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IMDG Class 5: Oxidisers and Organic Peroxides

IMDG Class 5 of the International Maritime Dangerous Goods Code covers two distinct categories of dangerous oxidising substances: Division 5.1 Oxidising Substances that yield oxygen and intensify the combustion of other materials (ammonium nitrate fertiliser, calcium hypochlorite swimming-pool chlorinator, hydrogen peroxide solutions, sodium chlorate herbicide, potassium permanganate, perchloric acid), and Division 5.2 Organic Peroxides that contain the bivalent -O-O- structural element and are thermally unstable, decomposing exothermically with a tendency to detonate or to burn rapidly (benzoyl peroxide, methyl ethyl ketone peroxide MEKP, di-tert-butyl peroxide, dicumyl peroxide). The two sub-classes have very different physical hazards (5.1 is a passive accelerant of fire elsewhere; 5.2 is an active decomposition source) but share the requirement for strict separation from combustibles, organic materials and acids. Division 5.2 Organic Peroxides require temperature control: each entry has a Self-Accelerating Decomposition Temperature (SADT) above which the decomposition becomes self-sustaining; the control temperature (typically SADT minus 10°C) and emergency temperature (typically SADT minus 5°C) are mandatory MDGF entries that drive refrigerated-container service for the entire voyage. Class 5.1 incidents are some of the largest port casualties on record because ammonium nitrate (Class 5.1) can mass-detonate when fuel-contaminated and externally heated: Texas City 1947, Toulouse AZF 2001, Beirut 2020. ShipCalculators.com hosts the principal computational tools that support Class 5 cargo handling: the IMDG segregation calculator, the IMDG packing group 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 5 has two sub-divisions

Class 5 is unusual in that the two sub-divisions are almost separate classes:

  • 5.1 Oxidising Substances are inorganic compounds (nitrates, chlorates, permanganates, peroxides of metals, hypochlorites) that, while not themselves combustible, release oxygen when heated. They make any nearby fire burn more intensely and faster, and can cause a fire that would otherwise self-extinguish to propagate.
  • 5.2 Organic Peroxides are organic compounds containing one or more -O-O- (peroxy) bonds. The peroxy bond is thermally weak and decomposes exothermically; if the heat of decomposition exceeds the heat dissipation capacity of the package, the substance enters runaway decomposition that can produce detonation, deflagration or rapid combustion depending on the specific entry.

The two sub-divisions therefore have different stowage rules, different EmS schedules and different documentation requirements, but they are grouped together because both involve the chemistry of oxygen-rich molecules and both react badly with combustibles.

Class 5.1 vs 1.5 vs IMSBC

Several substances at the boundary of Class 5 deserve careful classification:

  • Ammonium nitrate fertiliser: classified as Class 5.1 (UN 2067 fertiliser grade with less than 0.2% combustible material, UN 1942 with 0.2-0.4%) under most circumstances. As Class 1.5 (UN 0331, UN 0332) when manufactured as a deliberate explosive (blasting agent type B or E). Some bulk ammonium nitrate shipments are also subject to the IMSBC Code as a Group B cargo (chemical hazards). The same chemical species in different formulations can fall under three different regimes.
  • Sodium chlorate: Class 5.1 normally; if mixed with combustible material above threshold becomes Class 1 explosive.
  • Hydrogen peroxide: Class 5.1 in solution above 8% concentration; Class 8 corrosive subsidiary. Below 8% may be unregulated.
  • Calcium hypochlorite: Class 5.1; particularly hazardous because it self-heats in damp conditions and can ignite spontaneously.

The role of SADT for Class 5.2

The defining concept for Class 5.2 is the Self-Accelerating Decomposition Temperature (SADT): the lowest temperature at which the substance, in its packaged form, undergoes self-accelerating decomposition over a defined timescale. SADT is determined by UN Test H.4 (oven test) under standardised conditions.

For each Class 5.2 entry the IMDG Code specifies (or requires the manufacturer to specify):

  • SADT in °C.
  • Control temperature = the maximum temperature at which the substance can be safely transported. Typically SADT minus 10°C for liquids, SADT minus 5°C for solids.
  • Emergency temperature = the temperature at which emergency procedures must be initiated (refrigeration unit failure, jettison consideration). Typically SADT minus 5°C.

The control and emergency temperatures must appear on the MDGF and the outer packaging marking. The carrier must verify the refrigerated-container service can hold the cargo below the control temperature for the entire planned voyage including any deviation contingency.

Division 5.1: Oxidising Substances

Definition and examples

A substance is Division 5.1 if it yields oxygen at a rate sufficient to support combustion when heated, even if it does not itself burn. The classification test (UN Test O.1 for solids, O.2 for liquids) compares the substance’s reaction rate with cellulose to a reference oxidiser. The packing group is determined by the relative reaction rate.

