Magnesium

PYROPHORIC METALS

DOE-HDBK-1081-94

Spontaneous Heating and Pyrophoricity

because of the likelihood of widespread radioactive contamination during a fire. Where such

combinations are essential to operations, appropriate engineering controls and emergency

procedures should be in place to prevent fires or quickly suppress fires in the event the

controls fail.

Because extinguishing fires in combustible metals involves techniques not commonly

encountered in conventional fire fighting operations, it is necessary for those responsible for

controlling combustible metal fires to be thoroughly trained prior to an actual fire emergency.

The following material discusses the properties of various combustible metals, conditions in

which they become pyrophoric, storage and handling practices, processing hazards, and

methods of extinguishing fires involving these kinds of metals.

Magnesium

Properties

The ignition temperature of massive magnesium is very close to its melting point of

650 C (1,202 F). (See Table 2.) However, ignition of magnesium in certain forms

may occur at temperatures well below 650 C (1,200 F). For example, magnesium

ribbons and shavings can be ignited under certain conditions at about 510 C (950 F),

and finely divided magnesium powder can ignite below 482 C (900 F).

Metal marketed under different trade names and commonly referred to as magnesium

may be one of a large number of different alloys containing magnesium, but also

significant percentages of aluminum, manganese, and zinc. Some of these alloys have

ignition temperatures considerably lower than pure magnesium, and certain

magnesium alloys will ignite at temperatures as low as 427 C (800 F). Flame

temperatures of magnesium and magnesium alloys can reach 1,371 C (2,500 F),

although flame height above the burning metal is usually less than 300 mm (12 in.).

As is the case with all combustible metals, the ease of ignition of magnesium depends

upon its size and shape. As noted earlier, the specific area of a combustible substance

is the surface area of the substance exposed to an oxidizing atmosphere per gram of

the substance and is usually expressed in cm2 /g. A combustible substance that has a

high specific area is more prone to oxidize, heat, and ignite spontaneously. Thin,

small pieces, such as ribbons, chips, and shavings, may be ignited by a match flame

whereas castings and other large pieces are difficult to ignite even with a torch

because of the high thermal conductivity of the metal. In order to ignite a large piece

of magnesium, it is usually necessary to raise the entire piece to the ignition

temperature.

Pyrophoricity

Page 20

Rev. 0