Under 49 CFR 172.101 and the UN Model Regulations, sulfuric acid shipments are split into four proper shipping names: UN 1830 Sulfuric acid with more than 51% acid, UN 2796 Sulfuric acid with not more than 51% acid, UN 1831 Sulfuric acid, fuming, and UN 1832 Sulfuric acid, spent. The concentration boundary at 51% by mass is a regulatory qualifier rather than an abrupt physical-property cliff; density, viscosity, and corrosion behaviour change continuously through the concentration range. All four entries belong to Class 8, but only UN 1831 is assigned Packing Group I. UN 1830, UN 2796, and UN 1832 are Packing Group II in the UN Model Regulations. The UN number is not an administrative marker. It changes the proper shipping name on the dangerous goods declaration, the tank specification, the venting requirement, and the emergency-response information required under 49 CFR 172.202(b).

Because UN 1830 and UN 2796 share Class 8 and Packing Group II, the concentration qualifier is the primary regulatory distinction. In a spill, the emergency responder uses that qualifier to decide whether the acid is concentrated enough to generate excessive dilution heat. The shipper must not reclassify a 60% acid stream as UN 2796 merely because it has been diluted with water in the tank; the proper shipping name must reflect the material actually shipped.

What Separates UN 1830 from UN 2796 Once the Corrosivity Envelope Is Fixed?

The two entries are both corrosive liquids, but their material-of-construction and unloading profiles differ sharply. UN 1830 is handled in carbon steel storage and piping at ambient temperature, provided that the flow velocity is kept below approximately 0.9 m/s. Carbon steel passivates in concentrated sulfuric acid because a dense iron sulfate layer forms on the metal surface. If the acid is agitated or flows too quickly, the protective layer is disturbed, and localised corrosion rates increase. Published data for specific flow-velocity limits in transfer lines is limited; plant practice often sets the 0.9 m/s limit for carbon steel lines based on erosion-corrosion service experience. Transfer pumps for UN 1830 typically use PTFE-lined or PVDF-lined casings with silicon carbide mechanical seals to limit seal leakage.

UN 2796, in contrast, is not placed in bare carbon steel. Dilute sulfuric acid attacks carbon steel by hydrogen evolution, and the corrosion rate is a function of temperature and acid concentration. Storage and transport packs for UN 2796 are therefore made of high-density polyethylene, polypropylene, or glass-reinforced vinyl ester resin. A 37% H2SO4 solution has a density of approximately 1.285 kg/L at 15.5°C, measured by ASTM D4052-22. The heat of solution for concentrated sulfuric acid in water is approximately -95 kJ/mol at infinite dilution, so tank-cleaning operations that leave concentrated heel in a dilute line can generate enough heat to distort HDPE flanges and cause gasket leakage. For this reason, loading racks that handle both UN 1830 and UN 2796 use separate pumps, separate filters, and separate drain paths.

The density difference between 51% and 93% acid is approximately 1.41 kg/L versus 1.83 kg/L at 20°C. This changes the maximum fill weight for an intermediate bulk container and affects stacking limits. Both UN 1830 and UN 2796 are Packing Group II, but the packaging compatibility decision cannot be reduced to the packing group alone. A drum or IBC rated for UN 2796 may not be suitable for hot concentrated UN 1830 above 40°C unless the packaging material is verified for elevated-temperature service.

Oleum loads classified as UN 1831 require Packing Group I because the free sulfur trioxide content creates both liquid and vapour-phase corrosivity. The UN Model Regulations assign Packing Group I to substances that cause full-thickness destruction of intact skin tissue within an observation period of 60 min following an exposure period of 3 min or less. Oleum meets this criterion through combined H2SO4 and SO3 action. Common oleum grades contain 20%, 30%, or 65% free SO3, and the vapour pressure of SO3 above the liquid is sufficient to form a visible mist when atmospheric moisture is present. The mist can plug pressure-vacuum vents and attack flange gaskets. Service practice is to pad storage tanks with dry nitrogen to a dew point below -40°C. Transfer lines are often steam-traced because oleum freezing points vary with SO3 content and can fall within ambient plant temperature ranges.

At production-scale oleum loading racks, the liquid is moved through all-welded carbon steel pipe with PTFE gaskets, but elastomeric components are excluded because oleum initiates rapid cracking. Vent condensers and scrubber systems are installed to handle SO3 vapour displaced during tank filling. In the event of a release, water fog is used to knock down the SO3 mist, but the water addition must be continuous and controlled to avoid a violent boiling reaction. The proper shipping name on the declaration must include the word fuming; the shipper cannot use UN 1830 for oleum even if the total acid equivalent would be above 51%.

When Spent Acid Enters the Transport Chain as UN 1832

Spent acid under UN 1832 is not a single composition. Alkylation spent acid typically retains 85% to 90% H2SO4 and contains sulfonated hydrocarbons; steel pickling spent acid may contain 5% to 15% H2SO4 with dissolved ferrous sulfate. The absence of a concentration qualifier in the UN 1832 proper shipping name places the burden on the shipper to characterise the waste. In the United States, spent sulfuric acid may also be a hazardous waste under 40 CFR 261.22 if the pH is ≤2 or if it corrodes steel at a rate greater than 6.35 mm/yr at 55°C. The manifest and the DOT hazardous materials description are separate documents, but both may be required for the same shipment.

Tank trailers used for UN 1832 are usually lined or constructed of acid-resistant alloys because the contaminants can disrupt passivation. If a spent acid contains sufficient organic content, residual pressure can build from slow degradation reactions; therefore, venting and pressure-relief arrangements must be reviewed under 49 CFR 173.24(e). Unloading pumps for spent acid with suspended solids use recessed impeller or air-operated diaphragm designs with PTFE diaphragms. Progressive cavity pumps with EPDM stators are typically avoided because the stator can swell and seize after exposure to aromatic hydrocarbon contaminants. Settled solids in spent acid tank trailers are resuspended with air lances before unloading, and filtration is performed through acid-resistant polypropylene or PVDF housings. The shipper must not dilute a spent acid stream solely to move it from UN 1832 to UN 2796; mixing waste streams to alter classification is a regulated activity in many jurisdictions.

At battery electrolyte blending operations, UN 2796 shipments are produced by diluting concentrated acid with demineralised water in a batch tank. The blending system is designed with an acid-addition dip tube below the water surface, a recirculation loop returning to the top of the tank, and a cooling coil sized for the expected heat of dilution. A 30% H2SO4 solution has a density of approximately 1.22 kg/L at 20°C; verification by ASTM D4052-22 is used to confirm that the concentration is below the 51% threshold before the UN 2796 marking is applied. The recirculation loop is made of PVDF or polypropylene, and the unloading hose is bonded to prevent static discharge. These operational controls are not created by the UN number alone, but they reflect the engineering measures needed to keep the shipment within the concentration boundary stated on the dangerous goods declaration.

On the dangerous goods declaration, the sequence for a corrosive liquid is proper shipping name, hazard class, UN number, packing group, and additional information. For UN 1830 and UN 2796, the words with more than 51% acid or with not more than 51% acid are part of the proper shipping name and must appear exactly. For UN 1831, the word fuming is part of the name; for UN 1832, the word spent is part of the name. Emergency-response information must include the specific gravity, pH, and any known contaminants or inhibitors. A shipper of UN 1832 that does not have a full contaminant profile must not use the UN 1830 or UN 2796 entries for convenience, because the contaminant data are material to the responder’s selection of neutralisation and containment measures.