Direct answer: Hastelloy C276 is generally the stronger choice for hydrochloric acid because its much higher molybdenum content and tungsten addition improve resistance in reducing acid. Inconel 625 can be suitable for dilute HCl at controlled temperatures, but it should not be selected from alloy name alone.
Why C276 Usually Performs Better in HCl
| Factor | Alloy 625 / UNS N06625 | C276 / UNS N10276 |
|---|---|---|
| Nickel base | High | High |
| Molybdenum | About 8–10% | About 15–17% |
| Tungsten | Not a principal addition | About 3–4.5% |
| HCl selection direction | Dilute or less severe conditions after data review | Broader useful resistance in reducing acid |
| Other strength | Excellent seawater resistance and useful mechanical strength | Excellent broad chemical-process corrosion resistance |
Hydrochloric acid is reducing, non-oxidizing when pure, and highly aggressive to many passive alloys. C276 was designed with high molybdenum and tungsten for this type of environment. Special Metals reports useful C276 resistance up to approximately 3% HCl at boiling, up to 5% at 80°C, and across all concentrations up to about 66°C; these are selection guides, not design guarantees.
Conditions That Can Reverse a Simple Ranking
- Acid concentration and metal temperature
- Ferric or cupric ions, dissolved oxygen and other oxidizers
- Chlorides, fluorides and mixed acids
- Velocity, crevices, deposits and shutdown conditions
- Weld condition, contamination and surface finish
Recommended Selection Process
- Define normal, startup, cleaning and upset chemistry.
- Use manufacturer iso-corrosion charts at the actual concentration and temperature.
- Review weld metal and heat-affected-zone performance, not base metal only.
- Run coupon or loop tests when published data do not reproduce the service.
- Set an acceptable corrosion-rate and design-life criterion.
When Alloy 625 Still Makes Sense
Alloy 625 may be preferred when HCl exposure is dilute or intermittent and the equipment also needs seawater resistance, high strength, fatigue performance or availability in a specific product form. C276 is normally favored when hydrochloric acid corrosion is the dominant risk.
FAQ
Is C276 immune to hydrochloric acid?
No. Its corrosion rate still depends on concentration, temperature, contaminants, flow and fabrication condition.
Can Alloy 625 handle 10% HCl?
No responsible answer is possible without temperature and chemistry details. Review iso-corrosion data and validate severe service by testing.
Does welding affect selection?
Yes. Filler metal, dilution, heat input, oxide removal and iron contamination can control real equipment performance.
Primary References
Engineering note: Values and acceptance criteria must be checked against the edition of the governing standard named in the purchase order. Application-specific design approval remains the responsibility of the purchaser or project engineer.
Post time: Sep-11-2026