NATIONAL BUREAU OF STANDARDS REPORT

6325

EXPLORATORY EXPERIMENTS

WITH

MAGNESIUM FIRES

by

A. F. Robertson & W. H. Bailey

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT

1002-12-^873 February 20, 1959

MBS REPORT

6325

EXPLORATORY EXPERIMENTS

WITH

MAGNESIUM FIRES

by

A. F, Robertson & W. H. Bailey

for

Bureau of Ships

Department of the Navy

Code 538

Index Number NS-183-001

IMPORTANT NOTICE

NATIONAL BUREAU Of

intended Cor use within \ to additional evaluation a Hating of thia Report) eitl the Office of the Director however, by the Governm

to reproduce additional c

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

or progreaa accounting document!

la formally publlahed It la aubjected

ting, reproduction, or open-literature

irmiaalon la obtained In writing from

. C. Such permlselon la not needed,

call/ prepared If that agency wlshee <NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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EXPLORATORY EXPERIMENTS

WITH

MAGNESIUM FIRES

ABSTRACT

Experiments are reported which provide a qualitative

measure of the burning behavior of a variety of magnesium

alloys. Of the alloys tested those prepared by casting

exhibited the greatest flammability. It is suggested that

heavy sections be used in the fabrication of magnesium tool

parts since in this way ignition can be made more difficult.

A few experiments were performed which seem to indicate that

under certain conditions nitrogen and carbon dioxide in

gaseous form may be quite useful for control of such fires.

Liquid nitrogen was found to be effective for control of

magnesium fires.

INTRODUCTION

The use of magnesium alloys in the fabrication of tools

and other devices likely to be required for shipboard use

has become increasingly common. This together with the fact

that the Navy has for a considerable time avoided the purchase of tools containing magnesium poses problems in the

procurement of acceptable ones for marine use. Because of

this it has seemed desirable to make a brief review of some

of the fire hazard problems associated with the use of magnesium and its alloys. This study has been accomplished by

means of both a limited experimental study, and a review of

the technical information relating to fire hazards of magnesium alloys. In reporting on this work it has seemed desirable to present the results in two separate papers. The

present one is confined to the experimental studies which,

while only of an exploratative nature, are considered useful

as an indication of the magnitude of the fire hazard problem.

2

MATERIALS

The materials used in the study were those submitted for

this purpose by The Bureau of Ships, U. S. Navy. They were

in the form of rough forgings, castings, and machined parts

and had been submitted by various manufacturers for examination

These are shown in figure 1. The manufacturers of the various

specimens are identified in Table I.

Table II lists the alloy designations corresponding to

these specimens, principal alloying elements, and some

physical properties. The material contained in this table

was taken from information furnished with the specimens as

well as that available in reference / l7. For test purposes,

specimens were cut from the samples shown.

TEST METHODS

Three types of test were used in this study, one to

measure the ease of ignition, another to provide an indication of the ability of the alloy to support combustion and a third to show the possible usefulness of nitrogen and carbon

dioxide as fire extinguishing agents. It must be emphasized,

however, that all of these studies were exploratative in

nature and thus the results obtained should be considered

indicative of probable behavior rather than conclusive in

nature

.

Ignition Behavior

When the study was started it was considered likely that

a good correlation could be shown to exist between the length

of exposure to a flame ignition source required to produce

glow, ignition, or melting of the specimen and its properties

including geometry, and thermal characteristics. Accordingly,

an apparatus was set up to permit clamping specimens of the

thickness involved by 7/16 in. wide and 1-7/8 in. length in

a water cooled steel holder. This holder was mounted in such

a way that about 3A—in. of the 1-g- in. exposed length could

be placed within the flame of a meker burner. The elapsed

times between flame exposure and glow, melting, and ignition

of the specimen were recorded. In addition the duration of

the burning process and length of the specimen protruding

from the holder after termination of flaming were no.ted-. The

exposure time for each specimen was only that sufficient to

achieve ignition after which the specimen was removed from the

burner flame. No specimens were exposed for periods greater than

one minute. This test method was only used on specimens 1A to D.