Common Division 5.1 entries:

  • UN 1438 Aluminium nitrate.
  • UN 1454 Calcium nitrate.
  • UN 1486 Potassium nitrate.
  • UN 1490 Potassium permanganate.
  • UN 1495 Sodium chlorate.
  • UN 1942 Ammonium nitrate (fertiliser, > 0.2% combustible).
  • UN 2014 Hydrogen peroxide, aqueous solutions, > 20% but ≤ 60%.
  • UN 2015 Hydrogen peroxide, aqueous solutions, > 60%.
  • UN 2067 Ammonium nitrate fertiliser, < 0.2% combustible.
  • UN 2880 Calcium hypochlorite, hydrated.

Stowage and segregation

Division 5.1 stowage rules emphasise separation from combustibles:

  • Away from heat sources (engine room, exhaust, hot machinery).
  • Away from organic materials (wood dunnage, paper packing, oils, fuels).
  • In dry, well-ventilated spaces (calcium hypochlorite is particularly sensitive to humidity).
  • Not stowed below decks that have flammable cargo above (fire propagation acceleration risk).
  • For ammonium nitrate: large lot quantity limits, often max 200 tonnes per ship, with mandatory pre-arrival notification and dedicated anchorage at most ports.

Segregation requirements are the strictest for any non-explosive class:

  • From Class 1: ‘separated longitudinally by an intervening complete compartment or hold from’. The combination is the Texas City scenario.
  • From Class 3 Flammable Liquids: ‘separated by complete compartment or hold from’ for most combinations.
  • From Class 4: ‘separated longitudinally by complete compartment from’. 4.3 + 5.1 with water ingress is a triple hazard.
  • From Class 5.2 Organic Peroxides: ‘separated by complete compartment from’. Two oxidisers together accelerate decomposition.
  • From Class 6.1: ‘separated from’.
  • From Class 7: special routine.
  • From Class 8 acids: ‘separated longitudinally by complete compartment’. Acid contact with hypochlorite releases chlorine gas.

The IMDG segregation calculator implements the full table.

EmS

  • F-H: standard 5.1 fire schedule. Water is the principal extinguishing agent; flood with water to cool and to dilute the oxidiser. Distinguish from F-G (Class 4 ‘water-cautious’) - 5.1 takes water freely.
  • F-A: some specific 5.1 entries with general fire schedule.

Spillage:

  • S-Q: 5.1 standard. Contain with sand or earth (not organic absorbent). Recover into salvage drum. Decontaminate area with copious water.

Division 5.2: Organic Peroxides

Definition and types

A substance is Division 5.2 if it contains the -O-O- structure and is thermally unstable per UN Test H series. The classification distinguishes seven types A through G by decreasing severity:

  • Type A: capable of detonating or deflagrating rapidly as packaged. Prohibited for transport. Must be desensitised or reformulated.
  • Type B: capable of detonating or deflagrating rapidly but not as packaged.
  • Type C: capable of decomposing rapidly but not detonating.
  • Type D: only partial detonation, or slow decomposition.
  • Type E: cannot detonate or deflagrate rapidly; only burns.
  • Type F: slow decomposition under fire conditions only.
  • Type G: thermally stable; exempt from Class 5.2 (but the IMDG Code may still classify it under another class).

Most commercial organic peroxides are types B through E. Type B requires the most stringent transport conditions; type F the least.

Common Class 5.2 entries (with type letter):

  • UN 3101 Organic peroxide type B, liquid.
  • UN 3102 Organic peroxide type B, solid.
  • UN 3103 Organic peroxide type C, liquid.
  • UN 3104 Organic peroxide type C, solid (commonly benzoyl peroxide formulations).
  • UN 3105 Organic peroxide type D, liquid.
  • UN 3106 Organic peroxide type D, solid.
  • UN 3107 Organic peroxide type E, liquid.
  • UN 3108 Organic peroxide type E, solid.
  • UN 3109 Organic peroxide type F, liquid.
  • UN 3110 Organic peroxide type F, solid.

The 4-digit UN number identifies only the type and physical state; the actual chemistry varies. The MDGF and marking must specify the exact peroxide name and its SADT, control temperature and emergency temperature.

Temperature control

For Class 5.2 entries with control temperature ≤ 25°C, transport requires:

  • Refrigerated container (reefer) set to maintain the cargo below the control temperature. Standard reefer service supports cargo control temperatures down to about -25°C.
  • Backup refrigeration for long voyages. The IMDG Code requires a backup refrigeration unit or alternative cooling capability for shipments exceeding 24 hours of expected transit.
  • Continuous temperature monitoring with alarm at the emergency temperature.
  • Crew training on what to do if temperature rises: increase cooling, jettison if jettison is feasible and appropriate, abandon ship if neither.

