Container Loading Taboos for Dangerous Goods: Why Class 4.1 and Class 8 Cargoes Cannot Be Co-loaded

In the container loading process of dangerous goods maritime transportation, “segregation taboos” are an insurmountable red line for ensuring transportation safety. The International Maritime Dangerous Goods Code (IMDG Code) clearly defines co-loading restrictions for different classes of dangerous goods through strict segregation level classifications (such as “separated from”, “isolated”, “isolated by a complete compartment or hold”, etc.). Among them, the principle of “absolute prohibition of co-loading” for Class 4.1 (flammable solids) and Class 8 (corrosive substances) is one of the strictest taboos in all loading specifications. This taboo is not a subjective assumption but a rigid rule formed based on the hazardous properties, chemical reaction mechanisms, and numerous accident lessons of the two classes of goods. This article comprehensively analyzes the underlying logic of this loading taboo from five dimensions: hazardous nature, reaction risks, regulatory basis, accident warnings, and practical control.

1. Essential Analysis of Hazardous Properties of Class 4.1 and Class 8 Cargoes

To understand the root cause of the co-loading taboo, it is first necessary to clarify the respective hazardous attributes of Class 4.1 and Class 8 cargoes—the former is a “flammable kindling”, and the latter is a “combustion-supporting catalyst”, and their combination forms a dangerous closed loop of “ignition-combustion support”.

(1) Class 4.1 Cargoes: “Triple Hazardous Attributes” of Flammable Solids

The IMDG Code defines Class 4.1 cargoes as “solids which are readily combustible or may cause or contribute to fire through friction; self-reactive substances or polymeric substances which may undergo a strongly exothermic reaction”. Their hazardous properties are concentrated in three aspects:

1. Low Ignition Point and High Flammability: Most Class 4.1 cargoes have an ignition point below 150°C, and some even below 50°C (e.g., red phosphorus has an ignition point of approximately 240°C, sulfur about 260°C, while magnesium powder has an ignition point of only 520°C, which is highly flammable when exposed to open flame or high temperature). More dangerously, the heat released during the combustion of such cargoes can maintain the combustion chain reaction, allowing continuous burning even if the initial ignition source is removed.
2. Secondary Hazards of Combustion Products: The combustion process not only releases a large amount of heat but may also produce toxic or corrosive gases. For example, sulfur dioxide produced by sulfur combustion has irritant toxicity, and carbon monoxide released by celluloid combustion can cause asphyxiation, exacerbating accident hazards.
3. Friction Sensitivity and Dust Explosion Risk: Powdered Class 4.1 cargoes (such as aluminum powder and magnesium powder) are prone to generating dust during loading/unloading or transportation vibration. When the dust concentration reaches the explosion limit (e.g., the explosion limit of aluminum powder is 35-50mg/m³), even static electricity or tiny sparks can trigger a dust explosion, whose power is comparable to that of high explosives.

Typical Class 4.1 cargoes include: red phosphorus (UN1338), sulfur (UN1325), metallic magnesium (UN1869), celluloid products (UN2000), guanidine nitrate (UN1479), etc., which are widely used in chemical, pharmaceutical, military, and other fields.

(2) Class 8 Cargoes: “Dual Destructive Effects” of Corrosive Substances

Class 8 cargoes are defined as “substances which, by chemical action, will cause severe damage to, or destruction of, metal, skin, or other biological tissues”. Their hazardous properties are mainly manifested in the superposition of “chemical corrosiveness” and “combustion-supporting catalytic property”:

1. Strong Chemical Corrosiveness: According to different corrosive media, they can be divided into acidic corrosion (such as sulfuric acid, hydrochloric acid), alkaline corrosion (such as sodium hydroxide, potassium hydroxide), and other corrosion (such as formaldehyde solution). Such substances can directly damage metal containers and packaging materials, leading to cargo leakage; contact with skin can cause burns, and intrusion into the human body can corrode internal organs.
2. Combustion-Supporting and Oxidative Catalytic Effects: Most strongly corrosive substances have oxidizing properties. For example, concentrated sulfuric acid (UN1830) has strong dehydrating and oxidizing properties; concentrated nitric acid (UN2031) is both a corrosive substance (Class 8) and an oxidizing substance (secondary hazard Class 5.1), which can significantly reduce the ignition point of combustible substances and accelerate combustion reactions.
3. Heat Release Property When in Contact with Water: Some Class 8 cargoes (such as solid sodium hydroxide) release a large amount of heat when in contact with water, which is sufficient to ignite nearby flammable substances, forming a “corrosion-heat release-ignition” chain reaction.

