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How to choose fastener surface treatment? Zinc, Dacromet, hot-dip galvanising and black oxide compared

2026-09-12

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

Why surface treatment is key in fastener selection

The same fastener can last several times longer — or fail within months — depending on its surface treatment. Outdoor humidity, coastal salt spray and chemical environments corrode unprotected carbon steel screws within months; electronics need low-permeability finishes that do not interfere with signals; automotive underbodies and rail vehicles prioritise heat and vibration corrosion resistance. Choosing the right fastener surface treatment is not just about looks: it keeps bolts, nuts and washers delivering clamping force over the intended service life. When requesting quotations, Hong Kong buyers should state the service environment, temperature range and appearance requirements, otherwise suppliers may default to the cheapest ordinary zinc plating. A useful way to frame the decision is to separate the three functions a coating can perform. The first is barrier protection, where the coating physically seals the steel from moisture and oxygen; zinc plating, hot-dip galvanising and zinc-flake coatings all work this way. The second is sacrificial protection, where the coating is more electrochemically active than the steel and corrodes preferentially at a scratch or cut edge, which is why a galvanised part does not rust through at a drill mark while a painted part will. The third is a specific functional property, such as electrical conductivity for earthing, low outgassing for vacuum service, or a defined friction coefficient for torque-controlled assembly. Most selections are settled by asking which of the three matters most at the specific location, because a finish optimised for one will often be poor at another. It is also worth remembering that a coating is not a substitute for choosing the right base material. A zinc-plated carbon steel screw will never match a 316 stainless screw in a chloride environment, no matter how thick the plating; the coating buys time, and the material decides whether the joint survives the intended service life. Where a project's design life is ten years or more and the site is coastal, the sensible answer is almost always stainless or a hot-dip galvanised part rather than a heavier plating on carbon steel.

Comparison of four common surface treatment processes

Electroplated zinc (white, blue-white or yellow passivated) is about 5-15 micrometre thick, low cost and bright, suited to dry indoor environments with neutral salt-spray life typically 24-96 hours. Hot-dip galvanising per GB/T 13912 reaches 45-85 micrometre or more, giving 500-1,500 hours salt-spray life for outdoor, bridge and power-tower applications. Dacromet at 8-12 micrometre delivers 500-1,000 hours salt-spray resistance and withstands 300 degC, with no hydrogen-embrittlement risk, making it popular for high-strength bolts and automotive parts. Black oxide (blackening) changes dimensions negligibly, gives a matte black finish and good oil retention, but offers weak corrosion protection; it is used for indoor machinery, firearms or anti-glare applications. Selection should balance coating thickness, corrosion rating and temperature limit. Set out as a comparison table by duty: indoor dry, decorative appearance, low cost — electroplated zinc in clear or blue-white, 5-15 micrometre, 24-96 hours NSS; indoor or sheltered with occasional condensation and a requirement for a specific colour — yellow zinc or a trivalent passivate, 8-15 micrometre, 96-200 hours NSS; outdoor general construction with a multi-year design life — hot-dip galvanised to GB/T 13912, 45-85 micrometre, 500-1,500 hours NSS; coastal, chemical or high-temperature duty, and any high-strength bolt above grade 10.9 — Dacromet or a comparable zinc-flake system, 8-12 micrometre, 500-1,000 hours NSS and 300 degC capable; precision machinery needing minimal dimensional change or a specific friction coefficient — black oxide plus oil, or a phosphate finish, with sacrificial protection left to the base material. Two additional notes are easy to overlook. First, hot-dip galvanising is not suitable for small fasteners below roughly M8 or for closely toleranced threads, because the coating thickness closes up the fit and the parts may have to be tapped or chased after coating, which damages the protection exactly where it is needed. Second, a coating that lowers friction changes the torque-preload relationship, so any torque specification must be revisited when the finish changes; this matters most where a joint is specified with a zinc-flake coating on one order and plain zinc on the next.

