NBSIR 73-345

COMBUSTION OF METALS IN OXYGEN

PHASE II: BULK BURNING EXPERIMENTS

Alan H. Tench

Hans M. Roder

Alan F. Clark

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

December 1973

Prepared for:

NASA-Marshall Space Flight Center

Astronautics Laboratory

Materials Division

Huntsville, Alabama 35812

NBSIR 73-345

COMBUSTION OF METALS IN OXYGEN

PHASE II: BULK BURNING EXPERIMENTS

Alan H. TenchHans M. Roder

Alan F. Clark

Cryogenics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

December 1973

Prepared for:

NASA-Marshall Space Flight Center

Astronautics Laboratory

Materials Division

Huntsville, Alabama 35812

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

TABLE OF CONTENTS

Page

Abstract ^

1. Introduction ^

2. Description of Apparatus 4

2. 1 The Combustion Charaber 5

2. 2 Valving and Gas Controls ^

2. 3 The Experimental Site 8

2.4 The Ignition System 9

3. Experimental Procedure H

3. 1 General 11

3. 2 Visual Monitoring 1^

3. 3 Liquid Oxygen Procedure 13

4. Description and Assessment of Results 14

4. 1 Stainless Steel Experiments 1^

4. 2 Quenching Experiments ^1

4. 3 Combustion Studies on Al, Ordinary Carbon Steels,

and Ti 27

4.4 Liquid Oxygen Results ^1

4. 5 TV and Filmed Records 31

4. 6 Conclusions - Rules of Thumb -^2

5. Recommendations for Future Work ^2

5. 1 Specific Experiments to be Conducted

5. 2 More General Investigations 34

5. 3 Modifications to the Apparatus 35

6. References 38

Appendix A - Safety Memo Al

Appendix B - The Thermite Reaction Bl

iii
LIST OF TABLES

Page

Table 1. Summary of the Bulk Burning Experiments 15

Table 2. Miscellaneous Thermal Data on Metals and

Alloys 29

LIST OF FIGURES

Figure 1. The Combustion Chamber 6

Figure 2. The Experimental Layout 7

Figure 3. Typical Samples 16

Figure 4. Typical Remains After a Burn 17

Figure 5. Sample Tray, Tank Accident 23

Figure 6. Bottom Port View, Tank Accident 24

Figure 7. Copper Sand Tray SS Melt 25

Figure 8. Quenching of the Fe-Al Reaction 28

Figure 9- Liquid Oxygen Collection Apparatus 37

iv
ABSTRACT

This program was started to study combustion

of the ordinary metals in oxygen, to provide information

useful to designers of bulk oxygen handling equipment,

particularly from the point of view of safety. We have

studied the ignitability and combustability of carbon

steels, stainless steels, aluminum alloys, titanium and

copper metals, in oxygen atmospheres up to 60 psig.

We encountered a violent reaction involving burning

stainless steel and aluminum, a thermite reaction

apparently, and have investigated this from a quantitative point of view. We have tried throughout to

study the quantitative aspects of the combustion

processes and related quenching effects. Ordinary

steel and stainless steel burn quite readily at these

low oxygen pressures, and the stainless steel -aluminum

reaction produces intense heat with severe damage to

neighboring structures. We are at the stage where

further investigations could be carried out quite

expeditiously and on a sound statistical basis.

Keywords: alloys; aluminum; combustion; ignition; oxygen; safety;

stainless steel; steel; titanium.
1. INTRODUCTION

The burning of structural metals in oxygen is of interest to the

industrial, medical, aerospace and defense fields. Sufficient understanding of the basics of metal combustion might have prevented such

tragic oxygen accidents as those in the Apollo program and at the

Brooklyn Memorial Hospital. To better understand the fundamentals

of the heterogeneous metal combustion process, we are presently

engaged in a research program "Combustion of Metals in Oxygen".

This program is sponsored by NASA-Marshall Space Flight Center

under Contract H92145A with Mr. John G. Austin as project monitor.

The program was to be conducted in two phases. Research conducted

under Phase I is reported in References [l], [2] and [3]. Phase II includes

three parts:

1. Effects of bulk shape, mass and surface on combustion!

2. Ignition theory for minimum size and temperature for spot

ignition;

3. Electron behavior in metal combustion.

The present report summarizes the progress on Part 1 of Phase II,

which was the only part of Phase II initiated and funded. A sustained

effort for this part of the program has been under way only since July

1973.

1.1 Goals

(i) Documentation of Bulk Burning Behavior . Firstly, we

would seek simple information, describable in terms of direct visual

observations. For instance, we found that 304L stainless steel, if

brought to its melting point and allowed to spread out in a layer not

more than 1 /4 inch thick on a poorly conducting surface, would burn to

completion if the oxygen pressure was over 50 psig. This was determined by simple observation, with no special apparatus.

2
e.g. , remote high temperature thermometers. Thus, we sought to

study effects of mass, shape, surface state, orientation, oxygen

pressure, temperature, and flow of the metal on the combustion process.

(ii) Interpretation of results . This would proceed in two

directions. First, we would try to set up a sound statistical basis for

the above observations, in particular, since they were often of the Yes

or^ No sort, e.g. , did a complete burn take place, or did it not? Then,

by least squares methods, we would try to get equations to fit the processes and to define suitable parameters. Second, we would use an

analytical approach, to study the oxides produced on burning. Most

metals have more than one oxide and apparently several may be in

existence at any one time in the burning process. X-ray diffraction

methods would be used in this connection. Phase I of this project has

devoted considerably sophisticated efforts to a direct understanding of

the metal combustion process by spectroscopic and other analytic

methods, but we did not anticipate using these in Phase II.

(iii) Application of results to safety procedures . We would

seek to study those factors which were rate -limiting to the combustion

process and, in particular, exerted a pronounced quenching effect.

1. 2 Experimental Procedures

(i) Large quantities (of the order of lOOg) of metals would be

used, of different sorts, with both gaseous oxygen and liquid oxygen as

potential oxidizers. Low oxygen pressures (100 psig or less) were to

be used, and ignition was to be promoted by supplying considerable

quantities of heat to the metals.

(ii) Observations were to be largely visual , hence, of a simple

sort, largely to determine the overall nature of the combustion that

took place. Repeatability of the experiments was to be emphasized.

3
(iii) The dangerous nature of the experiments was realized,

and they were to be carried out on a special safety site, already

available at NBS, Boulder. Remote control and monitoring of the

experiments was to be carried out.

