Hastelloy C-276 vs Inconel 625 — Which Nickel Alloy for Corrosive Service?

Hastelloy C-276 — Composition and Properties

Hastelloy C-276 (UNS N10276) is a nickel-molybdenum-chromium alloy with the following nominal composition: Nickel 57% (balance), Molybdenum 15–17%, Chromium 14.5–16.5%, Iron 4–7%, Tungsten 3–4.5%, Cobalt max 2.5%.

The very high molybdenum content (15–17%) is the defining feature of C-276. Molybdenum is the primary element responsible for resistance to reducing environments — particularly hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and phosphoric acid. The combination of Mo, Cr and W gives C-276 outstanding resistance to pitting, crevice corrosion and stress corrosion cracking across a wide range of temperatures and concentrations.

Mechanical properties: Tensile strength 690 MPa minimum, Yield strength 0.2% 283 MPa minimum, Elongation 40% minimum. Density 8.89 g/cm³. Maximum service temperature in oxidising environments approximately 1040°C.

Inconel 625 — Composition and Properties

Inconel 625 (UNS N06625) is a nickel-chromium-molybdenum alloy with niobium, with nominal composition: Nickel 58% minimum, Chromium 20–23%, Molybdenum 8–10%, Niobium + Tantalum 3.15–4.15%, Iron max 5%.

The key difference from C-276 is the higher chromium content (20–23% vs 14.5–16.5%) and the addition of niobium. High chromium gives 625 excellent resistance to oxidising environments and high-temperature oxidation. Niobium strengthens the alloy through precipitation hardening at elevated temperatures.

Mechanical properties (annealed): Tensile strength 827 MPa minimum, Yield strength 414 MPa minimum, Elongation 30% minimum. Density 8.44 g/cm³. Service temperature capability up to 980°C in oxidising atmospheres — significantly higher than C-276 in oxidising conditions.

Corrosion Resistance Comparison

Reducing environments (HCl, H₂SO₄, wet process phosphoric acid): Hastelloy C-276 wins clearly. The 15–17% molybdenum content makes C-276 the benchmark alloy for HCl service. Inconel 625 has insufficient molybdenum for concentrated reducing acid service.

Oxidising environments (HNO₃, high-temperature oxidation, hot sulphuric acid above 60%): Inconel 625 wins. The high chromium forms a stable oxide layer. C-276 is not recommended for strongly oxidising acids.

Chloride pitting and crevice corrosion in seawater: Both perform well, but C-276 has a slight edge due to higher Mo. Both are used for seawater handling at ambient temperature.

Mixed acid environments (H₂SO₄ + HCl, or process streams with alternating oxidising and reducing conditions): C-276 is the safer choice due to its wider chemical resistance envelope.

High-temperature service above 650°C: Inconel 625 is preferred — it maintains strength and oxidation resistance at temperatures where C-276 becomes borderline.

Weldability and Fabrication

Both alloys are weldable using GTAW (TIG), GMAW (MIG) and SMAW processes. Both are supplied as ASTM B622 / B575 / B619 / B626 welded and seamless pipes, B443 sheets and plates, B564 forgings.

Hastelloy C-276 welding: Matching filler ERNiCrMo-4 (AWS A5.14). No PWHT required for most applications. The alloy is sensitive to sensitisation in the 600–800°C range — prolonged exposure must be avoided during fabrication.

Inconel 625 welding: Matching filler ERNiCrMo-3 (AWS A5.14). No PWHT required. 625 is very weld-stable and is actually used as overlay/cladding material on carbon steel pressure vessels to provide corrosion resistance at lower cost than solid 625 construction.

Both alloys work-harden rapidly during cold forming — intermediate annealing may be needed for complex shapes.

Real-World Applications

Hastelloy C-276 applications: Flue gas desulphurisation (FGD) scrubbers in power plants, HCl acid production equipment, sulphuric acid coolers, pulp bleaching equipment, pharmaceutical reactor vessels, chemical waste treatment systems, sour gas (H₂S-containing) production equipment.

Inconel 625 applications: Offshore riser clamps and subsea components (seawater + cathodic protection), exhaust systems and heat shields in aerospace, gas turbine combustion liners and transition ducts, furnace hardware at temperatures above 700°C, nuclear reactor spacer grids and springs, high-strength flexible risers for deepwater oil production, weld overlay on carbon steel pressure vessels.

Both appear in the same industries — oil and gas, chemical, marine — but in different parts of the plant depending on the specific corrosive challenge.

Cost and Availability from Creative Metal Industries

Both Hastelloy C-276 and Inconel 625 are premium-priced exotic alloys. In general, Inconel 625 is priced slightly lower than C-276 due to its lower molybdenum content (Mo is expensive). However, pricing varies significantly with form (pipe vs plate vs bar) and prevailing nickel and molybdenum prices.

Creative Metal Industries, Vadodara stocks both alloys in pipes, plates, sheets, bars, buttweld fittings and flanges from approved mills including Haynes International, Special Metals (now PCC), VDM Metals and ThyssenKrupp. All material supplied with EN 10204 3.1 Mill Test Certificates with full chemical and mechanical test results.

For project requirements or urgent needs, contact our team at +91 99982 80619. We provide quotes within 24 hours with full material traceability documentation.