Typical Class 8 cargoes include: sulfuric acid (UN1789), sodium hydroxide solution (UN1824), hydrochloric acid (UN1782), hydrofluoric acid (UN1790), formaldehyde solution (UN1198), etc., which are high-frequency transportation categories in chemical trade.

2. Co-loading Risks: Evolution Path from “Physical Contact” to “Catastrophic Reaction”

The danger of co-loading Class 4.1 and Class 8 cargoes is essentially the chemical reaction amplification effect of “flammability” and “corrosiveness/oxidation”. Any contact between them in the confined space of a container may trigger an evolution from local reaction to full-scale out-of-control in four stages:

(1) Stage 1: Packaging Damage and Substance Contact

Vibration, shock during transportation, or collision during loading/unloading can easily cause damage to cargo packaging. Class 4.1 cargoes are mostly packaged in flammable or fragile materials such as paper drums and plastic woven bags (e.g., sulfur is often packaged in polypropylene woven bags), while Class 8 cargoes are usually in glass, plastic, or rubber-lined steel drums (e.g., sulfuric acid in plastic-lined iron drums). When corrosive liquids of Class 8 cargoes leak, they first erode the packaging of Class 4.1 cargoes, exposing flammable solids; conversely, powder of Class 4.1 cargoes may penetrate into the sealing gaps of Class 8 cargoes, creating conditions for subsequent reactions.

For example, if a plastic drum containing sodium hydroxide solution (UN1824) cracks due to collision, the leaked alkaline liquid will quickly dissolve the woven bag packaging of sulfur, bringing sulfur powder into direct contact with sodium hydroxide. Meanwhile, a small amount of water generated by the leakage reacts with sodium hydroxide to release heat, forming an initial risk environment.

(2) Stage 2: Spontaneous Combustion and Combustion Support Caused by Corrosion

The corrosiveness of Class 8 cargoes intensifies the flammability risk of Class 4.1 cargoes in two dimensions:

1. Heat Release from Corrosion Ignites Flammable Solids: Chemical reactions between strongly corrosive substances and Class 4.1 cargoes are generally accompanied by heat release. For instance, when concentrated sulfuric acid contacts sulfur, a sulfonation reaction occurs, releasing heat that can raise the local temperature to over 300°C—far exceeding sulfur’s ignition point of 260°C—directly igniting the sulfur; when hydrofluoric acid contacts metallic magnesium, a displacement reaction occurs to generate hydrogen gas while releasing heat, and the mixture of hydrogen and air forms an explosive mixture that ignites when exposed to heat.
2. Oxidative Corrosion Reduces Combustion Threshold: Oxidative corrosive substances (such as concentrated nitric acid) undergo redox reactions with Class 4.1 cargoes, making the chemical properties of flammable solids more unstable. For example, concentrated nitric acid reacts with red phosphorus to produce phosphoric acid and nitrogen dioxide. During the reaction, red phosphorus is oxidized to more flammable phosphorus oxides, and the generated nitrogen dioxide (a combustion-supporting gas) further accelerates combustion.

(3) Stage 3: Combustion Spread and Toxic Gas Release

The “greenhouse effect” in the confined container space sharply amplifies combustion hazards. The heat released by burning Class 4.1 cargoes cannot diffuse effectively, causing the temperature inside the container to rise rapidly (up to over 1000°C), which in turn triggers the following chain reactions:

• The increased temperature further enhances the corrosiveness and oxidizability of Class 8 cargoes, accelerating leakage and forming a positive feedback loop of “combustion-combustion support-more intense combustion”;
• Toxic gases produced by combustion (such as sulfur dioxide from sulfur combustion, phosphorus pentoxide from red phosphorus combustion) mix with vapors of Class 8 cargoes (such as hydrogen chloride gas from hydrochloric acid) to form more toxic composite gases. Once the container seal fails, it poses a fatal threat to crew and port personnel;
• High temperatures may cause container structural deformation and door lock failure, leading to burning cargo falling or leaking and triggering chain accidents in adjacent containers.