The experiments conducted so far have been straightforward,

and can be repeated readily, to allow for an adequate statistical interpretation. Efforts have been made all along to achieve speed and

efficiency of operation, to allow a large number of experiments to be

carried out. We are to the stage now where, with a few small modifications, we could run routinely several burns per day (8 or so) and

could carry out quite searching investigations into various aspects of

metal combustion, e.g., oxygen pressure dependence, quenching

effects, effects of different geometries, and reactions between various

metals (in particular, the stainless steel -aluminum reaction we

encountered, which is a violent and dangerous sort of event)

all with an adequate statistical basis and capable of a valid statistical

interpretation.

The remainder of this report is in four sections, as follows:

(i) Description of apparatus,

(ii) Experimental procedures,

(iii) Description and assessment of results, and

(iv) Recommendations for future work.

2. DESCRIPTION OF THE APPARATUS

The apparatus consists of two parts, plus the recording equipment, (i) the combustion chamber , to contain the metal sample to be

ignited in a controlled atmosphere. It is termed also the tank or the

vessel .

(ii) the ignition apparatus , a radio frequency induction heater, to heat

the metal sample to ignition temperature.

4
2. 1 The Combustion Chamber

This is a stainless steel pressure vessel, shown in Figure 1.

It is cylindrical, with capped ends, approximately 5 feet in diameter

by 5 feet in height, and of approximately 1200 liters capacity. The

walls are of 1/4 in thick 347 stainless steel plate, with welded seams.

It was made here at NBS, Boulder, a number of years ago, to carry

out hydrogen-oxygen explosion experiments. Standard ASME calculations show it to have a safe working pressure of 115 psig, and we

anticipate using it to oxygen pressures of 100 psig. However, we were

able to obtain sufficiently interesting results at 60 psig, and have not

yet operated over that pressure.

Four observation ports are attached to the sides of the vessel.

They are fitted with plexiglass windows, allowing a view of the center

of the vessel. A larger single port is present at the top of the vessel,

closed by a l/2 in thick SS flange, and at first we operated through

these five ports. This proved inconvenient, so that after the incident

which necessitated major repairs, we had a large port (10 in x 18 in)

welded into the side, giving good access to the interior. A bolt-on door

was made to cover this port of 3/4 in 606 1 aluminum with an 0-ring

groove milled in it and secured by 5/l6 in cap bolts (22 in number).

A sixth port was present, originally, at the bottom of the

vessel. It received the 1 in diameter filling and evacuation line and

incorporated a rupture disc safety valve. This port was destroyed in

the incident mentioned and was then welded shut with only a 1 in drain

plug in its place.

2. 2 Valving and Gas Controls

The schematic is shown in Figure 2. Filling and evacuation are

carried out through a 1 in diameter line entering at the top port. This

5
Figure 1. The Combustion Chamber

6
Vent Stack

Figure 2. The Experimental Layout

7
line leads to (i) the vacuum pump to evacuate the vessel, (ii) to the

oxygen or other gas bottles, and (iii) to the vent line to relieve the

vessel pressure to atmospheric. Also, it supports the safety valve and

provides access for the pressure gauges.

Three working valves are present. The vacuum valve is energized

to open the line from the vacuum pump to the vessel; this valve is opened

only when the vacuum pump is operating to evacuate the vessel. The

vent valve is placed between the vessel and the atmosphere and is

designed (a) to hold up to 100 psig of oxygen in the vessel and (b) to open

under this pressure when its solenoid is energized. This vent will not

hold a vacuum in the vessel against atmospheric pressure so that a hold valve is provided between the vent valve and the atmosphere which

automatically closes when vessel pressure drops below atmospheric.

This hold valve is not energized and opens automatically when the vent

valve is open and the vessel pressure is above atmospheric. All three

of these valve are either of all-metal or of PTFE-metal construction

to comply with oxygen compatability requirements.

2. 3 The Experimental Site

The vessel is installed behind a safety wall on a special site at

NBS, Boulder, built to conduct potentially explosive experiments

(especially where large quantities of liquid gases are used). The

Layout is shown in Figure 2. The wall is of 12 in thick reinforced

concrete with sidewings to direct a "potential blast towards a nearby . uninhabited hillside. Small ports are present in the wall near ground

level for control wires and pipelines to pass through to enable the

experiments to be controlled and monitored from the safe side of the

wall.

8
A trailer is present on the safe side and is used as headquarters

and operating location when experiments are being actually run. Oxygen

and nitrogen bottles are located on the safe side of the wall and a feedline runs through to the vessel. The vessel stands on a concrete pad

and is covered by a porch-style roof but otherwise is open to the elements

and is operated at outdoor temperature. The vacuum pump and the

three working valves are on the unsafe side where they were placed

originally^ but eventually we would move these to the safe side (see

Section 5. 3).

2.4 The Ignition System

The metal samples require some means to ignite them; to

date we have used exclusively a radio frequency induction heater. The

requirements for ignition are that it be clean (non-contaminating),

efficient (rapid heating) and consistent. We have contemplated using

other means of ignition, e.g., chemical (thermite) or direct current

heating and may eventually do so, but to date the RF heating has given

good results and meets these three requirements. The heater is a commercial item already available at NBS, Boulder, operating at 690kc

and capable of delivering up to 3000 watts of power. It is a simple,

rugged device, water cooled and operating on 220 VAC and has worked

well. The main difficulty encountered has been that it was designed for

short range work, i. e. , the sample to be heated is supposed to be only

a few inches from the heater. Since we place the samples in the center

of the vessel (approximately 5 feet in diameter), we had to design some

sort of transmission line to conduct power from the RF generator to

the sample a distance of 30 in or so. At first we tried parallel lines,

but these (made of 1/4 in diameter copper tubing) proved inefficient

9
and there was considerable power loss. Eventually we moved to a coaxial

line made from two lengths of copper tubing (regular hard copper plumbing

line), one piece inside the other but separated to provide insulation. At

first the line was built for minimum impedance (the outside diameter of

the inside tube was as close as possible to the inside diameter of the outside tube). However, later studies indicate that minimum impedance is

not desirable for maximum power transmission and that the two diameters

above should be in the ratio of 1. 65 to 1 rather than 1 to 1. With this in

view, we have designed and partly built a bigger transmission line which

should be more efficient and moreover more trouble free, since the

present line is subject to shorting out due to the closeness of the inner

and outer tubes.

The sample to be heated is placed within the turns of a wound

coil connected to the terminals of the RF generator. Water circulates

through this coil for cooling purposes (otherwise the coil^too^ becomes

heated and will eventually melt). Coils are easily made from soft copper

tubing, and we settled on 3/l6 in diameter soft copper tubing as being

generally suitable. With standard tubing connectors, these coils are

readily mounted and demounted on the end of the transmission line.

For maximum coupling to the sample, the coil should be as closely invested to the sample as possible without actual physical contact

(this latter results in short circuiting the turns of the coil with reduced

coupling). It is an easy matter to wrap coils for each size and shape of

sample using the sample itself as a mandrel if need be.

