Class D fire extinguishers are the only type effective for combustible metal fires. These extinguishers use dry powder agents, such as sodium chloride or graphite, that smother burning metal without reacting with it. Using the wrong extinguisher on a metal fire, particularly one containing water, can cause a violent explosion.
Why Metal Fires Need Their Own Extinguisher
Combustible metals burn at extremely high temperatures and react dangerously with water, carbon dioxide, and many common extinguishing agents. Alkali metals like sodium, potassium, and lithium react with water to generate hydrogen gas, which ignites instantly and can turn a small fire into a catastrophic one. Standard ABC extinguishers, CO2 extinguishers, and foam extinguishers are all ineffective or actively dangerous on metal fires.
Class D extinguishers work differently from other types. Rather than spraying outward under pressure, they deposit powder gently over the burning metal using gravity. This is intentional. A high-pressure blast would scatter burning metal fragments and spread the fire. The powder forms a crust over the burning surface, cutting off oxygen and absorbing heat until the metal cools below its ignition point.
Common Combustible Metals
The list of metals that require Class D protection is longer than most people expect. The most commonly encountered ones include magnesium, sodium, potassium, lithium, titanium, and zirconium. Aluminum, zinc, and nickel catalyst are also combustible when in powder or dust form, even though their solid bulk forms are considered safe. Magnesium is particularly dangerous because its ignition temperature is near its melting point of about 1,200°F, and in dust or ribbon form it can ignite almost instantly.
Beyond pure metals, many metal compounds and alloys also qualify. Lithium hydride, sodium hydride, potassium hydride, lithium aluminum hydride, and sodium borohydride are all examples of reactive metal compounds that burn fiercely and require Class D suppression. Workplaces that handle any of these materials need to have the correct extinguisher within reach, not locked in a supply closet down the hall.
Dry Powder Agents and Which Metals They Cover
Not all Class D extinguishers are interchangeable. Different dry powder agents are formulated for different metals, and using the wrong agent on the wrong metal can fail to control the fire or make it worse.
- Sodium chloride powder is the most widely used Class D agent. It works well on magnesium, sodium, potassium, and their alloys. When applied to burning metal, the salt melts and forms a sealed crust that excludes oxygen.
- Copper powder is specifically effective for lithium fires. Copper rapidly absorbs heat from burning lithium and lowers the temperature below ignition point. Sodium chloride is less effective on lithium, making copper the preferred choice.
- Graphite powder is another common agent that works by smothering. It’s effective on several combustible metals and is often used in foundries and machining operations.
This is why Class D extinguishers are not given a general numerical rating like other extinguisher classes. Instead, each unit is labeled with a specific list of metals it can handle. Before purchasing one, you need to know exactly which metals are present in your workspace.
How to Identify a Class D Extinguisher
Class D extinguishers are marked with a yellow star containing the letter “D.” There is no pictogram symbol for Class D fires on newer labeling systems, so you’ll only see the older letter-and-star format. The body of the extinguisher typically lists the specific metals the agent is rated for, which is the most important thing to check before relying on it.
These extinguishers are not standard equipment in most buildings. They need to be specially ordered, and they’re typically found only in facilities that handle reactive metals: machine shops, laboratories, foundries, and manufacturing plants. If your workplace processes combustible metal dust or shavings, fire codes governed by NFPA 484 (the standard for combustible metals) require appropriate Class D suppression to be available.
Using a Class D Extinguisher Effectively
Most portable fire extinguishers discharge completely in 8 to 30 seconds with a range of roughly 6 to 10 feet. Class D units are no exception, so positioning matters. You need to be close enough to apply the powder directly onto the burning metal, but you also need to apply it gently. The goal is to let the powder settle and form a blanket over the fire, not to blast it in a way that scatters molten metal.
Start by applying powder around the edges of the fire and work inward, building up a layer that seals the burning material from air. Don’t disturb the crust once it forms. Metal fires can re-ignite if the crust breaks and exposes hot metal underneath. Even after the visible flames are gone, the metal beneath can remain dangerously hot for a long time.
Risks Beyond the Flames
Burning metals produce toxic metal oxide fumes that pose serious respiratory hazards. Magnesium fires produce an intensely bright white flame that can damage eyesight. Alkali metal fires release hydrogen gas during any contact with moisture, including humidity in the air, creating secondary explosion risks. If a metal fire grows beyond what a portable extinguisher can handle in those first few seconds of discharge, evacuation is the safer choice. These are not fires that respond well to improvisation.