IMDG Class 4: Flammable Solids, 4.1/4.2/4.3

IMDG Class 4 is one of the most operationally demanding categories in the IMDG Code, not because individual substances are as acutely hazardous as some Class 1 or Class 6.1 entries, but because the three divisions demand entirely different emergency responses. Water extinguishes a Class 4.1 fire. Water accelerates a Class 4.3 fire. The crew who switches from a 4.1 package label to a 4.3 one without re-reading the stow plan can turn a containable incident into a catastrophe. The Tianjin port explosion of August 2015 cost 173 lives partly because responders applied water to calcium carbide, generating acetylene, which fed a chain of secondary detonations. The lesson repeats every decade.

This article covers Class 4 under the IMDG Code 2022 Edition (Amendment 41-22, mandatory from 1 January 2024), which implements SOLAS Chapter VII and aligns with the UN Model Regulations 23rd revised edition. Classification test methods come from the UN Manual of Tests and Criteria, 8th revised edition (2023). The IMDG Class 4.3 water-reactive calculator returns gas type and evolution rate for a given UN entry, and the IMDG EmS lookup retrieves the applicable Fire and Spillage schedules.

Three divisions, three fire-fighting doctrines

The division boundary in Class 4 is not a bureaucratic subdivision; it reflects three distinct physical mechanisms that produce radically different emergency procedures.

Division 4.1 substances ignite when exposed to an external ignition source such as a spark, a match flame, or friction. Once lit, they sustain combustion. The mechanism is the same as a Class 3 flammable liquid except the fuel is solid. Water suppresses 4.1 fires by cooling and blanketing. Self-reactive substances in 4.1 add a second mechanism: exothermic decomposition that can accelerate without any external ignition, but even these respond to cooling water once ignited.

Division 4.2 substances generate heat internally. Pyrophoric materials react with atmospheric oxygen fast enough to reach ignition temperature within seconds of air exposure. Self-heating materials accumulate heat slowly, over hours or days, when stored in bulk quantities that prevent dissipation. The common factor is auto-ignition: no external flame or spark needed. Water response is substance-specific. White phosphorus can be quenched with water after smothering, because it needs to be isolated from oxygen. Alkyl metals react violently with water and must be fought dry.

Division 4.3 substances react with water itself to release flammable gas, and the reaction is usually exothermic, so the water both generates the fuel and provides the ignition energy. Sodium metal dropped in water produces hydrogen gas and enough heat to ignite it. Calcium carbide produces acetylene. Aluminium phosphide produces phosphine. The EmS for these entries carries an explicit no-water proviso that overrides default fire-fighting doctrine.

The comparison table at the end of this article sets the operational parameters side by side.

Regulatory framework and the SOLAS mandate

Class 4 transport by sea is governed by SOLAS Chapter VII, Part A (Regulation 2), which requires compliance with the IMDG Code for packaged dangerous goods. The IMDG 41-22 edition entered mandatory force on 1 January 2024 (the 2020 Edition had been mandatory from 1 January 2022 as Amendment 40-20). Port state control authorities inspecting under the Paris MOU or Tokyo MOU check compliance against the current mandatory amendment; carrying a ship’s copy of an outdated amendment is itself a deficiency.

The IMDG Code draws its classification criteria from the UN Model Regulations, Part 2, Chapter 2.4 (Class 4). Test methods come from Part III of the UN Manual of Tests and Criteria: Series N tests cover Class 4.1, 4.2, and 4.3. The IMDG Code cross-references the test series by letter: N.1 through N.5 cover the relevant screening and classification criteria.

Class 4 substances must carry a Dangerous Goods Declaration (DGD) per IMDG Code Chapter 5.4, a Container/Vehicle Packing Certificate where packed in a CTU, and proper marks and labels per Chapter 5.2. IMDG marking, labelling, and placarding covers the visual identification system in full. The SOLAS Chapter VII article covers the treaty obligation that makes IMDG compliance mandatory at sea.

Division 4.1: Flammable Solids

Definition and scope

IMDG Code Section 2.4.2 defines Division 4.1 as covering three distinct groups:

1. Flammable solids: solids that, under the conditions encountered during transport, are readily combustible, or may cause or contribute to fire through friction.
2. Self-reactive substances and mixtures: thermally unstable substances liable to exothermic decomposition even without oxygen.
3. Desensitized explosives: solid or liquid explosives that have been wetted or diluted with water, alcohol, or other phlegmatizing agents to suppress explosive properties for transport.

The breadth of this grouping produces an unusual range within a single division. UN 1350 (Sulphur) is a routine commodity cargo: it has Packing Group III, ships in bulk or in 25 kg bags, and its Class 4.1 classification reflects a relatively modest burning rate. UN 3223 (Self-reactive solid, type C, temperature controlled) requires a refrigerated container, a specified control temperature printed on the DGD, and emergency arrangements in port that include 24-hour access to a temperature-monitoring system.