The amount of power generated by the heater is controlled by a rheostat, hand operated and originally mounted on the heater itself. This

rheostat was demounted and reconnected to the heater by a transmission

cord so that the power output could be controlled at some distance from

the heater, in particular from the safe side of the blast wall. This

10
allowed the heater to be set up and energized (but with zero output) at

the unsafe side of the blast wall where it is located next to the vessel,

and then powered up from the safe side of the wall to start the

experiment.

The coaxial transmission line enters the vessel through a hole

drilled in the plexiglass window of one of the observation ports, and the

generator is placed alongside the vessel.

3. EXPERIMENTAL PROCEDURE

We have been much concerned with the safety aspects of this

work, and Appendix A contains an account of the procedures approved

by the Cryogenics Division Safety Committee and of the general requirements to be met in this respect. The incident described in Section 4

showed the strong advisability of taking all reasonable safety measures.

3. 1 General

The sample to be ignited is placed within the coil either

supported below on a platform or wrapped with two turns or so of

asbestos paper and put into the coil to make a push fit held by friction

but not in electrical contact with the coil. This latter procedure was

used for all stainless steel samples where it worked well. These SS

samples appear to heat uniformly and to melt almost all at once so that

they drop in a mass from the coil in a consistent way. We were not

anxious to use a potentially contaminating agent, such as the asbestos

paper, but apparently it has little effect in the sort of experiments tried so far and has proven to be convenient and effective. Projected experiments include an investigation of the possible contaminating effects of

this method.

Ordinary steel samples (1020 and 1075) proved harder to handle

with respect to ignition using the RF heater. They tend to swell and

11
buckle on heating, contacting the coil, resulting in short circuiting and

a direct cooling^ since the coils are made of water cooled copper tubing.

We have not been able to attain the consistency of ignition with these

ordinary steels that we have with the stainless steels, even though these

ordinary steels burn much more readily when once ignited.

After placing the sample in the coil, the heater is switched on but

not yet powered up. The vessel is sealed and the vacuum pump started

to evacuate the vessel. No effort is made to attain a high vacuum, and

usually we run the pump till the gauges show a reading around 25 in

Hg below atmospheric. At our altitude (5400 feet above sea level),

atmospheric pressure is about 26 in Hg. Our aim at this early stage

of the project is merely to remove most of the ambient atmosphere so

that subsequent filling gives a reasonably pure atmosphere of oxygen.

On attaining a suitable vacuum, the pump is shut off and the RF heater

is powered up momentarily to see that it is working properly as judged

by the plate current reading on the power tube.

We move then to the safe side of the blast wall and open the gas

valve(s) between the oxygen cylinders and the vessel. These valves

are regular gas bottle reduction valves as used on welding kits and are

used to fill the vessel to the desired pressure.

Heating power is then applied to the sample by operating the

rheostat described in Section 1 on the safe side of the wall. The experiment is observed by means of TV cameras described in 3. 2, When the

sample is heated adequately, the RF power is turned down and the burn

allowed to proceed spontaneously.

On completion of the experiment, the vent valve solenoid is

energized to allow vessel pressure to fall to atmospheric. We then go

around the wall, switch off the heater and open a port to allow

the atmosphere into the vessel. The solid combustion products are

recovered and the operation is complete.

12
3. 2 Visual Monitoring

For safety reasons we retire to the safe side of the wall

whenever there is substantial pressure of oxygen in the vessel (a. 20 psig)

and an attempt is made to ignite a sample. Accordingly, to observe the

processes, we use TV cameras aimed through the observation ports.

One is connected directly to a commercial TV set on the safe side of the

wall and is used for observations during the actual experiment. The

other is used as a recording camera to record results on video tape

and has been a great asset, since replays of the events are possible

and it is inexpensive to operate.

Also, we use movie cameras (B&W and color) which^with their

superior resolution^ are of great aid in studying the burning processes.

3. 3 Liquid Oxygen Procedure

It is our intention to study the effects of liquid oxygen on metal

burning processes. Accordingly, we need some means of introducing a quantity of liquid oxygen into the vessel. In view of the dangers of

handling liquid oxygen, we decided to liquefy oxygen within the vessel

itself by passing liquid nitrogen through a cooling coil in the vessel when

the latter is filled to about 20 psig of gaseous oxygen. A simple set-up

was made with an inlet and outlet on the top flange for vacuum insulated

lines. Liquid nitrogen is pumped through these to and from a 3/8 in

diameter copper cooling coil of seven turns placed in the vessel.

This results in rapid condensation of oxygen on the coil at rates on the

order of one liter of liquid per minute.

We made two tries to collect sufficient liquid oxygen to permit

dropping a burning piece of stainless steel therein. However, difficulties

arose in the retention of the liquid, since it evaporated as soon as it

dropped into the collection vessels (poorly insulated), and we were not

13
able to reproducibly carry out the intended procedure, A special

collection vessel is to be built as described later. The aim is to see

which of two processes predominates - the quenching effect of the cold

liquid or its oxidizing effect.

The above illustrates the technical difficulties in this sort of

work. We have to cater to extremes of temperature, considerable

variations of pressure, an oxygen atmosphere with strong sources of

ignition, and the need to view the process at all times,

4. DESCRIPTION AND ASSESSMENT OF RESULTS

A summary of the burning experiments conducted is given in

table 1, a picture of the typical samples is given in figure 3, while the

remains after a typical burn are shown in figure 4. In the short time

we have had the apparatus operating, our procedures have been determined by feasibility mostly, in regard to the ease of ignition of various

metals. Using the RE heater as described, we found soon that the

stainless steel samples were much the easiest to ignite in a consistent

way, strange as this may seem. Although ordinary steels seem to

ignite and burn more readily than stainless steels, they are considerably

harder to get to ignition temperature when using the RE heater, as indicated in 3. 1. The stainless steels retain their original shape up to

melting point where they seem to melt all at once. They are held in

the coil by friction and on melting drop out of the coil in one slug, usually.

In addition, these stainless steels seem to couple better with the RE

heater and to heat quicker than the ordinary steel.

In view of the above, we studied the stainless steels almost

exclusively for a while in order to obtain some valid results with the

equipment on hand and also in view of the importance of stainless steels

(hereafter also abbreviated as SS) in oxygen-handling equipment. In

14
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17
the course of this work, we encountered what is apparently a thermite reaction (see Appendix B)^in which there is a violent reaction

between SS and aluminum.

4. 1 Stainless Steel Experiments

The material used was 1 in diameter 304L stainless steel rod

cut into 1 in lengths to give a sample weight of 100-1Q3 g. This is a true stainless steel, a low carbon steel (0. 03% C) with nominally 19%

Chromium and 10% Nickel. It is used primarily for its high corrosion

resistance and not for any particular properties of strength or hardness

as in the case of high carbon chromium steels, e. g. , 336 or 338. It is

one of the stainless steels commonly used in scientific apparatus, oxygen

systems, and structural work.

