NBSIR 75-922 (Q)

Safety Evaluation of Soda-Acid

Fire Extinguishers

Bruce W. Christ and John H. Smith

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

October, 1975

Failure Analysis Report

Prepared for Bureau of Engineering Safety

Consumer Product Safety Commission

Washington, D. C. 20207

NBSIR 75-922

SAFETY EVALUATION OF SODA-ACID

FIRE EXTINGUISHERS

Bruce W. Christ and John H. Smith

Mechanical Properties Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

October, 1975

Failure Analysis Report

'This document has been prepared for the use of the

Bureau of Engineering Safety, Consumer Product

Safety Commission, Washington, D.C. 20207.

Responsibility for its further use rests with that

agency. NBS requests that if release to the public

is contemplated, such action be taken only after consultation with the Office of Public Affairs at The National Bureau of Standards."

Prepared for Bureau of Engineering Safety

Consunner Product Safety Commission

Washington, D. C. 20207

U.S. DEPARTMENT OF COMMERCE, Rogers C.B. Morton, Seer9tary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Acting Director
^4 J.
TABLE OF CONTENTS

Page

SUMMARY ii

I. INTRODUCTION 1

II. BACKGROUND INFORMATION - 2

III. FIRE EXTINGUISHER WHICH EXPLODED 5

IV. EXAMINATIONS ON USED FIRE EXTINGUISHERS 6

A. VisualExami nation 6

B. Metal 1 ograph i c Examination 8

C. Susceptibility to Corrosion 12

D. PressurizationTests 13

V. DISCUSSION 14

A. Speculation About Failure Mechanism 14

in Dome Metal of Fire Extinguishers

B. Classification of the Fire Extinguisher 16

Behavior

VI. CONCLUSIONS 17

i

SUMr4ARY

A safety evaluation of soda-acid fire extinguishers was carried

out. One extinguisher which exploded was photographed at low magnification, and three extinguishers removed from service were evaluated by

metal lographic examination and by pressurization tests. The objective

of these evaluations was to determine if the susceptibility of soda-acid

fire extinguishers to catastrophic failure during use -were a generic

problem.

The fire extinguisher which exploded separated into two pieces

via a circumferential tear at the dome end. The path of the tear was through dome metal — it followed what appeared to be a brazed joint

between dome and collar.

One of the three extinguishers removed from service was cut

up to facilitate metallographic examination. Definite evidence of

interior deterioriation due to deformation and corrosion was found

along a circumferential path following a joint between the dome and

collar. Visual examination of the other two extinguishers removed from

service revealed interior deterioriation in the same region. However,

these two extinguishers performed well during pressurization tests — one withstood 25 pressurizations such as occur during discharge in service,

then burst at 850 psig, as compared to the required test pressure of

500 psig. The second extinguisher burst at 700 psig. Failure of both

extinguishers occurred by bursting a disk out of the cap.

It was concluded that deterioriation at the dome end is likely

to. occur in many soda-acid fire extinguishers. Susceptibility to such

deterioriation seems attributable to a combination of several factors,

including design, use of dissimilar metals, manufacturi ng practice,

handling in service, time in service and frequency of discharge. Hov;ever, even extinguishers removed from service which exhibited conspicuous

interior deterioriation performed satisfactorily in pressurization

tests. Hence, the results of this evaluation are inconclusive as regards the question of whether or not the susceptibility of soda-acid

fire extinguishers to catastrophic failure is a generic problem.

1 i

SAFETY EVALUATION OF

SODA-ACID FIRE EXTINGUISHERS

I. INTRODUCTION :

Reference: Bureau of Engineering Safety, Consumer Product

Safety Commission, Washington, D.C. 20207. This investigation was conducted at the request of Mr. S. Greenwald, Office of Consumer

Product Safety - li^S, and Mr. James Talentino, Bureau of Engineering

Safety, Consumer Product Safety Commission. The request was made on June 26, 1975.