- 3

-

Sustained Burning

Because of difficulties, which will be described later,

in correlating the ignition test results with theoretical

considerations, as well as those resulting from the irregular

shapes of many specimens, it seemed undesirable to attempt to

prepare specially machined specimens from each of the various

alloys furnished for study.. Instead, a minimum of three

roughly cut specimens as shown in figure 2 were prepared on

the band saw from each of the alloys supplied® Qualitative

tests were then performed on the ease with which the small

end of the specimen could be ignited in a meker burner flame®

After ignition the specimen was held in a vertical position

with the small burning end down and observations were made on

its persistence in burning® If burning was observed to continue, another specimen was tested but this time with the

ignited end upwards® The third specimen was then tested in

the position considered as possibly evidencing sustained

combustion but at the same time presenting considerable

difficulty of achieving this condition®

Extinguishment

It was considered desirable to do some work on problems

associated with the control of magnesium fires. In most cases

small fires involving specimen 5B magnesium alloy AZ91C were

used. This material was selected because of its free burning

characteristics. The fires were usually formed on an asbestos

millboard sheet of one-half inch thickness and about 60-lb/ft3

density. Since considerable variation occured in the methods

of extinguishment studied it seems best to describe these

together with the results obtained.

RESULTS

The results of the various experimental studies are

presented below:

Ignition Behavior

Table III presents the data obtained during these studies.

It will be observed that differences in behavior between the

various alloys are not very great. There seems, however, to be

some evidence that the cast alloys are somewhat easier to ignite

and more vigorous in their burning characteristics than the

extruded alloys. Figure 3 presents the data for each of four

alloys in diagrammatic form. No attempt was made to verify the

extrapolations shown in this figure as dotted lines.

Sustained Burning

Attempts to correlate time to ignition with theoretical considerations were without success probably largely because of the

limiting assumptions necessary for simple analysis* Because of

this, in addition to previously mentioned considerations, it seemed

impractical to perform similar ignition studies on all of the alloys

submitted for inspection* As a result, simplified qualitative tests

were used for the remainder of the specimens*

These studies of sustained burning were performed manually*

The specimen being held by a large pair of forceps* The results of

the observations are summarized in Table II. In general, it

appeared that the cast alloys again showed easier ignition and a

greater tendency toward sustained burning than the other materials

studied* Some specimens of alloy AZ91C were observed to continue

burning when the ignited end of the specimen was held vertically

upwards. On the other hand, alloy AZ31B gave evidence of being

the most difficult of the magnesium alloys to ignite* None of the

alloys in which aluminum was the major alloying element were

ignited during these tests* Each of these however, melted after

prolonged heating.

During these tests it was observed that in many cases the

specimen could be more rapidly ignited if it was periodically

removed from the heating flame* It was also observed that a burning specimen whitl introduced into the gas flame seemed to

cool off and the brilliant magnesium fire seemed to diminish*

Because of this it was considered desirable to perform a few experiments on the use of gases for control of such fires*

Extin^ui shment

The literature seems quite clear in its advice against the

use of nitrogen or carbon dioxide for control of magnesium fires,

references 2 and 3* However, theoretical considerations indicate

that the heat of reaction in these materials is but 25 and 60 percent, respec tively, of that observed when magnesium burns in air*

It was considered likely that mention of the occurrence of explosions when these gases were applied resulted from mechanical

agitation of the fire resulting from vigorous application techniques*

Accordingly, experiments were first performed with carbon dioxide

snow* A small amount of snow was dumped on an open magnesium fire*

No violent reaction and little effect on the fire was observed

until the snow was raked into the fire* This resulted in sputtering

and spreading of the fire* Snow and gaseous carbon dioxide were

then dumped, into a bucket* A magnesium fire was started and

lowered into the bucket. As the burning metal entered the carbon

dioxide atmosphere the flaming oxidation reaction immediately

stopped. The molten metal was observed to cool off but continued

to glow with a dull red color* In addition to magnesium oxide, free

carbon, and perhaps some carbon monoxide was formed*
.

-
- 5 -

This reaction in carbon dioxide still took place at a temperature

high enough to cause the magnesium to burn from the molten form,

however, no flame was observed and the reaction temperature was

greatly reduced*

In using nitrogen on the experimental fires, initial studies

were made by fitting a carbon-dioxide fire extinguisher horn to a

nitrogen supply. When gas was delivered at low velocities from

this horn and directed towards a small magnesium fire it was

found to be possible to extinguish the flame. The fire rekindled

however on removal of the N2 supply. It was apparent that some

other arrangement to permit application gas from all directions

around the fire would provide more rapid and complete control.