Classification: the burning rate test (UN Test N.1)

For ordinary flammable solids, the classification criterion is the burning rate test from UN Manual of Tests and Criteria Part III, Section 33.2.1 (Test N.1). The procedure differs by form:

For powders and granules, the substance is pressed into a groove 250 mm long and ignited at one end. If the flame propagates 200 mm in under 120 seconds, and that burning rate exceeds 2.2 mm/s through the wetted zone of a second test where the central 80 mm has been wetted with water, the substance is Class 4.1.

For solid strips (like stick-form naphthalene or wax), ignition is applied at one end and the time for 200 mm burn is measured. A burning rate above 2 mm/s places it in Class 4.1. Substances that burn more slowly than the threshold are not classified.

Metals and metal alloys in powder, granular, or paste form require an additional assessment under Test N.1, taking into account specific surface area, particle size distribution, and moisture sensitivity. The IMDG packing group calculator applies these thresholds against UN entry data.

Classification: self-reactive substances (UN Test Series A through H)

Self-reactive classification uses seven type letters (A through G) assigned through Test Series A through H from UN Manual of Tests and Criteria Part II, Section 20. The type letter governs the permissible quantity per package, the temperature control requirement, and the maximum quantity per transport unit.

Type A self-reactive: detonates or deflagrates rapidly as packaged. These are prohibited from transport by sea under the IMDG Code; they must be reclassified or reformulated. In practice, the same substance at a lower concentration or in a different formulation may qualify as Type B.

Type B: does not detonate but may deflagrate rapidly, or may show a strong thermal effect in packaging. Maximum net mass per package: 10 kg for solids or 5 kg for liquids. Requires temperature control below the specified control temperature; the SADT for Type B substances is determined at 55 °C screening.

Types C and D: intermediate decomposition severity. Type C does not detonate and has only a weak thermal effect; Type D detonates weakly or deflagrates slowly. Maximum net mass per package: 25 kg (Type C) or 50 kg (Type D), both temperature-controlled.

Types E and F: slow decomposition. Type E does not show detonation, strong thermal effect, or rapid deflagration. Type F shows only a slight effect or none. These two types may not require temperature control; the IMDG special provisions for each UN entry confirm the requirement.

Type G: thermally stable (SADT 60 °C or higher), no temperature control required, no special transport restrictions beyond standard labelling.

The significance of this system is the SADT. The Self-Accelerating Decomposition Temperature is the lowest temperature at which self-heating leading to decomposition begins in a test vessel representing the commercial package. IMDG Code Section 2.4.2.3 requires:

• Control temperature: 10 °C below the SADT (for packages with SADT between 20 °C and 60 °C) or 15 °C below SADT (for SADT under 20 °C).
• Emergency temperature: 5 °C below the control temperature.

These two temperatures appear on the DGD and on the container. If the cargo hold temperature rises above the control temperature during passage, the master must take cooling action. If it reaches the emergency temperature, disembarkation or jettison procedures apply per the ship’s Dangerous Goods Document procedures and company SMS.

Desensitized explosives

The third category within 4.1 covers substances that, in their dry state, would be Class 1 (explosives), but have been wetted or diluted to suppress explosive character. The most commercially significant are:

UN 2555: Nitrocellulose with water, minimum 25% water by mass. Dry nitrocellulose with nitrogen content above 12.6% by mass is UN 0340/0341/0342/0343 (Class 1). Wetted at 25% or more, it ships as UN 2555, Class 4.1, PG II. The critical condition is maintaining moisture: if the nitrocellulose dries out during transport, Class 1 properties can return.

UN 1356: 2,4,6-Trinitrotoluene (TNT) wetted with not less than 30% water by mass. In dry form, TNT is UN 0209, Class 1.1D.

UN 1571: Barium azide, wetted with not less than 50% water by mass.

For desensitized explosives, the DGD must state the actual water content (or other phlegmatizing agent percentage) and the minimum content required. Container packing procedures must verify that the moisture content has not dropped below the minimum before sealing.

Notable Class 4.1 substances and UN numbers

Sulphur (UN 1350) is the highest-volume Class 4.1 commodity in maritime trade, shipped in millions of tonnes annually from Middle East and Central Asian production to fertilizer manufacturers in Asia and South America. It ships as PG III, either in bulk under the IMSBC Code (where it has its own IMSBC schedule) or in large bags as Class 4.1 under IMDG. The bulk IMSBC sulphur schedule has additional requirements around dust explosion risk that the packaged IMDG entry does not capture.

Naphthalene (UN 1334) is a polycyclic aromatic hydrocarbon used in moth repellents and as a chemical feedstock. PG III, Class 4.1. Its relatively high melting point (80 °C) means it can liquefy in hot cargo holds, producing a combustible pool that behaves partly like a Class 3 liquid.

Matches (UN 1944, safety matches; UN 1945, wax matches; UN 2254, matches, fusee) are among the oldest Class 4.1 entries. Limited quantities are permitted under IMDG Code Section 3.4 for safety matches, reducing labelling burdens for consumer-grade small packages.

