RapidBolt
The short answer
Choose hot-dip galvanising for long-life structural bolting that is rarely disturbed. Choose GEOMET zinc-flake where you need high salt-spray performance without hydrogen embrittlement risk or thread clearance problems. Choose Xylan or PTFE fluoropolymer where joints are broken and remade often, or where galling and consistent torque control matter more than raw barrier thickness.
The coating question is rarely "which is best". It is "how often will this joint be broken, and what happens to the asset if the nut will not come off".
Side-by-side comparison
| Criterion | Xylan / PTFE | GEOMET (zinc flake) | Hot-Dip Galvanised |
|---|---|---|---|
| Type | Fluoropolymer film | Zinc/aluminium flake, chrome-free | Metallurgically bonded zinc |
| Typical thickness | 15–40 µm | 8–12 µm | 45–85 µm |
| Salt spray (indicative) | 500–1,500 h | 720–1,000 h | Very high, thickness dependent |
| Friction control | Excellent, low & consistent | Good, controllable | Poor, variable — galling risk |
| Anti-galling / breakout | Excellent | Moderate | Poor |
| Hydrogen embrittlement risk | None (no plating) | None (no plating) | Low, process dependent |
| Thread fit impact | Minimal | Minimal | Significant — nuts must be tapped oversize |
| Max service temperature | ~250°C (grade dependent) | ~300°C | ~200°C |
| Best suited to | High-cycle process joints | Offshore & coastal fasteners | Structural steel, anchors, tank saddles |
Fluoropolymer: Xylan and PTFE
Fluoropolymer coatings such as Xylan 1070 and Xylan 1424 combine a resin binder with PTFE lubricant. Their headline benefit is not barrier protection — it is friction control. A consistent, low coefficient of friction means that applied torque translates predictably into bolt preload, which matters enormously on gasketed joints where uneven load causes leaks.
The second benefit is breakout. On flanges that are opened every shutdown — heat exchangers, filter housings, valve bonnets, LPG filling manifolds — a Xylan-coated stud will still undo years later. A bare or galvanised stud in the same duty often has to be cut off, which turns a two-hour job into a two-day job.
Zinc flake: GEOMET 321 and 500
GEOMET is a chrome-free zinc and aluminium flake system applied by dip-spin and cured. It delivers offshore-grade salt-spray performance in a film only 8–12 µm thick, so thread fit is essentially unaffected and no oversize tapping is needed. Because it is not electroplated, there is no hydrogen embrittlement risk — which is why it is widely accepted on high-strength fasteners where electroplated zinc is prohibited.
GEOMET is often topcoated to add lubricity and colour coding. This is the sensible default for exposed offshore and coastal bolting where you want maximum corrosion life without the dimensional consequences of galvanising.
Hot-dip galvanising
HDG remains unbeaten on cost per year of life for structural bolting. The zinc layer is metallurgically bonded and thick enough to provide decades of sacrificial protection, which is why it dominates infrastructure and civil applications, tank saddles, pipe supports and anchor bolts.
Its weaknesses are dimensional and tribological. The coating is thick and uneven, so nuts must be tapped oversize, which reduces thread engagement. Friction is high and inconsistent, so torque-to-preload correlation is poor. And zinc-on-zinc threads gall readily, making disassembly unreliable. HDG is the right choice for bolts you install once, not for joints you service.
Selection by service condition
- Coastal Australian plant, exposed, rarely disturbed → HDG or GEOMET 500
- Offshore topsides, splash zone adjacent → GEOMET with topcoat, or a corrosion-resistant alloy instead of a coating
- Heat exchanger and shutdown flanges → Xylan 1424
- LPG filling and decant manifolds (high cycle) → Xylan or PTFE — see LPG & gas distribution
- Structural steel, anchors, saddles → HDG to AS/NZS 4680
- Above 300°C → no organic coating; use an appropriate alloy or a ceramic/aluminium system
The mistake that causes most coating failures
Specifying a coating by name without specifying the exposure. "Coated" on a purchase order tells the supplier nothing. A useful specification states the atmosphere (coastal, offshore, buried, enclosed), the service temperature, how often the joint will be broken, and whether torque control is critical. With those four facts, coating selection is straightforward.
It is also worth remembering that a coating is a compromise. Where the environment is genuinely aggressive and the joint is critical, the honest answer is often to change the base material rather than the coating — to duplex or super duplex or a nickel alloy.
How EMG specifies and supplies coatings
We supply coated fasteners across fluoropolymer, zinc-flake, electroplated, hot-dip galvanised, ceramic and pre-treatment systems, with coating certification supplied alongside EN 10204 3.1 material certification. Where a project specification names a coating that does not suit the duty, we will say so before manufacture rather than after installation.
Send us the exposure, temperature and maintenance interval for your joint and request a technical quote — we will recommend the coating system and back it with the reasoning.
