Incoloy 825 Pipe Fittings: Sour Service, Phosphoric Acid, and Seawater

Alloy Overview

Incoloy 825 (ASTM B366 WPNIC-825 / EN 2.4858, NiCr21Mo, X2NiCrMoCuTi28-21) is a nickel-iron-chromium alloy with 38–46% Ni, 19.5–23.5% Cr, 2.5–3.5% Mo, 1.5–3.0% Cu, and Ti ≥0.6%×C. The high nickel content (above 40%) provides immunity to chloride stress corrosion cracking — a weakness of 316L and 904L. The Mo and Cr combination gives good pitting and crevice corrosion resistance. The titanium stabilises the alloy against sensitisation. ASME P-No. 45.

Key Applications

• Phosphoric acid (H₃PO₄): 825 has excellent resistance to phosphoric acid in all concentrations, including contaminated "wet process" phosphoric acid containing HF, H₂SO₄, and chloride. Used in fertiliser plant (MAP, DAP) evaporators and transfer piping.
• Sour oil and gas: NACE MR0175 / ISO 15156-3 qualified without special restrictions in the annealed condition. Good resistance to H₂S-bearing produced water and sour crude.
• Seawater and brine: Higher Ni than 904L gives better SCC resistance. Used in desalination high-pressure brine and in offshore produced water injection lines where 2507 is over-specification.
• Sulphuric acid: Like 904L, the Cu content provides resistance across intermediate concentrations. Less robust than C-276 in very aggressive H₂SO₄ but better than 316L or 321.
• Flue gas scrubbers (FGD): Where chloride levels are moderate and C-276 cost is hard to justify.

825 vs 904L — When to Step Up

Both 904L (EN 1.4539) and Incoloy 825 contain approximately 4–5% Mo and 1–2% Cu, and both target acid and chloride service. The key difference is nickel: 904L has 23–28% Ni; 825 has 38–46% Ni. This higher Ni in 825 provides: (1) immunity to chloride SCC at essentially all temperatures — 904L can suffer SCC in concentrated hot chloride; (2) better resistance to reducing media (HCl, dilute H₂SO₄); (3) ISO 15156-3 sour service qualification without the hardness restrictions that 904L sometimes faces. Specify 825 when chloride SCC is a risk, when sour service qualification is explicitly required, or when operating temperatures in chloride exceed 60°C.

Welding 825

Incoloy 825 uses ERNiCrMo-3 filler (AWS A5.14) — the same filler as Inconel 625. This is important: the 625 filler (ERNiCrMo-3) is over-alloyed relative to 825 base metal, but this is intentional — it provides a weld deposit with higher Cr and Mo than the base metal, ensuring the weld is not the weak link in corrosion resistance. No PWHT required. Inter-pass temperature ≤150°C. Post-weld pickling is recommended for acid service applications.

Cost Position

Incoloy 825 typically costs 30–50% more per kg than 904L and 30–50% less per kg than Inconel 625. In applications where the sour service qualification and SCC immunity of 825 are required but the premium corrosion resistance of 625 (PRE >50, ASME P-No. 43) is not needed, 825 represents an economical middle ground. Always compare on a fitted and installed basis — 825's lower yield strength than duplex grades means heavier wall sections may be required in pressure-critical applications, partially offsetting the raw material cost advantage.