A class D fire is a fire involving burning metal, such as magnesium, titanium, sodium, potassium, lithium, and zirconium. These fires can ignite around 650°C and climb past 3,000°C once metals like titanium or zirconium are fully burning. Water, CO2, foam, and ABC powder don’t just fail on a class D fire. They make it worse. Only a class D dry powder agent, matched to the specific metal, can put it out safely.

Picture this. A machinist opens the swarf bin under a CNC lathe at the end of his shift. A curl of magnesium swarf at the bottom is still glowing, smouldering quietly since the last cut, insulated by the metal piled on top of it. He grabs the nearest extinguisher. It’s CO2.

The second the CO2 hits the magnesium, the fire gets worse. A blinding white flame shoots up. Black soot covers the area. The CO2 isn’t putting the fire out. It’s feeding it.

This is a class D fire, and almost none of the rules from a normal fire apply here.

This guide covers what a class D fire actually is, why metal form matters more than the metal itself, the exact reactions that turn normal extinguishers into fuel, the right agent for each metal, how to apply it, and where this risk shows up in Indian industry.

For the basics on extinguisher use, see our guide on the PASS method fire extinguisher technique. For the other 5 fire types, check the classes of fire guide.

Class D fire meaning: direct definition

A class D fire is any fire fuelled by a combustible metal, such as magnesium, sodium, potassium, titanium, lithium, or zirconium. Magnesium ignites around 650°C, and once a metal fire is fully established, temperatures can cross 3,000°C, especially with titanium and zirconium. Under IS 15683:2018, EN 2, and ISO 3941, class D fires get their own category because they react dangerously with the agents used on every other fire class.

Three things define a class D fire:

• Ignition starts around 650°C, but a fully burning metal fire can reach above 3,000°C
• Water, CO2, foam, and ABC powder don’t just fail. They actively make these fires bigger
• There’s no single class D agent that works on every metal. The extinguisher has to match the metal involved

Combustible metals vs pyrophoric metals: a difference that changes everything

Not every dangerous metal behaves the same way, and this difference decides your entire response.

Combustible metals need outside heat to ignite. A magnesium bar or a titanium billet sitting in a warehouse is stable. These metals only become a class D risk once they’re cut, ground, or reduced to swarf, chips, or dust.

Pyrophoric metals ignite on their own, just from contact with air or moisture, at room temperature. No spark, no heat source needed. Fine lithium powder, sodium-potassium alloy (NaK), and finely divided zirconium fall into this group. The moment they’re exposed to air, they catch fire.

Class D extinguishers are built for combustible metal fires. Pyrophoric materials need extra precautions on top, like inert atmosphere handling and sealed containment.

Why metal form matters more than metal type

This is the one idea that explains almost everything about class D fires, and most guides skip it entirely.

Why does magnesium swarf ignite from a small spark while a solid magnesium bar sits safely for years? The metal is identical. The difference is surface area.

A solid block has very little surface area compared to its mass. Heat applied to it spreads into the bulk and dissipates. The surface never gets hot enough to ignite.

The same metal as chips, shavings, or powder has a huge surface area for the same mass. Heat can’t escape fast enough, so the surface heats up to ignition point. Once one spot catches, it radiates heat outward and the fire spreads.

This is backed by US Department of Energy data, which shows the same metals in thick blocks or castings are far harder to ignite than thin sections, fine particles, or molten metal.

So every class D risk check should ask: is this metal being machined or ground? Is dust or swarf building up anywhere? A billet sitting in storage is not a fire risk. The same metal as swarf in a bin is.

Common class D fire fuels and where they show up

Magnesium, the most common one

Magnesium alloys are light and strong, so they’re everywhere: automotive engine blocks, gearbox casings, wheel rims, steering columns (Maruti, Tata, Mahindra), aerospace frames, helicopter parts, and electronics housings.

