Industry News

Dacromet vs zinc plating for outdoor fasteners

2026-07-18

Compiled by JIALI Industrial (164.hk) — a Hong Kong fastener supplier specialising in screws, bolts, nuts and selection know-how.

Zinc plating and what it can and cannot do

Low cost and basic rust proofing make zinc plating the best value for indoor work and general outdoor service. The process is electroplating: the part is cleaned, pickled, plated in a zinc electrolyte and then passivated, producing an even bright coating typically 5 to 15 micrometres thick. Performance depends almost entirely on that thickness and on the passivation system. Bright or blue-white zinc at 5 to 8 micrometres gives white rust at 12 to 24 hours of neutral salt spray and red rust at 24 to 48 hours. Clear trivalent passivation at 8 to 12 micrometres extends that to roughly 24 to 48 hours of white rust and 48 to 96 hours of red rust. Yellow or black passivation at 12 to 15 micrometres reaches 48 to 96 hours of white rust and 96 to 200 hours of red rust. These figures set the boundary of the process very clearly: good enough for indoor equipment, cabinets, enclosures, furniture and machinery in dry or oily conditions, and not good enough for coastal exposure where the required life is measured in years. Two further limitations matter. Thickness is limited by the plating process itself, so much above 15 micrometres is impractical and the edges and recesses of a complex part will always carry less coating than a flat face — and it is those thin areas that corrode first. And the process involves acid pickling and hydrogen generation, which is why parts above roughly 320 HV, meaning 10.9 and 12.9 grade, need a controlled de-embrittlement bake within four hours of plating. Zinc plating remains the right answer for a large share of fasteners; the mistake is using it where the environment demands more.

Dacromet and how it differs

Better salt-spray performance and no hydrogen embrittlement make Dacromet the choice for automotive, outdoor and demanding work. Dacromet, and its zinc-flake relatives such as Geomet, is not electroplating at all: the part is dipped or sprayed in a dispersion of zinc and aluminium flakes in a chromate binder and then cured at around 300 degrees Celsius, forming a dense layered coating typically 8 to 12 micrometres thick. Three properties follow from that process. First, salt-spray performance is far higher per unit thickness, giving red rust at roughly 500 to 1000 hours against 24 to 96 hours for a comparable electroplated layer, because the overlapping flake structure is a much better barrier than a continuous metal layer. Second, there is no hydrogen embrittlement risk, since the process involves neither acid pickling of the finished part nor electrolytic hydrogen generation, which makes Dacromet the standard choice for 10.9 and 12.9 grade high-strength bolts. Third, heat resistance reaches about 300 degrees Celsius, meaning the coating survives conditions where zinc would oxidise rapidly, and it also resists alkaline and chloride attack better than plain zinc. The trade-offs are equally clear. Unit cost runs higher than zinc plating, roughly two to four times for a simple part. The coating is not as bright, so it is not chosen for decorative work. Electrical conductivity is lower than zinc plating, which matters for earthing components. And because it is a thin dip coating, thread tolerances must be considered, since a coating on a closely toleranced thread can interfere with assembly — which is why the coating thickness must be agreed with the thread class at the order stage rather than afterwards.

Scenario by scenario selection

Comparing the two processes against real requirements makes the trade-off concrete. Indoor machinery and furniture in a dry factory — zinc plating at 5 to 8 micrometres, because the environment does not justify the cost of a premium coating. General outdoor service with rain washing and low chloride — zinc plating at 12 to 15 micrometres with yellow passivation, or Dacromet where a longer interval between inspections is wanted. Coastal outdoor within a few kilometres of the sea — Dacromet or hot-dip galvanising, since zinc plating at any thickness will not survive the chloride load. Automotive underbody and chassis components — Dacromet or zinc flake, which tolerate road salt and frequent wetting and avoid embrittlement. High-strength bolts at 10.9 grade and above — Dacromet as the default, because electroplating introduces a hydrogen embrittlement risk that cannot be fully eliminated by baking. Springs and other hardened parts above 320 HV — Dacromet or mechanical zinc, for the same reason. Equipment exposed to temperatures up to 200 to 300 degrees Celsius — Dacromet, since zinc oxidises rapidly at elevated temperature. Decorative visible fasteners on interior fittings — bright zinc or nickel plating, where appearance governs and Dacromet would be specified for a quality the application does not need. Parts that must carry electrical current or bond a circuit — zinc plating, which conducts where a flake coating does not. And for a project with a mix of positions, the sensible approach is usually to use zinc plating on the sheltered and internal work and Dacromet on the exposed and high-strength work, rather than one coating everywhere.

Pitfalls and inspection checklist

Three pitfalls recur with these coatings. Substituting plain zinc plating for Dacromet to reduce cost, which typically shows up as red rust within the first year and converts a planned maintenance interval into an unplanned repair. Specifying Dacromet on a closely toleranced thread without allowing for coating build-up, so that the assembled nut binds or the bolt cannot reach its torque. And assuming the salt-spray figure alone settles the choice, when the coating thickness, the thread class and the substrate hardness all constrain what can actually be supplied. Three further traps are worth noting: ignoring coating thickness on threads and recesses, where the coating is thinnest and corrosion starts; treating a supplier's generic salt-spray statement as evidence for the specific part, when the test should relate to the coating system actually used; and assuming that a Dacromet part needs no handling care, when the cured coating can still be chipped by impact and the damage then concentrates corrosion. At incoming inspection, work through five checks. Confirm the coating system and its class against the order, since bright appearance alone does not distinguish Dacromet from zinc. Take a coating thickness reading on a sample, using the specified method, and check at least one thread or recess position rather than only a flat face. Request the salt-spray report that relates to the system supplied, and note the test duration so it can be compared with the design life. For 10.9 and 12.9 grade parts, confirm that the plating route avoided electroplating or that de-embrittlement records exist. And carry out a trial assembly with the mating nut or tapped hole before the batch is released, since coating build-up on threads is the failure that survives every paperwork check. Need advice on Dacromet versus zinc plating, or a salt-spray report for a specific part? Contact JIALI Industrial (164.hk) with your environment and service life requirement.

Dacromet vs zinc plating for outdoor fasteners

Zinc plating and what it can and cannot do

Dacromet and how it differs

Scenario by scenario selection

Pitfalls and inspection checklist

Related Reading