Metal powders require individual assessment. Magnesium powder (UN 1418) is PG II, with subsidiary risk Class 4.3 because it reacts with water to produce hydrogen. Iron oxide, scale, or sinter in coarse form may not be classified at all; in fine powder form the same material may be Class 4.2 (self-heating by oxidation). Particle size and surface area are determinative.

Charcoal (UN 1361, animal charcoal; UN 1362, activated carbon) is Class 4.2 when freshly produced and inadequately cooled, and Class 4.1 once it has stabilized. The specific characterization depends on the self-heating test result.

Stowage for 4.1

Standard 4.1 stowage under IMDG Code Section 7.2.7 (Segregation Table): on deck or under deck. Temperature-controlled entries (Types B and C self-reactives) must be stowed in a location where the hold temperature can be monitored and where refrigeration equipment is accessible.

Key stowage restrictions from IMDG Code Chapter 7.3 (Special Stowage Provisions):

SW1 applies to most 4.1 entries: stow “away from sources of heat.” For self-reactive substances under temperature control, the special provision typically adds SW22 (“Temperature controlled”) and specifies the control and emergency temperatures in the substance’s individual schedule in IMDG Code Volume 2.

Segregation for 4.1

IMDG Code Chapter 7.2 Segregation Table assigns 4.1 the following from-class requirements:

• From Class 1 (explosives): “Separated longitudinally by an intervening complete compartment or hold” from most explosive divisions; complete separation from 1.1, 1.2, and 1.5.
• From Class 4.2: no special segregation (“away from” applies as a default between incompatible classes, but 4.1 and 4.2 do not carry an explicit additional requirement in the standard table).
• From Class 5.1 (oxidizers): “separated by complete compartment or hold.” A flammable solid next to an oxidizer is a fire-acceleration pairing.
• From Class 5.2 (organic peroxides): “separated longitudinally” for Type B-D self-reactive substances, reflecting the similarity of hazard between 4.1 self-reactives and 5.2 entries.
• From Class 8 (corrosives): “separated from” for most 4.1 entries; some individual UN entries carry stricter provisions.

The IMDG segregation calculator automates these matrix lookups, including special provisions from individual UN entries that override the class-level defaults.

Division 4.2: Substances Liable to Spontaneous Combustion

Definition and sub-categories

IMDG Code Section 2.4.3 defines Division 4.2 as substances liable to spontaneous heating under normal transport conditions, or to heating up when in contact with air, and being then liable to catch fire. Two sub-categories produce quite different transport behavior:

Pyrophoric substances ignite spontaneously within five minutes of exposure to air at 20 °C. The classification test is UN Test N.2 (pyrophoric test for liquids) and N.3 (pyrophoric test for solids), both from UN Manual of Tests and Criteria Part III, Section 33.2. If any portion of a solid sample ignites during the five-minute observation window, the substance is pyrophoric. Pyrophoric liquids are placed in 4.2 (not 4.3 or 4.1) if the spontaneous ignition mechanism is oxidation in air rather than water reaction.

Self-heating substances do not ignite that fast. They oxidize slowly, accumulating heat when the surface-area-to-volume ratio of the bulk storage is high enough to generate more heat than dissipates. The classification criterion is UN Test N.4 (self-heating substances test), conducted at 140 °C for 24 hours. A cube of 25 mm side length igniting at 140 °C indicates the substance is classified as 4.2 PG II or III; a smaller sample (100 mm cube) igniting at 140 °C indicates PG II; 100 mm cube igniting at 120 °C indicates PG I.

Pyrophoric substances in maritime trade

White phosphorus (UN 1381) is the canonical pyrophoric substance in maritime operations. It ignites on contact with air and burns with a dense, acrid white smoke producing phosphorus pentoxide. It’s shipped under water: the physical separation from air is the transport safety measure. The standard package is a metal drum filled with water, with the white phosphorus blocks submerged. If the drum loses integrity and drains, ignition is immediate.

The IMDG Code lists white phosphorus as PG I, subsidiary risk 6.1 (toxic), and assigns EmS F-J and S-P. The fire schedule F-J instructs: do not use water on the burning substance directly; instead, flood the hold and allow submersion to quench oxygen access. The spillage schedule S-P prohibits letting the substance dry out and instructs covering any spilled material with wet sand immediately.

Alkylaluminium compounds (UN 3052, alkylaluminium halides, liquid; UN 3393, organometallic substance, liquid, pyrophoric) are used as polymerization catalysts. They react violently with water as well as igniting in air, giving them both 4.2 and 4.3 character, but the primary classification is 4.2 (pyrophoric) when the pyrophoric behavior is the dominant hazard. These ship in specialized tank containers under nitrogen blanket.

Alkyllithium compounds (UN 3394, organometallic substance, liquid, pyrophoric, water-reactive) similarly have dual hazard. Butyllithium solutions in hexane are shipped as 4.2 with subsidiary 4.3 and 3. The hexane (Class 3) constitutes additional vapor hazard.