Machining magnesium produces hazardous swarf. It ignites around 650°C in solid form, but fine swarf and dust can catch fire at lower temperatures. Once burning, magnesium can hit temperatures above 2,500°C, and it can keep burning even in molten form.

Flame colour: brilliant white. So bright it can damage your eyes if you look directly at it without protection. Brighter than a welding arc.

Sodium and potassium

Both are soft, silvery metals stored under mineral oil or in argon or nitrogen because they react with moisture in the air. Sodium shows up in pharma API manufacturing and chemical synthesis. Potassium is used in chemical synthesis and fertiliser production.

Both ignite on contact with water. NaK, a sodium-potassium liquid alloy, is pyrophoric and catches fire the moment it touches air. It needs inert atmosphere handling at all times.

Sodium flame: intense yellow-orange. Potassium flame: violet to lilac.

Titanium

Solid titanium doesn’t ignite easily. But titanium powder and grinding dust are serious class D hazards, and titanium fires burn above 3,000°C, hot enough that some class D agents stop working. That’s why graphite-based agents, not sodium chloride, are correct for titanium.

Titanium is used in aerospace parts (DRDO, HAL, Brahmos, private aerospace), medical implants, marine hardware, and chemical processing equipment. Solid titanium ignites around 870°C, but dust ignites at much lower temperatures.

Lithium

Lithium metal (not lithium-ion batteries) is used in primary batteries, chemical synthesis, and emerging solid-state battery research. It ignites in air and reacts violently with water.

Fine lithium powder is pyrophoric. Bulk lithium is a class D hazard. Copper powder is the only agent that really works here.

Flame colour: crimson red.

Zirconium and hafnium

Zirconium is used in nuclear fuel cladding and corrosion-resistant chemical plant parts. Hafnium shows up in nuclear and semiconductor applications. Both are class D hazards once machined or powdered. Solid zirconium ignites around 760°C, and zirconium dust is pyrophoric. Graphite-based agents are correct for both.

Why normal extinguishers make class D fires worse

This isn’t a case of the wrong extinguisher just not working. The wrong agent can actively accelerate a metal fire and cause an explosion.

Water on alkali metals (sodium, potassium, lithium):

2Na + 2H₂O → 2NaOH + H₂↑

The metal strips oxygen from water and releases hydrogen gas. The reaction generates heat, and the hydrogen ignites instantly. Scale this from a classroom demo to kilograms of burning sodium in a plant, and you get a violent explosion that throws burning metal outward.

Water on magnesium:

Mg + H₂O → MgO + H₂↑

Same mechanism. At magnesium fire temperatures, above 650°C, this reaction is violent and the hydrogen ignites immediately.

CO2 on burning magnesium:

2Mg + CO₂ → 2MgO + C

Burning magnesium is hot enough to strip oxygen straight out of CO2, using it as fuel instead of being put out by it. The fire gets bigger. Black soot forms. This is the exact scenario from the start of this guide.

CO2 on sodium and potassium:

4Na + 3CO₂ → 2Na₂CO₃ + C

Same idea. The alkali metal pulls oxygen from the CO2, the CO2 feeds the fire, and black carbon deposits form. Never use CO2 on an alkali metal fire.

ABC dry chemical powder on metal fires:

ABC powder, made of monoammonium phosphate, isn’t built to form the heat-absorbing crust a metal fire needs. On some burning metals, the phosphate can react badly with the hot surface. ABC powder will not put out a class D fire.

Foam on sodium or potassium:

Foam is water-based, so it triggers the same violent hydrogen reaction as water. Never use foam on an alkali metal fire.

The only safe option for any class D fire is a class D dry powder agent matched to the metal that’s burning.

The right agent for each metal

Dry powder vs dry chemical: a mix-up that costs lives

These two product types sound almost the same, look similar, and get confused constantly, sometimes with fatal results.

If your facility has both types, clear labelling, separate storage, and proper training are the only things standing between a worker and the wrong extinguisher in an emergency.

