Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
Monel 400 HF Acid Resistance - The Hydrofluoric Acid Default Alloy
Monel 400 (UNS N04400, ASTM B164) is the industry-default alloy for hydrofluoric acid and the qualified material for HF alkylation internals per API RP 751. HF strips chromium-rich passivation on stainless steel and on nickel-chromium-molybdenum alloys; the nickel-copper oxide film on Monel 400 holds across the full HF envelope. Service-proven across anhydrous HF and 5 to 95 percent aqueous HF, ambient to 95 deg C, deaerated and aerated. Monel 400 hydrofluoric acid attack rates in deaerated service stay below 5 mpy at ambient and below 20 mpy near boiling. Monel® is a registered trademark of Special Metals Corporation; TorqBolt is not authorized by, affiliated with or endorsed by Special Metals.
Why Monel 400 Holds in Hydrofluoric Acid
HF is a reducing acid that complexes chromium as soluble chromium trifluoride (CrF3), stripping the Cr2O3 passivation layer on austenitic stainless, Inconel 625 and Hastelloy C-276. Monel 400 contains zero chromium. The film is a tightly adherent nickel-copper oxide that converts at the surface to NiF2 and CuF2, both sparingly soluble in HF and re-depositing as a passive sheath. Specified for HF alkylation, anhydrous HF storage, fluorocarbon piping and aluminium-etching tanks for nine decades. Above 95 deg C: Inconel 600 or Hastelloy B-3. See corrosion resistance and vs stainless steel.
HF Concentration and Temperature Envelope
Monel 400 covers the full HF concentration band. Anhydrous HF is the alkylation feed and fluorocarbon raw material. Dilute aqueous (5 to 20 percent) is the pickling and etching grade. Concentrated aqueous (40 to 70 percent) is the bulk commercial grade. Hot concentrated HF above 70 percent at elevated temperature is the most aggressive band; rate monitoring is mandatory.
| HF concentration | Service condition | Monel 400 rate |
|---|---|---|
| Anhydrous HF (100%) | HF alkylation feed, 30 to 50 deg C | under 2 mpy |
| 5 to 20% aqueous | Pickling, etching, ambient | under 3 mpy |
| 20 to 40% aqueous | Process service, ambient to 60 deg C | under 5 mpy |
| 40 to 70% aqueous | Bulk commercial grade, to 60 deg C | under 8 mpy |
| 70 to 95% aqueous | Hot concentrated, 60 to 95 deg C | 10 to 30 mpy |
| Vapour-phase HF | Above the liquid, water-wet | elevated; clad protection above tray decks |
Source: Nickel Institute (Monel 400 in HF), API RP 751. Deaerated, freshly-pickled coupons; field rates vary with velocity, O2 and inhibitor practice.
Temperature envelope: Monel 400 in HF holds to 95 deg C within design margins. Above 100 deg C: Inconel 600, Hastelloy B-3 or Monel 400 clad over carbon steel. HF alkylation reboilers and overhead condensers run at 80 to 95 deg C on solid Monel 400 tube with documented multi-decade service life. Vapour-zone corrosion above the liquid is more aggressive than submerged service.
| Temperature band | Monel 400 rate | Design action |
|---|---|---|
| Ambient to 40 deg C | Below 3 mpy, deaerated | Solid Monel 400 piping, valves, bolting |
| 40 to 65 deg C | 3 to 8 mpy | Solid Monel 400; API RP 751 thickness budget |
| 65 to 95 deg C | 8 to 25 mpy | Solid Monel 400; periodic UT audit |
| Above 95 deg C | Accelerates; vapour-phase attack intensifies | Monel 400 clad over CS; Inconel 600 for tray internals |
| Above 150 deg C | Outside Monel 400 service window | Inconel 600 or Hastelloy B-3 |
Aeration Sensitivity: Deaerated HF is Specified
The largest variable in Monel 400 HF corrosion rate is dissolved oxygen. Deaerated HF (under 1 ppm O2) is the specified state for HF alkylation and anhydrous HF storage. Oxygen ingress through pump seals acts as the cathodic depolariser and accelerates attack 2 to 5 times. Field practice: nitrogen blanketing on storage, double mechanical seals with API Plan 53A barrier fluid, controlled-purge before opening flanges. See HF alkylation.
HF Alkylation Industry Context per API RP 751
HF alkylation combines isobutane with light olefins over anhydrous HF catalyst to make high-octane alkylate for the gasoline pool. Reaction at 30 to 50 deg C, 7 to 14 barg, HF at 99.5 percent. API RP 751 names Monel 400 and Monel 400-clad CS for piping, reactor internals, acid-side heat exchanger tubes, valve bodies and stud-bolt assemblies. Flange-joint: spiral-wound PTFE-filled Monel 400-wound gaskets, stud-bolts to F468, heavy hex nuts to F467. See HF alkylation.
