What the Family Contains
The name Hastelloy covers a family of nickel-base corrosion-resistant alloys rather than a single grade. Each member is built around a different balance of molybdenum, chromium and, in some cases, tungsten, and the choice between them is driven by the chemistry of the process stream, not by a single ranking of performance.
The grades most often specified in process equipment are C-276 (UNS N10276) for mixed oxidising and reducing duty, C-22 (UNS N06022) where stronger oxidising capability is needed, C-4 (UNS N06455) for similar duty with lower tungsten content, and B-3 (UNS N10675) for reducing acids where chromium is not wanted.
Grade Selection Table
| Grade | UNS | Chromium, % | Molybdenum, % | Tungsten, % | Typical duty |
|---|---|---|---|---|---|
| C-276 | N10276 | 14.5-16.5 | 15.0-17.0 | 3.0-4.5 | Mixed acids, wet chlorine, flue gas |
| C-22 | N06022 | 20.0-22.5 | 12.5-14.5 | 2.5-3.5 | Strongly oxidising media, bleach plants |
| C-4 | N06455 | 14.0-18.0 | 14.0-17.0 | None | Chemical processing with thermal stability |
| B-3 | N10675 | 1.0-3.0 | 27.0-32.0 | None | Hydrochloric and sulphuric acid, reducing |
The higher chromium content of C-22 widens the range of oxidising conditions it can tolerate; the near-absence of chromium in B-3 is deliberate, because chromium is detrimental in strongly reducing hydrochloric acid service.
Chemical Processing and Fine Chemicals
Reactors, heat exchangers, columns, piping and pump components in plants handling chlorinated organics, acid mixtures and halogenated intermediates are the largest single field of use. The reason is not simply acid resistance: it is that these plants alternate between oxidising and reducing steps, and a material with resistance to only one of them fails at the changeover point. Wet chlorine, hypochlorite and chlorine dioxide handling equipment is specified in the C-276 and C-22 grades for the same reason.
In fine-chemical and pharmaceutical production, the material is also selected for product purity. Because the alloy resists attack in cleaning and sanitising agents, it minimises metallic contamination of the process stream, which matters where trace metals affect product quality or analytical results.
Pollution Control and Flue-Gas Desulphurisation
Wet flue-gas desulphurisation is one of the most demanding applications for any corrosion-resistant alloy. The absorber environment combines acidic condensate, chlorides from the fuel, fluorides, entrained solids and high gas velocities, and the process cycles between wet and dry conditions across the unit. C-276 and C-22 are widely used for absorber internals, mist eliminators, spray headers, dampers, expansion joints and stack liners, with lower-cost materials such as 2205 duplex stainless steel used in the milder upstream locations.
Oil and Gas, Sour Service and Downhole Tools
In upstream and downstream oil and gas, the driving conditions are hydrogen sulphide, chlorides, carbon dioxide and elemental sulphur, often at elevated temperature and pressure. The alloys are used for downhole tubulars and tools, wellhead components, valves, subsea hardware, refinery piping and heat exchangers in crude units. For sour service, compliance with NACE MR0175 / ISO 15156 is established through the material listing in that standard, and the selection depends on the partial pressure of hydrogen sulphide together with the chloride level and temperature.
Marine, Desalination and Water Treatment
Seawater is a chlorinated, aerated, high-chloride environment that causes pitting and crevice corrosion in stainless steels even at ambient temperature. The alloys are used for seawater piping, heat exchanger tubing, pump shafts, valve trim, desalination plant components and offshore firewater systems. Velocity and oxygen content control the design: continuously flowing, aerated seawater is less aggressive toward these grades than stagnant seawater under deposits, so drainage, flushing and the elimination of crevices are important details.
Aerospace, Heat Treatment and Furnace Equipment
Where the requirement is mechanical integrity at temperature combined with oxidation and carburisation resistance, the same family is used for furnace rolls, retorts, muffles, radiant tubes, thermocouple protection sheaths and heat-treating fixtures. In aerospace and rocket hardware the material appears in exhaust and thrust components where a combination of moderate high-temperature strength and corrosion resistance is needed. For sustained load-bearing service above roughly 700 C, a solid-solution strengthened or oxide-dispersion strengthened nickel-base grade is normally the better selection.
How to Specify the Right Grade
Specification should start from the corrosion data for the actual medium, not from a generic grade comparison:
- Define the full chemistry, including trace oxidisers, chlorides, fluorides and pH range.
- Define maximum and minimum temperature, and whether the vessel cycles between wet and dry.
- Confirm the product form and the governing specification, for example ASTM B575 for plate and sheet or ASTM B622 for seamless pipe and tube.
- Require corrosion-test evidence or documented field experience for the specific duty when the material is a marginal candidate.
- Confirm whether post-weld solution annealing is required, since that decision affects fabrication sequence and cost.
Frequently Asked Questions
Q: Is Hastelloy one material or several?
A: It is a family of nickel-base alloys. C-276, C-22, C-4 and B-3 are distinct grades with different chromium, molybdenum and tungsten contents, and they are not interchangeable.
Q: Which grade is best for hydrochloric acid?
A: The nickel-molybdenum grade B-3 is designed for reducing acid service such as hydrochloric acid, because the absence of chromium improves performance in that specific medium. C-276 is used where the acid is contaminated with oxidisers.
Q: Can these alloys be used in seawater piping?
A: Yes, they are common choices for seawater and desalination equipment. Design should avoid stagnant conditions and crevices, and should control flow velocity within the limits used for the grade.
Q: What product specifications apply?
A: Plate and sheet are ordered to ASTM B575, rod and bar to ASTM B574, seamless pipe and tube to ASTM B622, welded pipe to ASTM B619 and welded tube to ASTM B626, with ASME equivalents used for code-stamped equipment.
Q: Do these grades require post-weld heat treatment?
A: The low-carbon, low-silicon corrosion grades are normally acceptable in the as-welded condition. Where service is highly aggressive or the material has been heavily cold formed, solution annealing is specified instead.
