Incoloy Alloy Corrosion Resistance Analysis for Chemical Processing Industry

Incoloy is not a single alloy but a family of Ni–Fe–Cr based austenitic alloys​ split into two clear branches: the 800-series (800 / 800H / 800HT)​ optimized for high-temperature oxidation, carburization and creep, and the aqueous-corrosion branch (825 / 925 / Alloy 20, sometimes grouped under Incoloy-type grades)​ optimized for sulfuric, phosphoric, sour-gas and chloride environments. In chemical plants, mixing these two branches is the most common specification error: 800H is excellent at 650–900°C flue gas but mediocre in 40% H₂SO₄; 825 is excellent in 40% H₂SO₄ at 80°C but not a creep carrier above ~550°C.

This article analyzes the corrosion behavior of the chemically relevant Incoloy grades with quantitative boundaries, and defines where each grade fits in a chemical-processing flowsheet.

1. Family Split: High-Temperature vs Aqueous-Corrosion Incoloys

Key point: “Incoloy” alone does not imply acid resistance. Only 825 / 925 / Alloy 20 carry the Cu–Mo–Ti corrosion package.

2. Chemistry Drivers of Corrosion Resistance

Typical composition windows (ASTM B424 / B423 / B463 etc.):

PREN (Pitting Resistance Equivalent Number), using PREN = %Cr + 3.3(%Mo) + 16(%N):

• 316L: ~24–26

• Incoloy 825: ~32–35

• Alloy 20: ~29–32

• 925: ~32–35 (same Mo as 825, higher Ni)

→ 825/925 sit between 316L and super-austenitic 254SMO (PREN~43) / Hastelloy C-276 (PREN~65). They resist pitting in seawater-cooled acid exchangers​ but are not a substitute for C-276 in strong HCl or HF.

3. Behavior in Specific Chemical Media

3.1 Sulfuric Acid (H₂SO₄) — the defining environment for 825 / Alloy 20

• 10–40% H₂SO₄, ≤ 80°C: 825 corrosion rate typically < 0.1 mm/year; Alloy 20 similar or slightly better due to 3–4% Cu. 316L often 0.5–1.0 mm/year in same duty.

• 40% H₂SO₄, 100°C: 825 still serviceable (rate rises but generally < 0.3–0.5 mm/year depending on aeration); above 70% concentration or > 90–100°C, both 825 and Alloy 20 degrade rapidly → switch to Hastelloy C-276 / C-22​ or silicon SS.

• Presence of Cl⁻ in H₂SO₄: 825 preferred over Alloy 20 in higher Cl⁻ because Mo 2.5–3.5 + Ni>38% gives better pitting margin.

• 800H in H₂SO₄: not recommended; Ni–Fe–Cr without Mo/Cu gives poor reductive-acid resistance.

3.2 Phosphoric Acid (wet-process, with F⁻ and SO₄²⁻)

• 825 is a standard material for wet-process H₃PO₄ evaporators, agitators, pump casings​ at ≤ 80–90°C and moderate F⁻ (< 500 ppm). High fluoride breaks passive film → coupon test mandatory.

• Alloy 20 also used; 825 slightly better in chloride-contaminated phosphoric streams.

3.3 Nitric Acid & Mixed Oxidizing Acids

• 800H/800HT and 825 both resist dilute HNO₃​ via Cr passive film; for pure concentrated HNO₃, 304L/310L or high-Cr stainless​ often cheaper and adequate.

• HNO₃ + H₂SO₄ mixed acid: 825/Alloy 20 acceptable at moderate T; Cr handles nitric side, Cu/Mo handle sulfuric side.

3.4 Hydrochloric Acid (HCl) & Hydrofluoric Acid (HF)

• Pure HCl > 5–10%: 825/Alloy 20 not recommended. Use Hastelloy C-276, C-22, or zirconium/glass-lined.

• Trace HF in wet-process H₃PO₄: tolerable at low F⁻; high HF requires C-276 or rubber-lined carbon steel.