Ni and Cu Leaching Rates by Service Condition
Monel 400 corrodes in HF by dissolving Ni and Cu as soluble fluorides. Leach products report to the process stream and are tracked in product-purity audits. The Ni:Cu ratio broadly tracks alloy chemistry (about 2:1); selective Cu dissolution occurs in oxygenated service.
| Service | Rate, mpy (mm/yr) | Ni / Cu leach |
|---|---|---|
| Anhydrous HF, 30 to 50 deg C, deaerated | under 2 (under 0.05) | low |
| 20 to 40% aqueous, ambient, deaerated | 2 to 5 (0.05 to 0.13) | low |
| 40 to 70% aqueous, 60 deg C, deaerated | 5 to 8 (0.13 to 0.20) | moderate |
| 70 to 95% aqueous, 80 deg C, deaerated | 10 to 20 (0.25 to 0.50) | elevated |
| Aerated HF, any concentration | x2 to x5 deaerated rate | x2 to x5; selective Cu dissolution |
| Crevice or weld root, 40 to 70% HF | 2 to 4 times bulk rate | elevated; post-pickle required |
Sources: Nickel Institute HF literature; API RP 751 Appendix; NACE T-5A-30. Coupon exposure qualification is the design basis.
Companion Materials: Gaskets, Bolts, Valve Trim
A Monel 400 HF flange joint is a system. Specification:
| Component | Specification for HF service |
|---|---|
| Pipe and tube | Solid Monel 400 to ASTM B165 seamless |
| Flanges | Solid Monel 400 to ASTM B564 or clad over CS; see flanges |
| Gaskets, ambient | Virgin PTFE envelope or solid PTFE; no graphite, aramid or silica filler |
| Gaskets, cycling | Kalrez 6375 or Aflas perfluoroelastomer; spiral-wound with PTFE filler and Monel 400 windings per ASME B16.20 |
| Stud bolts | Solid Monel 400 to ASTM F468; see stud bolts |
| Heavy hex nuts | Solid Monel 400 to ASTM F467; see heavy hex nuts |
| Valve body and trim | Solid Monel 400 or Monel 400 weld-overlay on CS body per API 600 / 602 |
| Welding filler | ERNiCu-7 (GTAW, GMAW) or ENiCu-7 (SMAW) per AWS A5.14, A5.11; post-weld pickle |
Frequently Asked Questions
Why use Monel 400 for HF? The nickel-copper film is stable where chromium-rich passivation collapses. NiF2 and CuF2 are sparingly soluble and re-deposit as a passive sheath. API RP 751 names Monel 400 for HF alkylation.
Does HF attack Monel 400? Slowly. Deaerated HF gives under 5 mpy at ambient and under 20 mpy approaching 95 deg C. Aerated HF runs 2 to 5 times faster.
What HF concentrations does Monel 400 cover? Anhydrous (100 percent), dilute aqueous (5 to 40 percent), concentrated (40 to 70 percent) and hot concentrated (70 to 95 percent). HF alkylation uses anhydrous HF at 99.5 percent.
What gaskets work with Monel 400 in HF? Virgin PTFE for low-temperature; Kalrez or Aflas for cycling; spiral-wound with PTFE filler and Monel 400 windings per ASME B16.20. No graphite, aramid or silica filler.
Related Pages
Applications: HF alkylation, corrosion resistance, seawater corrosion, sour service NACE MR0175. Standards: ASTM B164, B165, B564, F468, F467, UNS N04400. Forms: bolts, stud bolts, flanges, seamless pipe. Comparisons: vs stainless steel, vs Hastelloy C-276.
Request a Quote for Monel 400 HF Service Components
For Monel 400 piping, flanges, stud-bolts, nuts and valve internals to B164, B165, B564, F468, F467 for HF alkylation, anhydrous HF storage, fluorocarbon plant and aluminium etching, send HF concentration, temperature, O2 target, form and quantity to info@torqbolt.com or WhatsApp +91-22-66157017. TorqBolt QA-QC returns EN 10204 3.1 or 3.2 certificate, mill source and Ex-Mumbai price in one working day. See pricing.
References
API RP 751. Nickel Institute No. 282 (Monel 400 in HF). NACE T-5A-30. ASTM B164, B165, B564, F468, F467. AWS A5.14, A5.11. ASME B16.20.
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