Materials of Construction for Sulfuric Acid: What You Must Never Use is an exclusion matrix bounded by concentration, temperature, flow, and oxidant load. Sulfuric acid changes from reducing to oxidizing behavior within the 65–85 wt% window, and a material that withstands 98 wt% acid at 25 °C can fail rapidly in 20 wt% acid at the same temperature. Selection must therefore be governed by ASTM G31-12a immersion corrosion data, ASTM A240/A240M alloy designations, ASTM D543-20 polymer resistance testing, and ASTM D471-16a elastomer property retention. The following sections state the materials that must never be used for defined H2SO4 service envelopes.

Dilute and Intermediate Acid: Carbon Steel Is Excluded Below the Passivation Threshold

Carbon steel in sulfuric acid is not universally resistant; it relies on an iron sulfate film that forms only in concentrated acid. When the acid concentration falls below 70 wt%, the film dissolves faster than it precipitates, and uniform corrosion initiates. ASTM G31-12a immersion coupons in aerated 10–30 wt% H2SO4 at 25 °C typically show corrosion rates exceeding 1.0 mm/y. Under flowing conditions, the boundary moves upward: below 80 wt%, velocity removes FeSO4 corrosion products and prevents stable passivation. Carbon steel must never be used for storage tanks, piping, or pump bodies in 0–70 wt% sulfuric acid. In the 70–85 wt% band, it must not be used above 30 °C unless continuous ultrasonic thickness monitoring and hydrogen grooving inspection are specified. The exclusion also applies to carbon steel bolts, vents, and nozzles; vapor-phase attack above dilute acid is often more aggressive than liquid-phase attack because condensation produces thin, warm acid films.

In concentrated acid above 93 wt%, carbon steel can be used at ambient, but the passivation film is fragile. At temperatures above 40 °C, iron sulfate becomes increasingly soluble, and corrosion along the vessel floor can exceed 0.5 mm/y, particularly where sludge accumulates. In oleum above 20% free SO3, carbon steel is not automatically acceptable because free sulfur trioxide can react with moisture to form aggressive condensate at vents and manways. The same exclusion applies to carbon steel in acid-transfer lines where flow velocity exceeds 1.0 m/s; high shear strips the film. For these boundaries, ultrasonic thickness mapping per API 510 is used to track corrosion rate, but material exclusion is the primary control.

Because 316L (UNS S31603) contains 2.0–3.0 wt% molybdenum, it is often incorrectly considered a corrosion upgrade over 304L in sulfuric acid service. In practice, 304L and 316L share the same active-passive transition. Their passive range is confined to concentrations below 5 wt% and above 90 wt% at ambient. In the 20–80 wt% band, the chromium oxide film is unstable, and the steel corrodes in the active state. ASTM G31-12a coupons of 316L in 40 wt% H2SO4 at 50 °C commonly record uniform corrosion above 0.5 mm/y, with pitting at crevices, flange faces, and heat-tint zones. 304L is more restricted and should not be considered for sulfuric acid service above 10 wt% at any temperature above ambient. The alloy must never be used for pipework, pump casings, or tanks in 20–80 wt% acid above 30 °C. For 98 wt% acid, 316L is acceptable only at ambient and must not be used above 60 °C, where the passive film fails and weld decay becomes possible. Cast CF8M is subject to the same restrictions under ASTM A351/A351M.

Rotating components impose a tighter boundary than static coupons. A 316L impeller in 40 wt% acid at 50 °C fails first at the impeller eye and cutwater, where turbulence raises local wall-loss rates several times above static coupon values. Pump bodies and magnets should therefore be excluded from 316L service in the same concentration band even when tank linings appear intact. In acid-cooling heat exchangers, 316L tubes must not be used in 10–60 wt% acid because heat-transfer surfaces run hotter than the bulk acid and develop active corrosion under scales.

What Makes Duplex and Titanium Options Unsafe in Hot 50–80 wt% Acid?

