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Coated Fasteners
A comprehensive engineering reference for offshore piping engineers, corrosion engineers, EPC procurement teams and maintenance specialists — covering Xylan fluoropolymer, Dacromet and Geomet zinc-aluminium flake, hot-dip galvanised, electroless nickel-PTFE, electroplated zinc and mechanically applied zinc coatings for carbon steel, stainless steel and alloy fasteners. Covering K-factor engineering, salt spray performance, hydrogen embrittlement risks, installation rules and full coating documentation.
Coating Systems, Protection Mechanisms
& Engineering Selection
Barrier · Cathodic · Dry-Film Lubrication · K Factor
Why Coated Fasteners
Bare carbon steel fasteners (A193 B7 / A194 2H) corrode rapidly in outdoor, offshore, marine and chemical plant environments — converting to iron oxide within weeks in seawater spray zones and within months in industrial atmospheres. Coating extends functional fastener life from less than one year (uncoated CS in marine) to the full design life of the plant (typically 20–30 years). Beyond corrosion protection, specialty dry-film coatings (Xylan, PTFE) provide critical dry-film lubrication — reducing the nut factor K from 0.20 (uncoated dry) to 0.10–0.13 (Xylan), enabling more precise and repeatable torque-to-preload relationships during flange bolt-up.
Three distinct protection mechanisms are available: (1) Barrier protection — the coating physically seals the steel surface from moisture and oxygen; (2) Cathodic (sacrificial) protection — zinc-rich coatings (Dacromet, HDG, electro-zinc) are anodic to steel and corrode preferentially, protecting the steel even when the coating is scratched; and (3) Dry-film lubrication — PTFE or MoS₂ particles embedded in the coating matrix reduce thread friction and provide controlled, consistent K factors.
The torque equation for bolt assembly is T = K × F × d. Xylan 1070 reduces K from 0.20 (uncoated) to 0.10–0.13. This means the same wrench torque produces approximately 54–100% higher bolt preload on a Xylan-coated fastener than on an uncoated one. Using standard (uncoated) torque tables for Xylan-coated B7 studs will almost certainly yield the bolt — the preload will exceed the bolt’s proof load. Xylan-coated fasteners must always be tightened using the Xylan-specific torque tables from the coating manufacturer or the flange joint integrity analysis. This is one of the most common and dangerous site assembly errors on offshore platforms using Xylan-coated bolt kits.
Coating Systems — Engineering Descriptions
K-Factor Engineering, Salt Spray
& Coating Performance Data
DFT ISO 2808 · Xylan vs Dacromet vs HDG
| Coating System | DFT (µm) | Salt Spray (hr) | K Factor | Temp Range | HE Risk | RoHS |
|---|---|---|---|---|---|---|
| Xylan 1070 (PTFE) | 12–25 | ≥1000 | 0.10–0.13 | −70 to +260°C | None | Yes |
| Xylan 1424 (PTFE high-load) | 12–25 | ≥1000 | 0.12–0.15 | −70 to +260°C | None | Yes |
| Xylan 2510 (High-Temp) | 12–25 | ≥800 | 0.10–0.13 | −70 to +370°C | None | Yes |
| Dacromet 320 | 8–10 | 720–1000 | 0.12–0.16 | −40 to +300°C | None | Cr(VI) present |
| Geomet 321 (Chrome-Free) | 8–10 | ≥720 | 0.12–0.16 | −40 to +300°C | None | Fully RoHS |
| Hot-Dip Galvanising (HDG) | 45–80 | 500–1000+ | 0.18–0.22 | −20 to +200°C | Risk ≤8.8 — verify | Yes |
| Electroless Ni-PTFE | 12–25 | 500–1000 | 0.08–0.12 | −70 to +230°C | None | Yes |
| Electro-Zinc (Trivalent Cr) | 8–20 | 96–240 | 0.18–0.22 | −20 to +100°C | High ≥1040 MPa | Trivalent: Yes |
| Mechanical Zinc (Class 50) | 50–75 | 200–500 | 0.16–0.20 | −20 to +150°C | None (no acid) | Yes |
K factors are indicative and depend on surface finish condition, coating thickness uniformity, assembly speed and mating surface. For critical flange joints (Class 600 and above), determine the project-specific K factor by direct measurement with a calibrated bolt load cell on the actual fastener combination. Salt spray results per ISO 9227 neutral salt spray (NSS) to first rust appearance. HE = Hydrogen Embrittlement risk applies to finished fasteners ≥1040 MPa when processed with acid pickling and electroplating without bake-out treatment per ASTM F1941.