On June 26, 1975, Mr. S. Greenwald, Mr. James Talentino and

Dr. Bruce Christ of the NBS Mechanical Properties Section conferred

about how zo proceed with the evaluation of three used-but-undamaged*

soda-acid fire extinguishers which had been left at NBS on June 18,-

1975 by Mr. Talentino. Attention was being given to these used

soda-acid fire extinguishers because a similar one had exploded in use and resulted in a fatality. The fatality occurred on May 6, 1975

during a fire-fighting demonstration by firemen in the vicinity of

San Antonio, Texas (Reference 1). It was reported that the three

used fire extinguishers were from a collection which had been

voluntarily turned in by owners subsequent to the accident (Reference

1). These three fire extinguishers are identified as follows:

Manufactured by Company A, all model SA SG, Numbers G 319542, F 701019

and F 700745.

The fire extinguisher which exploded (Number F 407396) had been

left at iNBS on June 18, 1975 for some photographing. This fire extinguisher

was returned to Hr. Talentino on June 26, 1975. Several photographs of

this fire extinguisher were transmitted via memo to Nr. Talentino on July 8, 1975 (Reference 2). Analysis of this failed fire extinguisher

was of a preliminary nature, due to a constraint suggested by the

Consumer Product Safety Commission for legal reasons, "Do not alter its

physical condition or visual appearance in any way." (Reference 3).

According to Mr. Talentino, the objective of the Consumer

Product Safety Commission in this matter was to determine if the fire

extinguisher failure which led to the fatality were a generic problem

for soda-acid fire extinguishers — the three used fire extinguishers

were made available to test in any way which might provide information

* Throughout this report, the phrase "used-but-undamaged" referes to

extinguishers removed from service. Details of service history, such

as time in service and frequency of discharge are unknown. However,

it may be assumed that each extinguisher was discharged annually in

accord with instructions on the label fastened to the extinguisher shell.

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related to this objective. As a result of the discussion on June 26, 1975,

the following course of action was agreed upon:

1. cut up one used fire extinguisher for purposes of metal lographic

examination, and

2. carry out pressurization such as occurs in service on another used

fire extinguisher in an attempt to promote a burst failure.

This report contains a selection of results from the preliminary

analysis (Reference 2) which was carried out on the fire extinguisher

that led to the fatality. This report also contains results obtained

from one used extinguisher which was cut up for purposes of metal lographic

examination, and results from one used extinguisher which was subjected

to pressurization such as occurs in service. Results of hydraulic burst

tests are also included. These tests were agreed to betvyeen

Mr. Talentino and Dr. John Smith of NBS when service-type pressurization

did not lead to a burst.

II. BACKGROUND INFORMATION :

The Consumer Product Safety Commission supplied NBS with the

following documentation:

1. A copy of a memo to Richard Armstrong, Director, BES from

Carl Blechschmidt, Director, Office of Product Defect Identification.

Attached to this memo was information about the fatal accident

(Reference 1). Also attached was literature about fire extinguishers

from the National Association of Fire Equipment Di stributiors , Inc.

(hereafter referred to as NAFED) (Reference 4).

(ANSI Z171 . 1-1969) '.'Standards For Safety - Soda-Acid Fire

Extinguishers, dated October 14, 1969 (Reference 5).

Some highlights from this background information are cited below.

A. Accident Report (Reference 1) ^

1. The fire extinguisher which exploded and caused a fatality on May 6, 1975 had been hydrostatical ly re-tested in November, 1973.

A statement about the re-test pressure was not found in

Reference 1

2. The fire extinguisher which exploded had sprayed water for

approximately 10 to 20 seconds before the explosion.

B. Literature From NAFED (Reference 4) .

1, Periodic hydrostatic re-test occurs every 5 years for soda-acid

fire extinguishers. An "old test pressure = 75'- of shell psi

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rating" is cited, and a "new test pressure" = 100% of shell psi

rating" is also cited.