Practical selection by application environment

For general indoor machinery and furniture, electroplated zinc is sufficient. Outdoor construction, curtain walls and solar racking should prefer hot-dip galvanising or 304 / 316 stainless steel. Coastal, chemical and high-temperature sites suit Dacromet or stainless steel, and direct contact between dissimilar metals should be avoided to prevent galvanic corrosion. For high-strength 10.9 / 12.9 grade bolts needing corrosion protection, Dacromet is safer than electroplated zinc because it avoids hydrogen-embrittlement risk. At acceptance, always request coating-thickness reports and ASTM B117 or ISO 9227 salt-spray data to confirm the surface-treatment class matches the design life. Working through Hong Kong conditions specifically: an air-conditioned plant room or an indoor electrical panel needs nothing more than clear zinc, and specifying anything more expensive wastes budget. A sheltered car park soffit or an open service corridor, where condensation forms overnight but rain never reaches the part, sits in the middle band and is usually served by yellow zinc or a zinc-flake coating. An exposed rooftop plant frame, a canopy or a rooftop solar racking system faces full rain, sun and salt-laden air and should be hot-dip galvanised or stainless. A seafront promenade railing, a pier structure or a coastal pumping station faces chloride deposition throughout the year and justifies 316 stainless for exposed fixings, with Dacromet on any carbon steel structure that cannot practically be made of stainless. A swimming pool hall is a special case: the combination of warm humid air and chloramine vapour attacks standard galvanising quickly, and 316 stainless with a passivated finish is the reliable choice. Where dissimilar metals must be joined, such as a stainless bolt through a galvanised steel bracket, use a nylon or EPDM isolating washer and keep the cathode area small relative to the anode so the galvanised component is not consumed rapidly. For a finish that must be inspected over time, keep a reference sample of each coating type so that site staff can compare colour and texture and spot a coating that is thinning or discolouring earlier than expected. Need advice on fastener surface treatment selection? Contact JIALI Industrial (164.hk) with your environmental conditions and specifications for a fast match.

Common pitfalls and acceptance points in coating selection

Pitfall one is comparing coatings by thickness alone. A thicker coating is not automatically better: hot-dip galvanising is thick but unsuitable for small threads, while an 8 micrometre zinc-flake coating can outperform a 15 micrometre electroplated layer because of the different protection mechanism. Compare by corrosion performance and compatibility, not by micron count. Pitfall two is ordering by colour. Blue-white, yellow and black finishes can all be the same zinc plating with a different passivate, and the colour says nothing about corrosion performance; the passivate type does. Pitfall three is applying an electroplated finish to a high-strength part without considering hydrogen. Grade 10.9 and above should be baked, or better, moved to a zinc-flake coating. Pitfall four is ignoring the effect of the coating on fit. Thick coatings close up thread clearances, and a coated nut may not run freely onto a coated bolt unless the thread allowance is specified. Pitfall five is a salt-spray claim quoted without stating the standard, the test duration, or whether the reported time is to white rust or red rust. A 500-hour claim that refers to white rust is a very different claim from 500 hours to red rust. At acceptance, a practical checklist is: obtain the coating type and thickness report against the specification; obtain salt-spray data that names the standard (ASTM B117 or ISO 9227), the duration and whether white or red rust is the endpoint; check a sample for uniform coverage, including inside the thread and under the head, where coverage is often weakest; confirm the finish does not introduce a hydrogen risk for the property class ordered; and store coated parts dry and separated, because coated fasteners stacked in a damp bin can develop white rust in storage before they are ever installed. For long-life outdoor projects, agree a periodic inspection interval and a touch-up procedure for damaged areas, since a scratch on a galvanised part is recoverable with a suitable zinc-rich repair compound. Need advice on fastener surface treatment selection? Contact JIALI Industrial (164.hk) with your environmental conditions and specifications for a fast match.

How to choose fastener surface treatment? Zinc, Dacromet, hot-dip galvanising and black oxide compared

Why surface treatment is key in fastener selection

Comparison of four common surface treatment processes

Practical selection by application environment

Common pitfalls and acceptance points in coating selection

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