3.3 Class 8: Corrosive Substances

3.3 Class 8: Corrosive Substances

Quick Answer: ADR Class 8 covers substances which, by chemical action, cause full-thickness destruction of intact living skin tissue or degrade vehicle metals at a rate exceeding 6.25 mm per year at 55°C. Packing Groups I, II, and III correlate with skin exposure times from under 3 minutes up to 4 hours. Critical operational hazards include violent exothermic reactions when mixed with water or neutralizing agents, and the rapid generation of explosive hydrogen gas upon contact with vehicle metals.

Definition and Dual Hazard Mechanism of Class 8

Under ADR 2.2.8.1.1, Class 8 corrosive substances are defined as substances which, by chemical action, cause full-thickness destruction of intact skin tissue, or in the case of leakage, cause damage to or destroy other goods or means of transport.

Class 8 poses a dual threat during road transport operations:

• Destruction of living tissue: Rapid chemical necrosis of the skin, subcutaneous flesh, and mucous membranes. Contact with the eyes is catastrophic, causing rapid corneal opacity, irreversible ulceration, and permanent blindness within seconds.
• Destruction of transport equipment: Aggressive chemical corrosion of structural chassis beams, trailer floor plates, brake pipes, electrical conduits, containment drums, and load securing chains.

Packing Group Classification Criteria

Allocation to Packing Groups I, II, and III is based on OECD Test Guideline 404 (in vivo skin irritation/corrosion) or validated in vitro testing (e.g., OECD 430, 431, or 435):

The metal corrosion test under ADR 2.2.8.1.5.3 evaluates whether a substance corrodes the prescribed steel or aluminium test materials. Even if a chemical formulation does not attack human skin, a corrosion rate exceeding 6.25 mm per year automatically classifies it as a Class 8, Packing Group III dangerous good.

Chemistry of Corrosives: Strong Acids vs. Strong Alkalis

Corrosive substances are broadly divided into acidic and alkaline (basic) families, each displaying distinct chemical behaviors:

• Strong Acids:
  • Sulfuric acid (UN 1830): A heavy, oily liquid that acts as an aggressive dehydrating agent, ripping water molecules out of organic matter and carbonizing wood, paper, and flesh.
  • Hydrochloric acid (UN 1789): Releases dense, pungent, choking hydrogen chloride fumes that corrode surrounding metal surfaces even without direct liquid contact.
  • Nitric acid (UN 2031): A powerful oxidizing acid that can spontaneously ignite organic material (such as sawdust or textiles) and releases highly toxic, reddish-brown nitrogen dioxide (NO2) gas when reacting with metals.
• Strong Alkalis (Bases):
  • Sodium hydroxide solution / Caustic soda (UN 1824) and Potassium hydroxide solution / Caustic potash (UN 1814).
  • Alkalis cause liquefactive necrosis by saponifying fatty acids in cell membranes and denaturing proteins. Unlike acids, which precipitate protein crusts (coagulative necrosis) that somewhat slow chemical penetration, alkalis penetrate deeply, silently, and persistently into subcutaneous tissue. Alkalis in the eyes pose an extreme risk of complete blindness because they dissolve corneal collagen.

Critical Chemical Reactions and Driver Safety Rules

Transport drivers must understand three hazardous chemical reactions associated with Class 8 goods:

1. Acid-Base Incompatibility: Mixing acids and alkalis triggers a violent, instantaneous exothermic neutralization reaction. The immense heat generated causes rapid boiling, ejecting scalding corrosive spatter across the vehicle. Acids and alkalis must be kept strictly segregated during loading.
2. The "Water-into-Acid" Trap: Adding water directly to concentrated acid causes the initial water droplets to sit atop the dense acid, where they instantly boil into superheated steam. This localized steam eruption violently propels concentrated acid into the air. This explains why ad-hoc dilution is dangerous. Drivers do not mix spilled cargo or attempt neutralisation. It is not a reason to withhold prompt skin/eye irrigation after exposure; follow current first-aid advice and emergency instructions.
3. Hydrogen Gas Generation: When corrosive acids come into contact with structural vehicle metals (such as mild steel, iron, galvanized zinc, or aluminum), some acid/metal combinations can release elemental hydrogen gas (UN 1049). Hydrogen is an odorless, extremely flammable gas with a wide explosive range (a wide flammable range) that accumulates in closed cargo spaces and can be ignited by any small spark or static charge.

Vehicle Safety Equipment and Personal Protection (ADR 8.1.5)

Under ADR 8.1.5, transport units carrying Class 8 corrosive substances must carry specialized protective and environmental equipment:

• Personal protective equipment per crew member: ADR requires protective gloves and eye protection, alongside the warning vest and portable lighting apparatus. Select materials for the actual hazard; an apron or coverall may be required by the task risk assessment but is not a universal 8.1.5 item.
• Eye-wash bottle: At least one dedicated bottle filled with sterile saline solution or potable water, stored in the cab and kept immediately accessible.
• Environmental protection kit: A shovel, a drain seal and a collecting container compatible with the intended limited use to cover road gullies and storm drains, preventing corrosive run-off from contaminating municipal sewers or aquatic ecosystems.

Source check: 17 October 2026 — ADR 2025, English text published by the Slovak Ministry of Transport, 2.2.8 and 8.1.5.