(4) Stage 4: Difficulty in Fire Fighting and Secondary Disasters

Extinguishing co-combusted Class 4.1 and Class 8 cargoes is far more difficult than extinguishing single-class cargoes. On one hand, Class 8 corrosive substances damage conventional fire-fighting equipment: after sulfuric acid leakage, water sprayed by water-based fire extinguishers reacts with sulfuric acid to release heat, intensifying combustion; dry powder fire extinguishers can suppress combustion but cannot neutralize corrosive substances, resulting in a high risk of re-ignition. On the other hand, molten substances produced by combustion (such as magnesium oxide melt from burning magnesium) undergo secondary reactions when in contact with water (e.g., magnesium melt reacts with water to produce hydrogen explosion), posing significant safety hazards to fire-fighting operations.

3. Regulatory Basis: Rigid Definition of Segregation Requirements in the IMDG Code

The co-loading taboo for Class 4.1 and Class 8 cargoes is not a summary of industry experience but a mandatory legal requirement explicitly stated in the IMDG Code. Its core basis is reflected in the dual constraints of “segregation levels” and “special provisions”.

(1) “Separated from” Requirement for Segregation Level

The IMDG Code classifies dangerous goods segregation into 4 levels (1-4). The segregation level for Class 4.1 and Class 8 cargoes is “Separated from” (Segregation Level 2), specifically defined as “cargoes shall be separated by a minimum distance of 3 meters horizontally and vertically, or separated by a compartment or hold containing non-dangerous goods”. However, in container transportation scenarios, since containers are confined single spaces, the “3-meter distance” or “compartment segregation” cannot be achieved. Therefore, the “separated from” requirement is essentially equivalent to “prohibition of co-loading”.

This requirement is further clarified for specific UN numbers in the “Dangerous Goods List” of the IMDG Code: for example, the remarks column for sulfur (UN1325) of Class 4.1 clearly states “segregated from Class 8 corrosive substances”; sulfuric acid (UN1789) of Class 8 requires “kept away from flammable solids”. Regardless of the primary hazard class or secondary hazard class, as long as they involve Class 4.1 and Class 8 attributes, this segregation principle must be followed.

(2) Supplementary “Absolute Prohibition” in Special Provisions

In addition to general segregation levels, the IMDG Code formulates “Special Provisions” for some high-risk cargoes, imposing stricter prohibitive regulations on the co-loading of Class 4.1 and Class 8 cargoes:

• For self-reactive substances in Class 4.1 (such as UN3233, self-reactive solids), Special Provision SP108 explicitly requires “must not be co-loaded with any corrosive substances in the same container”, because their self-reactive properties will accelerate decomposition under the action of corrosive substances, leading to explosions;
• For oxidative corrosive substances in Class 8 (such as UN2031, concentrated nitric acid, secondary hazard Class 5.1), Special Provision SP230 stipulates “prohibited from mixed transportation with flammable solids”, as the superimposed risk of oxidation and flammability exceeds the scope of conventional control.

In addition, maritime authorities of various countries often adopt a “stricter than rules” attitude in implementation. For example, China’s Regulations on the Transportation of Dangerous Goods by Sea clearly stipulates “Class 4.1 and Class 8 cargoes shall not be loaded in the same container, even if the packaging is intact”; the EU requires containers carrying such cargoes to be individually affixed with “no co-loading” warning signs to strengthen practical control.

