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IMDG Class 8: Corrosive Substances

IMDG Class 8 of the International Maritime Dangerous Goods Code covers substances that, by chemical action, cause severe damage when in contact with living tissue or that materially damage other freight or means of transport in the event of leakage. The class is divided into three packing groups by the exposure time required to produce full-thickness destruction of intact skin: Packing Group I (very dangerous, full-thickness destruction within 3 minutes after up to 60 minutes of observation), Packing Group II (medium, full-thickness destruction between 3 minutes and 1 hour), and Packing Group III (low, full-thickness destruction between 1 hour and 4 hours, OR a corrosion rate on steel or aluminium exceeding 6.25 mm per year at a test temperature of 55°C). The class includes inorganic acids (sulphuric, hydrochloric, nitric, hydrofluoric, phosphoric, perchloric), strong bases (sodium hydroxide caustic soda, potassium hydroxide caustic potash, lime), salts of heavy metals (mercury, lead, copper, zinc compounds in corrosive solution), and many organic acids and bases (formic acid, acetic anhydride, dimethylamine, ethanolamine). Stowage and segregation under IMDG Code Chapter 7 emphasises separation from foodstuffs, from incompatible chemicals (acid + base produces violent neutralisation; acid + cyanide releases hydrogen cyanide; acid + sulphide releases hydrogen sulphide), and protection of vessel metallic structure from corrosion damage that can compromise structural integrity over the duration of the voyage. The standard emergency response is EmS F-A (or F-D for combustible packaging) and S-B (small spillage) or S-C (large spillage). Many Class 8 cargoes also carry the marine pollutant designation, attracting MARPOL Annex III obligations alongside IMDG. ShipCalculators.com hosts the principal computational tools that support Class 8 cargo handling: the IMDG segregation calculator, the IMDG packing group calculator, the IMDG EmS lookup, the container IMDG class lookup, the IMDG limited quantity calculator, 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

What Class 8 covers

A substance is Class 8 if it has either:

  • A destructive effect on living tissue (skin destruction within the test interval per UN Test C.1).
  • A corrosion rate on steel or aluminium exceeding 6.25 mm per year at 55°C test temperature (per UN Test C.2).

The first criterion captures direct human-health corrosivity; the second captures structural corrosion that may compromise the vessel even if the cargo never contacts a person. Many Class 8 substances satisfy both criteria; some satisfy only one (a substance that corrodes steel rapidly but is mild on skin is still Class 8 because of the structural risk).

Why Class 8 matters operationally

Class 8 is one of the most commonly shipped IMDG classes by tonnage and by container count. Bulk acid shipments by chemical tanker and packaged consolidations of caustic, acid, paint, adhesive and cleaning products move continuously through every major port.

The operational risks are:

  • Crew exposure: a leaking drum of strong acid in a closed cargo hold can produce skin burns, respiratory injury and eye damage to anyone entering the hold without PPE. The IMDG MFAG (Medical First Aid Guide) provides treatment guidance.
  • Cargo damage: a Class 8 leak on top of an unrelated container can corrode through the container roof and damage the cargo inside. Insurance claims for cargo damage from Class 8 leaks are common.
  • Vessel structural damage: persistent leakage onto deck plating, into bilges, or into ballast tanks can corrode steel structure to the point of requiring expensive repair. The cumulative effect of many small leaks over a vessel’s life is a maintenance line item for chemical tankers.
  • Reaction with other classes: acid contact with cyanide (Class 6.1) releases hydrogen cyanide gas; acid contact with sulphide (Class 4.2) releases hydrogen sulphide; acid contact with hypochlorite (Class 5.1) releases chlorine gas. Any of these is a potentially fatal exposure scenario for the crew.

Distinction from MARPOL Annex II

Bulk Class 8 cargo is regulated under MARPOL Annex II (Noxious Liquid Substances) and the IBC Code (International Bulk Chemical Code). The IMDG Code regulates packaged Class 8 cargo in containers, drums, IBCs and tank containers.

Many of the same acids and bases ship in both forms: bulk sulphuric acid in a chemical tanker is Annex II / IBC; drums of sulphuric acid in a container are IMDG Class 8. The substantive hazards are the same; the regulatory regime depends on the carriage form.

