Alloy 825 and Alloy 625 Pipe Fittings: Choosing Between Two Nickel Alloys for Corrosive Service
Alloy 825 (UNS N08825) and Alloy 625 (UNS N06625) are both nickel-iron-chromium alloys with molybdenum additions that provide broad corrosion resistance in reducing and oxidising acid environments, chloride-containing media, and seawater. They are often considered as alternatives in the same application — but their compositions, corrosion performance, mechanical properties, and costs are significantly different, and the choice between them materially affects both performance and project budget.
Composition Comparison
Alloy 825 is an austenitic nickel-iron-chromium alloy: Ni 38–46%, Cr 19.5–23.5%, Fe balance (~30%), Mo 2.5–3.5%, Cu 1.5–3.0%, Ti 0.6–1.2%. The iron content (~30%) and titanium stabilisation make 825 a "nickel-rich austenitic stainless" rather than a true nickel base alloy — it is closer in composition to 904L than to 625. The copper addition (1.5–3.0%) specifically improves resistance to reducing acids (sulphuric and phosphoric). Alloy 625 is a true nickel-base alloy: Ni ≥58%, Cr 20–23%, Mo 8–10%, Nb+Ta 3.15–4.15%, Fe ≤5%. The high molybdenum (8–10% vs 3% in 825) and the niobium addition give 625 its superior corrosion resistance and high-temperature strength. PREN for 825 is approximately 35–40; PREN for 625 is approximately 50–55 — a substantial difference that reflects their very different chloride pitting and crevice corrosion resistance.
Corrosion Resistance: Where Each Excels
Alloy 825 was developed specifically for sulphuric and phosphoric acid service. Its copper content (similar to Alloy 20) gives excellent resistance to sulphuric acid at intermediate concentrations (10–70% H₂SO₄) and to wet-process phosphoric acid (WPPA) at moderate impurity levels. In clean WPPA at temperatures below ~70°C, 825 often matches Alloy 20 at lower cost. In seawater and chloride environments, 825 has adequate resistance at ambient temperature but is susceptible to crevice corrosion at temperatures above approximately 50–60°C — its PREN of ~38 is close to the threshold for seawater service. Alloy 625's high molybdenum content makes it substantially more resistant: the critical pitting temperature (CPT) in seawater is approximately 60–70°C for 625 vs 40–50°C for 825. For offshore seawater service, subsea wellhead connections, and any application with aggressive chloride crevice conditions, 625 is the appropriate choice and 825 is marginal. In sulphuric acid above 70% concentration (the transition to oxidising behaviour), both alloys are susceptible — Hastelloy B-2 or glass-lined equipment is required.
High-Temperature Strength
Alloy 625 has significantly higher elevated-temperature strength than 825, due to solid-solution strengthening by Mo and Nb, and age-hardening by Ni₃Nb (γ'') precipitation in the annealed-then-aged condition. The room-temperature yield strength of 625 (solution annealed) is approximately 275–310 MPa; at 650°C it retains approximately 180 MPa. Alloy 825 yield strength (annealed) is approximately 240–275 MPa at room temperature; at 650°C it drops to approximately 90–100 MPa. This difference is relevant for pipe fittings in high-temperature corrosive service: a 625 fitting can carry higher pressure at elevated temperature than an equivalent 825 fitting, which may allow a thinner wall and lower material cost to partially offset 625's higher alloy price.
Weldability
Both alloys are weldable by GTAW and GMAW using matching filler metals (ERNiFeCr-1 for 825; ERNiCrMo-3 for 625). Alloy 825 is considered easier to weld — its lower molybdenum content reduces the risk of secondary phase precipitation in the weld metal during cooling, and the titanium stabilisation reduces sensitisation risk. Alloy 625 weld metal is prone to niobium segregation during solidification, which creates Nb-rich Laves phase in the interdendritic regions of the as-welded microstructure — Laves phase reduces weld metal ductility and toughness. For critical 625 welds (high pressure, dynamic loading, or cryogenic service), a post-weld homogenisation anneal at ~1150°C dissolves the Laves phase. This requirement adds cost and scheduling complexity to 625 fabrication that is not present for 825.
Cost and Decision Guide
Alloy 825 pipe fittings cost approximately 40–60% of equivalent 625 fittings by weight — the nickel and molybdenum cost premium of 625 is substantial. The decision guide: specify 825 for sulphuric and phosphoric acid service at moderate temperature (below ~70°C); for seawater service below ~50°C where crevice geometry is controlled; and where budget is constrained and service conditions are at the lower end of nickel alloy requirement. Specify 625 for offshore seawater service above 50°C; aggressive chloride crevice conditions; elevated-temperature service above ~400°C; and concentrated acid streams where 825 experience data shows inadequate performance. Do not substitute 825 for 625 in specifications written for 625 without a formal corrosion engineering review — the performance gap is real and well-documented in offshore and chemical industry field experience.