Stainless steel handles a remarkable range of conditions, but some environments are simply too hot, too corrosive or too demanding for it. When that happens, engineers turn to nickel alloys. These materials keep their strength at high temperatures and resist corrosion in conditions that would quickly destroy ordinary steels. They are more expensive, so knowing which alloy suits which job is important. This guide compares three of the best-known families: Inconel, Monel and Hastelloy.

Why nickel?

Nickel forms a stable structure that stays tough over a wide temperature range and resists many forms of corrosion. By alloying nickel with chromium, molybdenum, copper, iron and other elements, metallurgists create alloys tailored to particular environments: oxidising, reducing, chloride-rich, acidic or high-temperature.

Note that Inconel, Monel and Hastelloy are trade names. The same alloys are also sold under generic designations, such as UNS numbers, and under other brand names. When ordering, it is best to specify the UNS number or the standard grade.

Inconel: heat and oxidation resistance

Inconel alloys are nickel-chromium based. Their strength is their ability to resist oxidation and keep mechanical strength at high temperatures.

Alloy 600 is a nickel-chromium-iron alloy used for furnace components, heat treatment equipment and chemical processing where resistance to high-temperature oxidation and chloride stress corrosion cracking is needed.
Alloy 625 adds molybdenum and niobium, giving high strength and excellent resistance to pitting and crevice corrosion. It is widely used in marine, aerospace, chemical and oil and gas applications.

Monel: seawater and hydrofluoric acid

Monel alloys are nickel-copper based. Alloy 400 is the best-known grade. It has outstanding resistance to seawater and brackish water, and it is one of the few alloys that performs well in hydrofluoric acid. It is used in marine engineering, pumps and valves, heat exchangers, and chemical and hydrocarbon processing. It is tough, ductile and maintains good strength from sub-zero temperatures up to moderate heat.

Hastelloy: aggressive chemicals

Hastelloy alloys are nickel-molybdenum or nickel-chromium-molybdenum based and are designed for the most aggressive chemical environments. Alloy C276 is a common choice. It resists a wide range of corrosive media, including strong oxidisers, hot contaminated mineral acids, chlorides and wet chlorine gas. It is used in chemical processing, pollution control equipment such as flue gas scrubbers, and pulp and paper bleaching plants.

Quick comparison

High-temperature strength and oxidation: Inconel alloys lead.
Seawater and hydrofluoric acid: Monel 400 is a proven choice.
Mixed, highly aggressive chemicals: Hastelloy C276 offers very broad resistance.
Cost: all are significantly more expensive than stainless steel, and prices move with nickel and molybdenum markets.

Forms available

Nickel alloys are supplied as sheet, plate, bar, wire, seamless and welded pipe, tube, fittings, flanges and fasteners. Sheet and plate are often cut to size, which reduces waste on expensive material.

How to choose

Define the environment precisely: chemicals, concentrations, temperature, pressure and whether conditions are oxidising or reducing.
Check industry standards and the client specification for approved materials.
Consult corrosion data or a materials engineer for critical duties.
Consider fabrication: welding procedures and filler metals must suit the alloy.
Insist on certification: a 3.1 inspection certificate confirming chemistry and mechanical properties.

Fabrication tips

Nickel alloys work-harden quickly, so machining needs sharp tools, rigid set-ups and steady feeds. Welding should follow qualified procedures with matching filler metals, and surfaces must be kept scrupulously clean, because contamination from sulphur, lead or other low-melting elements can cause cracking. Keep nickel alloy material segregated from carbon steel in the workshop to avoid iron contamination that could later rust.

Sourcing nickel alloys

Nefton Exports Industries supplies nickel alloy sheets and other alloy products for chemical, petrochemical and power industries, with Materials Test Certificates. Send us the grade, size and quantity you need for a quotation.