The three packing groups

PG I: Very dangerous

Full-thickness destruction of intact skin within 3 minutes of exposure (or equivalent corrosion rate). Examples:

  • UN 1790 Hydrofluoric acid solutions, > 60% strength.
  • UN 2031 Nitric acid, > 70% strength.
  • UN 1830 Sulphuric acid, > 51% strength but ≤ 51%.
  • UN 1832 Sulphuric acid, spent (used in industrial processes).
  • UN 2032 Nitric acid, fuming (red fuming).

PG I requires the most rigorous packaging (UN-certified inner and outer per Section 6.1 or 6.7), strict segregation distances, and dedicated emergency response equipment on board.

PG II: Medium danger

Full-thickness destruction between 3 minutes and 1 hour. Examples:

  • UN 1789 Hydrochloric acid (muriatic acid).
  • UN 1805 Phosphoric acid solutions, > 80% strength.
  • UN 1813 Potassium hydroxide solid (caustic potash).
  • UN 1823 Sodium hydroxide solid (caustic soda).
  • UN 1824 Sodium hydroxide solution.

PG II is the workhorse category for industrial chemical shipments. Most consolidations of acid drums and caustic drums fall in PG II.

PG III: Low danger (but still corrosive)

Full-thickness destruction between 1 hour and 4 hours, OR meets the corrosion-rate criterion. Examples:

  • UN 2796 Battery acid (sulphuric acid ≤ 51%).
  • UN 2794 Wet cell batteries with corrosive electrolyte.
  • UN 1715 Acetic anhydride.
  • UN 2789 Acetic acid solutions, glacial or solutions > 80%.
  • UN 1779 Formic acid, > 85%.

PG III still requires Class 8 placarding and segregation but can use lighter packaging (UN-certified to lower drop heights) and has more flexible stowage.

The IMDG packing group calculator returns the PG for any UN entry.

Stowage and segregation

Standard Class 8 stowage

  • On deck or under deck in approved cargo spaces.
  • Drum drip trays for liquid Class 8 (especially PG I).
  • Stainless steel or rubber-lined cargo handling equipment for direct contact during loading and discharge.
  • Adequate ventilation for under-deck stowage to dilute any acid mist.
  • Watertight stowage for water-sensitive Class 8 (some acid mixtures are water-reactive when concentrated).
  • Vertical orientation for drums (drum bungs at the top to prevent leak under tilt).

Tank containers

Class 8 liquids in tank containers use various T-codes depending on the cargo aggressiveness:

  • T11 for general organic acids and dilute mineral acids.
  • T14 for stronger inorganic acids.
  • T22 for the most aggressive acids (anhydrous HF, concentrated nitric, oleum).
  • T20-T23 for hot acid service (nitric acid hot, oleum hot).

Tank construction:

  • Stainless steel 316L for many acids; resistance varies by acid concentration and temperature.
  • Carbon steel for oleum (concentrated H2SO4·SO3) - paradoxically, dilute sulphuric attacks carbon steel but very concentrated does not because of passive film formation.
  • Nickel alloys (Hastelloy, Inconel) for HF and chlorinated acid mixtures.
  • PTFE-lined or PFA-lined for the most aggressive species.

Mismatch of tank material and cargo is a classic incident cause. The bill of lading must specify the cargo grade and the tank construction must match.

Segregation rules

Class 8 segregation under IMDG Chapter 7.2:

  • From Class 1: ‘separated longitudinally by an intervening complete compartment’.
  • From Class 2.3 toxic gases: ‘separated from’ (acid + cyanide cargo can release HCN).
  • From Class 4 substances: ‘separated from’ for most combinations (acid + alkali metal releases hydrogen).
  • From Class 4.3 water-reactive: ‘separated longitudinally’. Acid spills can reach 4.3 substance and trigger gas release.
  • From Class 5.1 oxidisers: ‘separated by complete compartment’. Acid + hypochlorite = chlorine gas; acid + permanganate = chemical fire.
  • From Class 5.2 organic peroxides: ‘separated longitudinally’.
  • From Class 6.1 cyanides: ‘separated longitudinally’.
  • From Class 6.1 cyanide-containing salts specifically: this is a SPECIAL segregation in some IMDG editions because of the very high lethality of HCN release.
  • From Class 7: ’no requirement’ for most combinations.
  • From foodstuffs: ‘separated from’ minimum; ‘separated longitudinally’ for PG I.

