Coatings, compared honestly
Six finishes, each with a page that states what it is bad at as plainly as what it is good at. Choosing on price alone is the most common reason a joint fails early.
| Coating | Thickness | Max temp | Salt spray |
|---|---|---|---|
| PTFE fluoropolymer (Xylan 1424) | 25 – 38 µm (hai lop: lot + phu) | 260 °C | 1 000 – 3 000 h (ASTM B117) |
| Hot dip galvanizing | 43 – 86 µm (A153 Class C: trung binh toi thieu 50 µm) | 200 °C | > 1 000 h — nhung tuoi tho thuc te tinh bang chuc nam ngoai troi |
| Electroplated zinc | 5 – 25 µm tuy cap | 120 °C | 12 – 96 h trong xanh · 96 – 200 h cromat vang |
| Zinc flake (Geomet / Dacromet) | 8 – 12 µm | 300 °C | 480 – 1 000 h tuy so lop |
| Phosphate and oil | 5 – 15 µm (photphat mangan) | 200 °C | 24 – 72 h — bao ve tam thoi, khong hon |
| Black oxide | ~ 1 µm | 250 °C | < 24 h — chi khi con lop dau bao ve |
PTFE fluoropolymer (Xylan 1424)
PTFEThe blue coating you see on every offshore flange. A fluoropolymer film that does two jobs at once: it stops corrosion and it gives a low, repeatable friction coefficient so torque actually converts into bolt load.
Offshore and subsea flanges, sour service, stainless bolting, and any joint tightened with a calibrated torque or tensioning tool.
Hot dip galvanizing
HDGStuds dipped in molten zinc at about 450 °C. The thickest, longest-lasting sacrificial coating there is — and the one that forces you to overtap the nut, because 85 µm of zinc does not fit in a standard thread.
Structural steelwork, anchor bolts, pipe supports, transmission towers — anything outdoors, exposed, and not routinely dismantled.
Electroplated zinc
ZnA thin electrodeposited zinc layer, usually 5 to 12 µm, with a chromate passivation on top. Cheap, dimensionally accurate, and honest about what it is: indoor protection, not marine protection.
Indoor equipment, machine assembly, dry utility piping, and anywhere the fastener is not exposed to weather.
Zinc flake (Geomet / Dacromet)
Zn flakeZinc and aluminium flakes in an inorganic binder, cured in an oven rather than plated. Because no electrolysis is involved there is no hydrogen to embrittle the steel — which is why it is the coating of choice for class 10.9 and 12.9.
Class 10.9 and 12.9 fasteners, wind turbine bolting, automotive and any application where hydrogen embrittlement is a stated concern.
Phosphate and oil
Phos + oilA manganese or zinc phosphate conversion layer sealed with oil. It is not really a corrosion coating — it is a running-in surface that holds oil, prevents galling on first assembly, and keeps studs from rusting in the crate.
Insulated or lagged flanges, indoor process piping, and studs that will be coated or painted later.
Black oxide
Black oxideA one-micron black iron oxide conversion layer. It changes the colour and nothing else — treat it as a cosmetic and anti-glare finish that needs oil to resist even light humidity.
Indoor machinery, tooling, jigs and fixtures where appearance matters more than corrosion life.
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