These SS samples were wrapped in asbestos paper and pushed into

a six or seven turn coil of 3/16 in diameter copper tubing so that the

ends of the cylindrical sample were even with the ends of the coil. With

the RF generator operating at full power (nominally 3000 watts) these

samples would melt in about 100 seconds and then fall, ignited, out of

the coil almost completely. At the end of the burn, there were usually

a few grams of oxidized metal left adhering to the asbestos wrapping in

the coil, so we assume that at least 95g of the original sample had melted

out of the coil and had come into good contact with the oxygen present.

Since the experiments were viewed on the TV camera, we could

get a fair idea of how fiercely the sample burned and for how long. More

difficult to determine was the amount of burnt material produced and the

amount of unburnt material left. Considerable smoke was produced on

burning resulting in deposition of a film of powdered oxide over the

interior of the vessel, and there seems to be no easy way to collect this

for weighing or other analysis. The same problem was evident when

spattering occurred. This is part of the general problem of containing

18
burning metal. A highly refractory material is required which will not

shatter on sudden contact with the molten mass and which will not provide a significant quenching effect. We settled for sand trays, but these

were not ideal since sand was incorporated in the burning mass„ Sandstone slabs were tried, but they cracked and let the burning metal fall

through. We intend to look into high performance ceramics (of the

rocket nose-cone type).

Thus, our initial experiments with SS 304L consisted of dropping

burning slugs on to a sandbath, in various pressures of oxygen from

1/2 atmosphere to 60 psig. In no case did we seem to get ignition

before melting. As far as could be determined by visual inspection of

the process by TV monitor, sustained burning occurred only after the

sample melted and fell from the coil. About 20 samples were treated in

this way, and^ although no statistical treatment has been attempted, we

have reached the following tentative conclusions:

(i) Sustained burning will not occur generally below 5 0 psig of

oxygen. Below this pressure, it was usual for the sample, on hitting

the sandbath, to burn on its surface for a short while (seconds rather

than minutes) and then extinguish. The remains contained a large

contiguous mass of metal^ apparently only remelted stainless steel. The

lower surface, where it had rested on the sand tray, was usually quite

bright and of metallic appearance. The upper surface was usually

covered with a layer of dark oxide of variable thickness. The samples

tended to remain in one piece and were weighed after trying to scrape

off the adhering sand and adding the small amount of oxide adhering to

the coil. Most samples showed small weight gain, e. g. , they went from

102 g to 108 g (note that we lost oxide in the form of smoke, but we do

not believe these losses to be large). Accordingly, we conclude that

molten SS 3 04L will not burn readily below 5 0 psig of oxygen.

19
(ii) Above 5 0 psig of oxygen sustained burning will generally

take place if the molten sample is allowed to spread out and form a

fairly thin layer . When dropped on to the sand bath, the dense molten

metal (density around 8 g/cc) tended to form a depression in the sand

resulting in its containment in a mass of small surface area with a resultant suppression of burning. However, if it fell so that it spread

out fairly evenly over about 15 square inches, then there was complete

burning, apparently, to judge by the absence of particles of metal

visible to the naked eye. The product was dark colored and quite brittle,

being easily broken by hand, Masses of up to 190 g were obtained this

way, although these contained an undetermined amount of sand believed

to be smallo Thus, above 50 psig of oxygen, complete oxidation of

molten SS 304L will occur if adequate exposure to the oxygen is allowed.

Where the sample formed a depression in the sand and failed to

spread out and burn to completion, we noted a consistent phenomenon

during burning. One or more spark showers would occur during burning

in the form of spattering from some location in the mass. These

showers lasted for a minute or soo On examining the burnt sample

afterwards, we found associated with the location of the shower, a piece

of unburnt metal of a spheriodal shape embedded in the oxide layer.

Apparently, it had been molten but had not burnt due to the formation of

a thick layer of oxide (ca 1. 0 cm) around it. These unburnt residues

varied in weight from 2 g to 70 g. Their surface was smooth and bright,

and they seemed to be unoxidized stainless steel.

Complete burning took about 3-4 minutes. Little change in

oxygen pressure was seen as judged by inspection of the pressure gauges

during burning, and there seemed to be no great rise in temperature

within the vessel as judged by its surface temperature at the end of the

20
experiment. Where spark showers occurred, they seemed to happen

towards the end of the burn, and where more than one occurred, they

usually were not simultaneous. Presumably, there was a build-up of

the oxide layer followed by sudden rupturing.

4. 2 Quenching Experiments

In view of the above conclusions that we could get consistent

burning at 60 psig of oxygen where the 304L sample was allowed to

spread out, we investigated the effects of allowing the sample to fall on

to a metal plate to see whether this plate would conduct away heat at

such a rate as to retard the burning, i. e. , to quench the sample.

The first series of tests involved a plate of 606l type aluminum.

This alloy has nominal composition 1. 0% Mg, 0. 2% Cu, 0. 2% Cr, and

0. 6% Si. It is designed for general construction purposes where good

corrosion resistance is needed and is much used in truck body construction. The plate had dimensions 4 in x 4 in x 1/4 in and weighed 183 g.

It was placed on a sandbath about 1 in below the SS 304L sample (1 in x

1 in, weight approximately 103 gms, as before). The SS sample was

ignited as previously by RF heating and fell out of the coil on to the

plate. Oxygen pressure was maintained at 60 psig in all of these trials.

The first three runs were made all with the same plate and in

each case there seemed to be substantial quenching, especially when the

SS fell squarely on to the plate and did not flow on to the sand. In one

case, the sample gained only 2 g in weight, in another 9 g, and in the

third case 23 g where there had been considerable flow off the plate.

The plate was little affected either in shape or weight, and the SS samples

were pried off easily.

The above runs seemed a good indication that the shape and weight

of aluminum involved would quench adequately the shape and weight of

SS 304L involved at 60 psig of oxygen. In this sort of Yes o_r No test.

21
a trial on three samples often is considered statistically valid. However,

we decided to run a test on a total of 5 samples, i. e. , two more after the

above three. The fourth sample behaved in quite a different way to the

first three.

Tank Incident. (Fourth Sample) This trial was conducted in the

same way as above at 60 psig of oxygen. The SS sample melted and

dropped on the plate in the usual way. As seen on the TV monitor,

there was a short pause with little burning activity. Suddenly, there

was a resumption of burning and molten metal streamed off the supporting platform on to the bottom of the tank where it burned a hole (c. 15cm

by 20cm), releasing the oxygen violently and incapacitating the vessel

completely. Burning continued on the platform for some time after the

pressure dropped. On going around to the tank, we saw that there had

been considerable destruction in and around the bottom port with the

fill and gauge lines in considerable disarray. The photographs (figures

5 and 6) give an indication of the damage.

