Combustible Metals

The title of the article sounds like oxymoron, but beleive us, it is quite possible and from the intense white light of burning magnesium to the characteristic violet flame of potassium, combustible metals have long captured the fascination of scientists and industrial professionals alike. However, these metals also present unique and often challenging fire hazards that demand careful consideration and specialized safety protocols. Understanding the specific conditions that can ignite these metals and the crucial role of tailored extinguishing techniques is paramount in ensuring the safety of personnel and the protection of industrial environments. In this comprehensive guide, we are trying to describ distinct properties and ignition risks associated with combustible metals, shedding light on effective safety practices and the use of specialized dry powder extinguishing agents as a vital line of defense against potentially hazardous metal fires.

Specific gravity, 1.74; melting point of 1202°F (650°C).

Ignition temperature of magnesium is near the melting point, although ignition of some forms may occur at lower temperatures. As a dust cloud or in ribbon form, magnesium can be ignited almost instantly. Loose shavings ignite fairly readily. Magnesium is less easy to ignite the surface of a compact pile of chips. Magnesium fines wet with oils may ignite spontaneously. Fines wet with acids, water, water-soluble oils, or oils containing fatty acids will generate hydrogen. Powders form explosive mixtures with air that may be ignited by a spark. Fines will also react with chlorine, bromine, iodine and oxidizing agents.

Specific gravity, 4.51; melting point, 3040°F (1670°C). At red heat, 1300°F (704°C), the metal actively decomposes steam. Following strong nitric acid treatment, the metal may explode by the light impact or friction of handling with tongs.
The powder can be ignited in pure carbon dioxide above 1260°F (682°C), in nitrogen above 1475°F (801°C), and in air at 626°F (330°C) to 1094°F (590°C). Powdered titanium immersed in water or wet with water at ordinary temperatures has been ignited by chemical reaction.

Special Class D dry powder extinguishing agents are effective in smothering the fire and forming a crust that acts as a heat sink, thus cooling the metal and preventing further combustion.

Specific gravity, 6.51; melting point 3326°F (1830°C).

Fine particles in dust layers or clouds are pyrophoric, and may be ignited by heat, static or friction.
Particles can be ignited in nitrogen gas above 986°F (530°C) and in carbon dioxide above 1040°F (560°C). Zirconium samples may explode during or following treatment with strong nitric acid or carbon tetrachloride. Samples have also exploded while being dissolved in a mixture of sulfuric acid and potassium acid sulfate. Particles form
explosive mixtures with barium nitrate, potassium chlorate, and other oxidizing materials.

In dry powder form, zirconium or zirconium-copper alloys in glass containers may explode by impact or friction if the container breaks. Zirconium ignites more easily than magnesium and is slightly less hazardous than uranium. Zirconium powder is commonly handled wet because it is then more difficult to ignite, although once ignited it will burn more violently. At least 25 percent moisture by weight should be present.

Specific gravity, 0.97; melting point, 208°F (98°C).

Sodium metal is often used to remove excess water from flammable liquids during the distillation process. It is also used in the production of other sodiumbased compounds and in the reduction of organic esters.
Sodium metal reacts violently with water to produce hydrogen gas and sodium hydroxide. The exothermic heat of this reaction can lead to auto-ignition of the hydrogen gas and/or the metal itself and create a severe fire or explosion hazard. Due to its low melting point, explosions of molten sodium may occur during reaction with water.
Residual liquid present following the reaction contains a high percentage of sodium hydroxide, a caustic, corrosive material.

In finely divided form, sodium ignites spontaneously in air. Molten sodium reacts with most gases and liquids except the noble gases and nitrogen and combines vigorously with hydrogen to form the hydride. Solid sodium reacts strongly with water, alcohol, polyhalogenated hydrocarbons, halogens, acidic oxides, sulfuric acid, mercury, and certain alloys of lead, tin, zinc and bismuth.

Specific gravity, 0.53; melting point, 356°F (180°C).

As the hydride, lithium offers the most concentrated method of storing and transporting hydrogen.
Ignition and burning occur when lithium is heated to about 356°F (180°C). Lithium reacts less vigorously than sodium with water or air, and usually does not ignite. Lithium reacts strongly with chlorinated, fluorinated, and brominated organic compounds, halogens, and sulfuric acid. In the presence of moisture, lithium reacts exothermically with nitrogen at ordinary temperatures. Above the melting point, lithium rapidly forms
the nitride. Near its melting point, lithium ignites in air and burns with a characteristic intense, brilliant white flame. Following treatment with nitric acid, lithium may explode on very light impact or friction. Lithium also reacts violently with hydrogen peroxide.

Specific gravity, 0.86; melting point, 145°F (63°C).

Similar to sodium in its pyrophoricity, chemical activity, and hazards although somewhat more reactive.
See sodium section. Potassium is violently reactive with sulfuric acid and most halogens.
Potassium will detonate in contact with liquid bromine.

Specific gravity, 11.6; melting point, 3090°F (1700°C).

Thorium is pyrophoric and radioactive. As a dry powder it has a low ignition temperature. Thorium
powder is shipped under helium or argon gases in special containers. When poured through air, Thorium is subject to ignition by electrostatic spark. Thorium should be handled cautiously with a nonsparking spoon or scoop. Containers and tools should be grounded. Ignition has occurred due to chemical reaction between finely divided thorium and water at ordinary temperatures.

Specific gravity, 13.36; melting point, 4032°F (2222°C).

