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IMDG Class 4: Flammable Solids and Combustibles

IMDG Class 4 of the International Maritime Dangerous Goods Code covers three sub-divisions of dangerous solids and certain low-melting-point substances united by their tendency to ignite or spontaneously combust under transport conditions. Division 4.1 Flammable Solids are solids that can be readily ignited by friction or by a small external flame, plus self-reactive substances that decompose exothermically (matches, sulphur, naphthalene, nitrocellulose, self-reactive substances types B through F). Division 4.2 Substances Liable to Spontaneous Combustion are pyrophoric or self-heating substances that auto-ignite in air without an external ignition source (white phosphorus, alkali metals stored under inert gas, some organic peroxide-treated cottons, sulphide ores). Division 4.3 Substances Which in Contact With Water Emit Flammable Gases are the most operationally hazardous of the three because water - the universal fire-fighting agent - is the trigger for the hazard, not the cure (sodium, potassium, calcium carbide, magnesium powder, aluminium phosphide). Each sub-division has distinct stowage rules under IMDG Code Chapter 7, distinct EmS Emergency Schedules (F-A and F-G for 4.1, F-G for 4.2, F-G/F-H for 4.3 with no-water provisos), and characteristic incident profiles. The most challenging Class 4 incidents on record are 4.3 events: vessel fires that worsen when the master orders flooding because crew did not consult the SoF / DG manifest first. ShipCalculators.com hosts the principal computational tools that support Class 4 cargo handling: the IMDG segregation calculator, the IMDG packing group calculator, the IMDG Class 4.3 water-reactive calculator, the IMDG EmS lookup, the IMDG limited quantity calculator, the container IMDG class lookup, the IMDG tank container calculator and the IMO IMDG general calculator. A full listing of related computational tools is available in the calculator catalogue.

Contents

Background

Why Class 4 has three sub-divisions

Class 4 is unusual among the IMDG classes in that it covers three quite different physical phenomena, all of which produce a fire hazard but by different mechanisms:

  • 4.1 Flammable Solids: solid that can be ignited externally (friction, hot surface, small flame) and that propagates flame readily. Same mechanism as a Class 3 flammable liquid except in the solid phase. Also includes solid self-reactive substances and desensitised explosives.
  • 4.2 Spontaneously Combustible: substance that ignites itself, in air, without any external ignition source. Mechanism is rapid oxidation of finely divided metal or pyrophoric organic compound; if the heat is generated faster than it dissipates, the substance reaches its auto-ignition temperature and combustion begins.
  • 4.3 Water-reactive: substance that reacts with water to release flammable gas (hydrogen most commonly, also acetylene, phosphine, methane). The water reaction is normally exothermic, providing the ignition energy for the released flammable gas. Net effect is rapid fire propagation when water contacts the substance.

The three sub-divisions therefore have radically different fire-response procedures: 4.1 fires accept water; 4.2 fires generally accept water but must be approached with awareness of pyrophoric splash; 4.3 fires reject water absolutely and must be fought with dry powder, sand, dolomite or graphite.

Class 4 vs Class 1

There is overlap at the boundary between Class 4.1 (self-reactive substances) and Class 1 (explosives). The distinguishing factor is the UN classification test result:

  • A self-reactive substance that detonates in the standard test is Class 1 (explosive).
  • A self-reactive substance that deflagrates rapidly but does not detonate is Class 4.1 type B.
  • A self-reactive substance that decomposes exothermically without rapid deflagration is Class 4.1 type C, D, E or F (decreasing severity).

The same substance may be Class 1 or Class 4.1 depending on the formulation: nitrocellulose with less than 12.6% nitrogen content can be Class 4.1; with higher nitrogen content it becomes Class 1.

Class 4 vs IMSBC

For solid bulk cargoes, the equivalent regime to IMDG Class 4 is the IMSBC Code (International Maritime Solid Bulk Cargoes Code). Many self-heating bulk cargoes are regulated under IMSBC Group B (chemical hazards) rather than IMDG Class 4.2:

  • Coal: IMSBC Group B, with self-heating subsidiary hazard.
  • Direct-reduced iron (DRI): IMSBC Group B, with self-heating, water-reactive properties.
  • Sulphide ores (zinc concentrate, lead concentrate): IMSBC Group A (liquefaction) plus B (chemical) - some are also Class 4.2 in packaged form.

Packaged shipments of these substances (in IBCs or drums on container vessels) fall under IMDG Class 4.2, not IMSBC. The two regimes coexist for the same chemical species depending on the packaging form.