No attempt was made to develop a portable applicator of this type

although it seemed likely to be easily accomplished. Instead, a small open topped drum was fitted with a window and nitrogen supply,

burning magnesium was then placed within the drum, the cover

closed and nitrogen admitted* The fire was promptly brought under

control. The reaction which continued for a while with formation

of the nitride eventually was terminated due to heat losses.

Metallic magnesium was recovered from this fire.

Two further experiments were performed with small magnesium

fires. The first consisted of starting a fire on the asbestos

millboard and then placing a closed container over this fire

holding it down tightly to the millboard. The fire was gradually

extinguished and the reaction ceased. A second experiment consisted of the formation of secondary fires around burning magnesium

thus reducing the oxygen supply* This was done in two ways. First,

sawdust was dumped on a magnesium fire. There was an immediate

flareup of the dispersed dust but after this the dust burned slowly

from around the fire zone. The magnesium fire was observed to cool

off and the brilliant flame disappeared, this condition continued

as long as the wood fire continued around the magnesium core. The

second experiment involved placement of dry newspapers over the

magnesium fire. There was some delay in igniting and burning

through these and similar delay in control of the magnesium fire.

However, the intensity of the fire was greatly reduced after the

paper fire got well under way.

All these experiments were, however, performed with very small

fires and it seemed, therefore , that at least a few studies should

be performed with larger fires prior to reporting on the results

observed. Accordingly, a fire involving five pounds of magnesium

chips was formed in a tub of four-foot diameter by two feet high.

Under the application conditions used, it appeared difficult, if

not impossible, to maintain a CO2 atmosphere in such a tub without

the use of a cover. Both N2 and CQ? 'were found to reduce the flaming

and reaction temperature of such fires when a cover was used.
'

' '
- 6 -

However, because of the insulating effect of the asbestos lining

and the fact that the size of the fire was so comparable to that

of the volume of the tub, these fires were rather slow in cooling

off and the usefulness of such control measures seemed of questionable value.

A few experiments were performed on the use of liquefied N2

for control of such fires. Small scale tests involving up to 1/2 lb

of metal were very successful when the N2 was applied in the form

of a heavy spray. One experiment was then performed on a fire

involving five lbs of metal turnings. These were burned in the tub

previously described. This fire was extinguished with the use of

about 1-1/2 quarts of liquid N2

» DISCUSSION

The ignition and burning experiments seem to indicate significant differences between these properties of alloys in which

magnesium forms the principal element. The most pronounced differences seem to be between the cast and extruded form of the alloys,

the former exhibiting the most easily ignited form. It is understood that the corrosion behavior of these alloys exhibits quite

similar behavior. Reynolds and Williams, reference have shown

a correlation to exist between these two behavior characterestics,

in this light the observed differences seem reasonable.

The differences shown between the burning behavior of the

alloys was undoubtedly influenced to a considerable extent by the

testing methods used. It appears likely that all the alloys which

were observed to ignite could under suitable conditions burn

vigorously. However, it seems unlikely that metals containing

aluminum as the principal alloying element present a fire problem

even when magnesium is present in the alloy in concentrations of

as much as seven percent.

It appears likely that there should be considerable merit,

from the standpoint of fire safety, that the thickness of magnesium

parts, which could be exposed to an ignition source, be made as

heavy as practical. In this way heat losses from the area exposed

to the igniter may be large resulting in the necessity of prolonged,

application of large ignition sources prior to ignition. It is

possible that use of the Dow No. 17 anodizing treatment followed by

zinc chromate primer plus finish paint applied to magnesium parts

could as reported in reference 5? result in a considerable increase

in resistance to ignition.

The brief studies of possibilities of control of magnesium

fires seem to indicate that the use of nitrogen- and carbon dioxide

has been neglected apparently as a result of the fact' that magnesium

reacts with these gases at high temperatures The heat produced

- 7 -

from these reactions is however significantly lower than. that

resulting from direct oxidation of magnesium. The equilibrium

reaction temperatures are also much lower although the metal may

still remain in molten form.