| Application / Environment | Recommended Coating | Substrate | Reason |
|---|---|---|---|
| Offshore topsides flanges | Xylan 1070 | A193 B7 / A194 2H | NORSOK M-630; 1000+ hr; K=0.10–0.13; no galling |
| Chemical plant (corrosive atm.) | Xylan 1070 or EN-PTFE | A193 B7 / B8M | Chemical resistance; dry-film lube; no galling |
| Automotive Grade 10.9 / 12.9 | Dacromet / Geomet 321 | Grade 10.9 steel | No HE risk; consistent K; RoHS; thin film |
| Structural / civil outdoor | Hot-Dip Galvanising | A307 / Grade 8.8 | Thick Zn; cathodic protection; low cost/unit |
| Power plant high-temperature | Xylan 2510 | A193 B16 / B7 | +370°C rating; anti-seize at temperature |
| Food / pharma SS fasteners | EN-PTFE (Nedox) | A193 B8M SS 316L | FDA-compliant; anti-galling; uniform on threads |
| Cryogenic (−46°C+) | Xylan 1070 | A320 L7 / B8M | Remains flexible at −70°C; no embrittlement |
| Indoor / sheltered light duty | Electro-Zinc (Trivalent) | Grade 8.8 / B7 | Low cost; adequate non-aggressive environments |
| High-strength ≥Grade 10.9 | Mechanical Zinc Cl.50 | Grade 10.9 / 12.9 | No HE; thicker than electroplate |
| NACE sour service | Xylan 1070 on NACE substrate | A193 B7 (≤22 HRC) | Coating does not affect NACE compliance |
| Duplex flange bolt kits | Xylan 1070 on Duplex 2205 | A182 F51 / F53 | Anti-galling critical; 1000+ hr salt spray |
| RoHS / ELV regulatory | Geomet 321 or Xylan | Any | No hexavalent chrome; fully RoHS compliant |
Hydrogen Embrittlement Risk,
Installation Rules & Re-Use Guidance
No Anti-Seize on Xylan · HDG Nut Matching · Re-Use Cycles
Hydrogen Embrittlement in Coated Fasteners
Hydrogen embrittlement (HE) is a catastrophic delayed brittle fracture mode that affects high-strength steel fasteners (≥1040 MPa, approximately Grade 10.9 and above, or A193 B7 under high stress) caused by atomic hydrogen absorbed into the steel lattice during acid cleaning (pickling) or electroplating. The fastener may appear intact at assembly and fail hours or days later under sustained preload stress. Key facts:
- Acid pickling generates hydrogen — when steel is immersed in hydrochloric or sulphuric acid for oxide removal prior to plating, the acid attacks the steel surface and generates nascent hydrogen that diffuses into the steel crystal lattice
- Electroplating generates hydrogen — the cathodic reduction reaction during zinc electroplating generates hydrogen at the steel surface which can diffuse in
- Bake-out releases absorbed hydrogen — per ASTM F1941, fasteners must be baked at 190°C for a minimum of 4 hours within 4 hours of plating, while the hydrogen is still mobile; delayed bake-out is less effective
- Xylan, Dacromet/Geomet and Mechanical Zinc are HE-safe — none of these processes involve acid pickling or electrodeposition; they produce no absorbed hydrogen
- HDG carries some risk — the acid flux cleaning before galvanising can introduce hydrogen; ASTM F2329 specifies hydrogen embrittlement relief requirements for HDG fasteners; Grade 8.8 is generally accepted for HDG; Grade 10.9 HDG requires ASTM F2329 compliance
For any fastener above 1040 MPa (Grade 10.9, 12.9, A193 B7 at elevated preload levels), electroplating by acid-based processes without bake-out treatment is prohibited. The bake-out per ASTM F1941 is 190°C for a minimum of 4 hours, within 4 hours of plating removal. Delayed bake-out (after 4 hours) becomes progressively less effective as hydrogen diffuses deeper into the lattice. The safest coatings for high-strength fasteners are Dacromet/Geomet, Xylan and Mechanical Zinc — none require acid pickling or electrodeposition. If electroplating is unavoidable, specify ASTM F1941 bake-out in the purchase order and require a bake-out certificate (temperature, duration, timing after plating) in the documentation package.
