Inconel, Hastelloy or Monel? Choosing the right nickel alloy family

Brand names vs UNS numbers

Inconel, Hastelloy and Monel are historical trademarks that survived as shorthand. What you actually order is a UNS number and a product standard: Alloy 625 is UNS N06625 per ASTM B443, Alloy C-276 is UNS N10276 per ASTM B575, Monel 400 is UNS N04400 per ASTM B127. When you write an enquiry, always lead with the UNS number and the standard — brand names alone leave room for costly confusion.

Monel: nickel-copper for seawater and hydrofluoric acid

Monel 400 (about 67 % Ni, 30 % Cu) and its precipitation-hardened version K-500 are the default choices for seawater pumps, valve trim, fasteners and heat-exchanger components, and for hydrofluoric acid service where few other metals survive.

• Best at: seawater, brine, HF acid, alkalies
• Weak against: oxidizing acids (nitric), ferric chlorides
• Watch out: K-500 loses corrosion resistance if the aged condition is over-temped; weldments may need re-ageing

Inconel: nickel-chromium for heat and oxidation

The Inconel family (600, 601, 625, 718, 690) is built around a protective chromium oxide scale. These grades keep strength and resist oxidation, carburisation and chloride stress-corrosion cracking at temperatures where stainless steels scale away — furnace fixtures, heat-treat baskets, steam systems, nuclear components.

• Best at: high temperature, oxidation, chloride SCC, creep strength
• Weak against: hot reducing acids such as concentrated hydrochloric
• 625 and 718 add molybdenum/niobium for strength and localized-corrosion resistance

Hastelloy: nickel-molybdenum for reducing acids

Hastelloy B-3 is nearly pure Ni-Mo and dominates in hot, concentrated hydrochloric acid. The C family (C-276, C-22) adds chromium on top of high molybdenum, which buys broad-spectrum resistance: wet-process phosphoric acid, flue-gas scrubbers, oxidizing chloride media, mixed acids.

Family	Key elements	Typical UNS	Choose when
Monel	Ni-Cu	N04400, N05500	Seawater, HF, alkalies
Inconel	Ni-Cr (+Fe, Mo, Nb)	N06600/601/625/718/690	Heat, steam, oxidation
Hastelloy	Ni-Mo / Ni-Cr-Mo	N10675, N10276, N06022	Hot reducing acids, scrubbers

Quick selection rules

• Temperature above 600 °C or steam? Start with Inconel family.
• Wet HCl, H2SO4 of mid concentration, or complex plant acids? Start with Hastelloy C family.
• Seawater handling or HF? Monel 400 / K-500 remains hard to beat.
• Chloride pitting or crevice attack? Compare PREN: 625 (~50) beats 825 (~30) and everything below.

Where buyers go wrong

• Specifying Alloy 625 for a reducing-acid duty that Alloy 825 or C-276 handles at lower cost.
• Ordering 'Hastelloy' without the UNS and product standard — you may receive a substitute grade.
• Copying a competitor's material list instead of mapping the actual corrosion mechanism.
• Ignoring the weld consumable: a perfect base metal with the wrong filler is a failure waiting to happen.