Sodium sulfide, hydrated, with not less than 30% water by mass (UN 1849) is Class 4.2 PG II. The self-heating mechanism is oxidation of the sulfide ion by residual dissolved oxygen. Dry sodium sulfide is a stronger self-heater; the 30% water content depresses the oxidation rate enough for PG II assignment.

Self-heating substances and the DRI problem

Direct-reduced iron (DRI) is the maritime self-heating example that generates the most incident reports. DRI is metallic iron reduced from ore but not melted; it retains high surface area and contains residual carbon and sulfur compounds that oxidize readily in air and react with water to generate hydrogen. It ships in large bulk carrier parcels of 20,000 to 40,000 tonnes.

DRI falls primarily under the IMSBC Code as a Group B cargo, and the direct-reduced iron IMSBC schedule covers the bulk requirements. Packaged DRI fines (bags, big bags, drums) fall under IMDG Code Class 4.2 when the quantity per package is above the threshold for self-heating assessment. The interaction between the two codes means DRI vessels frequently carry IMSBC schedules in the ship’s library and IMDG Code Class 4.2 DGDs for the smaller packaged consignments.

Copra (UN 1363, dried coconut meat or coconut oil residue) is PG III, Class 4.2. The self-heating mechanism is lipid oxidation: the residual coconut oil oxidizes at rates governed by temperature, moisture, and air access. Ventilation is the primary control: copra stowed under forced ventilation dissipates heat; copra stowed tight in a sealed hold accumulates it. A series of copra fires in the 1990s and 2000s drove revisions to the cargo care provisions in IMDG Code Volume 2 for UN 1363.

Charcoal (UN 1361, animal or vegetable origin; UN 1362, activated) that has been freshly produced and improperly cooled retains residual volatiles and active surface sites that oxidize and self-heat. Fully aged charcoal that passes the Test N.4 screening may not be classified at all. The critical transport parameter is the “freshness” of the production batch: material shipped within four weeks of kiln exit carries higher self-heating risk than material stored for three months.

Oil-contaminated rags, waste, or absorbents can also be 4.2. UN 1856 (Rags, oily) and UN 1364 (Cotton waste, oily) are assigned Class 4.2 when the oil content and fiber surface area combine to produce measurable self-heating by Test N.4. Vessels carrying workshop or engine-room waste ashore must verify classification; the wrong UN number on an oily-rag container creates both MARPOL and IMDG liabilities.

Classification test: UN Test N.4

Test N.4 determines whether a self-heating substance qualifies as Class 4.2 and, if so, which packing group. The procedure:

Three wire-mesh cubes are packed tightly with the test substance: cube sizes 25 mm, 100 mm, and 100 mm. The 25 mm cube runs at 140 °C for 24 hours. The 100 mm cubes run at 140 °C and at 120 °C respectively, each for 24 hours. A thermocouple at the center of each cube records temperature.

Classification thresholds under IMDG Code Section 2.4.3.3:

• Ignition or center temperature exceeding oven temperature by more than 60 K in the 25 mm cube at 140 °C: PG I (most dangerous; maximum 0.5 kg per inner package for many entries).
• Ignition or 60 K rise in the 100 mm cube at 140 °C but not in the 25 mm cube: PG II.
• Ignition or 60 K rise in the 100 mm cube at 120 °C but not at 140 °C: PG III.
• No ignition or 60 K rise in any configuration: not Class 4.2.

The packing group assignment from this test directly controls the maximum net quantity per package (IMDG Code Section 4.1.1.3) and the segregation requirements.

Stowage and segregation for 4.2

IMDG Code Chapter 7.3 special stowage provisions for Class 4.2 include:

SW12: “Stow away from sources of heat.” Applies to most 4.2 entries. SW13: “Stow away from living quarters.” Pyrophoric entries producing toxic combustion products (white phosphorus, organometallics).

Segregation from Class 4.2 to other classes follows the IMDG Code Table 7.2.4, which assigns “away from” as a minimum separation for 4.2 from Class 1, and “separated from” for 4.2 from Class 5.1 and 5.2 (oxidizers accelerate 4.2 self-heating dramatically).

Pyrophoric substances require packaging under inert gas. Any package integrity failure during loading, sea passage, or discharge can produce immediate ignition. The ship’s dangerous goods officer must verify that no package shows deformation, damp patches, or hissing sounds before accepting pyrophoric cargo.

EmS for 4.2

EmS fire schedule F-G applies to most 4.2 entries and carries the standard instruction: cool containers with water spray but do not apply water directly to burning pyrophoric substances, and have crew evacuate the area if a strong reaction is in progress.

Pyrophoric entries such as UN 1381 carry bespoke schedules (F-J for white phosphorus) that override F-G with specific flooding or smothering instructions. The IMDG EmS lookup calculator returns the applicable schedule for any UN number.

Division 4.3: Substances Dangerous When Wet

Definition and mechanism

IMDG Code Section 2.4.4 defines Division 4.3 as substances that, by interaction with water, are liable to become spontaneously flammable or to give off flammable gases in dangerous quantities. The “dangerous quantities” threshold is determined by Test N.5.