Installation Rules for Coated Fasteners
- Xylan — no additional lubrication: The PTFE coating is the lubricant. Adding grease, anti-seize compound or PTFE tape on top of Xylan changes the K factor unpredictably and invalidates the torque specification. Apply torque per the Xylan-specific K factor table only. Using standard uncoated torque values will over-stress the bolt.
- Xylan — coat both bolt and nut: Xylan must be applied to both the stud bolt threads and the nut bearing face threads. Coating only one component gives inconsistent K factor because the uncoated component controls the friction. NORSOK M-630 and most offshore operator specs require both coated.
- HDG — always use HDG nuts with HDG bolts: HDG adds 45–80 µm of zinc per surface to the thread form. An HDG bolt has effectively oversize threads that will not engage correctly with a standard (non-HDG) nut. Always use oversize-tapped HDG nuts or specify nuts re-tapped after galvanising.
- Dacromet/Geomet — no top coat needed: The zinc-aluminium flake coating provides its own corrosion protection. Do not apply additional paint or grease over Dacromet as this prevents the cathodic protection mechanism from functioning.
- Re-use limits: Xylan 1070 retains effective lubrication for 5–8 assembly cycles before the PTFE layer wears through at the thread crests. Inspect threads and bearing face after each disassembly under magnification; replace or re-coat if PTFE layer is visibly absent from thread flanks. Dacromet and Geomet provide approximately 3–5 re-use cycles before the thin coating wears through at thread contact zones.
Applications by Industry, QC Testing
& Documentation Package
DFT · Salt Spray · Adhesion · K-Factor Test · IFI-125
Applications by Industry
Xylan 1070-coated A193 B7/A194 2H stud bolts and heavy hex nuts are the standard specification for all offshore topsides process piping flange fasteners per NORSOK M-630, Shell DEP 34.00.10, BP Engineering Technical Practice ETPs and most major operator piping material class documents. The Xylan coating protects the carbon steel fastener in the marine atmospheric environment for the full platform design life (typically 25 years) and provides the controlled K factor needed for accurate flange joint bolt-up torque specification. All offshore Xylan-coated fastener kits require a full coating quality documentation package — DFT report, salt spray certificate, K-factor test report — as standard deliverables alongside the EN 10204 3.1 fastener MTC.
Xylan 1070 and Dacromet/Geomet-coated A193 B7/A194 2H fasteners are specified throughout refinery and petrochemical plant outdoor process piping for flanged connections, heat exchangers, air coolers, pressure vessels and any equipment exposed to atmospheric corrosion or process fluid splash. The coating extends fastener service life from 2–5 years (uncoated CS in industrial atmosphere) to the full 20–30 year plant design life, dramatically reducing turnaround bolt replacement scope and cost.
Xylan 2510 (high-temperature grade, +370°C rated) on A193 B16 (Cr-Mo-V alloy steel) stud bolts is specified for main steam isolation valve bonnets, HP turbine casing flanges, reheater header flanges and HP bypass valve studs in power plants where sustained temperatures above +260°C make standard Xylan 1070 inadequate. The Xylan 2510 coating provides dry-film lubrication enabling proper bolt-up torque at high temperatures, anti-galling protection for repeated maintenance access, and corrosion protection during wet layup periods when the plant is cold and condensation forms on fastener surfaces.
