Zeron 100 AFP is the supply state for fasteners that need predictable, repeatable torque-to-tension on duplex-on-duplex bolted joints, the joints that gall when uncoated. The base bolt is Zeron 100 in either SA or FG condition, which is then surface-treated with one of the qualified anti-galling coating systems (Xylan, Xylar, SermaGard, Dacromet, TakeCoat ceramic, or MolyKote). The result is a fastener that retains the full PREN ≥ 40 corrosion envelope of super duplex while reducing thread friction coefficient from ~0.30 (uncoated, dry) to 0.08-0.14 (coated, dry), the difference between "mechanic torques the nut and it seizes mid-makeup" and "mechanic torques the nut to specification and the joint goes home cleanly".
Friction coefficient 0.08-0.14 (vs 0.30 uncoated) · ASTM F1137 coating spec · Anti-galling for repeated-makeup duplex flanges
Where AFP earns its keep: any flange that gets disassembled for routine inspection (offshore deck flanges, ROV-serviced subsea connectors, FGD scrubber inspection ports), any high-cycle structural fastener (FPSO turret swivels, mooring chain stoppers), and any high-torque joint where torque-to-tension predictability matters more than the cheapest installed cost (API 6A wellhead bonnet bolts, NORSOK topside flange bolting). Coating thickness is controlled per ASTM F1137; pre-coat NDT verifies bolt geometry, post-coat dry-film thickness is measured on every lot, and friction coefficient can be measured per ASTM F2328 when buyer specifies.
Duplex stainless steels gall. The same nitrogen-stabilised microstructure that gives Zeron 100 its strength (0.25% N in solution + 50/50 austenite-ferrite) also makes the rolled-thread surface metallurgically active, under high contact pressure, two duplex surfaces in relative motion will weld at the asperity level (cold-welding) and tear when motion continues, producing the characteristic torn-and-galled thread surface. Once a joint galls, you typically can't disassemble without cutting; even on assembly, galling produces unpredictable torque-to-tension because the weld-tear cycle absorbs energy that should be going into bolt stretch.
The coating fix breaks the metal-to-metal contact. A polymer-based coating (Xylan, Xylar) places PTFE-laden resin between the threads; a metallic coating (Dacromet, SermaGard) places a zinc-aluminum or aluminum-flake layer; a ceramic coating (TakeCoat) places an aluminum-oxide film; a lamellar solid (MolyKote) places MoS2 in a binder matrix. All of them reduce the local friction coefficient by 50-70% versus bare duplex, and the energy of the torque-up goes into clean bolt elongation rather than thread destruction.
| Coating | Type | Service temp | μ (typical) | Best for |
|---|---|---|---|---|
| Xylan 1014 | PTFE-loaded fluoropolymer (blue) | -50 to +260°C | 0.10-0.13 | General offshore + onshore sour service |
| Xylan 1070 | PTFE fluoropolymer (black) | -50 to +260°C | 0.10-0.13 | UV-stable variant, surface-mount marine fasteners |
| Xylan 1400 | Modified fluoropolymer (grey) | -50 to +260°C | 0.08-0.11 | High-cycle flanges where lower μ needed |
| Xylan 1424 | High-load fluoropolymer (blue/grey) | -50 to +260°C | 0.08-0.11 | Cryogenic LNG service + sour |
| Xylar 2 / 101 | Inorganic + organic resin | -65 to +260°C | 0.08-0.10 | High-cycle ROV-serviceable subsea fasteners |
| SermaGard 1105 | Ceramic-loaded coating | -50 to +540°C | 0.10-0.14 | FGD scrubbers, refinery furnaces, geothermal |
| SermaGard 1280 | Aluminum-ceramic + topcoat | -50 to +540°C | 0.10-0.14 | High-temp + chloride combined service |
| Dacromet 320 | Zinc-aluminum flake | -40 to +300°C | 0.10-0.13 | Subsea + offshore primary fasteners (cathodic-protection-friendly) |
| Dacromet 500 | Zn-Al flake + sealer | -40 to +300°C | 0.10-0.13 | Heavy-duty marine + offshore |
| TakeCoat 1000 | Ceramic / Al2O3 | up to +1100°C | 0.08-0.12 | Geothermal wellhead, fired equipment, exhaust manifolds |
| MolyKote | MoS2 in binder | -70 to +450°C | 0.08-0.10 | High-pressure flange repeated-makeup, cryogenic LNG |
| Magnigard | Zinc-rich proprietary | -50 to +180°C | 0.10-0.14 | Marine atmosphere splash-zone fasteners |
| Standard | Coverage |
|---|---|
| ASTM F1137 | Zinc/aluminum coating, thickness, adhesion, salt-spray, accelerated corrosion |
| ASTM F2328 | Friction coefficient measurement, method for verifying torque-to-tension |
| ASTM B117 | Salt-spray corrosion test, coating durability acceptance criterion |
| ISO 9227 | Salt-spray test (European equivalent of B117) |
| ISO 15330 | Hydrogen embrittlement test, required for some Dacromet / metallic coating processes |
| NORSOK M-501 | Surface preparation and protective coating, offshore coating system spec |
| NACE MR0175 / ISO 15156-3 | Sour-service hardness limit, coating doesn't change base-metal compliance, only friction |
| EN 10269 | European fastener spec, allows coated CRA fasteners in pressure equipment |
Joint preload is the goal of any bolted connection. The mechanic applies torque, the bolt stretches, and stretch × spring rate = preload. The relationship between applied torque and resulting preload depends on the friction coefficient at the threads and bearing surface. With uncoated super duplex, μ ranges from 0.20 to 0.40 depending on surface finish, lubrication, and number of prior makeups, meaning torque-to-tension can vary 2× for the same wrench reading. With AFP-coated bolts at controlled μ = 0.10±0.02, the torque-to-tension uncertainty drops to roughly ±15%. For a 1500-class flange where target preload is 60% of bolt yield, that's the difference between "all 12 bolts within the 50-70% yield band" and "two bolts at 30% (joint leaks) and three bolts at 95% (close to yield, potential SCC initiation)".
