IMDG Class 4.3 is the hazard subclass for substances that emit flammable gases on contact with water at a rate exceeding the IMDG-defined threshold. The class covers alkali and alkaline earth metals (sodium, potassium, lithium, calcium), metal hydrides (lithium aluminium hydride, sodium borohydride, calcium hydride), metal carbides (calcium carbide), certain metal alkyls, and a small number of inorganic salts. The hazard mechanism is water-reaction generating hydrogen, acetylene, methane, or other flammable gases that can ignite spontaneously or pose a fire risk in confined spaces.
What defines Class 4.3
A substance is Class 4.3 if its reaction with water at standardised test conditions evolves flammable gas at a rate above 1 litre per kilogram per hour. The PG split is based on gas evolution rate:
| Packing group | Gas evolution rate (L/kg/hr at standard test) | Examples |
|---|---|---|
| PG I | ≥ 10, with spontaneous ignition | Lithium aluminium hydride, sodium metal, potassium metal |
| PG II | ≥ 1, no spontaneous ignition | Calcium carbide, lithium hydride |
| PG III | ≥ 0.1, no spontaneous ignition under standardised test | Sodium borohydride (in some forms), some less reactive metal hydrides |
PG I substances react violently with water and the evolved hydrogen typically ignites at the reaction surface. PG II substances react vigorously but usually do not ignite at the surface (though the gas can still ignite from secondary heat sources). PG III substances react slowly enough that gas evolution is manageable in normal handling.
Common Chinese-export Class 4.3 substances
| Substance | UN number | PG | Use |
|---|---|---|---|
| Calcium carbide | UN 1402 | II | Acetylene generation, steel making, calcium cyanamide synthesis |
| Lithium aluminium hydride | UN 1410 | I | Reducing agent in pharmaceutical and fine-chemical synthesis |
| Sodium borohydride | UN 1426 | I or III depending on form | Pulp bleaching, pharmaceutical reduction |
| Sodium hydride | UN 1427 | I | Strong base in fine-chemical synthesis |
| Sodium metal | UN 1428 | I | Sodium-ammonia reductions, certain dye intermediates |
| Magnesium granules | UN 1869 | II or III | Pyrotechnics, some metallurgy applications |
Calcium carbide is the volume Class 4.3 cargo from China, used historically for acetylene generation and still produced at scale. The other entries are specialty intermediates with smaller volumes but significant per-tonne value.
Packaging requirements
Class 4.3 packaging must exclude water, and water vapour. Specific requirements:
- Calcium carbide: hermetically sealed steel drums (UN 1A1) with inner moisture barrier. Drum closure inspection at load port to confirm seal integrity.
- Metal hydrides (lithium aluminium hydride, sodium hydride): small parcels in HDPE bottles inside metal cans, the cans inside fibreboard outer boxes. Inert-atmosphere packing standard for higher-purity grades.
- Sodium and potassium metal: under mineral oil in welded steel containers. The mineral oil prevents air and water contact. The container sealing is the critical packaging element.
- Sodium borohydride: the powder and pellet forms have different humidity tolerance. Powder in moisture-barrier drums; pellets in heat-sealed PE inside fibreboard drums.
The desiccant inclusion in Class 4.3 packaging is a routine quality check. Carriers and customs sometimes inspect packages and reject cargo where the desiccant has failed (turned colour) indicating moisture ingress.
Segregation at sea
Class 4.3 must be stowed:
- “Separated from” Class 3 (flammable liquids), Class 4.1 (flammable solids), Class 4.2 (spontaneously combustible), Class 5.1 (oxidisers), Class 5.2 (organic peroxides)
- “Away from” Class 8 (corrosives), particularly aqueous corrosives that could leak and contact the 4.3 cargo
- “Segregated from” Class 1 (explosives) and Class 7 (radioactives)
For shipboard stowage, Class 4.3 cargo is typically placed in dry holds away from any aqueous cargo or any cargo that could leak water. Ventilation is provided to disperse any small leakage of evolved hydrogen.
Documentation chain
Standard DG documentation with extras:
- Standard DG declaration with UN number, PSN, class, PG, quantity
- SDS section 14 carrying transport classification
- Packaging seal-integrity certificate from the factory (especially for calcium carbide and metal sodium)
- For volume calcium carbide cargo: a moisture-content certificate from the factory laboratory
Some Chinese carriers and forwarders also require a “no leakage” inspection certificate before loading. The certificate is issued by the freight forwarder’s surveyor inspecting the drums or containers at the load port.
Operator note: the calcium-carbide-in-humid-port problem
Calcium carbide reacts with atmospheric moisture, not just liquid water. In high-humidity conditions a sealed drum with a small seal defect can develop pressure over weeks at sea as moisture vapour reacts with carbide and generates acetylene inside the drum. Carbide drums arriving at humid destination ports occasionally show drum bulging on opening. The carrier reports this and the buyer should not unseal the drum without proper venting and gas-handling equipment.
The sail timing matters. Carbide cargo dispatching from Tianjin in winter (low humidity) and arriving at Singapore (high humidity) accumulates moisture exposure during voyage. Arrange unloading and use within 30 days of arrival; longer storage at humid ports increases the seal-failure risk.
Related terms
IMDG umbrella code. UN number. Packing group. Class 4.1, Class 4.2, the other Class 4 subclasses with strict mutual segregation. Segregation table.