The water reaction that drives this class is nucleophilic attack by water on electrophilic metal centers or metal-hydride bonds, releasing hydrogen or, for carbides, acetylene, or for phosphides, phosphine. The stoichiometry for the principal reactions:

Sodium metal:

Calcium carbide:

Aluminium phosphide:

In each case, the reaction is exothermic. For sodium, the heat of reaction is sufficient to ignite the hydrogen immediately. For calcium carbide, the acetylene requires an ignition source (but the heat of reaction warms the evolving gas and the local atmosphere, reducing the energy needed for ignition). For aluminium phosphide, phosphine itself autoignites at 38 °C: a temperature routinely encountered inside a warm cargo hold.

The phosphine production from aluminium phosphide (UN 1397) and zinc phosphide (UN 1714) makes these substances particularly dangerous. Phosphine is acutely toxic at 50 ppm and has an occupational exposure limit of 0.3 ppm (OSHA PEL). A breached container of aluminium phosphide in rain is a simultaneous fire and toxic-gas emergency.

Classification test: UN Test N.5

Test N.5 (water reactivity test) from UN Manual of Tests and Criteria Part III, Section 33.4.6 determines both the classification into 4.3 and the packing group assignment.

A small sample (about 25 g or 25 mL) of the substance is combined with water at 20 °C. A manometer or bubble counter records gas evolution rate over time. The packing group thresholds under IMDG Code Section 2.4.4.3:

• PG I: gas evolves spontaneously (without any requirement for external energy) AND the maximum gas evolution rate exceeds 10 L/kg/hour, OR the gas evolves at any rate and ignites spontaneously.
• PG II: gas does not evolve spontaneously but the maximum rate exceeds 20 L/kg/hour at 20 °C.
• PG III: neither of the above, but the maximum gas evolution rate exceeds 1 L/kg/hour at 20 °C.

Substances with rates below 1 L/kg/hour are not Class 4.3.

For metals in powder form, surface area and particle size dominate the test result. Aluminum powder in 100-micron average particle size may pass the 4.3 test at PG III; the same aluminum at 10-micron particle size may test at PG II. The IMDG Code handles this by listing separate UN entries for different particle sizes: UN 1396 (aluminum powder, coated) versus UN 1396 without coating, with different PG assignments.

Key Class 4.3 UN entries

Calcium carbide (UN 1402) is PG I, Class 4.3. It’s produced in electric arc furnaces and shipped to acetylene production plants worldwide. The global annual production is roughly 19 million tonnes (IEA industrial process data), with China producing over 90% of that. It ships in sealed steel drums inside 20-foot dry containers, with each drum tested for freedom from moisture before filling. The minimum water-free condition must be maintained through the sea voyage: a container that leaks rain on a calcium carbide cargo converts a commodity chemical into an acetylene generator.

Sodium metal (UN 1428) and potassium metal (UN 2257) are both PG I. Sodium ships under kerosene or mineral oil (the oil excludes air and water). The reaction of sodium with water is vigorous at ambient temperature and violent at higher temperatures; a single kilogram of sodium produces roughly 21 grams of hydrogen, enough to fill a one-cubic-meter balloon at 25% of the lower explosive limit. Potassium reacts even faster; its reaction with water generates superoxide (KO2) as a secondary product, which is itself an oxidizer.

Lithium metal (UN 1415) is PG I. It’s used in battery manufacturing, pharmaceutical synthesis, and specialty alloy production. Unlike sodium and potassium, lithium burns with a crimson flame and produces lithium hydroxide plus lithium oxide fume, which are corrosive. Lithium battery fires (Class 9 or, in some configurations, Class 4.3) have been the subject of multiple IMO Circular correspondence since 2016, but the UN 1415 metallic lithium entry remains Class 4.3 for pure metal in bulk.

Magnesium powder (UN 1418) is PG II, with subsidiary risk Class 4.1 (flammable solid), because it reacts with water to evolve hydrogen but also burns readily when ignited in air. Fine magnesium powder (under 300 micron particle diameter) additionally carries a dust explosion risk that requires earthing of containers during transfer operations. Magnesium granules (UN 2950) are PG III.

Aluminium phosphide (UN 1397), zinc phosphide (UN 1714), and magnesium phosphide (UN 2011) are PG I, subsidiary risk Class 6.1 (toxic) because the phosphine product is an acutely toxic fumigant gas. These substances are widely used in agriculture as rodenticides and grain-storage fumigants. Their maritime transport in pellet or tablet form requires UN-approved packagings with specific gastight closures. The 1998 death of six crew members aboard the MV Pionersk was attributed to phosphine from UN 1397 cargo held in a partially sealed hold.

Sodium borohydride (UN 1426) is PG I. It reduces water rapidly in acidic conditions and more slowly in alkaline conditions; the commercial compound is typically stabilized in alkaline solution for liquid form (which would ship as a different UN number). Solid sodium borohydride, used as a pharmaceutical reducing agent, reacts with water to evolve hydrogen and must be kept completely dry.

