Ask a fastener engineer what kills bolts in the field and the answer is rarely the steel. It is rust. A corroded bolt can lose a serious share of its load-bearing section long before anything looks wrong from the outside, and the bill arrives later as seized threads, seasonal re-torquing, or an unplanned maintenance call. This is why the surface treatment decision belongs at the purchase-order stage, not on the job site.

FLRS supplies fasteners in four main finishes: zinc plating, hot-dip galvanizing (HDG), PTFE coating, and Dacromet zinc-flake coating. This guide compares them on the numbers that actually drive the choice: salt spray hours, coating thickness, temperature limits, hydrogen embrittlement risk, and cost.

Zinc plating deposits a thin metallic zinc layer electrochemically, then seals it with a chromate passivation film that delays white rust. Thickness runs 5–25 μm, and salt spray performance per ASTM B117 or ISO 9227 lands between 24 and 120 hours depending on the passivation grade. Service temperature tops out around 120 °C, and it is the cheapest option in this lineup.

It is also the most forgiving on tolerances. The coating is thin and uniform enough that thread fit is unaffected, which is why zinc plating remains the default for indoor machinery, electrical enclosures, and general steel fabrication.

One thing to watch: the electrolytic process charges the steel with hydrogen. On Grade 10.9 and above, trapped hydrogen can cause delayed brittle fracture unless parts are baked after plating per ISO 4042. FLRS bakes all 10.9 and 12.9 electro-zinc product as standard practice.

Hot-dip galvanizing works differently. Cleaned fasteners go into a molten zinc bath at about 450 °C, where iron and zinc react to form alloy layers topped with pure zinc. The result is thick: 40–120 μm, five to ten times an electroplated layer, good for 500 to more than 3,000 salt spray hours and 10–30 years of outdoor service. Continuous service temperature is around 200 °C.

For outdoor structures such as bridges, transmission towers, port cranes, and solar mounting systems, HDG gives the most protection per dollar spent, especially where the joint will be hard to reach for maintenance later.

The trade-off is dimensional. A coating that thick fills thread gaps, so HDG bolts often need cleared threads or matched HDG nuts with oversized tapping. Fine threads and tight tolerances can suffer. FLRS handles this by supplying HDG fasteners with threads cleared or with paired HDG nuts so assemblies fit up correctly.

Dacromet is a water-based slurry of zinc and aluminum flakes, applied by dip-spin or spray and cured at high temperature into a dense lamellar film. The coating is thin, 5–10 μm with no effect on thread accuracy, yet it performs far above its thickness: 500 to over 1,200 salt spray hours, and it holds up past 300 °C.

The decisive point for many buyers is what is absent from the process: electricity. Because Dacromet is non-electrolytic, there is no hydrogen embrittlement risk at all. That, plus a stable torque-tension relationship under preload assembly, makes it the default finish for Grade 10.9 and above structural joints, automotive underbody hardware, wind turbine bolted connections, rail transit parts, and marine applications where a fractured bolt is not an acceptable failure mode.

One procurement note: conventional Dacromet chemistry contains hexavalent chromium. Where RoHS or REACH compliance applies, FLRS supplies chromium-free zinc-flake systems of equivalent performance (Geomet-type) on request.

PTFE is a sprayed and cured fluoropolymer film, 20–50 μm thick. Its selling points are chemical inertness and an extremely low coefficient of friction, around 0.05–0.10, which prevents galling and seizing even across repeated assembly cycles. Continuous service reaches about 260 °C, 290 °C short-term. Corrosion resistance is excellent but depends heavily on the substrate underneath the film.

Chemical plants, cryogenic systems, high-temperature flanges, and valves are the natural territory for PTFE-coated fasteners. The coating is relatively soft, though. Impact and abrasion during handling and installation can damage it, and for long-term outdoor exposure it is best paired with a corrosion-resistant substrate such as stainless steel rather than relied on alone.

Criteria	Zinc Plating	Hot-Dip Galvanizing	Dacromet	PTFE Coating
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Coating thickness	5–25 μm	40–120 μm	5–10 μm	20–50 μm
Salt spray (ASTM B117)	24–120 h	500–3,000+ h	500–1,200+ h	Excellent*
Max service temp.	~120 °C	~200 °C	300 °C+	260 °C
Hydrogen embrittlement risk	Yes (baking required for ≥10.9)	No	No	No
Thread precision impact	Minimal	Significant (may need re-tap)	Minimal	Moderate
Relative cost	Low	Moderate–High	High	High
Ideal environment	Indoor / dry	Outdoor / structural	Automotive / wind / marine	Chemical / high-temp

*PTFE corrosion resistance depends heavily on the substrate beneath the coating.

Every surface-treated batch from our Xi'an facility carries documented quality control. Salt spray testing follows ASTM B117 or ISO 9227, with reports available on request. Coating thickness is measured against order specifications by magnetic or eddy-current gauge. Adhesion is checked per ISO 2409 or ASTM D3359, and coated high-strength assemblies get torque-tension validation.

Paperwork matches the hardware. Each shipment can carry material certificates to EN 10204 3.1 or 3.2 with full chemical composition and mechanical results, plus dimensional inspection and salt spray records. Third-party inspection through SGS, Bureau Veritas, or TÜV can be arranged on request. Our in-house laboratory runs hardness, tensile, and yield-strength tests to confirm the substrate is unchanged after coating.

Zinc plating for indoor cost and precision. HDG when the structure lives outdoors for decades. Dacromet where high-strength joints cannot tolerate embrittlement risk. PTFE where chemicals and galling are the enemy. Most real projects sit between these cases, so if you are weighing options, send FLRS the operating environment, strength grade, and budget, and we will recommend a finish, back it with sample bolts, and attach the full test data.

About FLRS Shaanxi Flourish Industrial Co., Ltd. is a Xi'an-based manufacturer and exporter of industrial fasteners, flanges, pipe fittings, and pressure-vessel components. We produce to German, American, and ISO standards with grades from 4.8 to 12.9 in carbon steel, alloy steel, and stainless steel SS303/304/316. All products support custom drawing production, full material traceability, and third-party inspection.

• Website: https://mechanical-fasteners.com/
• Tel: +86-29-86613868
• Email: flrs@mechanical-fasteners.com