Reactive metals: alkali metals and pyrophoric powders

Two hazards, one class

This class holds thirteen substances, and they do not all belong to it for the same reason.

The first group is the alkali metals — lithium, sodium, potassium — plus calcium. Their hazard is water. CAMEO Chemicals, the NOAA/EPA reactivity database, documents the mechanism for the group:

“Their reaction with water is violently rapid and quite exothermic. It produces gaseous hydrogen and other products. The heat that is generated is sufficient to melt the unreacted metal, ignite the liberated hydrogen and ignite the metal itself.”

The harmonised EU classification says the same thing in code: lithium, sodium and potassium all carry H260 — in contact with water releases flammable gases which may ignite spontaneously — together with H314 for the corrosive hydroxide the reaction leaves behind. Calcium carries H261, the slower version of the same statement.

The second group is metal powders: aluminium, magnesium, zinc and zirconium. The EU list names them explicitly for the form — “aluminium powder (pyrophoric)”, “magnesium powder (pyrophoric)”, “zinc powder — zinc dust (pyrophoric)”, “zirconium powder (pyrophoric)” — and gives them H250, catches fire spontaneously if exposed to air, usually alongside H260 or H261 as well. OSHA’s criteria define a pyrophoric solid as one which, “even in small quantities, is liable to ignite within five minutes after coming into contact with air” (B.10.1).

That is the whole difference between a roll of aluminium foil and a drum of aluminium powder. Not the chemistry — the surface area.

The water-reactivity criteria are numeric

The three water-reactive categories are set by how much flammable gas comes off, and how fast (B.12, Table B.12.1):

Category	Criterion	Statement
1	Reacts vigorously with water at ambient temperature, the gas generally tends to ignite spontaneously, or ≥ 10 litres of gas per kg of chemical in any one minute	H260 — in contact with water releases flammable gases, which may ignite spontaneously
2	≥ 20 litres per kg per hour, and not Category 1	H261 — in contact with water releases flammable gas
3	Reacts slowly; ≥ 1 litre per kg per hour, and not Category 1 or 2	H261

The jump from Category 1 to Category 2 is a factor of thirty in time — a minute against an hour. That is the practical difference between a spill that catches fire while you are still looking at it and one that fills a closed cabinet with hydrogen over a shift.

How we classify this page’s substances

Unlike most classes on this site, membership here comes entirely from documented reactivity data — the CAMEO reactive groups Metals, Alkali, Very Active (3 substances) and Metals, Elemental and Powder, Active (10) — and not from a hazard code. There is no GHS hazard class called “reactive metal”, so there is no code to anchor on.

That has an honest consequence which this page states rather than hides. Four of the thirteen carry no physical hazard code at all.

Substance	Harmonised hazard statements	Why it is here
beryllium	H350i, H330, H301, H335, H372, H315, H319, H317	carcinogen and acute toxicant — no fire or water-reactivity code
cobalt	H350, H341, H360F, H334, H317, H413	carcinogen, mutagen, respiratory sensitiser — no fire code
nickel	H351, H372, H317	suspected carcinogen, sensitiser — no fire code
selenium	H331, H301, H373, H413	acute toxicant — no fire code, and it is not a metal but a metalloid

These four are in the class because CAMEO groups the finely divided element as an active metal powder, which is defensible as combustible-dust chemistry. But their harmonised classification is entirely health-based, and for a storekeeper that is the hazard that should drive the decision. Beryllium and cobalt are not stored the way sodium is stored; they are stored the way a carcinogen is stored, with exposure control as the governing constraint.

Cadmium is the one that sits in both camps: the EU entry is “cadmium (pyrophoric)” and it carries H250 and H350, H341, H361fd, H330, H372, H400 and H410.

All thirteen carry the signal word Danger. Nine carry the GHS02 flame; five carry GHS08.

Why water, foam and CO₂ are all the wrong answer

Most fire instincts are trained on carbon fires, and every one of them fails here.

Water is fuel. The emergency-response guidance for sodium and potassium states it in capitals — “DO NOT USE WATER OR FOAM” — and describes the reaction: sodium “reacts violently with water to give sodium hydroxide and hydrogen, which ignites spontaneously”; potassium “IGNITES WHEN EXPOSED TO WATER OR MOISTURE”.

Foam is mostly water, and fails for the same reason.

Carbon dioxide is the one that surprises people, because it is the universal “safe” agent for electrical and liquid fires. CAMEO’s group documentation is unambiguous: alkali metals “are nonflammable, but they are combustible”, and “they may also burn in carbon dioxide and in nitrogen”. A CO₂ extinguisher on a burning alkali metal is not an inert blanket — it is an oxidant.

What is left is the Class D set: dry chemical, soda ash, lime, dry sand, sodium chloride powder, graphite powder. OSHA’s requirement is about availability rather than technique — where combustible metal powders, flakes, shavings or similarly sized products are generated at least once every two weeks, 29 CFR 1910.157(d)(6) requires that “the travel distance from the combustible metal working area to any extinguishing agent is 75 feet (22.9 m) or less”.

A store holding these metals should be planned backwards from that sentence: where is the Class D agent, and how far is it from the drum?

Powder is a different problem from bulk

Everything above concerns the material sitting in a container. The moment it is airborne, it is a different hazard with a different regulatory home.

OSHA’s Combustible Dust National Emphasis Program (CPL 03-00-008) defines combustible dust as “a finely divided combustible particulate solid that presents a flash-fire hazard or explosion hazard when suspended in air or the process-specific oxidizing medium over a range of concentrations”, and lists “metal dust such as aluminum, magnesium, and some forms of iron dusts” first among the materials in scope. OSHA’s combustible dust guidance for hazard communication places aluminium and magnesium dusts in St 3, the most severe dust-explosion class.

Practically: a sealed drum of magnesium powder in a dry store is a storage problem. The same powder in a dust extraction duct, on a beam, or in a layer on top of a light fitting is a deflagration waiting for an ignition source, and no storage cabinet addresses it.

Segregation

Four classes are marked never store with, and they split into two arguments.

Acids — mineral acids and oxidizing acids. The metal-plus-acid reaction produces hydrogen directly, without needing water to arrive first, and it does so at a rate set by surface area — which for a powder is very high. With an oxidizing acid the reaction also produces heat and nitrogen oxides.

Oxidizer classes — oxidizers and organic peroxides. A metal fire is already hard to extinguish using only atmospheric oxygen. A chemical oxygen source removes the last constraint, and intimately mixed metal powder and oxidizer is the composition of a pyrotechnic rather than of a fire.

Seven classes sit at keep separate. One — water-reactives and pyrophorics — is marked generally compatible, and it is worth being precise about what that means: it is not that the pairing is harmless, but that it is the same hazard needing the same control. A dry, sealed, sprinkler-free store with a Class D agent at the door serves both.

⚠ One consequence of the two-population problem above: for beryllium, cobalt, nickel and selenium the class-level segregation is about the wrong hazard. Their controlling risk is inhalation, and the substance’s own SDS sections 7 and 8 govern.

The flame pictogram is GHS02 and the health-hazard pictogram carried by five of these substances is GHS08. Class-level segregation is a starting point rather than a verdict on any individual substance: check SDS sections 7 and 10, or look the substance up in the storage compatibility matrix. The prohibited classes have their own pages — oxidizers, oxidizing acids, organic peroxides and self-reactives — and the closest relative of this class is water-reactives and pyrophorics.