Division 4.1: Flammable Solids

Definition and examples

A substance is Class 4.1 if it is a solid that, in the dry state, can be readily ignited by brief contact with an ignition source (such as a burning match) and that burns rapidly enough to constitute a hazard. The class includes:

  • Combustible solids: matches (UN 1944, 1945), naphthalene (UN 1334), sulphur (UN 1350), camphor (UN 2717).
  • Self-reactive substances type B-F: organic peroxides and other compounds that decompose exothermically, with the type letter denoting decreasing severity. Type A self-reactives are classified as Class 1 (explosive); types B through F are Class 4.1.
  • Desensitised explosives: nitrocellulose wetted with at least 25% water (UN 2555), trinitrotoluene wetted (UN 1356, although also Class 1 dry).
  • Polymerising substances: substances that polymerise exothermically and uncontrollably (some isocyanates, some methacrylates).

Stowage and segregation

Standard 4.1 stowage:

  • On deck or under deck in approved cargo spaces.
  • Away from heat sources.
  • Self-reactive substances type B-D require temperature control (refrigerated containers, specified maximum temperature on the MDGF).
  • Type E and F are less temperature-sensitive but still benefit from cool stowage.

Segregation:

  • From Class 1: ‘separated longitudinally by an intervening complete compartment’.
  • From Class 5.1 oxidisers: ‘separated by complete compartment’ (4.1 + 5.1 is a fire-acceleration combination).
  • From Class 5.2 organic peroxides: ‘separated longitudinally’ for type B-F self-reactives.
  • From Class 8: ‘separated from’.

EmS

  • F-A: standard fire schedule, water acceptable.
  • F-G: with combustible packaging or with self-reactive material, requires boundary cooling and possibly evacuation.

Spillage schedules typically S-G or S-J for 4.1 entries.

Division 4.2: Spontaneously Combustible

Definition and examples

A substance is Class 4.2 if it is liable to spontaneous heating under normal transport conditions or to heating up in contact with air with the consequent risk of ignition. Sub-categories:

  • Pyrophoric: ignites within five minutes of exposure to air. Examples: white phosphorus (UN 1381), alkylaluminium compounds (UN 3052), alkyllithiums (UN 3394).
  • Self-heating: undergoes self-heating in large quantities and over hours or days. Examples: copra (UN 1363, dried coconut meat, self-heats due to oil oxidation), sulphide ores in finely divided form, some organic chemicals stored in bulk.

Stowage and segregation

Pyrophoric substances ship in inert atmosphere containers: cylinders or drums sealed under nitrogen or argon, with valve packings designed to maintain the seal under transport vibration. Loss of inert atmosphere produces immediate ignition; the cargo is therefore handled with extreme care during loading and discharge.

Self-heating substances require:

  • Adequate ventilation to remove heat.
  • Quantity limits per stowage location to limit the heat-source size.
  • Avoid stacking to allow heat dissipation from individual packages.
  • Temperature monitoring during long voyages.

Segregation is similar to 4.1 with additional restrictions from Class 8 (acid contact with metals can produce hydrogen plus heat) and from Class 5 (oxidiser contact accelerates combustion).

EmS

  • F-G: standard fire schedule with water cautious. White phosphorus is fought by smothering with sand and water cooling once smothered; alkyl metals are fought with dry powder only (water reacts violently).

Spillage schedules typically S-J or S-M.

Division 4.3: Water-reactive

Definition and examples

A substance is Class 4.3 if it reacts with water to evolve flammable gas at a rate determined by UN Test N.5. The principal water reactions are:

  • Alkali metal + water: Na + H2O → NaOH + ½H2 (with significant heat). Sodium (UN 1428), potassium (UN 2257), lithium (UN 1415).
  • Calcium carbide + water: CaC2 + 2H2O → Ca(OH)2 + C2H2 (acetylene). Calcium carbide (UN 1402).
  • Active metal powder + water: Mg, Al, Zn powders evolve hydrogen. Magnesium powder (UN 1418).
  • Phosphide + water: phosphide ions evolve phosphine (PH3, itself toxic and flammable). Aluminium phosphide (UN 1397) - widely used as a grain fumigant.
  • Hydride + water: NaH, LiH, CaH2 evolve hydrogen.

The IMDG Class 4.3 water-reactive calculator returns the gas type and approximate evolution rate for a given UN entry.