For SADT > 50°C the control temperature may be ambient and refrigeration is not required - but temperature monitoring is still recommended.

Stowage and segregation

Class 5.2 stowage:

  • On deck only for most type B and type C entries (fast jettison capability).
  • In refrigerated containers (the RF code on the container number indicates reefer service).
  • Away from accommodation (decomposition fumes can be toxic).
  • Away from heat sources (engine uptake, exhaust).

Segregation:

  • From Class 1: ‘separated longitudinally by complete compartment’.
  • From Class 5.1 oxidisers: ‘separated by complete compartment’.
  • From Class 5.2 of incompatible chemistry: ‘separated’ (some peroxides are mutually catalytic).
  • From Class 4 and Class 3 flammables: ‘separated by complete compartment or longitudinally’.

EmS

  • F-J: standard 5.2 fire schedule. Flood with water from a safe distance. Boundary cool adjacent stowage. Do not approach burning peroxide closer than safe stand-off (typically 50 metres for type B, 25 metres for type E/F).
  • S-R: 5.2 spillage. Move the affected container to a safe location if possible (the container itself may not be on fire); apply absorbent for liquid spill; flood with water to dilute and cool.

Notable casualties

SS Grandcamp, Texas City 1947

The April 1947 explosion of the Liberty ship SS Grandcamp at Texas City, killing approximately 580 people, was caused by Class 5.1 ammonium nitrate fertiliser (2,300 tonnes) catching fire at the dock. The fire heated the cargo until it underwent mass detonation. The casualty drove the classification of fertiliser-grade ammonium nitrate as Class 5.1 (it had been considered safe before Texas City) and the segregation rules now in IMDG Chapter 7.2. See also the IMDG Class 1 article for the broader context.

AZF Toulouse 2001

In September 2001 an ammonium nitrate storage facility at the AZF chemical plant in Toulouse, France, exploded killing 31 people and injuring 2,500. The investigation found that approximately 300 tonnes of off-spec ammonium nitrate had been stored mixed with chlorine-contaminated material in a building inadequately separated from a sodium dichloroisocyanurate (chlorinated organic) store; contact between the two oxidisers initiated detonation. The disaster drove the European Seveso III Directive revision and tighter EU rules on ammonium nitrate storage above ground.

Beirut port explosion 2020

The August 2020 Beirut port explosion (covered in the IMDG Class 1 article) involved approximately 2,750 tonnes of Class 5.1 ammonium nitrate stored without proper segregation for six years. A fire of unclear origin (possibly involving fireworks stored adjacent) ignited the cargo, producing one of the largest non-nuclear explosions in history. Beirut reinforced that ammonium nitrate stored in bulk requires the strictest segregation and the strictest fire prevention controls.

MV Cheshire 2017

The Spanish-flag bulk carrier MV Cheshire suffered a major fire in August 2017 in the Atlantic carrying approximately 41,000 tonnes of Class 5.1 ammonium nitrate fertiliser (UN 2067). The cargo self-decomposed in cargo hold 4, generating heat and toxic gases. The vessel was abandoned but did not detonate; it was towed to Algeciras and later scrapped. The investigation drove temperature monitoring requirements for large bulk ammonium nitrate shipments and revisions to the IMSBC Code for Group B oxidisers.

Documentation

MDGF requirements for Class 5.2

For organic peroxides the MDGF must include:

  • UN number and proper shipping name.
  • Type letter (B, C, D, E or F).
  • SADT in °C.
  • Control temperature in °C.
  • Emergency temperature in °C.
  • Concentration of the active peroxide.
  • Diluent and concentration (some entries are mixtures with desensitisers).
  • EmS reference (typically F-J, S-R).
  • 24-hour emergency contact.

For Class 5.1 the MDGF is standard but normally requires:

  • Marine pollutant indicator.
  • For ammonium nitrate: explicit fertiliser-grade indication (UN 1942 vs UN 2067 distinction matters for stowage).

Container Packing Certificate

For Class 5 cargo:

  • Container clean and dry, particularly free of any organic residue from previous cargo.
  • Drum closures torqued to specification.
  • For 5.2 in refrigerated container: reefer set point verified, alarm thresholds programmed, monitoring continuous.
  • Marking and placarding: yellow diamond with flame-over-circle symbol for both 5.1 and 5.2 (the 5.2 placard adds the type letter on the bottom band).

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 II (Class 1) and Part IV (SADT for Class 5.2), 7th revised edition, United Nations, 2019.
  • US Chemical Safety and Hazard Investigation Board, West Fertilizer Company Investigation Report, 2016.
  • French INERIS, Rapport d’enquête AZF Toulouse, 2002.
  • Lebanese Army, Investigation Report on the Beirut Port Explosion of 4 August 2020, 2021.