2. This literature provides documentation of numerous failures of

soda acid fire extinguishers, as well as foam, water cartridge

and loaded stream cartridge extinguishers. It is reported, for

example, that IT.'o of ihose fire extinguishers made with a stainless

steel shell fail ihe required 5-year reiest, and that a larger

fraction — 46';o — of ihose made with a soldered copper shell

fail the retest. Precise figures on ihe fraction of soda-acid

extinguishers which fail in service were not found in the liAFED

literature.

3. The 1974 literature from IIAFED indicates that soda-acid fire

extinguishers have been "discontinued." Apparently, "discontinued"

means these extinguishers are no longer made. Nevertheless, many

of these extinguishers do remain in service and com,e up for periodic

re-test.

C. Underwriters Laboratories Specification (Reference 5)

1. The construction of a soda-acid fire extinguisher essentially consists

of a cylindrical shell, with closures called a dome and a bottom

fastened at eiiher end. The dome end consists of the dome (Paragraphs

20, 24F and 24G), a collar (Paragraphs 30-35) and a cap (Paragraphs

47-57), as well as a hose outlet elbow (Paragraphs 41-43) and an interior-mounted strainer (Paragraphs 39-40). The cap is fastened

to the collar via threads.

2. " The kinds of materials and methods used in the manufacture of

soda-acid fire extinguishers are mentioned in Paragraphs 18-22

and 26-30. Joining techniques are described in Paragraphs 27 and

33.

3. Mechanical properties of the materials used for fire extinguisher

manufacture are mentioned in Paragraph 24B.

4. Concern about carbide precipitation in the stainless steel shell is

expressed in Paragraphs 28, 98G and 98H.

5. Concern about the corrosion behavior of the entire structure is

expressed in Paragraph 19.

6. Pressure capacity and rupture pressure of the extinguisher itself

are referred to in Paragraphs 23, 24 and 24A.

7. Test methods for evaluating the performance of soda-acid fire

extinguishers are described in Paragraphs 85-111.

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NOTE: A description of a method for burst testing was not found

in the Underwriters Laboratories Specifications.

80 The chemical charge for a soda-acid fire extinguisher is specified

in Paragraph 83. For a 2 1/2-ganon extinguisher, the charge

consists of

Sodium Bicarbonate — 1 1/2 pounds + 1 ounce

Concentrated Sulfuric Acid (specific gravity 1.83) — 4 fluid ounces 1/4 fluid ounce

The functioning of a soda-acid fire extinguisher is as follows:

a. The extinguisher is partially charged by dissolving sodium

bicarbonate in water — this solution is contained in the

shell.

b. The charging is completed by installing a bottle of sulfuric

acid inside the extinguisher, (See Paragraph 58 for a description of acid bottles). The acid bottle is positioned

upright in a support cage (Paragraphs 60-62) at the hose-end

of the extinguisher (Paragraph 9). The hose end is at the

top when the extinguisher is stored in an upright position.

c. A loose-fitting stopple (Paragraphs 63-65) closes the acid

bottle when the fire extinguisher is stored in the upright

position.

d. When the fire extinguisher is put into service, the vessel is

inverted so that the hose-end is at the bottom. When inverted,

acid dribbles from the bottle past the loose-fitting stopple

and into the bicarbonate solution. Chemical reaction occurs between the acid and bicarbonate solution which releases gas.

It is the pressure of this gas which drives the water solution

from the extinguisher. Since there is no valve on the soda-acid

fire extinguisher, discharge occurs until it is empty. The

gas point is defined as the time when the discharge changes

from a liquid to a gas (Paragraph 86).

NOTE: A gasket is used to prevent gas from escaping at the

threaded joint between the cap and collar (Paragraphs 56-57).

Concern is expressed that the cap not reach the dome when the

gasket is removed (Paragraph 32).

NOTE: Instructions on the labels of the four fire extinguishers

examined recommend that the extinguisher be discharged annually.

