HCl Packing Groups and UN 1789: Why Concentration Decides Your Label

Hydrochloric acid (aqueous hydrogen chloride; CAS 7647-01-0; EINECS 231-595-7) is carried as UN 1789, Class 8, with the proper shipping name “Hydrochloric acid, solution.” The regulatory logic behind HCl Packing Groups and UN 1789: Why Concentration Decides Your Label originates in the concentration-dependent corrosion behaviour of aqueous hydrogen chloride on standard steel and aluminium coupons. At 20 °C, a 37% w/w solution has a density of 1.18 g/cm³ and an HCl molarity of approximately 12.0 mol/L. A 10% w/w solution has a density of 1.048 g/cm³ and an HCl molarity of approximately 2.87 mol/L. Doubling or halving the mass fraction does not produce a simple linear change in corrosivity; the activity coefficient of hydrogen ions, the equilibrium partial pressure of HCl vapour, and the concentration of dissolved oxygen at the metal surface shift simultaneously.

The UN classification process for UN 1789 does not measure pH directly. Under UN Manual of Tests and Criteria, Part III, Section 37, the material is exposed to standard low-carbon steel and aluminium alloy coupons at 55 °C for seven days. The reported metal loss is converted to a corrosion rate in millimetres per year. A packing group is assigned based on the higher of skin-corrosion test data or metal-corrosion data. For hydrochloric acid, the metal-corrosion pathway usually controls because the solution penetrates intact skin only after a finite contact time, whereas low-alloy steel and aluminium alloys are attacked within hours. At 37% w/w and 55 °C, the seven-day metal-corrosion test generally yields a corrosion rate above 6.25 mm/year on the standard steel coupon specified by the test method. At 10% w/w, the steel rate may fall below the upper threshold while aluminium often remains above 0.63 mm/year, which supports a lower packing group assignment where a PG III row is used. The concentration therefore changes the transport label because the mass fraction determines which corrosion-rate band is crossed. Actual field corrosion rates vary with alloy heat, surface preparation, and dissolved oxygen; shippers must use coupon data from an accredited laboratory for classification.

What Happens Below 20% w/w Under ADR Special Provision 520?

ADR 3.3, special provision 520 exempts aqueous solutions of hydrogen chloride containing not more than 20% hydrogen chloride from the requirements of ADR. This is a hard regulatory cut-off, not a corrosion-test result. A 19.8% w/w solution moved in a 1,000 L intermediate bulk container inside an ADR contracting state is outside ADR scope even if the liquid would still corrode aluminium at more than 0.63 mm/year at 55 °C. The cut-off is commonly described as the 20% w/w boundary, although the special provision text refers to hydrogen chloride content by mass. The corresponding UN Model Regulations dangerous goods list entry for UN 1789 is assigned to Packing Group II. Outside ADR, shippers cannot automatically assume that dilution to 20% removes the Class 8 label; the US DOT hazardous materials table under 49 CFR §172.101 does not adopt ADR special provision 520 as a general exemption. If a lower packing group is claimed under 49 CFR, the classification must be supported by test data under 49 CFR §173.137.

Under EU CLP Regulation (EC) No 1272/2008, the concentration thresholds generate a second label layer. Hydrochloric acid is classified as Skin Corr 1B (H314) at concentrations of 25% w/w and above, and as Skin Irrit 2 (H315) with Eye Irrit 2 (H319) between 10% w/w and 25% w/w. The transport packing group and the CLP hazard category are not identical systems; however, Skin Corr 1B is generally aligned with PG II for the purposes of transport classification. The transport document for a 37% consignment therefore carries UN 1789, Class 8, PG II, while the SDS Section 2 carries H314 and EUH071. A 15% solution outside ADR may carry UN 1789, PG III on the dangerous goods declaration but the SDS still lists H315 and H319. The label on the package remains the Class 8 corrosive pictogram; the packing group is not a second pictogram but a written element on the transport document and, for certain packagings, appears as part of the marking adjacent to the UN number on the outer package.