Calcium, in turnings or cuttings form (UN 1401) and as calcium granules (UN 1855), reacts with water to produce calcium hydroxide plus hydrogen. The reaction rate for Ca is slower than for Na or K, placing it at PG II. Calcium powder (UN 1855) can be PG I depending on particle size.

Stowage for 4.3

IMDG Code Section 7.2.7 and special stowage provision SW1 require that all Class 4.3 packages are stowed “away from” sources of water. The specific provisions in Chapter 7.3 add:

SW2 applies to PG I entries: “Stow ‘on deck or under deck’.” The on-deck option must be in a closed freight container.

SW8 applies to water-sensitive substances that must not be wetted: “Stow away from any sources of moisture, including condensation. Ensure all packages are in good condition before stowage.” Condensation in a container cycling through temperature gradients (tropical days, cold nights at sea) can produce enough moisture to initiate a reaction in poorly sealed drums.

Critical ship-design interaction: Class 4.3 must not be stowed in a cargo space immediately above or adjacent to any ballast tank vent, bilge pipe outlet, fire main outlet, or sprinkler nozzle. A ship with a CO2 fixed fire-suppression system in a hold carrying 4.3 cargo must verify that the CO2 system does not also carry water-mist supplementary stages. CO2 itself does not react with Class 4.3 substances, but any moisture entrained in CO2 cylinders from condensation is a concern for PG I entries.

The “when wet, dangerous” or “dangerous-when-wet” placard (IMDG Code Section 5.2.2.2.1.2, label No. 4.3) must be affixed to all packages and containers. It’s a blue diamond with a white flame symbol. Its presence is the crew’s visual cue to override default water fire-fighting.

Segregation for 4.3

IMDG Code Table 7.2.4 assigns 4.3 the following key requirements:

• From Class 1 (explosives): “Separated longitudinally by an intervening complete compartment” from most divisions.
• From Class 5.1 (oxidizers): “Separated from.” An oxidizer leaking onto a 4.3 substance can initiate or accelerate the water-reaction chemistry.
• From Class 8 (corrosives): “Separated longitudinally.” Acidic corrosive drainage reaching a 4.3 substance can initiate hydrogen evolution without the water test threshold being met.

Class 4.3 also carries “separated from” requirements for all aqueous solution cargoes and for all substances liable to produce moisture in the stow.

EmS for 4.3

The fire schedule for most Class 4.3 entries is F-G. But the EmS Guide’s F-G entry for Class 4.3 carries explicit no-water instructions that distinguish it from F-G as applied to Class 4.1 or 4.2. The relevant extracts from the EmS Guide (IMO, 2021 edition) for 4.3:

“FIRE: Do not use water. Dry powder or dry sand recommended.”

For phosphide entries (UN 1397, UN 1714, UN 2011), F-G is supplemented by a toxicity warning: don breathing apparatus before approaching; phosphine concentration may be lethal at the cargo location even without a visible fire.

The spillage schedule S-N (“Keep dry”) applies to most 4.3 solid entries. Instructions: do not use water to wash spillage; collect with dry sand or other dry inert material; transfer to a marked, sealed dry container; deliver ashore to a licensed waste facility.

Division comparison table

Temperature control for self-reactive substances and some 4.2 entries

Self-reactive substances classified as Types B, C, D, and sometimes E require temperature-controlled stowage where the cargo temperature does not exceed the control temperature assigned in the IMDG Code Volume 2 schedule for that UN entry. The control and emergency temperatures derive from the SADT as described above.

In practice, temperature control means either a refrigerated container (reefer unit set to the control temperature) or a ventilated cargo space where the ship’s refrigeration system can maintain temperature. Most commercial operators use 20-foot or 40-foot refrigerated ISO containers. The reefer unit’s set-point must be verified and logged before loading, and the container must be connected to the ship’s reefer power from the moment it comes aboard.

The IMDG Code requires port facilities handling temperature-controlled Class 4.1 to have emergency procedures for reefer failure. If the reefer unit fails at sea, the DG manifest must identify which consignments carry emergency temperatures, allowing the master to prioritize monitoring. When the emergency temperature is breached, the relevant IMDG Code procedure calls for jettison if the substance cannot be safely cooled and the vessel is more than 12 nautical miles from the nearest land.

Some Class 4.2 substances, particularly organometallic compounds, also require temperature-controlled stowage, though the mechanism is preventing thermal runaway rather than maintaining SADT margin. These entries carry explicit temperature provisions in their IMDG Code Volume 2 schedules.

Marine pollutant status within Class 4

A number of Class 4 entries carry the Marine Pollutant mark (the MP mark, a fish and tree symbol) under IMDG Code Chapter 2.10. The principal marine pollutant entries within Class 4 include:

• UN 1397 (Aluminium phosphide): Marine Pollutant. The compound itself is acutely toxic to aquatic organisms, and phosphine dissolved in seawater produces phosphoric acid.
• UN 2011 (Magnesium phosphide): Marine Pollutant, same reasoning.
• UN 1381 (Phosphorus, white, dry or under water or in solution): Marine Pollutant; highly toxic to aquatic life.

