NATIONAL BUREAU OF STANDARDS REPORT

6340

A SURVEY OF SOME MAGNESIUM FIRE PROBLEMS

by

A. F. Robertson

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1002-12-1+873 March 16, 1959

NBS REPORT

63^0

A SURVEY OF SOME MAGNESIUM FIRE PROBLEMS

by

A. F. Robertson

for

Bureau of Ships

Department of the Navy

Code 538

Index Number NS-183-001

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I,y P repared if that a f' enc y wishes

U. $, DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A SURVEY OF SOME MAGNESIUM FIRE PROBLEMS

ABSTRACT

At normal pressures magnesium appears to burn in the vapor

phase resulting from, vaporization of the metal at temperatures

below that observed during combustion.. Although the heat of

combustion is quite comparable to that of wood, the reaction

temperature is much higher 0

The high temperature existing during such fires can result

in flash dissociation of water applied to them® The resulting

flareup of the fire and apparent explosion may scatter the molten

metal, spreading the fire.

Ignition of magnesium may be controlled by consideration of

the relative geometry of the metal and the Ignition source , the

objective being to dissipate heat more rapidly than the rate of

its supply* The use of an anodic coating is suggested as likely

to result in greatly increased periods between fire exposure and

ignition.

Techniques for extinguishment of magnesium fires appear to

have been based, largely on the formation of a protecting blanket

around the burning magnesium with concurrent cooling and possibly

formation of secondary fires. Experiments performed with small

fires suggest the possible use of CC>2 or N2 for control purposes.

The heat of reaction of magnesium, with these gases amounts to

about 60 and 25 percent, respectively, of that for the straight

oxidation process. This together with the fact that reaction

temperatures are greatly reduced should, under certain conditions

permit their effective use in limiting temperatures experienced

with magnesium fires. Liquid nitrogen, when properly applied,

provides an effective extinguishing agent.

Physical Properties

Magnesium is a light, white tough metal which tarnishes

with an oxide film on exposure to air. It is one “third lighter

than aluminum and in its alloys has been widely used in aircraft

construction. Recently it has been introduced, for fabrication

of portable power tools arid other implements. Pure magnesium

has a specific gravity of 1.7^+ at 68°F, melts at 1202°F_(650°C)

and boils at 204-3°F (1117° C) and atmospheric pressure As

a solid, it exhibits a vapor pressure of 0.01 atmosphere

at a temperature of 1126°F (608°C). In the liquid form the

vapor pressure is reported .337 as 5 mm at I207°F (653°C) and

100 mm at 1.6l8°F (88l°C).

- 2

Reactivity

Its affinity for oxygen is so great that heavy metal parts,

when once ignited, burn with great vigor and may be difficult to

extinguish. In the finely ground form it may burn with explosive

violence . Its combustion in air is accompanied with liberation

of an intense white light. This radiation from burning magnesium

may be considered more as an indication of the high temperature

at which the oxidation reaction takes place than as an indication

of high heat of combustion. This latter is about 1.0,650 BTU/lb/t/

and thus is of the same order of magnitude as for many other combustibles. The temperatures developed during combustion of magnesium at atmospheric pressure are reported as being usually

between the boiling point of the metal 2025°F (1107°C) and the

melting temperature of the oxide 50?2°F (2800c, C)/)+_7.

Because magnesium, boils at a temperature below that developed

during its combustion, a flame of considerable size is usually

developed. The dazzling white light accompanying this flame is

probably largely a result of the high temperature and the .resulting

emission from thermally excited oxide particles.

Ignition

The ignition of magnesium, as of other combustibles, becomes

possible only when, among other requirements, the rate of heat

generation exceeds that of heat loss. Thus, with commonly available ignition sources, the physical form of the metal will influence the ease with which it may be ignited. Finely divided

material is readily ignited with only small Ignition sources

while heavy magnesium, objects usually require large area ignition

sources. The high thermal conductivity of the alloys and pure

magnesium [_ 320 to 1092 BTU in/hr°F ft2 (0.11 to 0.376 cal/cm°C

sec) assists in the dissipation of heat from ignition sources

which are small compared to the size of the metal part being heated.

Fassell, et al / 6/ have studied the self-ignition behavior

of over 100 magnesium alloys. They report ignition temperatures

in the range of 86^+ to 1200°F (h-62 - 64-9°C). Their measurements

were made on small cylindrical specimens heated within an oven.