| Service | Typical coating choice | Why |
|---|---|---|
| API 6A wellhead bonnet bolts | Xylan 1424 or Dacromet 320 | HPHT torque + sour + repeated annual disassembly |
| NORSOK M-650 topside flange bolting | Xylan 1014 / 1070 | Standard offshore spec; Equinor / Aker BP buyer-approved |
| FPSO turret / swivel / mooring fasteners | Xylar 2 or Dacromet 500 | Cyclic load + repeated-makeup; long-life durability |
| ROV-serviceable subsea connectors | Xylar 101 or MolyKote | ROV makeup torque must be predictable; one-shot install |
| FGD scrubber + chimney bolting | SermaGard 1105 / 1280 | Wet-flue-gas chloride + 200-300°C; periodic disassembly |
| LNG export terminal flange bolting | Xylan 1424 or MolyKote | Cryogenic + chloride + repeated disassembly |
| Geothermal wellhead + steam separator bolts | TakeCoat 1000 or SermaGard | 500°C+ continuous service |
| Refinery / petrochem high-temp flange bolting | SermaGard 1280 | Up to 540°C continuous + sour-service |
| Splash-zone marine fasteners (offshore platform jackets) | Magnigard | Salt-spray atmosphere + cyclic wet-dry exposure |
No. AFP is a surface treatment on top of an SA or FG bolt. The base-metal MTC (chemistry, mechanical, hardness, Charpy, ferrite, CPT, sigma-phase) is unchanged from the underlying supply state. NACE MR0175 hardness is judged on uncoated thread root before coating. The coating cert is a separate document attached to the same shipment.
Avoid welding within 25 mm of a coated bolt, coating burn-off creates fume (Xylan, MolyKote produce HF and SO2 respectively when burned), local coating loss damages corrosion protection, and HAZ heat may reach the coated zone. If welding is unavoidable, mask the bolt; if structural welding is in the joint design, use uncoated SA or FG and apply coating only to threads via field-applied paste (e.g. Loctite 8014).
Add 5-7 working days to the base bolt lead time for Xylan / Xylar / SermaGard. Dacromet and TakeCoat ceramic coatings add 7-10 days. Specials with witnessed coating cert + friction-coefficient measurement per ASTM F2328 add another 3-5 days. Stock-coated standard sizes (M16, M20, M24 in Xylan 1014 blue) ship in 5-7 working days from PO.
For repeated-makeup applications: yes, strip, re-blast, re-coat. Each re-application slightly degrades bolt threads from blasting; budget 3-5 disassembly cycles before bolt replacement. For single-use installs (not designed for service disassembly), the coating is consumed in the first makeup and isn't intended for re-application; use a fresh bolt on reassembly.
Xylan 1424 on FG base. Reasons: 1424 has lowest measured μ in the Xylan family (better torque control under ROV install), the fluoropolymer is impervious to H2S in the dry state, the coating handles -46°C deepwater temperature, and Xylan is a routine Equinor / Petrobras / Aramco-approved spec. Dacromet 500 is the alternative when the buyer requires zinc-aluminum metallic coating for cathodic-protection compatibility.
Send the spec (form + size + length + base supply state SA or FG + coating system + buyer-spec coating standard + friction-coefficient requirement if any + quantity + destination port) to info@torqbolt.com or +91-22-66157017 / WhatsApp. We reply with stock availability, mill route, lead time, MTC + coating cert tier, and price within one business day (Mumbai IST).