Previous to this, we had one SS sample which had fallen directly

on to the sand tray in 60 psig of oxygen but had run towards the side of

the 1/32 in thick copper tray, melted a hole therein, burned a hole in the

supporting aluminum platform (1/8 in thick plate) and had fallen on to

the bottom of the vessel but with no untoward results (see figure 7).

Quenching had been rapid, and we could easily pry the sample remains

off the wall of the vessel. Apparently, in the case of the tank incident,

there had been a thermite reaction (iron oxides reacting with aluminum

metal to give iron plus aluminum oxides) which is known to be violent.

After repairing the tank, we were able to repeat this SS-Al type

of reaction and record it on TV and color film. In one trial, a SS sample

was dropped onto an aluminum plate 4 in x 4 in x 3/16 in thick, resulting

in a vigorous reaction at 50 psig of oxygen. During this, tank pressure

22
23
Figure 6. Bottom Port View, Tank Accident

24
25
rose to 55 psig, and the tank became quite warm to the touch. The

copper RF heating coil was melted completely during this burn. Note

that this coil is water cooled. There was evidence (red material present

in the residue) of copper oxidation.

A second burn using an aluminum plate 5 in x 5 in x 1 /8 in

(weight 139 g) at 60 psig of oxygen again resulted in a vigorous reaction,

recorded on color film.

Other burns, where the plates were bigger than the three above

or where the oxygen pressure was reduced, showed good quenching^

At 30 psig of oxygen, a SS sample was completely quenched

when dropped onto an aluminum plate 6 inx 6 inx 3/16 in. The plate

showed only slight effects. At 60 psig of oxygen, an aluminum plate

6-1/8 in X 2-3/4 in x 1 in also gave complete quenching. In these cases,

the SS sample showed little burning activity after it cleared the coil.

On hitting the plate, it cooled and solidified rapidly with a thin layer of

oxide on the upper surface and with the lower surface (in contact with

the plate) quite bright and metallic appearing.

A similar result occurred when a copper plate was used. This

plate was 3-1/2 inx 5-1/2 inx 1/8 in (weight 35 0 g) , and quenching

was thorough,

A 316 SS plate 4-1/2 in x 3-3/8 in x 1/8 in (weight 235 g) was

partly burned when a SS 304L sample was dropped onto it at 60 psig

of oxygen. The outer portions of the plate remain intact, but the center

portion appeared to be completely oxidized. The molten part of the plate

and the ignition slug flowed away from the rest of the plate before the

entire plate could melt and burn. It appears that stainless steels need

a great deal of heat supplied to them before they will burn vigorously

at 60 psig of oxygen. It is a matter of great interest to deternaine what

oxygen pressure will sustain burning of SS to which no heat otherwise

26
has been supplied. For instance, if a SS plate at room temperature is

suddenly ignited in one spot, what oxygen pressure is needed to sustain

the burning of the plate to completion?

We conclude that the Fe-Al reaction is reproducible if we have

oxygen pressure of 60 psig or more and if the aluminum plate is not over

3/16 in thick. A quantitative description of all of our Fe-Al experiments

is given in figure 8.

A rough estimate shows that 100 g of SS cooled from melting

point (c 1250°C) to 650°C (aluminum melting point) would yield 12, 500

calories. To melt 200 g of aluminum, starting from room temperature,

requires about 20, 000 calories. It would be worthwhile to continue

these Fe-Al reactions to investigate the mass dependence, i. e. , is there

a maximum mass of aluminum beyond which a fixed mass of burning SS

will not cause ignition? The dependence on oxygen pressure would be of

interest^ too.

4. 3 Combustion Studies on Aluminum, Ordinary

Carbon Steels and Titanium

Aluminum. Cylindrical aluminum slugs were treated in the

same way as the SS samples^ being wrapped in asbestos paper and heated

in the RF coil. They heated well, melting quickly and flowing out of the

coil in one piece. However, even at 60 psig of oxygen, there was negligible

oxidation, the molten mass merely solidifying to a normal appearing

form of the metal. This result is not suprising since the ignition temperature for Al is considerably higher than its melting temperature, see

table 2.

Ordinary Carbon Steels . As indicated previously, these steels

appeared to ignite and burn readily but did not couple with the RF heater

as well as the SS samples^ and moreover were liable to swell and buckle

so that the coils of the heater were shorted out^allowing the samples to

27
O 00 CD Sj- CM

' Bujujnq iv aquo AiHiavaOdd

28
29
quench. Some samples did burn, however, and one, a 3 in length of

7/8 in diameter 1075 rod, burned apparently to completion in less than

1/2 atmosphere of oxygen. This was recorded on black and white

film (movie) and is impressive in the steady way in which combustion

proceeds rather like, a candle burning in air. To study combustion of

these kinds of steels will require a different means of ignition unless we

increase oxygen pressure to where the sample will sustain burning in the

early stages of heating, thus obviating the need for further RF heating.

We should hear in mind that the coils of the RF heater are water cooled

so that any metal touching them will receive a severe quenching. We

tried the asbestos paper procedure with these steels, but their swelling

ruptured the paper leaving the metal in contact with the coils.

However, we have been able to appreciate that these steels are

much easier to burn than the SS steels as may be expected if we regard

corrosion as an oxidation process.

Titanium. Successful attempts were made to burn titanium metal.

A 1-1/8 in diameter titanium rod was used; a 1 in length (weight 78 g) was

wrapped in asbestos paper and treated in the same way as the SS 304L

samples. Although this sample heated well, we were not able to ignite

it or even to melt it under 30 psig of oxygen. A smaller piece

(1-1/8 in diameter x 1/2 in length; weight 38 g) was treated in the same

way at the same pressure with ignition and vigorous burning after only

30 seconds of RF power applied. Ignition seemed to take place in the

coil with the flaming mass dropping out on to the sand bath and burning

to form a considerable volume of oxide.

A titanium plate (Al 5%, Sn 2. 5%; 5 in x 5 in x 1/8 in; weight

217 g) burned completely and vigorously when a SS sample was dropped

onto it at 55 psig of oxygen. Pressure rose to 60 psig and considerable

30
heat was produced. The SS and titanium both appeared to burn to

completion with the titanium forming the characteristic layer of yellow

titanium dioxide as in the laser burning studies of Phase I,

4. 4 Liquid Oxygen Results

As indicated previously, we were able to liquefy oxygen in the

vessel quite readily but did not have a suitable collection apparatus

available to retain the liquid. An indication of a suitable set-up is given

in Section 5. 3, and we intend to drop burning SS samples into liquid

oxygen at 60 psig of oxygen gas pressure to see whether quenching or

enhanced burning occurs.

