Valve Selection for Acid Service — H₂SO₄, HCl, HNO₃ and Phosphoric Acid

Acid service valve selection is a materials engineering exercise — no single alloy works across all acids, concentrations, and temperatures. The chemical industry handles dozens of different acids at varying concentrations; using a standard carbon steel or SS 316L valve in the wrong acid service is a guaranteed failure. This guide covers the four most common industrial acids (sulphuric, hydrochloric, nitric, and phosphoric) and maps the correct valve material to each service condition.

Sulphuric Acid (H₂SO₄) Valve Materials

• Concentration 0–5%: Dilute H₂SO₄ — extremely corrosive to carbon steel and SS 316L. Use Alloy 20 (CN7M / N08020) or Hastelloy C-276
• Concentration 5–50%: Medium H₂SO₄ — Alloy 20 standard material; Hastelloy G-3 or G-35 also suitable
• Concentration 50–80%: High H₂SO₄ — Alloy 20 still suitable; lead-lined valves sometimes used in older plants
• Concentration 78–100%: Concentrated/oleum H₂SO₄ — Carbon steel is surprisingly compatible at room temperature (forms protective sulphate film). Do NOT use SS 316L (pitting), do NOT use Alloy 20 (higher corrosion at very high conc). Carbon steel WCB is acceptable for room-temperature concentrated H₂SO₄
• High temperature (>60°C): corrosion rates increase dramatically — upgrade material by one tier
• Summary: Alloy 20 is the workhorse material for sulphuric acid at all concentrations except very dilute (<1%) and very concentrated (>80% at ambient)

Hydrochloric Acid (HCl) Valve Materials

• HCl attacks virtually all common metals — carbon steel, SS 316L, and even many high-alloy steels
• Hastelloy C-276 (N10276): Best metallic material for HCl at any concentration; resists up to 20% HCl at temperatures below 50°C
• Titanium Grade 2: Excellent resistance to HCl at <10% concentration; attacked by HCl above 10% or above 60°C
• Monel 400 (N04400): Acceptable for dry HCl gas; attacked by wet (aqueous) HCl
• PTFE-lined ball and butterfly valves: Often preferred — PTFE has excellent resistance to all HCl concentrations up to 100°C
• FRP-lined valves: Alternative to PTFE for large-bore HCl headers (DN150–DN600)
• Note: Alloy 20 is NOT recommended for HCl — its performance is inferior to C-276 for HCl service

Nitric Acid (HNO₃) Valve Materials

• Nitric acid is an oxidising acid — it attacks most corrosion-resistant alloys through oxidising pitting
• SS 316L: Acceptable for dilute HNO₃ (<50%, <50°C) only; fuming nitric acid attacks 316L
• SS 304L / 310S: Better than 316L for HNO₃ service (molybdenum in 316 reduces HNO₃ resistance)
• Titanium Grade 2: Excellent resistance to concentrated nitric acid (up to 90% at 70°C)
• Hastelloy C-276: NOT recommended for HNO₃ — the tungsten and molybdenum content increase attack rate
• Zirconium: Excellent for HNO₃ but expensive and rare in valve format
• PTFE-lined: Excellent for all HNO₃ concentrations

Phosphoric Acid (H₃PO₄) Valve Materials

• Wet process phosphoric acid (from phosphate rock dissolution with H₂SO₄): Contains fluoride impurities — very corrosive
• Alloy 20 (CN7M): Standard material for wet process H₃PO₄ in fertiliser plants
• Duplex 2205 (UNS S31803): Good resistance to phosphoric acid circuits with chloride content
• High-silicon cast iron (14–15% Si, Duriron F4A): Used in older plants for concentrated H₃PO₄
• PTFE/FEP-lined: Suitable for all phosphoric acid concentrations and temperatures to 150°C
• Carbon steel: NOT suitable for phosphoric acid — corrodes rapidly