The IMDG segregation calculator implements the full table.

Emergency response

EmS schedules

  • F-A: standard fire schedule for Class 8. Most Class 8 substances are not themselves flammable; F-A applies for the combustible packaging fire scenario.
  • F-D: with combustible packaging additional fire load (acid in wood-stave drums, for example).
  • S-B: small Class 8 spillage. Contain with absorbent (not organic for strong oxidising acid - use sand or vermiculite). Neutralise CAREFULLY (acid with weak alkali, base with weak acid - the neutralisation reaction is exothermic and generates spatter). Recover into salvage drums for disposal ashore.
  • S-C: large Class 8 spillage. Same procedure as S-B but on larger scale; may require deck wash with dilute alkali (for acid spills) or dilute acid (for base spills) before final cleanup.

Personal protective equipment

The IMDG MFAG specifies for Class 8:

  • Acid-resistant gloves (PVC, nitrile or neoprene depending on cargo).
  • Acid-resistant boots and apron.
  • Eye protection (full-face shield for PG I; safety glasses minimum for PG III).
  • Self-contained breathing apparatus (SCBA) for any approach where vapour generation is possible.
  • Emergency eye-wash and shower stations on deck near Class 8 stowage.

First aid for skin contact

The IMDG MFAG protocol for Class 8 skin contact:

  1. Remove contaminated clothing immediately.
  2. Flush with copious water for at least 15 minutes (the universal corrosive first-aid).
  3. Do NOT apply neutralising agents (the neutralisation is exothermic and increases injury).
  4. Cover with sterile dressing after flushing.
  5. Evacuate to medical facility for any PG I or PG II exposure.

For HF specifically, the protocol adds calcium gluconate gel application after water flush; without calcium gluconate the fluoride ion penetrates deep tissue and causes severe ongoing damage.

Notable casualties

MV Stolt Tankers various

The Stolt Tankers fleet (chemical tankers operated by Stolt-Nielsen) has had several Class 8 incidents over decades, mostly minor and confined to deck-area spillage during loading/discharge. The company’s published safety record demonstrates that chemical tanker operations with rigorous procedure adherence keep Class 8 incident rates very low despite the constant exposure to aggressive cargoes.

MV Sun Sea acid leak 2011

The MV Sun Sea (a small general cargo vessel) suffered a minor acid leak from a deck-stowed drum in 2011 in Indian coastal waters. The leak was contained without injury but contaminated approximately 30 m² of deck and required hot work and re-coating during the next dry-docking. The casualty illustrated the importance of drum inspection during loading and the use of drip trays for PG II liquid Class 8.

Acid container leaks at terminals

Numerous minor incidents have occurred at container terminals where a Class 8 PG I or II tank container leaked during the terminal handover. Standard response: cordon off the area, deploy spill-response trained personnel with full PPE, neutralise carefully, decontaminate. No fatalities in modern recorded cases at major Western terminals; some lost-time injuries from skin splash during the response phase.

Documentation

MDGF requirements

For Class 8 entries:

  • UN number, proper shipping name, class (8), packing group (I, II or III).
  • Concentration of the active substance (acid concentrations matter for both classification and emergency response).
  • Container material compatibility information for tank container shipments.
  • EmS reference (typically F-A, S-B or S-C).
  • Marine pollutant indicator if applicable.
  • 24-hour emergency contact.

Container Packing Certificate

For Class 8 cargo:

  • Container clean and dry, no incompatible residue.
  • Drum bungs torqued to specification.
  • Drip trays for PG I.
  • Drums secured against shifting.
  • Marking and placarding correct (Class 8 placard: black ‘C’ over a hand and a metal piece being eaten by liquid, on white background with black bottom band).

See also

References

  • IMO, International Maritime Dangerous Goods Code, 2022 Edition (IMDG 41-22), International Maritime Organization, 2022.
  • IMO, International Bulk Chemical Code (IBC Code), International Maritime Organization, current edition.
  • IMO, EmS Guide: Revised Emergency Response Procedures for Ships Carrying Dangerous Goods, International Maritime Organization, current edition.
  • IMO, Medical First Aid Guide for Use in Accidents Involving Dangerous Goods (MFAG), 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.
  • ICS / OCIMF, Tanker Safety Guide (Chemicals), 4th edition, Witherby, 2014.