Fines are pyrophoric. The metal burns with little visible flame but with a high rate of heat release.
Chemical reacts with water to form hydrogen, which may ignite spontaneously.

Hafnium hazards are similar to those of zirconium. Crushing and sizing should be carried out under inert gas.

Specific gravity, 6.62; melting point, 1166°F (630°C).

Antimony ignites and burns in air above 780°F (416°C).

Aluminum is used both as a commercially pure metal and as an alloy.
In finely divided powder or dust form, aluminum and its alloys are combustible in air and present a serious combustion explosion hazard. Aluminum will react violently with many chemicals. Aluminum particles and smaller turnings will react with water to form hydrogen gas which is highly flammable and explosive in favorable concentrations. The rapid vaporization of water in contact with molten aluminum can result in rapid phase
transformation explosions. Halogenated extinguishing agents should not be used.

Specific gravity, 3.5; melting point, 1300°F (704°C).

In contact with water, barium liberates hydrogen but usually without ignition.

Specific gravity, 1.85; melting point, 2400°F (1316°C) also called glucinum.

Beryllium decreases the combustibility of molten aluminum and magnesium.
As a powder, ignition may occur if the metal is heated to about 1115°F (600°C). Burning occurs with an intense flame but can be extinguished by water. Beryllium powder should be kept away from air and moisture and stored in tight containers, preferably under argon gas. Beryllium and its compounds are very toxic and contact with skin or inhalation of dust or fumes should be avoided.

Specific gravity, 9.80; melting point, 520°F (271°C).

Bismuth burns in air with bluish flame when vaporized and oxidizes rapidly when molten. At red heat,
bismuth decomposes steam. Bismuth reduces carbon dioxide but does not react with nitrogen or hydrogen.

Specific gravity, 1.55; melting point, 1562°F (850°C).

In contact with water, calcium liberates hydrogen but usually without ignition. Calcium is considered
pyrophoric under some conditions and is normally shipped in lump form under argon.

Specific gravity, 8.65; melting point, 610°F (321°C).

Cadmium oxidizes when heated, giving off dense brown fumes that decompose steam above 750°F
(400°C). As a powder, cadmium decomposes hot water but without igniting

Specific gravity, 8.96; melting point, 1981°F (1083°C).

Copper is not known to produce explosive properties when in finely divided form.

Specific gravity, 11.34; melting point, 621°F (327°C).

At high temperatures lead volatizes and burns with white flame. At red heat, lead is rapidly oxidized by air and
at white heat by steam.

Specific gravity, 7.86; melting point, 2802°F (1540°C).

Iron presents a dust hazard under favoring conditions of particle size and dispersion in air. Iron can be easily
ignited in the form of dust or steel wool or as fine turnings or chips containing oil.
Structural steel has a specific gravity of 7.83 and melting point of 2605°F (1430°C). When heated above 600°F (315°C), it begins to lose strength rapidly.

Specific gravity, 7.43; melting point, 2246°F (1230°C).

Manganese dust can be ignited at 840°F (449°C) in air.

Specific gravity, 10.2; melting point, 4750°F (2620°C).
Molybdenum powder reacts vigorously with water vapor at 1300°F (704°C). Molybdenum presents a slight dust hazard under favoring conditions of particle size, dispersion, and strong ignition source.

Specific gravity, 7.19; melting point, 3407°F (1875°C).

Chromium presents a moderate dust explosion hazard under favorable conditions of partial size dispersion and ignition source.

is highly reactive and can ignite upon exposure to air or through friction. Phosphorus fires burn at high temperatures and can be challenging to extinguish. Specialized dry powder extinguishing agents are used to smother the fire and prevent it from spreading, ensuring the safety of personnel and the surrounding environment.

Specific gravity, 7.13; melting point 786°F (419°C).

In the form of dust, in contact with moisture, alkaline solutions, or acetic acid, zinc will heat spontaneously to ignition. Large pieces of zinc are difficult to ignite but once ignited will burn strongly.
In oxygen, oxidation of the metal takes place rapidly at 300°F (149°C) and ignition occurs at 930°F (499°C) with a bluish flame. Steam is decomposed on contact with zinc at 660°F (349°C). Acids or caustic soda also liberate hydrogen on contact with zinc. In air, zinc burns to the oxide, forming white or bluish smoke. As a dust cloud, zinc can be ignited at 1110°F (600°C).

Specific gravity, 19.2; melting point, 6115°F (3380°C).

As fine powder, tungsten may be pyrophoric. Hydrogen reduced powder may retain some absorbed hydrogen, presenting a dust explosion hazard. Tungsten reacts violently with molten nitrates, nitrites, and peroxides.

Specific gravity, 7.29; melting point, 450°F (232°C). Tin dust can be ignited in air at 1165°F (630°C).

Specific gravity, 11.85; melting point, 572°F (300°C). At red heat, thallium decomposes water, producing hydrogen. It is very toxic

Specific gravity, 6.24; melting point, 846°F (452°C).

Tellurium presents a moderate dust explosion hazard under favoring conditions of particle size, dispersion and ignition source.

Specific gravity, 16.62; melting point 5425°F (2996°C).

Tantalum presents a moderate dust explosion hazard under favorable conditions of particle size, dispersion, and ignition source.

Specific gravity, 2.6; melting point, 1386°F (752°C).

In contact with water, strontium liberates hydrogen readily, but usually without ignition.

Specific gravity, 2.33; melting point, 2588°F (1420°C).

A silicon dust cloud can be ignited in air at 1425°F (775°C). Pure silicon metal dust has been shown to be highly explosive under certain conditions.