In performing the experiments, the temperature of the specimen

at a time just prior to that at which, its rate of temperature

change rapidly increased, was taken as the ignition temperature.

This technique would be expected to and Indeed appears to have

resulted in temperatures slightly higher than those measured, by

Setchkin / 27 in which specimens were exposed for prolonged

periods at essentially constant temperature. During such tests,

it would, be expected that the surface to volume ratio of the

metal specimen would be an important variable affecting the ignition
s

‘
- 3 -

temperature observed. It seems quite likely that under suitable

conditions the ignition of powdered, magnesium stored in bulk

form could take nla.ce at ambient temperatures much below those

reported above /p_/. Reynolds and Williams have proposed /9J

that the self-ignition behavior of magnesium and other metals is

closely related to the low temperature oxidation corrosion of

the metal

Fire Prevention

As with other combustibles, there is little that can be

done to prevent the combustion of magnesium and its alloys when

exposed to heat or fire in the presence of oxygeno Means can,

however, be devised to reduce the ease with which ignition can

occur. The relatively high thermal conductivity of these metals

can result in high heat losses from the portion of a specimen

which may be exposed to an ignition source. Thus when exposed

in sheet form to a given ignition source, the ease of ignition

will be related in an inverse manner to the thickness of the

material*

Another factor influencing the combustion behavior of

magnesium is the ability of the oxide film, formed on the metal

surface when exposed to high temperatures, to support and contain the molten metal. Here again, thin sheets of magnesium

are more susceptible to sustained combustion than are heavy ones.

Work done at Underwriters' Laboratories [107 has resulted in the

recommendation that a minimum thickness of 0.080 in. be required

for magnesium sheet material proposed as enclosure for electrical

appliances. If, however, the magnesium is in the form of a rod

there is evidence that considerably heavier sections may support

combustion when burned in the inverted position.

Another method which has been proposed for delaying ignition

of magnesium parts involves the use /ll7 of a special anodizing

process followed by zinc chromate primer and paint finisho Six

inch square sheets of 0.1 in. thickness when so treated showed

delays to ignition, after application of a torch flame, of more

than thirty times those observed with similar but untreated

specimens. This treatment appears to form a tough but flexible

envelope which contains the molten magnesium. Methods such t as

these seem definitely worthwhile from the standpoint of fire

safety, but seem unlikely to result in prevention of all such

fires.

_ 1+ _

Fires - Extinguishment

During combustion of magnesium and many of its alloys, the

high temperatures involved results in melting and boiling of a portion of the metallic fuel* Because of this and its low

specific gravity, it seems probable that when a water stream

or that of any other liquified or solidified gas is directed

at the fire, the molten metal is spread about exposing it to

air and immediate violent burning. The hot magnesium may

cause dissociation of water vapor with which it comes in contact

releasing hydrogen as a decomposition product. It is possible

that, under suitable conditions, (including those of limited

water supply) recombination of this hydrogen with oxygen from

the ambient atmosphere can result in serious explosions.

At elevated temperatures the reactivity of magnesium is

so great that nitrogen and carbon dioxide are decomposed and

combine with the metal. The reaction with these usually inert

gases is a comparatively mild one and normally proceeds at lower

temperatures than the oxidation reaction. Experiments with

small magnesium fires indicate that, under certain conditions, the

reduction of reaction temperature and heat release may warrant

their use where complete flooding is practical. Thus experiments

with small magnesium fires showed that the brilliant flame was

immediately extinguished when burning magnesium was immersed in

an atmosphere of C02

. The molten magnesium was observed to cool

but continue to react with the formation of the oxide, carbon,

and possibly also carbon monoxide. Theoretical considerations

indicate that this reaction is accompanied with a release of heat

on the order 60 percent of that for the reaction in air. Similar

experiments involving the flooding of a compartment, within which

magnesium was burning, with N2 resulted in rapid reduction and

disappearance of the flame followed by cooling of the molten

metal. For small fires the continuing reaction with formation

of the nitride proceeded so slowly that the reaction was in some

cases terminated. The heat release during this reaction, when

maintained, has been estimated to be on the order of 25 percent

of that for the reaction in air.- The behavior of magnesium in

such atmospheres together with a cooling and smothering of feel

seems to form the basis of operation of combustible materials

which have been suggested /127 for control of magnesium fires.