4. Accident Warnings: Review of Catastrophic Consequences Caused by Co-loading

Numerous dangerous goods maritime transportation accidents in history have confirmed the necessity of this co-loading taboo with painful costs. The following two typical accidents clearly demonstrate the whole process from hidden danger to disaster caused by co-loading Class 4.1 and Class 8 cargoes:

(1) 2019 Container Fire Accident at a Southeast Asian Port

Accident Process: A freight company loaded magnesium powder (UN1869) of Class 4.1 and hydrochloric acid (UN1782) of Class 8 into the same container without any segregation measures. During transportation, the hydrochloric acid drum leaked due to vibration. Hydrochloric acid reacted violently with magnesium powder: Mg + 2HCl = MgCl₂ + H₂↑. The heat released by the reaction ignited the unreacted magnesium powder, and the generated hydrogen mixed with air to form an explosive mixture. The container first exploded with hydrogen, followed by continuous burning of magnesium powder. The temperature inside the container rose to 1200°C, igniting 3 adjacent containers, one of which contained Class 2.1 flammable gas (propane), eventually causing a chain explosion.

Accident Consequences: 2 dock workers were killed, 5 were seriously injured, 12 containers were burned down, and direct economic losses exceeded 20 million US dollars. The accident investigation showed that co-loading was the sole direct cause—if the two classes of cargoes had been loaded separately, even hydrochloric acid leakage would not have caused combustion and explosion.

(2) 2022 Shipboard Fire Accident in the Indian Ocean

Accident Process: A cargo ship was sailing in the Indian Ocean when a deck container suddenly emitted smoke. Crew inspection found that the container was loaded with celluloid (UN2000) of Class 4.1 and sodium hydroxide solution (UN1824) of Class 8. Due to the damaged packaging of sodium hydroxide solution, the alkaline liquid penetrated into the celluloid products. Celluloid underwent a hydrolysis reaction in the alkaline environment, releasing heat and gradually carbonizing and igniting. After the fire spread, the crew tried to put out the fire with water, which caused further diffusion of sodium hydroxide solution. Meanwhile, carbon monoxide produced by celluloid combustion caused 3 fire-fighting crew members to lose consciousness due to poisoning.

Accident Consequences: The container was completely burned down, the ship’s deck was damaged, and the poisoned crew members were rescued and out of danger. However, the ship was forced to make an emergency call at a nearby port, delaying the voyage for nearly a month, with demurrage, repair costs, etc., totaling 8 million US dollars.

These two accidents jointly confirm a core conclusion: there is no possibility of “lucky safety” in co-loading Class 4.1 and Class 8 cargoes. Even if the packaging is intact, any minor disturbance during transportation may trigger a fatal reaction.

5. Practical Control: Full-Process Specifications to Avoid Co-loading Risks

Avoiding the risk of co-loading Class 4.1 and Class 8 cargoes requires establishing a full-chain control system from “pre-loading assessment, in-loading implementation, to post-loading verification”, transforming regulatory requirements into actionable operating standards.

(1) Pre-loading: Risk Assessment and Plan Formulation

1. Dual Confirmation of Cargo Attributes: First, clarify the cargo’s class based on the UN number and the “Dangerous Goods List” of the IMDG Code—focusing on the “secondary hazard class”. For example, some Class 8 cargoes may also have Class 5.1 oxidizing properties (such as concentrated nitric acid), requiring compliance with dual segregation requirements of Class 8 and Class 5.1. For instance, UN2031 (concentrated nitric acid), although primarily Class 8, has a secondary Class 5.1 hazard, so its segregation requirements with Class 4.1 cargoes must meet both Class 8 and Class 5.1 restrictions.
2. Container Suitability Inspection: Select dangerous goods-specific containers that meet IMDG Code requirements, ensuring no damage, rust, or leakage, and intact internal anti-corrosion coating (especially when loading Class 8 cargoes). For powdered Class 4.1 cargoes, check whether the container has dust-proof sealing measures; for liquid Class 8 cargoes, confirm that the container bottom is equipped with leak-proof pallets and drainage grooves.
3. Formulate Special Loading Plans: Clarify the principle of “one class per container”. Class 4.1 and Class 8 cargoes must be loaded into different containers, and the two containers must meet the “separated from” requirement during ship stowage—minimum 3-meter horizontal distance and no vertical overlap. If ship space is limited, adopt the “non-dangerous goods segregation” method, i.e., load general cargo between the containers of the two classes of dangerous goods to form a physical buffer.

(2) In-loading: Strict Implementation of Segregation