When These Alloys Earn Their Cost
Nickel-molybdenum and nickel-chromium-molybdenum alloys should be specified when the process medium, the temperature and the contamination risk together put ordinary stainless steels out of reach. The trigger is rarely a single number. It is the combination of a strongly corrosive medium, an operating temperature that accelerates attack, and the cost of a shutdown if a line fails. That is the point at which grades such as C276 (UNS N10276), C22 (UNS N06022) or B3 (UNS N10675) become the economical answer rather than the expensive one.
Selection Logic: Match the Grade to the Duty
| Duty | Typical grade | Reason |
|---|---|---|
| Reducing acids - hydrochloric, sulfuric, hydrobromic | B2 (UNS N10665), B3 (UNS N10675) | Highest molybdenum content and minimal chromium give the best reducing-acid resistance; B3 for welded construction |
| Mixed acid and chloride-bearing streams with oxidising excursions | C276 (UNS N10276) | Combined chromium and molybdenum resistance to pitting, crevice attack and general corrosion |
| Flue gas desulfurisation and pharmaceutical reaction vessels | C22 (UNS N06022) | Outstanding pitting and crevice corrosion resistance with good weldability |
| Oxidising media combined with reducing conditions | C2000 (UNS N06200) | Balanced chromium, molybdenum and copper content for mixed duty |
| Phosphoric acid and strongly oxidising mixed acids | G30 (UNS N06030), G35 (UNS N06035) | High chromium content for oxidising solutions and pickling service |
| Strongly reducing acid with oxidising impurities | BC1 (UNS N10362) | Developed to tolerate oxidising impurities that B-family alloys will not accept |
| High-temperature combustion and furnace components | X (UNS N06002) | Oxidation resistance, creep strength and formability at elevated temperature |
Typical Process Equipment
These alloys are specified as fabricated equipment rather than as commodity stock, and the same grade range appears repeatedly across the process industries:
Heat exchangers, including shell-and-tube, plate and bayonet designs for acid and chloride duty.
Columns and towers for distillation and absorption in corrosive service.
Condensers handling acid vapours and wet chloride gases.
Reactors and autoclaves where pressure, temperature and corrosion act together.
Pressure vessels and storage tanks for aggressive intermediates.
Piping systems, including seamless pipe, welded pipe, fittings and flanges to the applicable ASTM product specifications.
Spargers, dip tubes, thermowells, agitators and similar internals that see the most aggressive local conditions.
Equipment of this type is usually built from plate, sheet, bar and pipe covered by ASTM B575, B333, B574, B564, B622, B619, B626 and B366, with ASME equivalents for pressure-retaining construction.
Industry Sectors and Operating Conditions
Chemical processing: reactors, piping systems, pressure vessels and heat exchangers where uniform attack, localised attack and stress corrosion cracking all have to be controlled at once.
Geothermal energy: brines carrying chlorides, sulfides and dissolved gases at temperature, where stainless steel is not an option.
Solar thermal and concentrated solar: high-temperature heat transfer and storage circuits that need stable corrosion performance over decades of cycling.
Oil and gas: sour, chloride-bearing and high-temperature process streams, including downstream refining duty.
Pharmaceutical: reaction vessels and process equipment requiring clean surfaces, weld integrity and resistance to aggressive reagents and cleaning cycles.
Fertiliser and phosphoric acid: evaporator tubing, heat exchangers and process components where high-chromium grades outperform other nickel alloys.
Fabrication, Welding and Supply Considerations
Formability: most chromium-bearing grades are readily cold and hot formed by deep drawing, press forming and stamping. Nickel-molybdenum grades work-harden quickly and need slower forming sequences and interstage annealing.
Welding: matching filler metals are available across the family and weldability is generally good. The exception is B2, where the heat-affected zone is more sensitive to grain-boundary attack; B3 was introduced partly to solve that problem.
Cleanliness: grinding dust, carbon steel contact and marking residues must be excluded from fabrication. Contamination shows up as localised attack months later.
Size range: piping can be supplied seamless or welded across a wide diameter and wall range, while plate, bar, strip and forgings cover vessel, internals and flange manufacture.
Documentation: mill test reports, grade certification and heat treatment records should be requested with the order, since the corrosion performance claimed depends on the analysis and the anneal actually delivered.
Common Questions
Q: When should a nickel alloy be chosen instead of a super duplex stainless steel?
When the medium is a reducing acid at strength or temperature that duplex cannot hold, or when chloride levels and temperature together put the stainless steel outside its safe window. Both the medium and the temperature have to be considered together.
Q: Which grade is the best general-purpose choice?
C276 (UNS N10276) is the most versatile of the family and the standard for petrochemical processing duty. Where the medium is strongly oxidising, the high-chromium grades are better; where it is purely reducing, the molybdenum-rich B grades are better.
Q: When is B3 the right answer rather than B2?
Whenever the equipment is welded or heavy-walled, and whenever long service at moderate temperature is expected. B3 offers better thermal stability and much better post-weld behaviour.
Q: What equipment sees the most demanding conditions?
Internals such as spargers, dip tubes and thermowells, plus heat exchanger tubing. They combine thin sections, the hottest temperatures and the most aggressive local chemistry, so grade selection for these parts deserves the closest review.
Q: Can these alloys be used for high-temperature furnace parts?
Only the grades designed for that duty. X (UNS N06002) is intended for oxidation resistance, creep strength and formability at elevated temperature; C22 is limited to about 677 °C and C276 is not intended for prolonged exposure in the intermediate high-temperature band.
Q: How is the right grade confirmed before ordering?
By matching measured medium, concentration, temperature, aeration and contamination data against published corrosion data and the applicable product specification, then confirming the fabrication and heat treatment route with the mill before the order is placed.