Marine pollutant status adds a marking and documentation requirement: the letters “MARINE POLLUTANT” must appear on packages above 5 L or 5 kg, and the DGD must state “MARINE POLLUTANT” prominently. Under MARPOL Annex III, discharge of marine pollutant packaged goods at sea is prohibited (with narrow exceptions for safety of the vessel or crew).

Subsidiary hazards and dual-classification entries

Many Class 4 entries carry subsidiary-risk labels that indicate a second IMDG class hazard. The subsidiary risk affects stowage, segregation, and emergency response:

Magnesium powder (UN 1418) is classified 4.3 primary, 4.1 subsidiary. It reacts with water (4.3) and also burns in air (4.1). Stowage must satisfy the more restrictive requirements of both divisions.

Organolithium compounds (UN 3394, 3393 series) often carry subsidiary risks from 4.2, 4.3, and 3 simultaneously. The hexane or ether solvent contributes Class 3 vapor hazard; the water reactivity contributes 4.3; the pyrophoric character drives the 4.2 primary classification.

UN 1371 (Dinitrosobenzen, dry) has been reclassified at various revisions; some entries in the 4.1 self-reactive group carry subsidiary risk Class 1 for specific formulations if the decomposition propagates in packagings above a certain size. Always cross-check the current IMDG Code Volume 2 entry for the exact UN number and formulation; the subsidiary risk can change between amendments.

Interaction with IMSBC Code for bulk cargoes

The IMDG Code and the IMSBC Code coexist for the same substances when packaging differs. The touchstone is the form of shipment:

Packaged goods (bags, drums, IBCs, boxes, intermediate bulk containers) on container ships or general cargo ships: IMDG Code applies.

Solid bulk cargoes in cargo holds without inner packaging: IMSBC Code applies, and the IMDG classification is replaced by the IMSBC Group designation (Group A: liquefaction risk; Group B: chemical hazard; Group C: neither).

For Class 4 substances this means:

Sulphur in bulk: IMSBC Code, Group C (no liquefaction or chemical hazard) with Group B characteristics noted for dust explosion under specific conditions. The IMSBC sulphur schedule covers this in detail.

Direct-reduced iron: IMSBC Code, Group B with self-heating and water-reactivity properties. The DRI IMSBC schedule article covers the specific temperature limits, moisture requirements, and monitoring intervals.

Seed cake (UN 1386/UN 1387) in bags: IMDG Code Class 4.2, PG II or III depending on oil content. Seed cake in bulk: IMSBC Code Group B with self-heating characterization. The rapeseed meal IMSBC schedule addresses the bulk form.

Charcoal in bags or drums: IMDG Code. Charcoal in bulk: IMSBC Code Group B.

The dual-regime situation produces practical difficulties when a master has a parcel of bulk DRI in the holds plus a few pallets of packaged aluminium phosphide as a containerized consignment. The holds carry IMSBC documentation; the container carries an IMDG DGD. Both sets of requirements apply simultaneously to the same voyage.

Incident case studies

Tianjin Binhai warehouse explosion, 12 August 2015

The Ruihai Logistics warehouse explosion killed 173 people and injured 798 more. The investigation report by the State Council of the PRC identified the principal substances as ammonium nitrate (Class 5.1, quantity approximately 800 tonnes) and calcium carbide (Class 4.3, quantity approximately 500 tonnes), stored without legal authorization and without the segregation required under Chinese and international standards.

First responders applied water to address what appeared to be a warehouse fire. The water reached the calcium carbide, generating acetylene. The acetylene ignited in the first major explosion. That explosion disrupted the ammonium nitrate stacking and the heat initiated ammonium nitrate decomposition, producing the second and far larger detonation recorded on seismographs at 2.9 magnitude by the China Earthquake Networks Center.

The IMDG Code implications: calcium carbide (UN 1402, Class 4.3, PG I) requires “separated from” segregation from Class 5.1 (oxidizers), and both classes carry prohibitions against water-based fire suppression in proximity to the other. The storage arrangement violated both requirements. The incident reinforced the IMO Circular on 4.3 awareness that had been issued after similar incidents in ports where calcium carbide is processed.

MV Ince Inebolu calcium carbide container fire, 2012

A container vessel in the Black Sea region experienced a fire in a 20-foot container declared as calcium carbide (UN 1402). Investigation established that rain had infiltrated the container through a damaged door seal. The acetylene generated inside the sealed container built up to an explosive concentration and ignited from an unknown source. The container had been accepted aboard with visible door-seal damage at the port of origin; the CTU packing check had not verified seal integrity.

The incident produced guidance from several port state authorities requiring CPC (Container Packing Certificate) verification to include a container integrity check for Class 4.3 cargoes, specifically noting rain-ingress risk.

