A Metal Which Burns
Increasing Use of Magnesium Alloys Requires Special Fire Precautions
MAGNESIUM is the lightest metal used in structural alloys, approximately a third lighter than aluminum and less than one-quarter the weight or iron or steel. A pound of magnesium for example makes a half-inch square bar 64 inches long, a pound of aluminum a similar bar 42 inches long, and a pound of steel a bar 14 inches long. This lightness combined with strength and toughness makes magnesium when allowed with other metals particularly adaptable for forgings, castings, sheets, and bars; and its use is rapidly increasing, particularly in the aviation, automobile, railroad, structural and metallurgical industries.
Magnesium Burns Intensely
Magnesium and its alloys burn very intensely in powder, ribbon, chip, or dust form. Heavier pieces are not particularly combustible because they rapidly conduct heat away; however, when an entire piece can be brought up to sufficient temperature (about 930° F. for pure magnesium), ignition will take place. Such an instance occurred in a Pennsylvania plant two years ago, when 500 pounds of magnesium alloy slabs one-quarter inch thick were being heated in a gas-fired furnace prior to rolling. Because of inaccurate pyrometer readings, the furnace temperature ran excessively high, probably about 990° F., and the metal ignited, was consumed, and did considerable damage to the furnace. Magnesium oxide carried into the air settled on machinery, which required extensive labor to clean.
Dangerous When Wet
Another type of magnesium fire occurred, during the past year, at a die-casting plant while reclaiming magnesium alloy scrap which had been kept wet with the thought of protecting it against fire. The charge was heated slowly to drive off moisture, but during the process a violent puff occurred and fatally burned two workmen. Because magnesium oxidizes in contact with water and produces hydrogen gas, it was thought that the explosion might have been caused by the ignition of a mixture of hydrogen and air. However, in a Factory Mutual test duplicating the reclaiming process, a 55-gallon drum filled with wet magnesium turnings was heated by an oil burner, but not enough hydrogen was liberated to be ignited, although the magnesium itself caught fire violently in 14 minutes. Smaller quantitative tests also show that the rate at which hydrogen is liberated from wet magnesium alloy chips, even while beat is being applied and up until ignition takes place, is too small to be of any hazard with an average amount of ventilation.
Metal Heats to Combustion
Actually what happens is that heat is liberated from wet magnesium alloy turnings. The amount is slight at ordinary temperatures. It increases as heat is applied until a point is reached where the temperature rises abruptly and combustion follows.
Water supplies the oxygen for combustion of magnesium more rapidly than air. Thus, wet magnesium burns more intensely than the dry metal, and once ignited will continue to burn under water. For this reason, all common water-type extinguishers are worthless on magnesium fires. Carbon tetrachloride in contact with hot magnesium gives off chlorine; carbon dioxide, foam and dry chemical extinguishing agents give off oxygen ; and both the chlorine and oxygen combine with the burning magnesium to increase the intensity of the fire.
Oil As An Extinguishing Agent
Some claims have been made for oil as an extinguishing agent, and it is a fact that where magnesium chips are covered with cutting oil, combustion is not so rapid because the air necessary for combustion is cut off from the surface of the metal by the thin oil film. Using oil to extinguish such fires, however, is not advisable because it is difficult to apply without cause the oil itself to ignite.
Fires in small quantities of scrap are best extinguished by powdered talc, cast-iron borings, or similar inert material such as fine dry sand, although sand being abrasive should not be used around machinery. When the material is in considerable bulk, there is no practical way of extinguishing the fire, Even a building in which it is stored cannot be protected because sprinklers are inadvisable. For example, at a Connecticut plant recently, sparks from a fire under a lead-melting pot set fire to an unsprinklered shed, in turn burning an adjoining shed in which 2,500 pounds of magnesium alloy turnings were stored in wooden barrels and boxes with burlap covers. The plant manager warned city firemen of the presence of the chips, but the firemen, instead of letting the fire in the shed burn or checking it with a small stream and protecting the near-by buildings, played large streams on the burning shed. Water came in contact with the hot turnings and produced explosive bursts of fire, somewhat endangering the near-by main building. The firemen, however, did good work in confining the fire, although the sheds and contents were destroyed at a loss estimated at $5,000.
House Cleaning Rule
Ordinarily, the value of magnesium and magnesium alloy scrap is low, so that the direct fire loss may be small. Turnings and chips should be removed frequently from machining operations, placed in metal containers with tight-fitting covers, and stored preferably in a detached non-combustible building entirely separate from other combustibles.
In machining magnesium, no water, even in mulsifiable cutting oils, should be used. If dry machining is not practical, a mixture of kerosene and lard oil or straight kerosene is preferable. Turnings should be constantly swept up, so that if ignition occurs at the cutting tool, as it may from friction, the fire will not spread. This once happened in a New Jersey plant, when a high-speed cutting tool of a milling machine came in contact with a steel wedge used to secure a magnesium alloy casting, striking sparks which ignited chips and caused a fire which resulted in costly damage to the machine.
Suggestions for Safe Handling
Magnesium dust in air is highly explosive. In a test bomb, it developed a pressure of 67 lbs. per sq. in.—nearly double that of aluminum or cornstarch, which are considered unusually severe. Because of this high explosibility, ordinary type dust collectors should not be used for removing magnesium dust from grinding operations. A special collecting system is needed, which utilizes a water spray to wash the dust into a trough where it will be carried away as sludge. This should be collected in drums, mixed with four or five parts of sand and disposed of on a dump. In this condition it may still burn but with little intensity. In actual grinding of magnesium alloys, only grinding wheels reserved expressly for this purpose should be used, because if the wheels are also used for ferrous metals, particles may adhere which might produce sparks.
Precautions for Molten Metal
Foundries and die-casting operations using molten magnesium or its alloys should be located in non-combustible, fire-resistive buildings. In melting magnesium, a flux generally composed of sulphur and borax is used which floats on the surface and prevents the molten metal from igniting. Foundries should be on the first floor, with high ceilings, so that in event of fire in the magnesium there will be less likelihood of damage to ceilings or of having fire spread. Concrete or other moisture-retaining floors are not desirable, and hardburned glazed brick or other equivalent material should be used instead.
Reprinted from Factory Mutual Record by courtesy of the Associated Factory Mutual Fire Insurance Companies.