We did conduct two preliminary tests. In the first one, we used

a 9 in diameter pyrex pie dish to collect the liquid oxygen. Burning of

the SS sample must have been vigorous because spattering of molten,

oxidized steel occurred all the way to the top interior part of the tank.

The dish, of course, cracked; the iron oxide had melted through it all the

way to the sand. In the second test, we used a polystyrene foam boat

lined with Al-foil to contain the liquid oxygen. A 3 in copper disk about

1/8 in thick was placed to prevent the steel from penetrating the plastic

foam on contact. The reaction again was vigorous. The copper disk was

severely oxidized on its top surface. The foam boat burned up completely

and_, as expected^ gave rise to a rather large pressure increase. The

Al foil was partially burned and partially melted,

4. 5 TV and Filmed Records

1) Black and White movie, of a 3 in length of 7/8 in diameter

1075 steel rod burning in 1/2 atmosphere of oxygen. Shows a steady

process rather like a candle burning. Length is 5 minutes.

31
2) Composite video tape of 10 burns, length 25 minutes. Includes

the thermite reactions.

3) Color movie of an SS-Al thermite reaction. Length 2 minutes.

4. 6 Conclusions - Rules of Thumb

1) Iron and iron alloys will burn readily at low pressures of

oxygen (60 psig or so),

2) Regular stainless steel burns easily and well.

3) Stainless steel serves to ignite other metals, notably aluminum

and titanium. Note ; SS and Al combinations in oxygen systems may increase the danger considerably. The SS-Al reaction is particularly

vicious.

4) We have not been able to get copper, aluminum, or brass to

burn by themselves at 60 psig of oxygen.

5) Oxide formation hinders combustion. If oxide is allowed to

accumulate around the burning metal, it exerts a smothering effect.

6) Thresholds exist both for combustion and quenching, e. g. , oxygen pressure or mass of quenching agent. This aspect needs a thorough statistical exploration.

7) To get good combustion at 60 psig or less of oxygen, iron

and iron alloys must be melted.

5. RECOMMENDATIONS FOR FUTURE WORK

We recommend that the bulk burning experiments be continued,

since we are at the stage now where, with small alterations to the

apparatus, we could run several burns per day (8 or so) and could carry

out searching investigations into various aspects of metal combustion.

32
5. 1 Specific Experiments to be Conducted

1) Burning of SS 304L up to 100 psig of oxygen. Run standard

tests with oxygen pressure over 60 psig. In particular, determine

whether we can obviate the incomplete burning where the metal slug does

not spread out to form a thin layer.

2) SS-Al thermite reaction. Do a good statistical study of varying

thicknesses of Al plate on the reaction. Try different Al alloys, e. g. , the Al alloys used in aircraft parts, the high Mg alloys, etc. , to see how

these enhance or depress the thermite reaction.

3) Thermite reaction with ordinary steels . If what we have seen

with SS-Al is indeed the thermite reaction, then it should proceed more

readily with ordinary steels, since they produce iron oxides more

rapidly (by burning more readily). We could study this thermite reaction

at much lower oxygen pressures and see what effect is to be had from

variable oxygen pressure.

4) Burning of Al plates in various orientations . Determine

whether orientation, e. g. , whether flat or upright, affects rate of burning

of Al plate.

5) Burning of various Al alloys . In particular, determine

whether Mg content affects burning behavior.

6) Determination of combustion temperatures . Use spectrograph

or other means.

7) Study weight changes more accurately. Use refractory

crucible.

8) Study liquid oxygen effects on burning metals . Drop burning

metal into liquid oxygen (see 5. 3 for details).

9) Extinguishing agents . Study effects of water, carbon granules,

etc. ,

33
10) Oxygen gas jets . Study effects of jets of oxygen gas on

burning metals.

11) High alloy stainless steels . These steels, e.g., ZS/ZIL,

(25% Cr, 21% Ni), have high corrosion resistance and^accordingly,

should not burn as readily as 304L.

5. 2 More General Investigations

1) Develop a reliable means of ignition for ordinary steels, so

that we can conduct the same sort of experiments with them as with SS.

We would try AC current heating here.

2) Chemical analysis of combustion products . In particular,

determine amount of unburned metal left. This is not a simple problem.

In the case of titanium in the laser assisted burning experiments, we

found by X-ray diffraction (study of powder patterns) that a variety of

oxides are present. A residual pellet of material progresses from

unburned metal in the center to fully oxidized titanium dioxide on the

surface. Other studies ( see reference 5) show that

titanium oxides have variable stochiometry explainable in terms of

variable occupance of lattice sites by the oxygen atoms, giving small but

significant changes in lattice parameters. Similar behavior is to be

expected in the other metal oxides and is a whole field of study in itself.

However, our first needs are simpler, and the first is to determine the

amount of unburned metal present in any residue. With stainless steels,

there are at least three metals present (Fe, Cr, and Ni) so that some

multiple leaching process may be necessary (dilute acids and alkalis,

cyanide solutions). This will be worked up on some known mixture, e. g. , powdered SS metal and various amounts of the metal oxides, to find a simple

reliable process to show how much metal has been burned. X-ray studies

can then be used to show which oxides are present in crystalline form.

34
3) Define valid statistical procedures so that our results are

more qualitative.

4) Addition of other gases , e. g, , N CI or F to see what effect

these have on combustion.

5) Investigate effects of sand and asbestos . Do these act as

fluxing agents and so affect the rate of combustion of metals.-^ Other

construction materials, e. g. , insulations^ may be investigated

also.

5. 3 Modifications to the Apparatus

Since the vessel has a safe working pressure of only 115 psia, we

feel that no major developments can be carried out to increase the pressure range. However, there are minor modifications to be carried out

to get speed and efficiency and to allow series of experiments to be done

to afford a sound statistical basis for our conclusions.

1 ) Reduction in vessel capacity. A reduction in the vessel capacity by at least one half would

be an advantage to conserve oxygen and to reduce filling time. At first

we planned to fill the lower half of the vessel with sand, but this would

add great weight and due to the porosity of the sand would not reduce the

capacity as desired. Our plan now is to use surplus fire extinguisher

tanks (c. 2 gallons capacity) and to pack the space around them with

sand. Original capacity can readily be restored if need be,

2) Alteration of the valving arrangement.

Metal oxide dust is produced in these burns and has a tendency to clog the valves, freezing them shut. We intend to replace

the present solenoid-operated valves with hand-operated valves on the

safe side of the blast wall. These will be metal- PTFE valves to

preserve oxygen compatibility.

35
3) Access to the vessel.

The door of the 10 in x 18 in port will be hinged in place and

closed by six clip-operated dogs to reduce the time needed to open and

close the vessel.

