layers, so treated parts are easy to paint or colour.

Selecting and Installing Zinc-Plated Fasteners

Specifying a zinc finish is a trade-off between thickness, tolerance, appearance, temperature and cost. Three checks prevent most field problems.

• Match the coating to the drawing: electroplating for tight tolerances and appearance, hot-dip galvanizing for the longest outdoor life, sherardizing or mechanical plating for high-strength bolts that must not pick up hydrogen, Dacromet where space is tight and temperature is high.
• Allow for thickness: a 45–55 μm hot-dip layer changes thread fit and hole clearance, while 4–8 μm coatings barely affect dimensions.
• Handle hydrogen properly: dehydrogenate immediately after electroplating, and store finished bolts dry so white rust does not form on fresh zinc.
• Keep inspection simple: visual check plus a magnetic or eddy-current thickness gauge covers most acceptance needs; agree the salt-spray hours per order.

Frequently Asked Questions

Which zinc process avoids hydrogen embrittlement on high-strength bolts?

Mechanical plating, sherardizing and Dacromet generate no hydrogen while the coating forms. Electroplating does, so high-strength bolts must be dehydrogenated immediately after plating.

How thick is a hot-dip galvanized coating on fasteners?

Typically 45–55 μm, and it is hard to control precisely. Electroplated zinc is thinner and more uniform, and Dacromet is only 4–8 μm thick.

How long does electrogalvanized zinc last in a neutral salt spray test?

Up to 72 hours in the standard version. Special sealants extend this beyond 200 hours, at roughly 5–8 times the cost of standard zinc plating.

Which zinc coating handles the highest temperature?

Dacromet resists heat up to 300 °C. Conventional zinc plating starts to peel at around 100 °C.

How hard is a sherardized layer?

About 250 HV. The layer is mainly the δ phase (FeZn17) with 7–11 % iron by mass, which gives good wear resistance.