Alloy 20 (UNS N08020): Sulfuric Acid Resistant Nickel Alloy for Chemical Equipment

ASTM B462 / ASTM B729 · Published: 2026-06-01 · Updated: 2026-06-02

Alloy 20 (UNS N08020, Carpenter 20Cb-3) was originally developed in 1951 specifically for sulfuric acid service — and remains the most commonly specified material for handling 20-40% sulfuric acid at temperatures up to 80°C. Its chemical composition is precisely balanced for this environment: 20% chromium (oxidizing acid resistance), 34% nickel (reducing acid resistance), 2.5% molybdenum (pitting resistance), and 3.5% copper (sulfuric acid catalytic protection). The niobium content (8×C minimum) stabilizes carbon as niobium carbides, preventing sensitization during welding — Alloy 20 is weldable without post-weld heat treatment.

The alloy is approved for ASME Section VIII pressure vessel construction up to 427°C and is included in NACE MR0175 for sour service. Primary applications: sulfuric acid storage tanks, pickling tanks, heat exchangers in sulfuric acid plants, chemical reactor vessels, and food processing equipment handling acidic products (vinegar, citric acid). Alloy 20 is also immune to chloride stress corrosion cracking (SCC) — a failure mode that limits austenitic stainless steels (304, 316) in hot chloride environments. For sulfuric acid service above 80°C or above 60% concentration, Alloy 20's corrosion rate increases — Hastelloy C276 or tantalum may be required for these extreme conditions.

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