Technical Support for Acid Buyers: Storage, Dilution and Corrosion Questions Answered

Purchased sulfuric acid is normally specified by concentration, iron content, and residue on ignition under ASTM E223. Bulk deliveries at 93–98 wt% H₂SO₄ exhibit a specific gravity of 1.80–1.84 at 20 °C and a freezing point that ranges from approximately −35 °C at 93 wt% to +3 °C at 98 wt%. The acid is classified as a corrosive liquid under UN1830 and requires dedicated storage, unloading, and dilution infrastructure designed for both the as-received concentration and any future downgrade to lower concentrations.

Carbon Steel Storage Limits for 93–98 wt% Acid

Under stagnant or low-velocity conditions, ASTM A36 carbon steel is widely used for bulk storage of 93–98.5 wt% sulfuric acid. The passivating iron sulfate film that forms at the shell surface is responsible for this compatibility. Published corrosion data for still 98 wt% acid at 25 °C indicate general corrosion rates below 0.1 mm/y, whereas the same steel can exceed 0.5 mm/y when the concentration falls below 88 wt% or when the metal temperature rises above 45 °C. Hydrostatic testing with water without immediate drying is a common cause of dilute acid corrosion on tank floors, because residual water combines with the first incoming acid to produce a low-concentration liquid that strips the iron sulfate film.

Tank shells for this service are typically specified to API 650 with a shell corrosion allowance of 6.4 mm. Continuous fillet welding is preferred in the floor-to-shell corner because crevices trap moisture and produce local acid dilution. Vapor spaces are not inerted, but they are fitted with a drying system because moisture absorption at the tank wall can create a thin dilute layer above the bulk liquid level.

Freezing point data are the basis for specifying external heat tracing for 98 wt% storage in outdoor tanks where winter ambient temperature falls below +3 °C. The same tank storing 93 wt% acid may not require heat tracing in most climate zones, but moisture absorption through an open vent can raise the freezing point of the surface layer. Heater skin temperature is limited to 30 °C because carbon steel corrosion accelerates above this threshold. Internal steam coils are generally avoided unless the coil skin temperature can be maintained below 30 °C, which is impractical for saturated steam.

Vent drying systems for 93–98 wt% sulfuric acid tanks are circulation units containing a desiccant bed arranged to maintain a dew point below −40 °C in the vapor space. Exposed tank surfaces above the liquid level are still lined or coated in many installations when airborne chlorides or high humidity are present. An internal coating system selected under NACE SP0294 may be used on the vapor-space underside of a fixed roof to prevent the formation of thin acid films that attack weld seams. The coating must be applied over a near-white metal blast-cleaned surface and remains subject to the same 30 °C temperature limit as the bare carbon steel shell.

How Does Heat of Dilution Constrain Acid-to-Water Addition?

Dilution of 98 wt% acid to process strengths below 10 wt% releases a large integral heat of solution. The addition of water into concentrated acid produces a low-density, low-boiling local mixture at the addition point and can eject acid mist; therefore, dilution systems are piped so that the acid enters below the surface of the dilution water and the water phase is recirculated through a static mixer. Process specifications for a motor-driven dilution skid typically include a PTFE-lined carbon steel mixer shell, a tantalum thermowell rated for 200 °C local excursion, and a PFA-lined downstream transfer line.

Dilution water is normally drawn from a supply meeting ASTM D1193 Type II or better. Chloride in municipal or well water can be concentrated in the diluted acid and can initiate pitting in downstream stainless steel instrumentation. A conductivity-based interlock system is positioned to stop acid feed if dilution water flow falls below the minimum turndown specified by the mixer manufacturer. This interlock prevents the formation of an overheated acid layer in the mixing vessel. Temperature sensors used for this interlock are double-walled PTFE-sheathed thermocouples with a response time below 5 seconds in flowing acid service.

Recirculation coolers in continuous dilution skids are commonly specified as graphite shell-and-tube units or PTFE-lined plate-and-frame exchangers. The coolant side is water or glycol, and the acid outlet temperature is controlled below 60 °C to protect downstream fluoropolymer-lined piping and instrumentation. A low flow trip on the recirculation cooler is interlocked with the acid feed valve because stagnant acid in a hot exchanger can reach vaporization conditions and raise local pressure above the flange rating specified by ASME B16.5.

Unloading lines constructed from Schedule 80 carbon steel are not exempt from velocity limits simply because the acid concentration is above 93 wt%. The protective iron sulfate film is shear-sensitive, and excessive velocity produces bright-line attack at elbows, weld toes, and pump discharge nozzles. A maximum line velocity of 0.5 m/s is commonly specified for carbon steel piping in 93–98 wt% acid transfer. Pump selection favors centrifugal units with ductile iron casings and alloy internals; throttle valves should not be used to control flow because throttling creates localized turbulence. Flow control is better achieved with variable-frequency drives on the pump motor or with an orifice plate designed for a pressure drop below 0.7 bar.