Apparently, the normal combustion of magnesium takes place

with the formation of solid reaction products which tend to form

an insulating layef between the metal and flame. If this layer

is moved away the flame is supported on the metal surface and

boiling starts as a result of increased heat transfer. Apparently,

it is this boiling which scatters the burning metal. Thus it

appears that mechanical disturbance of magnesium burning in air

is likely to result in spreading of the fire. Reaction of magnesium in an atmosphere of N2 or ^^2 however appears to take

place at temperatures low enough to prevent boiling and thus there

appears to be little, if any, evidence of sputtering and spreading

of such fires even when mechanically disturbed.

The use of liquid N2 on magnesium fires appears to have considerable merit. This is especially true when there is a partial

enclosure or surround at the base of the fire. The small amount

of work done with liquid N2 indicated need for careful study of

application techniques since use of inadequate rates resulted in

rekindling of the fire.

CONCLUSIONS

The following conclusions are based on the experiments performed and may not be necessarily applicable to alloys of other

types or fires of larger sizes.

1. Alloys containing magnesium as the principal alloying

element can, under suitable conditions, be ignited by means of

a meker burner flame.

2. Alloys containing aluminum as the primary alloying element

and magnesium in quantities as great as seven percent were not

found to be ignitable with a meker burner flame.

3. The rapidity of ignition of a magnesium part is a function

of the size and type of igniting source as well as the geometry of

the part being ignited. In general, it appears desirable to require

that parts of magnesium tools which may be exposed to an ignition

source have as heavy sections as practicable.

4. Magnesium parts prepared from castings appear to exhibit

greater flammability than similar specimens from extruded material.

5. Liquid nitrogen appears to have considerable merit for

extinguishment of magnesium fires.

- 8 - REFERENCES

1* "Magnesium Alloys and Products," Dow Chemical Co., 1955®

2. "NFPA Handbook of Fire Protection," Crosby-Fiske-Fors ter

,

Eleventh Edition 195^®

3. "Magnesium," National Board of Fire Underwriters, Special

Interest Bulletin No. 178, Sept. 1998.

k. "An investigation of the Ignition Temperatures of Solid

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

Mechanical Engineering, Stanford University. It is understood that this report will be published in 1959 by NASA 0

5® "Private Communicatiori' Dow Chemical Corp. (see also NBS

Report No. 6302)

TABLE I

Manufacturers of Various Specimens

Suecimen Number Manufacturer

1A, IB, 1C, 1 D Dow Chemical Co.

2A, 2B , 2C Thor Power Tool Co»

3A Chicago Pneumatic Tool Co.

kA Inger sail- Rand Co.

5A, 5B, 5C Mall Tool Co.

6A , 6B Cleco Div.,Reed Roller Bit

Co.

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TABLE III

Ignition Behavior of Magnesium Alloys

(average of two observations)

Thickness

Elapsed time from Exposure to Length of

Specimen

Consumed Alloy Glow Melt Fire Fireout

in. min min min min in.

AZ91C* 0.18 0,23 0.30 1.02 1

1/32 AZ63A* 0 . 1.8 0.22 0.29 1.04 1

zk6oa 0.20 0.28 0.33 0.89 1.1/16

AZ31B 0,17 0.22 0.35 0.98 13/16

AZ91C* 0,25 0.32 0.4l 1.37 13/16

1/1

6

AZ63A* 0.25 0.32 0.46 1.25 7/8

ZK60A Qo 29 0 « 3.8 0.49 1.22 3/8

AZ31B 0.28 0.40 0.53 1.46 5/8

AZ91C* Oo33 0.51 0.68 1 0 91 11/16

1/8 AZ63A* 0.36 0.54 O .76 1 .40 1/4

ZK60A 0.42 0.75 - — 0

AZ31B 0,36 0 ® 64 - - 0

AZ91C* 0 A9 0

1/4 AZ63A* 0.53 — — 0

ZK60A 0.64 — _ 0

AZ31B 0.64 - - - 0

Castings

Specimens

used

in

present

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tj. S. DEPARTMENT OF COMMERCE

Ijfcwln L. Stt-oasm, Secretory

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