Dacromet 320 and Geomet 321 (chrome-free) are the dominant coatings for Grade 10.9 and 12.9 automotive engine, chassis and suspension fasteners where the HE-free process is mandatory and the RoHS/ELV regulatory environment prohibits hexavalent chrome. Mechanically applied zinc per ASTM B695 Class 55 is used for Grade 10.9/A490 structural bolts in bridges, towers and heavy civil construction where the room-temperature tumbling process eliminates HE risk and provides the thick, robust zinc coating needed for long-term outdoor exposure.
Quality Control for Coated Fasteners
- Dry film thickness (DFT) — measured with an eddy-current gauge per ISO 2808 at minimum 5 locations per batch on representative fasteners. Xylan 1070: minimum 12 µm, maximum 25 µm per IFI-125. Report must list individual readings, average and compliance status.
- Salt spray test — per ISO 9227 neutral salt spray (NSS); coated fasteners exposed for the specified minimum hours and inspected for first rust appearance. Xylan 1070 offshore requirement: ≥1000 hr to first rust. Test certificate must state test duration, first rust appearance time and compliance per IFI-125.
- Adhesion test — cross-cut adhesion per ISO 2409; scribed coating must achieve rating 0 or 1 (no peeling from cross-cut pattern). Mandatory for Xylan and EN-PTFE coatings.
- K-factor (nut factor) test — per IFI-125 using a bolt load cell or strain-gauged instrumented fastener; minimum 3 test fasteners per batch; K factor must fall within the specified range (0.10–0.13 for Xylan 1070). Report must include individual readings, mean K value and compliance.
- Visual inspection — uniform coverage over all threads and bearing faces; no bare spots, runs, drips, delamination, colour variation or contamination. Documented per AQL sampling plan.
- HE bake-out certificate — for electroplated fasteners ≥1040 MPa: bake-out temperature (190°C), duration (minimum 4 hours), time from plating to bake commencement (must be within 4 hours) confirmed and recorded.
| # | Document | Standard | Minimum Requirement |
|---|---|---|---|
| 01 | Fastener MTC | EN 10204 3.1 / 3.2 | 3.1 standard; 3.2 (TPI) for offshore, NACE sour service, exotic alloy |
| 02 | Coating Application Certificate | IFI-125 / ISO 10683 / manufacturer | Coating grade (e.g. Xylan 1070), batch, DFT measured values, cure temp confirmed |
| 03 | DFT Measurement Report | ISO 2808 | Min 5 readings per batch; range and average; compliance with IFI-125 minimum DFT |
| 04 | Salt Spray Test Certificate | ISO 9227 (NSS) | Hours to first rust confirmed; must meet or exceed specification minimum (≥1000 hr Xylan 1070 offshore) |
| 05 | Adhesion Test Report | ISO 2409 (cross-cut) | Rating 0 or 1 confirmed; coating does not peel from cross-cut pattern |
| 06 | K-Factor (Nut Factor) Test Report | IFI-125 / ASME PCC-1 | 3 fasteners per batch; individual K values, mean K, compliance with specified range (e.g. 0.10–0.13) |
| 07 | HE Bake-Out Certificate (if electroplated) | ASTM F1941 | Mandatory for electroplated fasteners ≥1040 MPa; 190°C × min 4 hr within 4 hr of plating |
| 08 | ISPM-15 Phytosanitary Certificate | IPPC / FAO | All wood packaging for international export |
RR Hydraulics supplies specialty coated fasteners — Xylan 1070, 1424 and 2510 (PTFE dry-film), Dacromet 320, Geomet 321 (chrome-free, RoHS), Hot-Dip Galvanised (ASTM F2329), Electroless Nickel-PTFE, Trivalent Zinc Electroplate and Mechanical Zinc (ASTM B695) — on all substrate grades including A193 B7/A194 2H, A193 B8M/A194 8MA, A320 L7, Duplex 2205, Super Duplex 2507, Grade 8.8/10.9/12.9 and exotic alloys. Sizes M8–M72. Full coating documentation: DFT report (ISO 2808), salt spray certificate (ISO 9227), adhesion report (ISO 2409), K-factor test report (IFI-125), coating application certificate, HE bake-out certificate (where applicable), plus EN 10204 3.1/3.2 fastener MTC. 48-hour express dispatch on standard in-stock coated fasteners.