4) Coaxial transmission line . As indicated in Section 2. 4, we have started construction of

an improved coaxial RF transmission line (to carry power from the RF

heater to the sample within the vessel). This is expected both to increase

efficiency (of power transmission) and reliability (freedom from arcing

out) of the ignition process.

5) Liquid oxygen collection apparatus . We have had no difficulty in liquefying oxygen within the

vessel but must find a good means of retaining the liquid before dropping

molten metal into it. This will be done by using a fairly narrow deep

SS dewar to collect the liquid. The condensing coil will be placed towards

the periphery of the vessel, and an insulated funnel and delivery tube will

be used to conduct the liquid to the dewar, as shown in figure 9.

6) Observation ports . The present observation ports are inadequate, since they

are tunnels about 4 in long with a 4 in diameter opening at the inner end.

This restricts the view of the interior of the vessel as can be seen in our

TV and movie recordings, where often it is not possible to record the

whole scene as needed. We will reset the ports with the windows flush

with the vessel wall instead of several inches away as at present.

To record the liquid oxygen experiments, we will install an

observation port in the top of the vessel to look directly down into the

liquid oxygen collection dewar. Suitable openings for this port are

present in the flange already.

36
lines to cooling coil

Metal sample to be ignited,

held in heating coil by friction

RF Transmission

Line

Dewcr to collect LOX

Figure 9. Liquid Oxygen Collection Apparatus

37
ACKNOWLEDGMENTS

We wish to express particular thanks to Mr. Larry L. Sparks

and to Mr. John C. Moulder of this Division for their efforts on behalf

of this project. Mr. Sparks was responsible for getting the project

underway, in particular for developing the RF heating system and the

oxygen liquefaction process, both of which were highly successful,

Mr. Moulder was responsible for the success of the monitoring and

recording systems, both TV and film, and his ready expertise in this

and other regards is much appreciated. We are in debt to our project

monitor Mr. John G. Austin for direction and valuable advice.

6. REFERENCES

[1] C, C. Runyan, J, C. Moulder, and A. F, Clark, Time-Resolved

Spectra of Visible Radiation in Bulk Titanium Combustion, paper

presented at 15th Annual Rocky Mountain Spectroscopy Conference,

Denver (Aug. 1973).

f2] J. C. Moulder and A. F. Clark, Coherent Optical Filtering for

Phase Contrast Photography in Combustion Research, paper

presented at 15th Annual Rocky Mountain Spectroscopy Conference,

Denver (Aug. 1973K

[3] C, C. Runyan, J. C. Moulder, and A. F. Clark, Time-Resolved

Spectra of Bulk Titanium Combustion, to be published.

[4] A. F. Clark and J, G. Hust, A Review of the Compatibility of

Structural Materials with Oxygen, to be published, AIAA Journal.

[5] S. Andersson, B. CoUen, G. Krusse, U. Kuylensteirna, A,

Magneli, H. Pestmalis, and S. Asbrink, Acta Chem. Scand. 1

1

, 1653 (1957).

38
U.S. DEPARTMENT OF COMMERCE

National Buraau of Standards

Boulder. Colorado 8030S

D.t. : July 12, 1973

To : Alan H. Tench, 275. 03

From : 275. 06

Subject: Experimental Hazards Committee

Approval has been granted for the use of the metal combustion experimental

apparatus with the following exceptions.

1. Establish a maximum working pressure of 100 psig on

the experimental vessel.

2. Equip the vessel with a gas relief device which will prevent

the pressure from exceeding 100 psig.

3. All personnel shall be behind the blast wall when the vessel

is being pressurized with oxygen or when the oxygen in the

vessel is being removed by means of a vacuum pump.

R. W. STOKES

Chairman, Experimental

Hazards Committee

Cryogenics Division

Institute for Basic Standards

cc: A. F. Schmidt, 275. 00

J. E. Cruz, 275. 06

J. M. Arvidson, 275. 06

A. F. Clark, 275. 03

G. C. Straty, 275. 04

Al
Oat*

To

July 10, 1973

Safety Committee, Cryogenics

U.S. DEPARTMENT OF COMMERCE

National Buraau of Standards

Bouldar. Colorado 60302

From : Alan H. Tench, Metal Combustion Group

Subject: Request to conduct metal combustion experiments, using gaseous and

liquid oxygen.

The proposed experiments are part of work being carried on in the Metal

Combustion group, and are to be an extension of work already done therein.

There are two aims: (i) to study the burning of large quantities of metal

(kilograms), and (ii) to study the effects of nitrogen on the combustion

process. There is strong indication that nitrogen is necessary to cause

spattering during combustion of certain metals, and this aspect is to be

investigated in particular.

Accordingly, we need some container which will (i) provide a controlled

atmosphere, e. g. , pure oxygen, (ii) contain the combustion products for

further analysis, and (iii) provide a measure of containment of potentially

violent processes.

The proposed container for the experiments is now installed behind the

North wing of the blast wall of the NBS Boulder facility where it is proposed

to carry out the experiments. '

The container was furnished to us by R. O. Voth of Cryogenics and was

used by him for hydrogen-oxygen explosion experiments. A diagram of his

set-up is attached herewith. His calculations, based on ASME Handbook

recommendations, show a safe working pressure of 115 psi (the safe

pressure calculated on the basis of circumferential stress of the cylindrical

portion). The side observation ports have 1/2" thick lucite windows which

are calculated to have a safety factor of 2.9 at 100 psi, while the top cover plate

plate of 1/2" thick stainless steel is calculated to have a safety factors of

at least 3. 0 at 100 psi.

The container has been tested to 60 psi (June 19, 1973), using nitrogen gas

at ambient temperature. The test was to have been continued to 100 psi (in

10 psi steps), but a small plug in the teflon cap blew out due to being not

tightened down properly. We intend to complete the test within the next few

days.

A 2
page 2.

Volume of container is calculated to be 1250 litres, and its weight about

1000 lbs.

Three (3) pressure gauges are connected to the container (one next to the

container, one in the trailer, and one beside the gas tanks, on the safe side

of the blast wall). A solenoid-operated pressure relief valve is in place in

the vent line, and is operated manually from the trailer. Filling valves

(between the gas tanks and the container) are also solenoid operated by a manual switch in the trailer. A vacuum pump is connected to the container

to exhaust it prior to filling with the required gases. This pump has oxygen

compatible oil, and the pressure relief valve is oxygen compatible also.

An ideal gas calculation on oxygen shows that if we burn one gram-molecular

weight of Fe to FeO, and assume that the resultant heat is transferred at

once to 40 moles of oxygen at a constant volume of 1000 litres initially at

room temperature, then we can expect a temperature rise of 315° C with a corresponding pressure rise of approximately 15 psi. Accordingly, if we

burn 5 0 gms or so of Fe in the container, initially filled with oxygen at

atmospheric pressure and ambient temperature, we may expect no great

increase in pressure but would have to consider temperature effects on the

plexiglas, etc. .