Charcoal self-heating incidents

Charcoal ship fires are among the most frequent Class 4.2 incidents in the historical record, particularly on vessels serving West African or Southeast Asian trade routes where charcoal is shipped in 50 kg jute bags from small artisanal kilns. The freshness issue is acute: artisanal charcoal is often bagged within 24 to 48 hours of kiln exit, while residual heat and active surface sites are highest.

The IMO Sub-Committee on Carriage of Cargoes and Containers (CCC) reviewed charcoal incidents in 2016 and 2019 and noted that inadequate test certification and misdeclaration as “general cargo” (rather than IMDG Class 4.2) was a common factor. The current UN 1361 schedule requires adequate cooling before bagging and a self-heating test certificate before acceptance.

Documentation requirements for Class 4

Dangerous Goods Declaration

IMDG Code Chapter 5.4.1 specifies the DGD content for Class 4. Mandatory fields:

• UN number: UN followed by four digits (e.g., UN 1402).
• Proper shipping name: as listed in IMDG Code Volume 2, Column 2. Must not be abbreviated or paraphrased. For example, “Calcium carbide” is correct; “CaC2” is not.
• Technical name: for entries marked “n.o.s.” (not otherwise specified), the technical name of the substance(s) must appear in parentheses after the PSN.
• Hazard class or division: 4.1, 4.2, or 4.3.
• Packing group: I, II, or III.
• For self-reactive substances (4.1): the type letter (B, C, D, E, F, or G) and the control temperature and emergency temperature if applicable.
• Marine pollutant: if applicable, the words “MARINE POLLUTANT” or “ENVIRONMENTALLY HAZARDOUS.”
• EmS reference.
• Total quantity and net quantity.
• Number and type of packages.

Container Packing Certificate

The CTU (Cargo Transport Unit) Packing Certificate required by IMDG Code Section 5.4.2 must confirm that:

• The CTU has been inspected and is fit for purpose, including door seal integrity for Class 4.3 entries.
• Packing was done in accordance with IMDG Code Chapter 7.
• All packages are properly marked and labelled.
• For 4.3 entries specifically: moisture content of any packaging material was checked (wet dunnage or wet pallet wood can initiate a reaction).

Transport emergency card (TREM card) and emergency contact

IMDG Code Section 5.4.3 requires that an emergency contact (24-hour) appear on the DGD. For Class 4.3 and 4.2 PG I entries, flag state administrations and some port authorities additionally require a ship-specific TREM card or reference to the EmS Guide in the cargo documentation package.

Limitations

This article addresses Class 4 under IMDG Code 2022 Edition (Amendment 41-22). The IMDG Code is amended on a two-year cycle; readers working with specific shipments should verify the classification, packing, stowage, and segregation provisions against the current mandatory amendment applicable to the voyage date.

Classification of a substance as Class 4.1, 4.2, or 4.3 depends on test data from the UN test series (N.1 through N.5). Published UN numbers reflect formulations tested by the substance manufacturer or national authority at the time of initial classification. Reformulated substances, new particle size distributions, or different moisture contents may produce different test results, requiring reclassification. Shipper responsibility for correct classification (IMDG Code Section 2.0.1) means that a cargo accepted under a published UN number may still be misdeclared if the specific consignment differs from the tested formulation.

Self-heating test data (Test N.4) is temperature- and package-size-dependent: a substance that passes Test N.4 at one production scale may produce different results if the commercial package size changes. Results from a 100 mm cube may not represent behavior in a 1,000 kg bulk bag.

The IMSBC/IMDG interface for bulk-versus-packaged cargoes of the same substance is addressed in IMDG Code Section 1.1.2.4 and IMSBC Code Section 1.3, but borderline cases still require competent authority guidance. National maritime administrations (IMO member states) publish their own circular letters on specific substances where the regulatory boundary is unclear.

Water-reactive incident response described here reflects the IMDG Code 41-22 EmS Guide. Vessel-specific fire plans, flag state emergency procedures, and cargo insurer requirements may impose additional restrictions.

See also

• IMDG Class 1: Explosives
• IMDG Class 2: Gases
• IMDG Class 3: Flammable Liquids
• IMDG Class 5: Oxidisers and Organic Peroxides
• IMDG Class 6: Toxic and Infectious Substances
• IMDG Class 8: Corrosive Substances
• IMDG Class 9: Miscellaneous Dangerous Goods
• IMDG Marking, Labelling, and Placarding
• SOLAS Chapter VII: Carriage of Dangerous Goods
• IMSBC Code
• Direct-Reduced Iron IMSBC Schedule
• Sulphur IMSBC Schedule
• Rapeseed Meal IMSBC Schedule
• IMDG Class 4.3 Water-Reactive Calculator
• IMDG Segregation Calculator
• IMDG Packing Group Calculator
• IMDG EmS Lookup
• IMDG Limited Quantity Calculator
• Container IMDG Class Lookup
• IMDG Tank Container Calculator
• IMDG DG Finder
• Cargo Hold Heat Release Rate Calculator
• Coal Self-Heating Indicator Calculator
• Cargo Heat Spontaneous Combustion Calculator