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Monel 400 (UNS N04400 / W.Nr. 2.4360) is a single-phase, nickel-copper solid-solution alloy (Ni ≈ 63–70 %, Cu ≈ 28–34 %, Fe ≤ 2.5 %). Its corrosion resistance originates from two synergistic effects:
High nickel content (≥ 63 %): Confers immunity to chloride-induced stress corrosion cracking (SCC) and general resistance to reducing environments.
Copper addition (≈ 30 %): Provides specific tolerance to hydrofluoric acid (HF), moderate sulfuric / phosphoric acids, and improves resistance to seawater uniform corrosion.
It is not a universal corrosion alloy—it performs poorly in strongly oxidizing acids (hot HNO₃, H₂SO₄ with oxidants) and hot concentrated HCl. This article provides a complete, specification-based corrosion-resistance overview with quantitative guidance for engineering evaluation.
1. Seawater & Brine (Chloride-Containing Natural Waters)
|
Environment |
Typical Condition |
Corrosion Rate / Behavior |
Notes |
|---|---|---|---|
|
Flowing Seawater (Cl⁻ ≈ 19–35 g/L) |
5–35 °C, velocity 1–2 m/s |
< 0.025 mm/a (1 mpy) uniform corrosion |
Immune to Cl⁻ SCC; superior to 316L (which pits/fails by SCC > 60 °C) |
|
Stagnant / Low-Velocity Seawater |
≤ 40 °C, aerated |
Slight pitting possible in crevices; generally PREN ≈ 22–25 → acceptable for most marine use |
For severe crevice (hot stagnant brine > 60 °C) consider 6 % Mo SS or Alloy 625 |
|
Brackish / Estuarine Water |
Variable salinity |
Comparable to seawater; < 0.03 mm/a typ. |
— |
|
Salt Spray / Marine Atmosphere |
Coastal, industrial marine |
Excellent; patina forms, no rusting |
Used for fasteners, hardware, propeller shafts |
→ Primary marine alloy for pump shafts, impellers, condenser U-tubes, valve stems, and desalination heat exchanger tubing.
2. Hydrofluoric Acid (HF) — Key Differentiator
Monel 400 is the standard metallic material for anhydrous and aqueous HF in petroleum alkylation units.
|
HF Concentration |
Temperature |
Corrosion Rate (Typical) |
Remark |
|---|---|---|---|
|
Anhydrous HF (100 %) |
Ambient to 120 °C |
< 0.025 mm/a |
Unique among common structurals; 316L fails rapidly |
|
Aqueous HF (≤ 60–70 %) |
Boiling (~110 °C) |
0.025–0.075 mm/a (acceptable) |
Dependent on aeration / impurities |
|
HF with organics / alkylation mix |
30–80 °C |
Generally acceptable |
Verify no free oxidants (Fe³⁺, HNO₃) present |
⚠️ If HF contains strong oxidizing contaminants (e.g., HNO₃, dissolved Cl₂, Fe³⁺ > few hundred ppm), corrosion rate can increase sharply → review case-by-case.
3. Sulfuric & Phosphoric Acids (Reducing Acids)
Monel 400 tolerates dilute to moderate H₂SO₄ and wet-process H₃PO₄, especially when de-aerated. It does not replace Hastelloy C-276 for hot concentrated H₂SO₄ or HCl.
|
Medium |
Concentration |
Temperature |
Behavior |
Comment |
|---|---|---|---|---|
|
Sulfuric Acid (H₂SO₄) |
≤ 20 % |
≤ 60 °C |
0.05–0.13 mm/a (acceptable for many chem. plants) |
De-aerated → better; aerated → increased attack |
|
H₂SO₄ |
20–50 % |
≤ 50 °C (de-aerated) |
0.13–0.25 mm/a (marginal) |
Evaluate with coupon test |
|
H₂SO₄ |
> 50 % or > 80 °C |
Any |
Not recommended |
Use C-276 / C-22 or Si-cast iron |
|
Phosphoric Acid (H₃PO₄, wet process) |
26–32 % P₂O₅ + F⁻ + Cl⁻ |
≤ 80–90 °C |
Acceptable (0.05–0.15 mm/a) |
Common in fertilizer industry (extraction tanks, pipes) |
|
Hydrochloric Acid (HCl) |
≤ 10 % |
≤ 40 °C (de-aerated) |
0.13–0.50 mm/a (borderline) |
> 10 % or hotter → unsuitable → C-276 |
4. Alkaline Solutions (Caustic Soda / Potash)
|
Medium |
Concentration |
Temperature |
Behavior |
Note |
|---|---|---|---|---|
|
Sodium Hydroxide (NaOH) |
≤ 70 % |
≤ 100–150 °C |
< 0.025–0.05 mm/a |
Good resistance |
|
NaOH |
> 70 % or > 150 °C (molten) |
— |
Use Nickel 200 / 201 (Monel 400 can be used but life shorter than pure Ni) |
Ni-200/201 preferred for evaporators & molten caustic |
|
Potassium Hydroxide (KOH) |
Similar to NaOH |
Similar |
Comparable resistance |
— |
Monel 400 is not susceptible to caustic embrittlement / SCC (unlike austenitic SS in hot concentrated NaOH).
