Magnesium can be ignited, upon which it will burn with a brilliant white light at a
temperature of about 2800°C. However, it is only magnesium vapor that will burn. Thus, the metal
must be heated to a temperature high enough to produce a sufficient quantity of vapor to support
combustion. As a practical matter, this means the metal must be melted. Pure magnesium melts
at 650°C. Because the heat conductivity of magnesium is high, all of a massive piece must be
raised to the melting point for the piece to burn freely. Thus it is very difficult to start a fire with a
massive piece such as a casting. Even if a torch is used to raise a part of the casting to the
ignition temperature and burning starts, it will cease when the torch is removed, by heat conduction
to other parts of the casting and consequent lowering of the temperature below the ignition
temperature.
On the other hand, finely divided magnesium, such as some kinds of machining scrap,
and very thin ribbon, cannot conduct the heat away from a source and burning can be initiated with
a match, which has a flame temperature of about 900°C. Continuous burning of the chip or ribbon
will be sustained after the match is removed. Fire hazard is therefore principally associated with
finely divided magnesium, and is most apt to be encountered in machining or grinding operations,
rather than in the service of a part.
Once started, a magnesium fire can be extinguished by using the normal strategies of cooling,
removing oxygen, or letting the magnesium be totally consumed. Machining chips that have started
to burn can be cooled effectively with cast iron chips, and this technique is used quite
successfully. Since magnesium is an active chemical, it will react with oxygen preferentially to
many other materials. Hence, any extinguisher that contains oxygen as a part of its chemical
structure will probably support the combustion of magnesium rather than stop it. This is true of
water, and one should not use water to cool the magnesium without being aware of the fact that,
during cooling with water, reaction will occur, producing heat and hydrogen. The hydrogen, in turn,
may react with the air to produce explosions; Water can be used, but only in large quantities in
order to cool the mass of magnesium below the melting point, with recognition of the hydrogen
explosion hazards that are being introduced. Water should never be used for extinguishing a fire of
finely divided material, since the vigorous reaction with water will scatter the chips and spread the
fire.
Oxygen can be excluded by covering with a non-reactive material, such as melting fluxes or
other proprietary materials. Any oxygen-containing material, such as sand, should be avoided. In a
confined volume, such as a heat-treatment furnace, boron trifluoride is an extremely effective fire
extinguisher through the formation of magnesium fluoride, which excludes oxygen.
As with all oxydizable materials, an air suspension of fine magnesium power can explode, and
this is the most serious hazard associated with the burning of magnesium. Explosion will occur
only if the power is fine enough to remain suspended in the air for an appreciable period of time. As
a practical matter, this means that the power must be at least as fine as about 200 mesh, which
has an average particle size of 74 mm. Thus, the only time that an engineer is apt to encounter an
explosion hazard with magnesium is in handling dust created by a grinding operation.
Since magnesium will react very slowly with water, even at room temperature, to produce
hydrogen, the large surface area associated with finely divided machining chips can, when wet,
produce sufficient hydrogen to cause a hydrogen explosion hazard. The worst case is a large
amount of damp power since the temperature will then rise, leading to still greater hydrogen
production and even to ignition of the magnesium itself. Power should be stored dry. If it must be
wet, the amount of water should be copious and means provided for hydrogen dispersal.
In summary, fire is not a significant hazard for structures made of magnesium, since the
magnesium is too massive to support combustion under service, or even abusive, conditions. It is a
hazard mainly during the preparation of structures, such as during grinding, machining, or heat
treatment.
Below you will find a photo of a container filled with gasoline, then ignighted. All of the fuel burned
and the magnesium never ignighted.