Plan of Operation

We intend to proceed cautiously (in view of the foregoing paragraph), and

will start with gram quantities of metal at atmospheric pressure of oxygen

or less. The metal will be ignited by RF (radio frequency) heating, which

is introduced into the container by copper leads through the teflon cap in

the top flange. These leads are already installed and tested, and we have

heated a 10 gram sample of iron to a temperature considered sufficient to

ignite it (1300° C) at 1 atmosphere of oxygen.

As a starting rule, we intend to retire to the safe side of the blast wall

for any oxygen pressures above 15 psi over atmospheric. We intend not

to exceed an oxygen pressure of 20 psi over atmospheric until the apparatus

and procedures are thoroughly tested.

Degreasing of the container and all of its attached lines (including vent and

gauge lines) has been carried out using a combination of washing (or flushing)

with trichlor oethane, hosing with detergent solution and spraying with Freon.

To supply liquid oxygen to the container, we intend to condense gaseous

oxygen already in the container, by means of passing liquid nitrogen through

A 3
page 3.

a copper coil within the container, and collecting the condensed oxygen

in a suitable vessel. This procedure has been designed by Mr. L, L. Sparks,

and he succeeded in condensing nitrogen rapidly and araply so that there should

be no trouble in getting liquid oxygen in the same way. Thus, we are spared

the need to actually handle liquid oxygen and transfer it to the container, etc. , with a great increase in safety and convenience. However we will not do any

liquid-oxygen burning experiments until we have conducted a number of gaseous

oxygen burning experiments in view of the potential violence of the former.

Further safety approval will be sought before liquid-oxygen experiments are

carried out.

Operating Procedures

1, Ensure system clean. Install sample.

2, Open Valves 1 and 2. Close Valve 3,

3, Evacuate,

4, Close Valves 1 and 2, Open Valve 3,

5, Fill with oxygen (or other desired gas), to required pressure,

6, Turn on recording instruments. Ignite the sample,

7, Open valve 2 ( to relieve pressure),

8, Close 2, Fill with nitrogen to 10 psi over atmospheric pressure,

9, Repeat Steps 7 and 8 if necessary (to flush system),

10. Open Valve 2.

11. Remove sample.

Page 4 - Omitted - for diagram of revised site layout see figure 2,

A 4
page 5.

Figure 3.4.2 Assembly of Glass LH^ Vessel and Large Metal

Containment Vessel

(R. O. Voth Apparatus)

A5
APPENDIX B

The Thermite Reaction

If finely divided alunninum is ignited, it burns with great

brilliance to aluminum oxide, 7. 47 kcals are liberated per g of aluminum. Because of its high affinity for oxygen, aluminum is used to

liberate metals from oxides which are difficult to reduce and for the

attainment of high temperatures, e. g. , for the welding of iron. The

mixture of aluminum filings and iron oxides, e, g. , Fe^O,, used for

3 4

this purpose is known as thermite. Temperatures up to 2400° C may be

reached with it. The Goldschmidt process, the reduction of various

metal oxides to the metals with aluminum, is used commercially to prepare chromium metal, especially where carbon free metals are required.

The nobler metals, e. g. , copper, may give rise to an explosive reaction

in this process.

Table 3 gives the heats of formation of the commonest oxide of

various metals, and from this we calculate that 100 g of SS 304L will

give 174 kcals on complete oxidation. The thermite reaction of the

resultant oxides (Fe, Cr, Ni) is calculated to yield 148 kcals. The

metals liberated in this thermite reaction may then be re-oxidized,

yielding a further 174 kcals. Thus, a total of 496 kcals may be anticipated from the reaction of 100 g of SS 304L under the conditions described

in our experiments. In the process, 43. 4 g of aluminum are oxidized.

100 g of SS 304L on solidifying from its melting point and cooling

down to 660°C (Mp of Al) yields 12. 5 kcals. If we assume 10% oxidation,

we obtain a total of 29. 85 kcals yielded, and on this ground may postulate

that a minimum of 30 kcals is available to be transferred to the aluminum

plate. This amount is sufficient to start a thermite reaction, however,

the actual amount of heat required for ignition of the Aluminum plate may

be considerably smaller than 30 kcals because we cannot estimate the

efficienty of the heat transfer.

B 1
Table 3. Heats of Formation of the commonest oxides of various

elements, per gram atom of oxygen (16. 00 g of oxygen),

in kcals.

Element Oxide AHf Element Oxide AHf

Ca CaO 151. 9 Fe FegO^ 65. 5 Mg MgO ' 143. 7 Ni NiO 58. 4

Al AlsOg 133. 0 H HgO (g) 57. 8

Na NasO 99. 4 Co CoO 57. 2

Cr CrgOg 93. 0 Pb PbO 50. 8

Mn MnO 92. 0 Cu CuO 37. 1 Zn ZnO 83.2 Ag AggO 7.31

B 2
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. 1 . PU BLIC AT ION OR RE PORT NO. 2. Gov't Accession

BIBLIOGRAPHIC DATA tmo cxr) no oat

SHEET iN-DOlK - M--34D

3. Recipient's Accession No.

|

4. TITLE AND SUBTITLE

Combustion of Metals in Oxygen Phase II: Bulk Burning Experiments

5. Publication Date December 1973

6. Performing Organization Code

7. AUTHOR(S)

Alan H. Tench, Hans M. Roder and Alan F. Clark

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No. 2750439

11. Contract/Grant No. H92145A

12. Sponsoring Organization Name and Address NASA-Mar shall Soace Flieht Center

Hunts ville, Alabama 35812

13. Type of Report & Period Covered

Interim 4/72-9/72

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This program was started to study combustion of the ordinary

metals in oxygen, to provide information useful to designers of bulk

oxygen handling equipment, particularly from the point of view of

safety. We have studied the ignitability and combustability of carbon

steels, stainless steels, aluminum alloys, titanium and copper metals,

in oxygen atmospheres up to 60 psig. We encountered a violent reaction involving burning stainless steel and aluminum, a thermite reaction

apparently, and have investigated this from a quantitative point of view.

We have tried throughout to study the quantitative aspects of the combustion processes, and related quenching effects. Ordinary steel and

stainless steel burn quite readily at these low oxygen pressures, and the

stainless steel-aluminum reaction produces intense heat, with severe

damage to neighboring structures. We are at the stage where further

investigations could be carried out quite expeditiously and on a sound

statistical basis.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Alloys; aluminum; combustion; ignition; oxygen; safety; stainless steel;

steel, titanium.

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