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Short answer
A corrosive fluid is a liquid that destroys living skin on contact, that eats through steel or aluminum, or both. Regulators treat these as two separate hazards with separate tests: OSHA and DOT define skin corrosion by irreversible tissue damage, and define metal corrosion as a rate above 6.25 mm per year at 55 °C.
Definition and scope
In everyday lab talk “corrosive” usually means a strong acid or base. The regulations are narrower and more specific. OSHA’s Hazard Communication Standard (29 CFR 1910.1200) has one criterion for skin and a different one for metals, and the Department of Transportation (49 CFR 173.136) has its own Class 8 definition for shipping. A fluid can meet one definition and not another, so the label you see depends on which rule is being applied.
For the chemical-by-chemical view of acids, bases, first aid and spills, see corrosive chemicals in the lab. This page covers a narrower question: what the word actually means in the rules, and how each meaning is tested.
How corrosivity is tested
Corrosive to skin
OSHA Appendix A, section A.2, defines skin corrosion as irreversible damage to the skin: visible necrosis through the epidermis and into the dermis. Category 1 is split by how quickly the damage appears: 1A after an exposure of up to 3 minutes with observation up to 1 hour; 1B after more than 3 minutes and up to 1 hour, observed up to 14 days; 1C after more than 1 hour and up to 4 hours, observed up to 14 days.
Where no test data exist, A.2.2.5 says a substance is considered corrosive (Skin Category 1) if its pH is 2 or lower, or 11.5 or higher. That presumption can be overridden when acid or alkaline reserve data suggest otherwise, but only if confirmed by other data, preferably a validated in vitro test.
Corrosive to metals
OSHA Appendix B, section B.16, has a single category: a corrosion rate on steel or aluminum surfaces exceeding 6.25 mm per year at a test temperature of 55 °C (131 °F). Both metals are tested; if the first metal tests as corrosive, the test on the other is not required. The appendix names example specimens, including steel to ISO 3574 or SAE 1020 and non-clad aluminum 7075-T6.
Corrosive for transport
49 CFR 173.136 defines a Class 8 (corrosive) material as one that causes irreversible damage to human skin at the site of contact within a specified time, or a liquid (or a solid that may become liquid in transport) with a severe corrosion rate on steel or aluminum under 173.137(c)(2). Section 173.137 then assigns packing groups by skin-exposure time, and adds that Packing Group III also covers materials that do not irreversibly damage skin but corrode steel or aluminum faster than 6.25 mm (0.25 inch) a year at 55 °C. For how packing groups are used on shipments, see packing group.
Types or related concepts
The three regimes side by side, each row taken from the regulatory text:
| Regime | What it protects | Test or trigger |
|---|---|---|
| OSHA HazCom Appendix A, A.2 (skin corrosion) | People | Irreversible skin damage; Category 1A, 1B, 1C by exposure and observation time; pH presumption of 2 or lower, or 11.5 or higher, when no other data exist |
| OSHA HazCom Appendix B, B.16 (corrosive to metals) | Metal containers and equipment | Corrosion rate on steel or aluminum above 6.25 mm per year at 55 °C |
| 49 CFR 173.136 and 173.137 (Class 8) | People and metal packaging in transport | Irreversible skin damage within a set time (Packing Groups I, II, III by exposure time); Packing Group III also by steel or aluminum corrosion above 6.25 mm per year at 55 °C |
Under 173.137 the skin-based packing groups are: Packing Group I, damage within up to 60 minutes of observation after exposure of up to 3 minutes; Packing Group II, damage within 14 days after exposure of more than 3 and up to 60 minutes; Packing Group III, damage within 14 days after exposure of more than 60 minutes and up to 4 hours (test per OECD Test No. 404). Hazard communication classes are covered in GHS classification of chemicals and GHS labels and pictograms.
Where it matters for labs, clinics and supply decisions
Because “corrosive” can mean harm to skin, harm to metal, or both, the classification on a safety data sheet answers two different questions. Skin classification drives protective equipment and exposure response; see chemical-resistant glove selection and the example of hydrofluoric acid safety. Metal classification matters for what a fluid is stored or shipped in, since the test is defined on steel and aluminum specimens. Segregation of corrosives from incompatible materials is covered in chemical storage compatibility and segregation rules.
The metal test does not say whether a given stainless steel, plastic or glass item is suitable. For equipment and container materials, the safety data sheet and the equipment manufacturer’s own compatibility information are the sources to use. Waste rules use their own corrosivity characteristic; see RCRA hazardous waste codes.
Common misunderstandings
- “Corrosive means low pH.” The pH of 2 or lower, or 11.5 or higher, is only what OSHA uses absent other information, and A.2.2.5 allows it to be overridden by acid or alkaline reserve data confirmed by other data. High pH bases are corrosive too.
- “Corrosive always means it burns skin.” Under 49 CFR 173.137, a material can fall in Packing Group III purely because it corrodes steel or aluminum, without irreversible skin damage.
- “One number defines it.” Some secondary articles quote 6.35 mm per year or a pH of 12.5. The regulatory text for metals (OSHA B.16 and 49 CFR 173.137) says 6.25 mm per year, and the OSHA skin presumption is 11.5 or higher. Check the rule you are actually subject to.
- “Tested on steel means tested on everything.” The test uses steel and aluminum specimens only.
FAQ
What makes a fluid corrosive?
Either irreversible skin damage (visible necrosis through the epidermis into the dermis) or a corrosion rate above 6.25 mm per year on steel or aluminum at 55 °C. Either can qualify it, depending on the rule.
What pH is considered corrosive?
When no other information exists, OSHA treats a substance as skin-corrosive at pH 2 or lower or 11.5 or higher. That presumption can be overridden with confirming data.
Is a corrosive fluid the same as an acid?
No. Strong bases can meet the skin criteria through the high-pH presumption, and a fluid can be classed corrosive to metals without being corrosive to skin.
Sources
Checked 2026-10-29.
- OSHA, Appendix A, A.2 Skin corrosion/irritation: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200AppA
- OSHA, Appendix B, B.16 Corrosive to metals: https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1200AppB
- 49 CFR 173.136, Class 8 definitions: https://www.ecfr.gov/current/title-49/part-173/section-173.136
- 49 CFR 173.137, Class 8 assignment of packing group: https://www.ecfr.gov/current/title-49/part-173/section-173.137
