Extinguishing Magnesium Fires

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

PYROPHORIC METALS

dust precipitator. NFPA 480 requires these types if precipitators to be vented to

prevent hydrogen buildup resulting from magnesium reacting with the water spray.

NFPA 480 also places restrictions on the mass flow rate of magnesium into the

precipitator as well as the amount of magnesium sludge accumulated in the

precipitator. The equipment should be restricted to magnesium processing only.

Molten magnesium in the foundry presents a serious fire problem if not properly

handled. Sulfur dioxide or melting fluxes are commonly used to prevent oxidation or

ignition of magnesium during foundry operations. The action of sulfur dioxide is to

exclude air from the surface of the molten magnesium; it is not an extinguishing

agent. Fluxes perform both functions.

Pots, crucibles, and ladles that may contact molten magnesium must be kept dry to

prevent steam formation or a violent metal-water reaction. Containers should be

checked regularly for any possibility of leakage or weak points. Steel lined runoff

pits or pits with tightly fitting steel pans should be provided, and the pans must be

kept free of iron scale. Leaking metal contacting hot iron scale results in a violent

thermite reaction. Use of stainless steel pans or linings will eliminate this possibility.

Heat treating ovens or furnaces, where magnesium alloy parts are subjected to high

temperatures to modify their properties, present another special problem.

Temperatures for heat treating needed to secure the desired physical properties are

often close to the ignition temperatures of the alloys themselves, and careful control

of temperatures in all parts of the oven is essential. Hot spots leading to local

overheating are a common cause of these fires. Large castings do not ignite readily,

but fins or projections on the castings, as well as chips or dust, are more readily

subject to ignition. For this reason, castings should be thoroughly cleaned before

heat treating. Magnesium castings in contact with aluminum in a heat treating oven

will ignite at a lower temperature than when they are placed on a steel car or tray.

Magnesium should not be heat treated in nitrate salt baths. Certain commonly used

molten mixtures of nitrates and nitrites can react explosively with magnesium alloys,

particularly at temperatures over 538 C (1,000 F).

Extinguishing Magnesium Fires

Magnesium and its alloys present special problems in fire protection. Magnesium

combines so readily with oxygen that under some conditions water applied to

extinguish magnesium fires may be decomposed into its constituent elements,

oxygen and hydrogen. The oxygen combines with the magnesium and the released

hydrogen adds to the intensity of the fire. None of the commonly available gaseous

extinguishing agents (CO2, for example) are suitable for extinguishing magnesium

fires. The affinity of magnesium for oxygen is so great that it will burn in an

atmosphere of carbon dioxide. Magnesium may also burn in an atmosphere of

nitrogen to form magnesium nitride. For these reasons, the common extinguishing

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Pyrophoricity