3.5 Chloride Stress Corrosion Cracking (Cl⁻ SCC)

• 300-series SS fail at > ~60°C + tensile stress + Cl⁻; 825/925/Alloy 20 with Ni > 38%​ are practically immune​ to classical Cl⁻ SCC in aqueous service to ~200°C. This is why 825 is used in seawater-cooled acid coolers where 316L would crack.

3.6 Pitting / Crevice in Seawater + Acid

• 825 PREN 32–35 resists pitting in < 2000 ppm Cl⁻, ≤ 40°C​ with good velocity; in hot brine (> 60°C, > 5000 ppm Cl⁻) pitting risk rises → super-duplex or 6Mo better; C-276 best.

3.7 Sour Gas (H₂S + CO₂ + Cl⁻) — Oil & Chemical Interface

• 825 (annealed, ≤ 35 HRC)​ and 925 (age-hardened, NACE MR0175 / ISO 15156 qualified)​ are listed for sour service.

• 925 adds Ys 760–965 MPa​ while keeping 825-class corrosion → used for high-strength wellhead/chemical-injection mandrels where 825 would be too soft.

• 800H is not​ a sour-service grade despite high Ni; lack of Mo/Cu limits aqueous resistance.

4. Temperature Ceilings in Chemical Service

5. Welding & Sensitization

• 825 / Alloy 20​ are Ti- or Nb-stabilized → after GTAW/GMAW with ERNiCrMo-3 (625-type)​ or ERNiCrMo-4 (C-276-type), HAZ stays immune to intergranular corrosion; post-weld solution anneal optional, not mandatory for most chemical piping.

• 800H/800HT​ welded with ERNiCr-3; fine for high-temp ducts, but not for acid lines.

• 925​ supplied solution-annealed or aged (620–760°C age); welding of aged 925 requires re-solution + age for full strength, but corrosion envelope unchanged.

6. Grade Selection Map for Chemical Plants

• H₂SO₄ 10–40%, ≤ 80°C, with Cl⁻: Incoloy 825​ (pipe, shell-and-tube exchanger, tank liner)

• H₂SO₄ 10–40%, ≤ 93°C, minimal Cl⁻, lower cost than 825: Alloy 20 (N08020)

• Wet-process H₃PO₄ with F⁻/SO₄²⁻, ≤ 90°C: 825​ (preferred over Alloy 20 if Cl⁻ present)

• Sour gas + high-strength fastener/mandrel (NACE MR0175): 925​ (not 825, not 800H)

• Steam header, pyrolysis transfer line, 650–900°C oxidizing/carburizing: 800H / 800HT​ (not 825)

• Seawater-cooled acid cooler shell: 825 tubes + carbon-steel/clad shell; avoid 316L tubes

• Strong HCl, HF, > 70% H₂SO₄ hot: none of the Incoloys — go Hastelloy C-276 / C-22 / glass-lined

7. Common Mis-specifications

1. Writing “Incoloy pipe” without grade → workshop may ship 800H for an H₂SO₄ line (fails in months).

2. Using 825 for reformer/cracker tube at 850°C​ → wrong branch; 800HT only, and even then creep-limited.

3. Substituting 925 for 825 in a large acid tank​ → pays 3× strength penalty for no corrosion gain.

4. Assuming 825 = C-276 in HCl → 825 has zero Cu/Mo advantage in non-oxidizing HCl; C-276 needed.

8. Summary

Incoloy alloys serve the chemical industry in two non-overlapping roles:

• 800 / 800H / 800HT​ = high-temperature structural oxidation/carburization resistance (600–1150°C gas side), not aqueous acid grades.

• 825 / Alloy 20​ = Ni>38% + Mo 2.5–3.5% + Cu 1.5–4% + Ti/Nb stabilization → excellent in H₂SO₄ (10–40%, ≤80–100°C), H₃PO₄, mixed oxid/reduct acids, Cl⁻ SCC immune, PREN 29–35.

• 925​ = 825 corrosion package + age-hardening (Al/Ti) for NACE sour service at Ys ~760–965 MPa.