5. Other Environments
|
Environment |
Rating / Behavior |
Note |
|---|---|---|
|
Nitric Acid (HNO₃) |
Poor — rapid attack even at low conc. & ambient T |
Use Inconel 600 or Alloy 20 for moderate HNO₃ |
|
Acetic / Formic / Citric Acids (organic, non-oxidizing) |
Good to moderate T (≤ 100 °C) |
Similar to dilute H₂SO₄ tolerance |
|
Atmospheric (Rural / Urban / Industrial) |
Excellent; tarnish patina, < 0.0025 mm/a |
— |
|
Sulfide-Containing Sour Gas (H₂S + CO₂ + Cl⁻, NACE MR0175) |
Acceptable if solution-annealed + hardness ≤ HRC 35 (typical 70–90 HRB) |
Not a sour-service "superalloy" but widely used for chemical injection lines & topside piping |
|
Molten Salts / Molten Metals |
Case-specific; generally not a primary application |
— |
6. Summary Corrosion Resistance Table
|
Medium / Condition |
Monel 400 Suitability |
Typical Corrosion Rate |
Preferred Alternative if Unsuitable |
|---|---|---|---|
|
Flowing Seawater / Salt Spray |
★★★★★ (Excellent) |
< 0.025 mm/a |
— (Alloy 625 if extreme crevice) |
|
Hydrofluoric Acid (all conc., ≤ b.p.) |
★★★★★ (Industry Std.) |
< 0.025–0.075 mm/a |
— |
|
Dilute H₂SO₄ (≤ 20 %, ≤ 60 °C, de-aerated) |
★★★★☆ (Acceptable) |
0.05–0.13 mm/a |
C-276 if oxidant present |
|
Wet-Process H₃PO₄ (+ F⁻/Cl⁻, ≤ 90 °C) |
★★★★☆ (Acceptable) |
0.05–0.15 mm/a |
C-276 if very aggressive |
|
NaOH (≤ 70 %, ≤ 150 °C) |
★★★★☆ (Good) |
< 0.05 mm/a |
Ni-200/201 for > 70 % or > 150 °C |
|
Conc. HCl (> 10 %) or Hot Conc. H₂SO₄ (> 70 %) |
☆☆☆☆☆ (Unsuitable) |
High attack |
C-276 / C-22 / Si-iron |
|
Hot Conc. HNO₃ |
☆☆☆☆☆ (Unsuitable) |
Rapid attack |
Inconel 600 / Alloy 20 |
|
Chloride SCC (any T with Cl⁻ water) |
★★★★★ (Immune) |
N/A |
— |
PREN (Pitting Resistance Equivalent Number) ≈ %Cr + 3.3×%Mo = ≈ 22–25 for Monel 400 (no Mo intentionally added; equation gives rough comparative index only).
7. Practical Corrosion-Allowance Guidance
Seawater piping / heat exchanger tubes: Design corrosion allowance 1.5–3.0 mm (typical); expected life > 20 y in normal flow conditions.
HF alkylation piping / vessels: Design allowance 1.5–2.0 mm; inspect periodically for localized attack at welds (though HAZ is not sensitized).
Chemical process (dil. H₂SO₄ / H₃PO₄): Allow 1.0–1.5 mm + verify by plant coupon test if impurity oxidants suspected.
8. Summary
Monel 400 excels in:
Seawater & chloride environments → SCC immune + low uniform corrosion
Hydrofluoric acid → only common structural alloy for all concentrations to bp
Dilute / moderately conc. non-oxidizing acids (H₂SO₄, H₃PO₄) and alkalis ≤ 70 % NaOH @ ≤ 150 °C
Cryogenic to ~538 °C service with no ductile-brittle transition
It must not be specified for:
Strong oxidizing acids (HNO₃, hot conc. H₂SO₄ + oxidant)
Hot concentrated HCl
Severe crevice corrosion in hot stagnant high-chloride brines (PREN limitation)
Correct application within this envelope makes Monel 400 one of the most cost-effective, reliable corrosion-resistant alloys for marine, petrochemical (HF), and process industries.