Agent-to-metal matching table

Sodium chloride powder melts on contact with the hot metal and forms a thick salt crust over the surface. This crust blocks oxygen and pulls heat away from the metal at the same time. It’s the most common class D agent around.

Copper powder is for lithium fires specifically. Copper conducts heat extremely well, so it cools the burning lithium fast. It also forms a non-combustible copper-lithium alloy, which stops the reaction at the surface. The US Navy originally developed this for naval lithium use.

Graphite powder (G-Plus, G-1, Therm-X) handles the hottest metal fires, including titanium and zirconium, where NaCl can’t stay stable. Graphite holds up above 3,000°C and forms a smothering crust without reacting with the metal. Standard kit in aerospace and foundry settings.

Ternary Eutectic Chloride (TEC) is built for magnesium and aluminium alloy fires in aerospace. It melts at a lower temperature than plain NaCl, so it forms a sealing crust faster on alloy fires.

Why class D extinguishers don’t have a number rating

Every other extinguisher class has a number, like 13A or 55B, showing the size of fire it can handle. Class D extinguishers don’t.

The reason is simple. Metal fires vary too much. A sodium chloride extinguisher rated for 10kg of burning magnesium tells you nothing about how it’ll perform on 10kg of burning titanium powder. Different metal, different burn temperature, different agent needed.

Instead, class D extinguishers are listed for the specific metals they’re tested on. When choosing one, match the listing to the metals actually present in your facility. One unit might not cover every class D hazard in the building.

How to apply a class D extinguisher correctly

Forget the PASS method here. Class D application works completely differently.

1. Discharge gently, not in a blast. Class D extinguishers are built to release agent at low velocity, blanketing the metal rather than blasting it. A high-pressure stream scatters burning metal particles, and each particle becomes a new fire.

2. Bury it, don’t knock it down. The goal isn’t to stop the visible flame fast. It’s to fully cover the burning metal in a thick, unbroken layer of agent that seals out oxygen completely. Apply continuously from the closest safe distance and build up the layer.

A few more things to remember:

• Don’t move or disturb the burning metal while applying agent
• Approach from the side, not straight into the flame
• A magnesium fire’s radiant heat can damage eyes from 2 metres without protection. Don’t watch it directly
• After the fire looks out, leave the powder crust alone. The metal underneath could still be over 600°C and can reignite if uncovered. Wait until it’s fully cooled to room temperature

Identifying the metal by flame colour

When you arrive at a class D fire and don’t know what’s burning, flame colour gives you a quick clue from a safe distance. Different metals need different agents, so getting this wrong wastes time and can make things worse.

Use this as a first check only. Confirm the metal from facility records or signage before picking your agent, and use flame colour as a quick cross-check.

No class D extinguisher available? Emergency response

If a class D extinguisher isn’t immediately at hand and the fire is small and contained, dry sand or dry earth can buy you time.

The sand or earth must be completely dry. Any moisture reacts with burning alkali metals exactly like water does, releasing hydrogen.

Shovel it gently over the burning metal. Don’t pour from height, and never use compressed air. The aim is full, gentle coverage without scattering burning particles.

This is an emergency measure only, not a replacement for the right class D agent. Call emergency services right away. Even if the fire looks out under the sand, leave it alone. The metal underneath may still be above ignition temperature.

Where class D fires happen in Indian industry

Class D fire risk shows up wherever combustible metals are processed, stored, or used, and that covers several major Indian sectors.

Aerospace and defence: DRDO, HAL, Brahmos, and private aerospace companies machine magnesium alloy parts and titanium structural components. Both generate hazardous swarf and dust during machining.

Automotive: Maruti Suzuki, Tata Motors, Mahindra, and Hero use magnesium alloy die castings in engine blocks, gearbox housings, steering columns, and brackets. Every machining centre handling these parts has a class D risk at the swarf bin.

Pharma: Indian API manufacturing uses sodium and potassium metal in reduction reactions. Handling pharma-grade sodium is a real class D risk in these plants.

