Reducing-acid, brine, sulphuric, hydrochloric, and chloride-bearing chemical service for the Cu-bearing super duplex grade
The 2 percent copper addition that distinguishes Ferralium 255 (UNS S32550) from Cu-free super duplex grades like 2507 (S32750) and Zeron 100 (S32760) is the metallurgical differentiator that opens chemical-processing service. Copper improves the alloy's resistance in reducing acids (sulphuric, hydrochloric, phosphoric, and the chloride-bearing organic acids) where the Cu-free super duplex grades suffer general corrosion. The cost premium of Ferralium 255 over standard duplex 22Cr is justified specifically by this copper benefit in the wet-process chemical industries: phosphoric-acid evaporators (where H2SO4 contamination is unavoidable), kraft-pulp digesters under upset conditions, dilute-HCl pickling lines, and the bottoms section of refinery vacuum towers handling chloride-stripped feed.
| Acid | Concentration | Temperature | Ferralium 255 acceptance |
|---|---|---|---|
| Sulphuric acid (H2SO4) | 0 to 30% | up to 80 °C | Acceptable; corrosion rate ≤ 0.1 mm/yr |
| Sulphuric acid | 30 to 70% | up to 50 °C | Marginal; consider 2507 or nickel-alloy |
| Sulphuric acid | > 70% | any | Not acceptable; specify alloy 20 / Inconel 825 |
| Hydrochloric acid (HCl) | 0 to 1.5% | up to 40 °C | Acceptable; corrosion rate ≤ 0.1 mm/yr |
| Hydrochloric acid | 1.5 to 5% | up to 25 °C | Marginal; consider C-276 |
| Phosphoric acid (H3PO4) wet-process | up to 54% P2O5 | up to 90 °C | Acceptable with chloride < 200 ppm; the Cu benefit shows here |
| Sodium hydroxide (NaOH) | up to 50% | up to 100 °C | Acceptable; super duplex routinely outperforms 316L in caustic |
| Acetic, formic, citric organic acids | any concentration | up to boiling | Acceptable; preferred over 316L for chloride-contaminated organic acids |
| Chlorinated brine (NaCl, CaCl2, MgCl2) | up to saturation | up to 232 °C (NACE limit) | Acceptable; the prime use case |
Above the acid-service envelope, the escalation path is to nickel-base alloys: alloy 20 (UNS N08020) for higher concentrations of sulphuric acid, Inconel 825 (UNS N08825) for sulphuric and phosphoric mixtures with chloride, Inconel 625 (UNS N06625) for the broadest envelope of acid plus chloride plus elevated temperature, Hastelloy C-276 (UNS N10276) for severe acid-plus-chloride service. See the Ferralium 255 vs Inconel 625 comparison page for the cost-vs-corrosion-rate trade-off.