These include graphite, powdered pitch and heavy mineral oil.

Other materials such aspowdered volcanic ash, flux, dry sand

and sodium chloride /I2/ are probably effective primarily to the

degree with which they reduce the access of oxygen to the burning metal and act, as heat sinks, to assist in cooling the molten

mass.
.
- 5

-

Experiments recently performed with liquid indicate

that when properly applied to magnesium fires it is highly

effective for their extinguishment. Fires involving as much

as five pounds of magnesium turnings as formed in a four-foot

metal tub were completely extinguished with less than a gallon

of liquid nitrogen.

Recently, several new extinguishing agents have been

proposed for use on magnesium fires. Trimethoxyboroxine has

been used for this purpose by the Navy /IJ>7. Work done for the

Air Force has resulted in the proposal that tricresyl phosphate

or a 9-17 percent mixture of boric acid in trethylene glycol/yEVJ

be used for such fires. Each of these, when applied, results

in the development of secondary fires which apparently have the

merit of removing oxygen from the atmosphere surrounding the

burning magnesium. In addition, these materials are said to

form a flux coating around the molten mass thus largely excluding oxygen and permitting the metal to cool below the ignition temperature.

The usefulness of N2 , CO2 for control of magnesium fires

appears to have been overlooked. The reason for this is probably

the fact that such gases can only be counted on reducing the heat

of the reaction and do not necessarily prevent its completion.

These gases do, however, reduce the temperature of the burning

magnesium and permit its safe attack with other extinguishing

agents which may be used to smother or cool the burning metal

to temperatures below the kindling point. The use of liquified

N2 as an extinguishing agent for such fires apparently has not

been considered.

REFERENCES

1. "Thermodynamics Properties of the Elements" Stull & Sinke

ACS Monograph (1956).

2. "The Thermodynamic and Physical Properties of the Elements"

L. Brewer in "The Chemistry and Metallurgy of Miscellaneous

Materials: Thermodynamics" L. L. Quill, McGraw Hill (1950)*

3. "Vapor Pressure Measurements on High Boiling Point Metals"

E. Baur and R. Brunner, Helvetica Chimica Acta. 17 pp<>

958 - 969 . ( 1935 ).

5.

"The Combustion of Metals" Aristid V. Grosse and Joseph B,

Conway, Research Institute of Temple University, First

Technical Report High Temperature Project Contract No. 9 onr

87301 (Oct 15, 1951).

5. "Magnesium Alloys and Products" The Dow Chemical Company

6. "Ignition Temperatures of Magnesium and Magnesium Alloys"

Wo M. Fassell, Jr., L. B. Gulbransen, J. R. Lewis and

J. H. Hamilton, J Inst Metals Trans. 3 PP« 522-528, (1951)

7. Private Communication - N. P. Setchkin

8. "Self-Ignition Temperatures of Materials from Kinetic

Reaction Data." D. Gross and A. F. Robertson, J Research

NBS , V 6l, pp. 413-417. (Nov 1958)

9. "An Investigation of the Ignition Temperatures of Solid

Metals, Summary Report, W. C. Reynolds, Department of

Mechanical Engineering, Stanford University. (June 15? 1957)

10 . "A Determination of the Acceptability of Magnesium Alloys

as Enclosures for Portable Electrical Appliances" G, Quayleo

Bulletin of Research No. 45, (1951). Underwriters' Laboratories,

Inc

11 . Private Communication, The Dow Chemical Co.

12. "Magnesium" NSC Data Sheet No. 4-26. Annon. National Safety

News 73 - 33 -40 . (Feb 1955).

13* "The Use of Trimethoxyboroxine for the Extinguishment of Metal

Fires Part 1 Magnesium" R. L. Tune, R. L. Gipe, H. B. Peterson

and R. R. Neill, Naval Research Laboratory Report 4933 (July 1957

l4. "Extinguishing Magnesium Fires" C. H. Yuill. Fire Engineering

108 pp. 1082-1083. (1955)

)MM-NBS-DC
f
U. S. OfcPARTMKNT OK COMMERCE

Lewis L. Sortin'*, Srrrt<ltity

NATIONAL lUmfeAU OF STANDARDS

A, V. Afltill, l)lrrctor THE NATIONAL IHJRKAIJ OE STANOARON

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