Chemical industry: Bulk sodium handling and NaK alloy in heat-transfer systems both carry class D risk.

EV and battery manufacturing: Lithium metal handling for solid-state battery development is growing fast in India, and it’s a new class D risk for facilities moving away from lithium-ion.

Research: IITs, CSIR institutes, and DRDO labs work with reactive metals in small quantities. The risk is often underestimated because volumes are low, but the chemistry doesn’t care about scale.

A formal class D fire risk assessment, covering which metals are present, in what form, and how much, is the starting point for picking the right agent and extinguisher. For the full picture across all fire types, see the classes of fire guide.

Preventing class D fires

Engineering controls

Use flood coolant approved for the metal being machined. Many standard cutting fluids react badly with magnesium. Collect swarf continuously instead of letting it pile up, and keep swarf bins sealed and away from heat.

For grinding, use wet grinding wherever possible to suppress dust. If dry grinding is unavoidable, use local exhaust ventilation with spark-arrest filters.

Store sodium and potassium under mineral oil or in sealed argon or nitrogen atmospheres. Never leave alkali metals exposed to air, and check storage containers regularly for moisture.

Remove combustible metal swarf from the facility daily. Don’t let it build up in bins, and follow the right disposal protocol for each metal since sodium swarf and magnesium swarf need different handling.

Administrative controls

• Run a formal combustible metal fire risk assessment for any zone handling class D metals in any form other than large bulk stock
• Choose class D extinguishers matched to each specific metal hazard, not one generic unit for the whole site
• Keep class D extinguishers clearly separated from ABC powder fire extinguishers so no one grabs the wrong one in an emergency
• Train staff on identifying class D fires, picking the right agent, and the application technique that’s different from every other extinguisher type
• Reference NFPA 484 and relevant BIS standards for facility-specific compliance

Class D fire vs class A, B, and C

For the full breakdown across all 6 fire classes, see the classes of fire guide.

Class D fire FAQs

What is a class D fire?

A class D fire is a fire fuelled by a combustible metal, including magnesium, sodium, potassium, titanium, lithium, and zirconium. Under IS 15683:2018, EN 2, and ISO 3941, these fires ignite around 650°C and can exceed 3,000°C once fully established. They react violently with water, CO2, foam, and ABC powder. Only a class D dry powder agent matched to the specific metal can safely put one out.

What metals are involved in class D fires?

Common class D metals include magnesium, sodium, potassium, titanium, lithium, zirconium, hafnium, and aluminium powder. They usually become a fire risk as swarf, chips, grinding dust, or thin sections, not as solid blocks. In India, magnesium (automotive and aerospace machining), sodium and potassium (pharma and chemical synthesis), and titanium (aerospace and defence) are the most common.

Why can’t you use water on a class D fire?

Water reacts with burning metals, especially alkali metals like sodium, potassium, and lithium. The reaction strips oxygen from water, releasing hydrogen gas and extra heat, and the hydrogen ignites instantly. For sodium, the reaction is 2Na + 2H₂O → 2NaOH + H₂↑. At real fire scale, this causes a violent explosion and throws burning molten metal outward.

Why does CO2 make magnesium fires worse?

Burning magnesium is hot enough to strip oxygen directly from CO2: 2Mg + CO₂ → 2MgO + C. Instead of being smothered, the magnesium uses CO2 as fuel. The fire intensifies and black carbon soot forms. Never use CO2 on a magnesium fire. The same reaction happens with sodium and potassium: 4Na + 3CO₂ → 2Na₂CO₃ + C.

What fire extinguisher is used for class D fires?

Class D fires need specialist dry powder extinguishers, different from ABC dry chemical units. The right agent depends on the metal: sodium chloride for magnesium, sodium, and potassium; copper powder for lithium; graphite-based powder (G-Plus, G-1) for titanium and zirconium; and Ternary Eutectic Chloride (TEC) for some magnesium-aluminium alloy fires in aerospace.

