Spontaneous Heating and Pyrophoricity
This section covers the pyrophoricity of combustible metals. Properties of various
combustible metals are discussed as well as the conditions in which they become pyrophoric.
Nearly all metals will burn in air under certain conditions. Some are oxidized rapidly in the
presence of air or moisture, generating sufficient heat to reach their ignition temperatures.
Others oxidize so slowly that heat generated during oxidation is dissipated before the metal
becomes hot enough to ignite. Certain metals, notably magnesium, titanium, sodium,
potassium, lithium, zirconium, hafnium, calcium, zinc, plutonium, uranium, and thorium, are
referred to as combustible metals because of the ease of ignition when they reach a high
specific area ratio (thin sections, fine particles, or molten states). However, the same metals
in massive solid form are comparatively difficult to ignite.
Some metals, such as aluminum, iron, and steel, that are not normally thought of as
combustible, may ignite and burn when in finely divided form. Clean, fine steel wool, for
example, may be ignited. Particle size, shape, quantity, and alloy are important factors to be
and vary widely from the combustibility characteristics of the alloys' constituent elements.
Metals tend to be most reactive when in finely divided form, and some may require shipment
and storage under inert gas or liquid to reduce fire risks.
Hot or burning metals may react violently upon contact with other materials, such as
oxidizing agents and extinguishing agents used on fires involving ordinary combustibles or
flammable liquids. Temperatures produced by burning metals can be higher than
temperatures generated by burning flammable liquids. Some metals can continue to burn in
carbon dioxide, nitrogen, water, or steam atmospheres in which ordinary combustibles or
flammable liquids would be incapable of burning.
Properties of burning metal fires cover a wide range. Burning titanium produces little
smoke, while burning lithium smoke is dense and profuse. Some water-moistened metal
powders, such as zirconium, burn with near explosive violence, while the same powder wet
with oil burns quiescently. Sodium melts and flows while burning; calcium does not. Some
metals (e.g., uranium) acquire an increased tendency to burn after prolonged exposure to
moist air, while prolonged exposure to dry air makes it more difficult to ignite.
The toxicity of certain metals is also an important factor in fire suppression. Some metals
(especially heavy metals) can be toxic or fatal if they enter the bloodstream or their smoke
fumes are inhaled. Metal fires should never be approached without proper protective
equipment (clothing and respirators).
A few metals, such as thorium, uranium, and plutonium, emit ionizing radiation that can
complicate fire fighting and introduce a radioactive contamination problem. Where possible,
radioactive materials should not be processed or stored with other pyrophoric materials