Lined Steel Tank Container Materials and Gasket Performance at 37% w/w HCl

Glass-lined carbon steel ISO tank containers with PTFE envelope gaskets are specified for concentrated HCl because the glass lining reduces the iron dissolution reaction that otherwise accelerates at temperatures above 40 °C. Unlined carbon steel transfer piping in 10% w/w HCl at 20 °C has a reported general corrosion rate in the range of 0.5 mm/year to 1.0 mm/year, but the same steel can exceed 6.25 mm/year at 20% w/w and 55 °C when dissolved oxygen is present. The oxygen reduction half-cell is the principal cathodic reaction; deaeration of the liquid lowers the mixed potential and reduces metal loss. Centrifugal pumps with silicon carbide mechanical seals and PTFE or PVDF wet-end components are preferred for bulk transfer because 316L stainless steel is susceptible to chloride pitting once the temperature exceeds 40 °C. Natural rubber-lined steel, bromobutyl-lined steel, and high-density polyethylene are also used for storage; titanium is not recommended for concentrated HCl because of high corrosion rates, and aluminium is incompatible in all transport-relevant concentrations.

Limited quantity provisions differ sharply across packing groups. Under ADR 3.4, UN 1789 assigned to PG II carries a limited quantity limit of 1 L per inner packaging and a gross mass limit of 30 kg per package. Where a PG III assignment is permitted, the limited quantity limit is 5 L per inner packaging and the same 30 kg gross mass limit applies. This difference changes the permissible packaging specification: a PG II consignment of 37% w/w HCl is typically packed in a UN 31H1 high-density polyethylene drum or a UN 31HA1 composite intermediate bulk container with a steel outer cage. The closures must be vented or pressure-relieved because concentrated HCl evolves hydrogen chloride vapour; a sealed closure can deform at 40 °C due to vapour pressure. Flexible intermediate bulk containers are not appropriate for hydrochloric acid; only rigid or composite intermediate bulk containers are permitted under the relevant packing instructions.

When a 10% w/w Solution Corrodes 6082 Aluminum Above 0.63 mm/year

A 10% w/w HCl solution has a pH of approximately -0.46, because the hydrogen ion concentration is 2.87 mol/L. In the UN metal-corrosion test, 6082 aluminium alloy exposed to 10% w/w HCl at 55 °C can lose material at a rate above 0.63 mm/year within the seven-day immersion period. The aluminium corrosion mechanism is not uniform; chloride ions destabilize the passive oxide film and produce deep pit propagation. The UN corrosivity test does not directly score pitting depth; it measures mass loss per unit area per unit time and converts the average loss to mm/year. This is why a dilute solution that appears less aggressive in steel service may still be assigned a corrosive packing group: aluminium is the deciding metal. Shippers attempting to move a 12% w/w solution under a reduced packing group must obtain test data from an accredited laboratory using the UN Manual of Tests and Criteria Part III Section 37 procedure, not from pH paper or a steel coupon alone.

Without Both Steel and Aluminum Coupons, the Packing Group Shift May Be Understated

The UN test prescribes standard low-carbon steel and aluminium alloy coupons. If the steel coupon already exceeds the PG II threshold, the aluminium result may be omitted for the upper packing group decision. For lower concentration solutions, the aluminium result becomes decisive because the steel rate may be moderate while the aluminium rate remains high. The use of ASTM G31-12a laboratory immersion corrosion equipment is common when testing hydrochloric acid fractions for classification. The test requires controlled temperature at 55 °C ± 1 °C, continuous immersion, and removal of corrosion products by chemical cleaning before the final mass-loss determination. The calculated corrosion rate must be reported as millimetres per year to two decimal places when the value is near a packing group boundary.

Before a packing group decision is finalized, the concentration of the bulk acid should be verified by titration against 0.1 N sodium hydroxide to a phenolphthalein endpoint, or by density measurement using an Anton Paar DMA 4500 density meter with a temperature-stabilized cell at 20 °C. A 20% w/w boundary requires a combined expanded measurement uncertainty of less than 0.1% w/w if the shipment is to be documented as exempt under ADR SP 520. Density alone is not sufficient because density curves flatten near the 20% region; titration is preferred for regulatory samples. ASTM D 1193 Type IV reagent water is used for dilution and blank correction. The retained sample should be stored in a vented polyethylene container because hydrogen chloride vapour attack on the steel cap of a glass bottle can occur within 48 hours at 30 °C.