Alloy 20 (Carpenter 20) Pipe Fittings: The Sulphuric Acid Grade Between 316L and 904L
Alloy 20 (UNS N08020, ASTM B366 WPNC) was specifically engineered for hot sulphuric acid service — a corrosive environment that defeats 316L and challenges 904L. Its niobium stabilisation eliminates sensitisation in welded piping, and its copper content (like 904L) provides the acid-inhibiting mechanism that carbon and low-alloy steels completely lack.
Composition and Grade Equivalents
| Element | Alloy 20 (N08020) | Note |
|---|---|---|
| Ni | 32–38% | High Ni provides chloride SCC immunity |
| Cr | 19–21% | Oxidising acid resistance |
| Mo | 2.0–3.0% | Pitting resistance in halide environments |
| Cu | 3.0–4.0% | Key to H₂SO₄ resistance — same mechanism as 904L |
| Nb | 8×C min | Stabilised — eliminates sensitisation in welded condition |
| UNS | N08020 | Also known as Carpenter 20, 20Cb-3 |
| ASTM fitting standard | ASTM B366 WPNC (Nb-stabilised) | — |
| ASME P-Number | P-No. 45 | Same group as Incoloy 825 |
| Welding filler | ERNiCrMo-3 (Inconel 625 filler) | Higher-alloy filler to compensate dilution |
Why Alloy 20 Was Developed
In the 1950s, the sulphuric acid and phosphoric acid industries needed a material that could handle hot concentrated acids without the rapid corrosion that destroyed 304 and 316 stainless. Carpenter Technology developed Alloy 20 (20Cb-3) specifically for this service. The copper addition (3–4%) provides the same corrosion-inhibiting mechanism observed in 904L — copper ions in the corrosion product layer slow the anodic dissolution rate in H₂SO₄.
Critically, Alloy 20 is niobium-stabilised (Nb ≥8×C), which eliminates the sensitisation problem that makes unstabilised austenitic grades (316L at higher carbon, or 316 standard) prone to intergranular attack after welding and heat treatment. This makes Alloy 20 safe to use in the as-welded condition in acid service without post-weld solution annealing.
Alloy 20 vs 904L vs Hastelloy C-276 for H₂SO₄
| Grade | H₂SO₄ Effective Range | Max Temp | Relative Cost | Limitation |
|---|---|---|---|---|
| 316L | <5% or >98% (fuming) | Ambient only for dilute | Lowest | Attacks rapidly in 10–80% at elevated temp |
| 904L | 0–80%, moderate temps | ~50°C for mid-range | Moderate | Falls off above 60°C in mid-concentration range |
| Alloy 20 | 0–85%, wider temp range | ~65–75°C for mid-range | Moderate-high | Not suitable for concentrated hot oleum |
| C-276 | Wide range, high temps | >100°C for many conc. | Highest | Not needed below 80°C where Alloy 20 works |
Applications in Chemical and Pharmaceutical Industries
Alloy 20 is the standard material for sulphuric acid storage tanks, pickling systems, acid metering, and phosphoric acid process equipment. It is also widely specified in pharmaceutical manufacturing — particularly in API production — where both H₂SO₄ and chlorides may be present and the high nickel content prevents SCC under sterilisation cycles. It should not be confused with 316L pharmaceutical grade: Alloy 20 is for acid-contact service, while 316L (1.4404) is for clean steam, WFI, and low-acid aqueous processes.
Welding Alloy 20 Fittings
Filler: ERNiCrMo-3 (Inconel 625 filler, 2.4831). This higher-alloy filler compensates for dilution during welding and ensures the weld deposit retains adequate corrosion resistance. Do not use ER308L or ER316L — these deposit a weld with far lower Ni and Cu than the base metal and will corrode preferentially in acid service. Preheat is not required. Interpass temperature ≤150°C. The niobium stabilisation means post-weld annealing is not mandatory, but full solution anneal at 1090–1120°C followed by rapid quench restores maximum corrosion resistance for the most aggressive service.
What to Specify on the PO
ASTM B366 WPNC (Alloy 20, Nb-stabilised), fitting type, NPS, schedule, EN 10204 3.1 MTR with full heat chemistry verifying Cu 3–4%, Mo 2–3%, Nb ≥8×C. If used in H₂SO₄ service, state service explicitly — this ensures the manufacturer does not substitute a non-Cu-bearing grade. For pharmaceutical service add crevice-free full-penetration weld requirement.