Sulfuric Acid Service Pipe Fittings: Material Selection from Dilute to Concentrated H₂SO₄

The Concentration-Corrosion Relationship

The corrosion of metals in H₂SO₄ shows a characteristic curve with concentration: very dilute H₂SO₄ (below approximately 2%) is essentially a mild acid and is handled by 316L stainless at ambient temperature — the passive film is stable and corrosion rates are low; intermediate concentrations (2–70%) are the most corrosive range for most metallic materials — the acid is a strong reducing acid that dissolves passive films on most metals. This is the most challenging range for material selection and often requires alloy upgrades; high concentrations (70–93%) have intermediate corrosivity — many metals can form protective sulfate films in this range. Carbon steel shows an unusual behavior: moderately resistant above approximately 70% at ambient temperature; and concentrated/fuming H₂SO₄ (above 93%, including oleum or fuming sulfuric acid) — carbon steel forms a stable iron sulfate (FeSO₄) passive film at the metal surface and shows very low corrosion rates in concentrated H₂SO₄ above approximately 93%. This is one of the very few cases where carbon steel is acceptable for a strong acid service, and it is the basis of the widespread use of carbon steel in sulfuric acid storage tanks and pipe fittings in acid plants and alkylation units handling 93–98% H₂SO₄.

Material Selection by Concentration Range

Dilute H₂SO₄ (below 5%, ambient to 60°C): 316L stainless is generally acceptable at ambient temperature and low concentrations. Above 5% or above 60°C, 316L corrosion rate increases unacceptably and alloy upgrades are required. Type 317L (higher Mo than 316L) extends the range slightly. For dilute H₂SO₄ at elevated temperature, rubber-lined carbon steel is a common economic choice; intermediate H₂SO₄ (5–70%): this range requires specialty alloys. Alloy 20 (UNS N08020 — 20% Cr, 34% Ni, 2.5% Mo, Cb-stabilised) was developed specifically for H₂SO₄ service in the 20–60% range at temperatures to approximately 65°C, and is the standard "20% acid" material in fertiliser and chemical plants; Hastelloy C-276 is used where both oxidising and reducing conditions are encountered, or where Alloy 20 is marginal (60–70% H₂SO₄); Zirconium (Gr702 or Gr704) is extremely resistant in H₂SO₄ across a wide concentration and temperature range — it forms a stable ZrO₂ passive film that resists both dilute and intermediate concentrations at temperatures up to 200°C. Zirconium fittings are expensive but provide superior life in the most aggressive intermediate-concentration H₂SO₄ services; high-concentration H₂SO₄ (70–93%): carbon steel becomes usable at the high end of this range. Duriron (high-silicon cast iron, 14.5% Si) is corrosion-resistant across 65–100% H₂SO₄ at temperatures to approximately 110°C — used for large-bore fittings in acid plant service where its brittleness (cast iron) is acceptable and the superior corrosion resistance justifies the installation care required; and concentrated/oleum (above 93%): carbon steel (ASTM A234 WPB) is the standard fitting material for 93–98% H₂SO₄ at ambient to 65°C. The FeSO₄ passive film is stable in this concentration range and corrosion rates are below 0.25 mm/year. Velocity must be controlled (below approximately 1.2 m/s in carbon steel) to prevent film disruption by turbulence at elbows and tees. Above 65°C or for oleum, carbon steel corrosion rates increase — alloy steel or Alloy 20 fittings are required.

Velocity Effects at Pipe Fittings

In both dilute H₂SO₄ and concentrated H₂SO₄ (carbon steel service), velocity at elbows and tee branches is a critical factor. At elbows, the flow direction change creates high local velocity at the intrados and turbulence downstream — both conditions that disrupt the protective surface film (FeSO₄ in carbon steel, passive oxide in stainless or nickel alloys) and expose fresh metal to the acid. The erosion-corrosion rate at elbow intrados in H₂SO₄ service can be 3–5 times the corrosion rate in straight pipe at the same velocity. This explains why elbow corrosion is often the first point of failure in H₂SO₄ piping even when the straight pipe sections show acceptable corrosion rates. Long-radius elbows (standard ASME B16.9 LR, radius = 1.5D) create less turbulence than short-radius elbows and are preferred for H₂SO₄ service. Tee branch connections are particularly vulnerable — the main-run to branch transition creates intense turbulence at the branch crotch that can perforate carbon steel tees in concentrated H₂SO₄ service within months if velocity is not controlled.

PTFE-Lined Fittings for Intermediate Concentrations

For intermediate H₂SO₄ concentrations (20–70%) where metallic fitting materials are either marginally acceptable or very expensive, PTFE (polytetrafluoroethylene) lined carbon steel fittings provide an attractive alternative: the carbon steel shell provides the structural strength and pressure rating; the PTFE liner provides complete chemical resistance to H₂SO₄ at all concentrations and most temperatures below the PTFE service limit (approximately 150°C continuous). PTFE is not corroded by H₂SO₄ at any concentration. The liner is formed to the fitting shape, bonded or interference-fitted into the fitting body. Lined fittings for H₂SO₄ service must be specified to the same dimensional standard as unlined fittings (ASME B16.9 OD and face dimensions) so that they are interchangeable. The primary limitation of PTFE-lined fittings is that they cannot sustain vacuum without the liner collapsing — if the process can go below atmospheric pressure (steam-out, slug flow), lined fittings require vacuum-rated liner thickness or vacuum support rings.