Stowage and segregation

The defining requirement is isolation from water. Stowage must:

  • Be in a watertight compartment.
  • Not be near scupper drains, ballast tank vents or fire-main outlets.
  • Have container roofs intact (rain-water ingress is the most common in-transit failure mode).
  • Be marked with the ‘when wet’ placard (white symbol of fire on blue diamond background).

Segregation:

  • From all aqueous substances (‘away from’ minimum).
  • From foodstuffs containing water (‘separated from’).
  • From Class 8 acids (‘separated longitudinally’, because acid drainage water can reach the 4.3 substance).
  • From other 4.3 substances of incompatible chemistry (sodium and acid phosphides should not be co-stowed).

EmS

  • F-G with caveats: do NOT use water on 4.3 fire. Use dry powder, sand, dolomite, graphite or specific class D extinguishing agents. Smother oxygen if possible.
  • S-N: spillage schedule. Do not use water; collect with dry sand or dry inert absorbent; transfer to a sealed dry container; dispose ashore.

The crew must be trained to recognise the ‘when wet’ placard and to override their default ‘water on fire’ response when the placard is present.

Notable casualties

MV Bow Mariner 2004

The chemical tanker Bow Mariner exploded and sank off the Virginia coast in February 2004 carrying ethanol (Class 3) plus residue of methyl tert-butyl ether (MTBE). Although primarily a Class 3 incident, the casualty involved a Class 4.3 cause: the residual cargo had been treated with magnesium-based desulphurisation agents, and crew using water to flush a tank generated hydrogen gas which subsequently exploded. 21 of 27 crew died. The investigation drove mandatory Class 4.3 awareness training for chemical tanker crews, particularly around tank-cleaning procedures.

MV Sewol 2014

The Korean ferry Sewol capsized and sank in April 2014 with the loss of over 300 lives. Although the immediate cause was overloading and unstable cargo securing, the casualty included incorrectly stowed Class 4.3 calcium carbide drums that contributed to the rapid fire propagation after the vessel listed. The Sewol disaster drove the global tightening of Ro-Ro passenger vessel cargo declaration standards and the requirement for specific Class 4 awareness for ferry crews.

Tianjin port explosion 2015

The Tianjin port explosion of 12 August 2015 (covered also in the IMDG Class 1 article) involved Class 4.3 calcium carbide alongside the principal Class 1 and Class 5.1 cargoes. The water sprayed on the developing fire by first responders reacted with the calcium carbide to generate acetylene, which contributed to the secondary detonations. The disaster reinforced the lesson that emergency responders must consult the cargo manifest before applying water and that storage facilities must clearly label the ‘when wet’ substances for first-responder visibility.

Documentation

MDGF requirements

For Class 4 entries the MDGF must include:

  • UN number, proper shipping name, class (4.1, 4.2 or 4.3), packing group.
  • For self-reactive substances: type letter and control temperature (the temperature above which the substance must not be allowed to rise during transport).
  • For water-reactive substances: explicit ‘water-reactive’ indication.
  • EmS reference (typically F-A, F-G, F-H).
  • 24-hour emergency contact.
  • Marine pollutant indicator if applicable.

Container Packing Certificate

Class 4 packing requirements:

  • Inner packagings designed to prevent leak (especially for 4.3 entries).
  • Cushioning and absorbent material between drums and container walls.
  • Drum closures torqued to specification.
  • For 4.3: container roof and side seals verified watertight.
  • Marking and placarding correct (the ‘when wet’ placard for 4.3 must be visible from a distance).

Routeing and notification

Classes 4.2 and 4.3 frequently require:

  • Pre-arrival notification (24-72 hours).
  • Avoidance of certain ports during typhoon or monsoon seasons (heightened water-ingress risk).
  • Specific anchorage or berth allocation away from fuel tankers and from population centres.

See also

References

  • IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022.
  • IMO, International Maritime Solid Bulk Cargoes Code (IMSBC Code), International Maritime Organization, current edition.
  • IMO, EmS Guide: Revised Emergency Response Procedures for Ships Carrying Dangerous Goods, International Maritime Organization, current edition.
  • United Nations, Recommendations on the Transport of Dangerous Goods, Model Regulations, 22nd revised edition, United Nations, 2021.
  • United Nations, Manual of Tests and Criteria, Part III, 7th revised edition, United Nations, 2019.
  • US Chemical Safety and Hazard Investigation Board, Bow Mariner Investigation Report, 2005.
  • Republic of Korea Maritime Safety Tribunal, Sewol Casualty Investigation Report, 2014.
  • State Council of the People’s Republic of China, Tianjin Port 8.12 Major Fire and Explosion Accident Investigation Report, 2016.