Hydrogen grooving is a distinct failure mode observed on carbon steel tanks in concentrated sulfuric acid service. The iron sulfate film is periodically disrupted by local sulfate reduction or by mechanical cleaning, and the exposed steel is attacked by acid with the evolution of hydrogen gas. Rising hydrogen bubbles create vertical grooves that fill with corrosion product and can be mistaken for erosion. Inspections under API 653 should include ultrasonic thickness measurements in the lower shell course and at the product line penetration because those areas see the highest flow and the longest wetting time.

Flange gaskets in concentrated acid service are normally flexible graphite with a 316 stainless steel insert, or PTFE envelope gaskets when low bolt load is available. Spiral-wound gaskets with PTFE filler are avoided in high-pressure acid lines because the filler can creep. Threaded joints are minimized because acid leakage concentrates by evaporation and leaves a solid acid crust. Mechanical seals for sulfuric acid pumps are specified with PTFE or silicon carbide faces and Hastelloy springs; packed pumps are acceptable only with continuous external flush.

If a Plant Uses 316L for Dilute Acid, Where Is the Compatibility Boundary?

Type 316L stainless steel (UNS S31603) has a narrow compatibility envelope in sulfuric acid. Published corrosion data support its use only in very dilute solutions at ambient temperature; beyond 5 wt% or above 25 °C, the passive chromium oxide film is not stable and active general corrosion can initiate. Alloy 20 (UNS N08020) extends service into a broader range of dilute acid, but its performance is concentration and temperature dependent and should be confirmed against isocorrosion diagrams from the alloy producer. PTFE-lined carbon steel and PFA-lined carbon steel are specified for concentration swings between 0 wt% and 98 wt% because they do not rely on a metallic surface film. Liner collapse under vacuum is the primary non-corrosion failure mode, so lined systems are fitted with vacuum breakers set below 0.2 bar differential.

Tantalum and high-nickel alloys such as UNS N10276 are reserved for high-temperature dilute acid service or acid containing oxidizing salts. These materials are not interchangeable based solely on generic alloy family designation. A material substitution should be validated by a corrosion test conducted in the actual process fluid at the maximum skin temperature, using coupons exposed under ASTM G31 for at least 96 hours.

Emergency eyewash and shower units within the sulfuric acid unloading area are installed in accordance with ANSI/ISEA Z358.1 and supplied with tepid water between 16 °C and 38 °C. Splash protection for operators includes acid-resistant clothing, face shield, and goggles meeting ANSI Z87.1. Respiratory protection is not required during closed-loop unloading; a half-mask acid-gas respirator is specified only if a transfer hose leak can produce an acid mist above the 1 mg/m³ exposure limit. Spill kits use soda ash or limestone, which should be applied as a dry powder to the periphery of a liquid spill. Water is not applied directly to a large sulfuric acid spill because the reaction heat can produce steam and acid aerosol.

When Sulfuric Acid Is Used as a pH Control Agent in Wastewater Treatment

In wastewater pH neutralization, 93 wt% or 98 wt% acid is injected into a side-stream mixer rather than directly into the main wastewater channel. Direct injection into a concrete channel can etch the concrete and release calcium sulfate precipitate. The side-stream dilution loop is specified with a PTFE-lined static mixer, a fluoropolymer-lined magnetic flow meter, and a pH analyzer using a high-temperature antimony or glass electrode. Acid feed is stopped by a pH set-point interlock at pH 2.5 to protect downstream piping and biological treatment units. Published operating guidance for wastewater treatment plants indicates that the acid quantity for neutralization is determined by measuring both total alkalinity and the flow rate, rather than pH alone.

Calcium carbonate scaling in the mixing zone is controlled by maintaining a local pH below 2.0 at the acid injection point, then neutralizing the side stream before return to the main channel. The acid day tank for wastewater service is kept below 15 °C if the ambient temperature exceeds 30 °C because the tank is often located in a chemical feed room with limited ventilation. Tank vents are routed to a scrubber or to an acid-resistant exhaust duct to limit the accumulation of sulfuric acid mist near electrical cabinets.

Tank truck unloading stations are equipped with dry-disconnect couplings and a pressure-sensing line that locks out the transfer pump if the vapor return line is blocked. A pneumatic diaphragm pump is often selected for small day-tank transfers because it can be dead-headed without exceeding the safe system pressure. The hose is identified in the plant maintenance system for replacement every 5 years or after 500 unloading cycles, whichever occurs first. After each unloading event, the transfer line is blown with dry nitrogen or dry air at a dew point below −40 °C to prevent the slow absorption of moisture at pipe walls.