Duplex stainless steel UNS S32205 has higher chromium and molybdenum than 316L, but its pitting resistance does not create passivity in hot intermediate sulfuric acid. At 50 wt% H2SO4 and 60 °C, ASTM G31-12a tests on 2205 frequently exceed 0.25 mm/y, and crevice attack occurs under deposits and at gasket surfaces. Titanium UNS R50400 must never be used in pure sulfuric acid at elevated temperatures; soluble titanium sulfate forms in 20–40 wt% acid above 60 °C, producing rapid general corrosion, and published case histories of plate heat exchangers describe through-thickness attack under such conditions. Titanium can be considered only in very dilute acid below 5 wt% at ambient or in strongly oxidizing mixed-acid streams, not as a general replacement for acid service.

Aluminium and copper alloys are excluded from practical sulfuric acid service. Aluminium dissolves rapidly in dilute H2SO4 because the amphoteric oxide film is not stable below pH 3, and cupric ions from brass or bronze valves produce galvanic attack on steel. Monel 400 is similarly unsuitable in aerated sulfuric acid, where oxygen assists anodic dissolution. These materials must never be specified for wetted pressure-containing components in 0–80 wt% acid unless a corrosion barrier is provided.

Protective coatings and paints do not move a material into the acceptable zone for sulfuric acid. Organic coatings applied to carbon steel in 20 wt% acid fail at pinholes and welds, and the corrosion front can spread beneath the coating. In concentrated acid, coatings must never be the sole corrosion allowance because the iron sulfate film is a passive layer that depends on uniform contact; a pinhole can produce a groove. The corrosion allowance required by ASTM A36/A36M carbon steel plate cannot compensate for active dissolution in dilute acid.

When Thermoplastics Soften or Sulfonate Above 60°C in Concentrated Acid

For thermoplastic liners and piping, the exclusion boundary is set by heat distortion and sulfonation rather than metal passivation. PVC is limited to dilute acid and must never be specified for 93 wt% acid above 40 °C; dehydrochlorination produces cracking and collapse. HDPE and PP are unsuitable for 98 wt% acid above ambient, and even at 50 °C ASTM D543-20 immersion testing shows loss of tensile elongation due to oxidation and sulfonation. PVDF provides broader resistance but is not rated for continuous service in 98 wt% acid above 100 °C. Only PTFE and PFA are considered universal lining materials for sulfuric acid up to 260 °C. Unlined fiberglass-reinforced plastic tanks using general-purpose polyester or vinyl ester resin must never be used for concentrated acid above 80 wt% without a corrosion barrier qualified to EN 13121-3.

For lined metallic equipment, the liner itself is not the only exclusion boundary. A PTFE liner over carbon steel can fail at a nozzle if the adhesive is attacked or if permeation of SO3 creates acid condensation behind the liner. Vacuum service imposes a tighter liner limitation because lift-off and permeation rate increase with temperature; PTFE-lined steel must not be used for concentrated sulfuric acid above 120 °C under vacuum unless the liner is mechanically anchored and tested per ASTM F423.

Elastomer Seal and Liner Rejection Criteria by Swell and Hardness

Because elastomeric gaskets and linings often control the failure boundary of a sulfuric acid system, their exclusion criteria are evaluated by swell and hardness retention. EPDM performs acceptably in dilute acid up to 50 wt% at ambient but must never be used in concentrated acid above 80 wt% or in oleum. Natural rubber and neoprene are similarly excluded from concentrated sulfuric acid because of surface attack and swelling. FKM is not an automatic upgrade: hot concentrated acid dehydrofluorinates the polymer, producing hardening, cracking, and compression-set loss. ASTM D471-16a immersion testing should be used to reject any elastomer with volume swell above 15% or Shore A hardness loss above 10 points after 168 h at the service temperature. For 98 wt% acid above 60 °C, only perfluoroelastomer or PTFE-encapsulated seals are appropriate. Published failure analyses of pump mechanical seals with standard FKM O-rings in concentrated acid describe loss of compression set and sealing force within days, not weeks.

In production-scale acid storage, failures cluster at interfaces rather than in straight shell plate. A PTFE-lined carbon steel tank can discharge product through a nozzle if the 304L flange face corrodes; a Hastelloy C-276 pump head can fail at the seal if the elastomer swells. The exclusion list for sulfuric acid must be applied to every wetted component: vessel shell, nozzle, gasket, fastener, instrument thermowell, and sight-glass fitting. Published inspection data from tanker loading and phosphoric acid dilution lines show that the first breach is usually the component whose material was not explicitly excluded by the specification.