What’s the difference between dry powder and dry chemical extinguisher?

Dry chemical (ABC or DCP) uses monoammonium phosphate or sodium bicarbonate and covers class A, B, and C fires, not class D. Dry powder (class D) uses sodium chloride, graphite, copper powder, or TEC, rated for specific metals only. The names sound similar and the units look similar, which makes mixing them up in an emergency a real risk. Class D units are usually yellow, ABC units are usually red.

Which class D agent should be used for a lithium fire?

Copper powder is the right choice for lithium fires. It conducts heat fast, cooling the burning lithium quickly, and forms a non-combustible copper-lithium alloy that stops the reaction at the surface. Sodium chloride works less well on lithium. Water and CO2 must never be used, since lithium reacts with water the same violent way sodium does.

Which class D agent should be used for a magnesium fire?

Sodium chloride (NaCl) powder is the standard agent for magnesium fires. At magnesium fire temperatures, NaCl melts and forms a thick salt crust over the burning surface, blocking oxygen and pulling heat away at the same time. Graphite-based agents work too. Water, CO2 fire extinguisher, foam, and ABC powder should never be used.

Can you use an ABC fire extinguisher on a class D fire?

No. ABC dry chemical extinguishers aren’t rated for class D fires and shouldn’t be used on metal fires. They can’t form the oxygen-blocking crust a metal fire needs, and the phosphate compounds in ABC powder can react badly with some hot metal surfaces. ABC powder won’t put out a class D fire and may make it worse.

What’s a pyrophoric metal and how is it different from a combustible metal?

A combustible metal needs outside heat to ignite and is stable in bulk form, only becoming a class D risk as dust, powder, or fine swarf. A pyrophoric metal ignites on its own from contact with air or moisture at room temperature, no heat needed. Fine lithium powder, NaK alloy, and finely divided zirconium are pyrophoric. Standard class D extinguishers handle combustible metal fires, but pyrophoric materials need extra protocols.

Why does metal form (dust vs solid) matter for class D fire risk?

A metal that’s completely safe as a solid block becomes a class D risk as swarf, and a severe risk as dust. The reason is surface area. A solid block dissipates heat into its mass, so the surface never reaches ignition temperature. The same mass as fine dust has up to 100 times more surface area, so heat can’t escape fast enough and the surface reaches ignition point quickly. A 1kg magnesium block has roughly 600cm² of surface area. The same mass as 100-micron dust exceeds 60,000cm².

What colour does magnesium burn?

Magnesium burns with a brilliant white flame, one of the brightest light sources from any common material. It’s bright enough to cause permanent eye damage if you look directly at it without protection, brighter than a welding arc. Magnesium fires also produce white magnesium oxide smoke, which makes them easy to spot from a safe distance.

What colour does sodium burn?

Sodium burns with an intense yellow-orange flame. This is the same effect behind yellow sodium street lamps, where excited sodium atoms emit light at around 589nm. In a sodium fire, this yellow is strong, steady, and dominant, making it one of the most reliable colour identifiers for any burning metal.

What is the PASS technique and does it apply to class D fires?

PASS (Pull, Aim, Squeeze, Sweep) is the standard technique for class A, B, and C extinguishers, explained fully in our PASS method fire extinguisher guide. It does not apply to class D fires. Class D extinguishers use a burial technique instead: low-velocity discharge that gently covers the burning metal in a continuous layer. A high-pressure sweep would scatter burning particles and spread the fire.

Can you use sand on a class D fire?

Yes, but only as an emergency measure when no class D extinguisher is available. The sand must be completely dry, since any moisture reacts with burning alkali metals just like water does, releasing hydrogen. Gently shovel dry sand over the burning metal to cover it fully. Don’t pour from height or use compressed air, and call emergency services right away. Sand is not a substitute for the correct class D agent.

What is NFPA 484?

