Engineering selection brief
Choose by the complete chemical environment—not by alloy reputation.
Alloy 20 (UNS N08020) is often the first material evaluated for sulfuric-acid equipment. Hastelloy C-276 (UNS N10276) generally offers a broader starting margin in hydrochloric acid, chloride-bearing streams and variable mixed-chemical service.
Decision boundary: concentration, metal temperature, chlorides, oxidizing contaminants, aeration, velocity, crevices and weld condition can change the result. An acid name alone is not enough to approve either alloy.
Start with Alloy 20 when
Sulfuric acid is the main duty
Use published concentration–temperature data to confirm the actual window. Its copper-bearing chemistry makes it a practical candidate for many sulfuric-acid systems, subject to contaminants and operating conditions.
Start with C-276 when
Chlorides or chemistry changes widen risk
Its high molybdenum content and tungsten addition generally provide a wider localized-corrosion margin in reducing acids, crevices and variable mixed streams—but still require service-specific verification.
Fast selection matrix
| Service condition | Alloy 20 | C-276 | Required check |
|---|---|---|---|
| Sulfuric acid | Strong candidate in many concentration–temperature windows | Also viable in many conditions; not automatically excessive | Acid concentration, temperature, impurities and acceptable corrosion rate |
| Hydrochloric acid | Do not assume suitability without condition-specific data | Usually the broader starting candidate | Concentration, temperature, aeration and oxidizing ions |
| Chlorides and crevices | Useful resistance; localized-corrosion limit must be checked | Generally the wider pitting and crevice-corrosion margin | pH, chlorides, deposits, gasket geometry and stagnation |
| Variable mixed stream | Use only when every credible condition is covered | Often shortlisted for a wider process envelope | Normal, cleaning, startup, shutdown and upset chemistry |
Do not use a universal “acid limit”
Claims such as “Alloy 20 is suitable up to 60% sulfuric acid” or “C-276 handles every hydrochloric-acid condition” remove the variables that control corrosion. Use an iso-corrosion diagram, published test data or representative coupon testing for the real concentration and metal temperature.
Why the chemistry changes the shortlist
Alloy 20
An iron-base, nickel-chromium-molybdenum-copper austenitic alloy. Copper is central to its sulfuric-acid performance; nickel, chromium and molybdenum contribute to general and localized corrosion resistance. Niobium stabilization supports fabrication resistance to sensitization.
Hastelloy C-276
A nickel-chromium-molybdenum-tungsten alloy. Its high molybdenum level and tungsten addition support performance in reducing media and localized-corrosion conditions, while chromium contributes resistance when oxidizing species are present.
The applicable purchase specification depends on product form. Plate, bar, pipe, tube, fittings and forgings should not be ordered from a chemistry label alone; identify the required ASTM or ASME specification and edition.
Four service questions that decide the material
1. Is sulfuric acid the stable primary medium?
Alloy 20 can be an efficient choice in many sulfuric-acid windows. C-276 also has published sulfuric-acid data and should not be dismissed as “not optimized.” Compare both at the same concentration, metal temperature, impurity level and corrosion-rate criterion.
2. Are hydrochloric acid or reducing chlorides present?
C-276 is commonly shortlisted because it retains useful resistance across a broad range of reducing-acid conditions. “Broad range” is not universal immunity. Ferric or cupric ions, aeration and temperature can change the mechanism. Alloy 20 requires direct evidence for the exact exposure.
3. Can crevices or deposits concentrate the chemistry?
C-276 generally offers the wider localized-corrosion margin, but deposits, dead legs, gasket crevices, incomplete drainage and low-flow zones can make the local chemistry much more aggressive than the bulk stream. Alloy selection and equipment design must be reviewed together.
4. Does chemistry change during cleaning or an upset?
A reducing production stream may become oxidizing during cleaning; evaporation may concentrate chlorides at a heat-transfer surface. Cover normal operation, cleaning, startup, shutdown and the most credible upset. Where tables do not reproduce the stream, specify testing with representative base metal and welds.
Fabrication can change field performance
- Welding: qualify filler metal, heat input, interpass temperature and cleaning for the alloy, thickness and code.
- Post-weld condition: Alloy 20 assemblies are often used as welded, but this does not eliminate procedure control. C-276 is readily weldable with suitable consumables.
- Surface condition: remove heat tint and contamination; cleaning cannot repair an incorrect weld procedure.
- Testing: when corrosion risk is uncertain, include representative welds and crevices in the qualification test.
Compare lifecycle cost—not alloy price alone
Alloy 20 may have a lower raw-material price in many market conditions, but dimensions, quantity, product availability, filler metal, fabrication, inspection and lead time affect installed cost. A higher-alloy material is not economical unless the additional corrosion margin is needed.
Use one comparison basis: design life, corrosion allowance, inspection interval, downtime consequence and replacement cost. This prevents both over-specification and a low initial price that creates early maintenance risk.
RFQ checklist: send these details first
Main chemicals, trace chlorides, oxidizers and solids
Normal and maximum metal temperature, pressure and concentration
Velocity, aeration, deposits, crevices and stagnant zones
Cleaning, startup, shutdown and credible process excursions
Form, dimensions, quantity, standard and specification edition
Welding, heat treatment, surface, NDE and certificate needs
Frequently asked questions
Is Alloy 20 always the best choice for sulfuric acid?
No. It is a strong candidate for many duties, but concentration, temperature and contaminants determine performance. Confirm the exact operating window.
Is C-276 immune to hydrochloric acid?
No alloy is universally immune. Concentration, temperature, aeration and oxidizing contamination still define the acceptable window.
Can composition alone predict service life?
No. Geometry, weld quality, surface condition, velocity, deposits and process excursions can control the outcome.
Build a condition-based quotation
Send the service conditions before choosing the alloy.
Provide chemistry, temperature range, product form, dimensions, quantity and specification. We will compare the requested Alloy 20 and C-276 forms against the stated order conditions.
Post time: Jul-01-2025