NFPA 484 is the National Fire Protection Association’s Standard for Combustible Metals. It covers storage, handling, machining, and fire protection requirements for combustible metal hazards in industrial facilities, including where class D extinguishers should be placed and how swarf and dust should be managed. It’s the main reference standard for class D fire risk management.

Do class D extinguishers have a numerical fire rating?

No. Standard extinguishers carry numerical ratings like 13A or 55B showing the size of fire they can handle. Class D extinguishers don’t, because metal fires vary too much. A sodium chloride extinguisher rated for magnesium tells you nothing about how it performs on titanium or lithium. Instead, class D extinguishers are listed for specific metals, and selection has to match the metals actually present.

Where do class D fires most commonly occur in India?

Class D fires occur wherever Indian facilities machine, grind, or handle combustible metals. The highest-risk sectors are automotive manufacturing (magnesium alloy die casting for Maruti, Tata, Mahindra), aerospace and defence (DRDO, HAL, Brahmos, working with titanium and magnesium), pharma API manufacturing (sodium and potassium in synthesis), EV battery development (lithium metal for solid-state batteries), and research labs handling reactive metals.

How do you prevent a class D fire in a machining facility?

Collect swarf continuously instead of letting it accumulate, use flood coolant approved for the specific metal, and keep swarf bins sealed and away from heat. Run formal combustible metal fire risk assessments, install class D extinguishers matched to the metals being machined, and keep them clearly separated from ABC units. Train all machining staff on class D recognition and the correct agent for their metals.

What is NaK alloy and why is it a class D fire hazard?

NaK is a liquid sodium-potassium alloy used in specialist heat-transfer applications, including some research reactor cooling systems. It’s pyrophoric, meaning it ignites the moment it contacts air or moisture at room temperature, with no external ignition source needed. NaK requires inert atmosphere handling at all times, and any spill that touches air ignites immediately. It’s one of the most hazardous class D materials in industrial use.

What is the safest way to store combustible metals like sodium and potassium?

Store sodium and potassium under mineral oil in sealed containers, or in sealed vessels under argon or nitrogen. Never store either metal in air. Check containers regularly for moisture ingress and oil levels, keep them in cool dry locations away from any water source, and store only the quantities you actually need. Keep them away from other materials and make sure class D extinguishers are nearby.

Can titanium catch fire? Under what conditions?

Yes. Bulk titanium, like forgings, plate, or thick bar, doesn’t ignite easily under normal conditions. But titanium powder, grinding dust, and thin sections are class D hazards. Solid titanium ignites around 870°C, while titanium dust ignites at much lower temperatures. Once burning, titanium can reach above 3,000°C, one of the hottest class D fire scenarios, and needs graphite-based agents to extinguish.

What’s the difference between a class D fire and a class B fire?

A class B fire involves flammable liquids like petrol, diesel, or solvents, burning at 200 to 700°C, and is handled with foam or DCP. A class D fire involves burning metal at 650°C to over 3,000°C, where standard class B agents like foam, DCP, and CO2 are dangerous. In CNC machining environments, a class B fire from cutting oil can ignite metal swarf nearby and escalate into a class D fire.

What happens if you use foam on a sodium fire?

Foam is water-based, so it reacts with burning sodium the same way water does: 2Na + 2H₂O → 2NaOH + H₂↑. The reaction releases hydrogen gas and generates extra heat, making the fire worse rather than putting it out. The hydrogen can ignite instantly, causing an explosion that throws burning molten sodium outward. Never use foam on any alkali metal fire.

Final word

Class D fires don’t happen often, but in the facilities where they do, there’s zero room for error. The wrong extinguisher doesn’t just fail. It can cause an explosion and throw burning molten metal at the person trying to help.

Knowing the metal involved, having the right agent for it, and using the burial technique correctly are the only things that matter here.

Speciality Geochem supplies fire safety equipment and consultation to Indian industrial, defence, and aerospace facilities, including class D fire risk assessment and extinguisher selection. For procurement enquiries, contact Speciality Geochem directly. For the full reference on all fire classes, see the classes of fire guide.