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III. FIRE EXTINGUISHER WHICH EXPLODED:

The fire extinguisher which exploded (number F 407396) was delivered

to NBS in iwo pieces — one piece had separated from the other via a circumferential tear near what appeared to be a brazed joint at the dome end.

Figure 1 shows a view of the components of the fire extinguisher which exploded.

The cap and collar — still fastened together at the threaded joint —

appear at the lower left in Figure 1. Attached to the cap-collar assembly

is the support cage which holds the acid bottle. The bottle and stopple

appear in the center foreground. The outer ring and spoke of the cap

handle were probably damaged by the impact when it "blew straight to

the pavement" during the explosion (Reference 1, page 2).

Close-up views of the cap-collar assembly appear in Figures 2a-2d.

These views show that a ring of dome metal (arrows in Figures 2a and 2b)

remains attached to the collar, probably by the joining metal (Paragraph

33 of Reference 5). The significance of the black arrow on the dome metal

in Figures 2a and 2b is not clear — this arrow was on the as-received

part. The circumferential tear seems to have occurred entirely in. the

dome metal, and the path of the tear seems to follow — at least

approximately — the tip of the joining metal interior fillet which

touched the dome metal. Indeed, what appears to be a fin-like projection

of the joining metal is pointed out (arrow) at the right center of

Figure 2a. The process used to join the collar to the dome — welding,

brazing or soft soldering — has not been definitely established. Many

of the white regions in Figures 2a-2d probably represent deposits of

sodium bicarbonate.

A view of the cap-collar assembly which includes some of the fracture

surface of the dome metal along an arbitrarily-selected portion of the

circumference appears the composite picture. Figure 3. In this picture,

which has a three-dimensional character to it, the dome metal as seen at

tha outside top of an upright extinguisher appears above the side arrows,

and the edge of the flange of the collar — which would normally be

inside the extinguisher — appears in elevation below the side arrows. Appearing as a layer upon the flange is the dome metal — which

disappears towards the top horizon. The fracture through the thickness

of the dome metal is shown in elevation, running between the side arrows. The fracture path does not run in a straight line, but follows a rather

wavy path. In fact, scallops in the fracture path through the dome

metal — which show up especially well at the right of Figure 3 — reveal some of the surface of the collar flange which was joined to the dome

metal. Terrace-like features" which may be ruptured metal or corrosion

product show up at the right-most scallop and also at the third scallop

from the right. A feature which may be a del ami nation between the dome

metal and the collar metal appears just to the left of center in

Figure 3. A smaller possible delamination appears somewhat further

to the left. The white regions in Figure 3 may be areas where sodium

bicarbonate has deposited. Conditions of photographing made it rather

difficult to show up joing metal at the interface between the dome

metal and the collar metal.

The circumferential tear which is clearly evident at the top of

the dome in Figure 1 is shown in some close-up views which appear in

Figures 4a, 4b, and 4c, Conspicuous bulging does not show up anywhere

along the circumference. Indeed, the separation seems to have occurred

in a uniform fashion along the entire circumference. The wavy path

followed by the fracture is clearly seen in Figure 4c. All three

figures seem to indicate that the fracture occurred entirely in the dome

metal. It is likely that the dome metal is a stainless steel, as suggested by its surface luster, its inability to attract a small,

hand-held magnet and the wording of the specification. Paragraph 20 of

Reference 5. However, conclusive identification of the dome metal via

chemical analysis was not obtained at this stage of the examination.

IV. EXAMINATIONS ON USED FIRE EXTINGUISHERS :

A. Visual Examination

The three used — but undamaged — fire extinguishers were

examined visually. Table I summarizes the data taken from the retest

labels found on these extinguishers. Since retest occurs every five

years (Reference 4), these data are helpful in inferring a minimum time

in service for an extinguisher. For example, fire extinguisher number

F 700745 was last retested in May, 1973, indicating that its minimum

time in service up to the date it was turned in in May, 1975 was

(5 years + 2 years) = 7 years.

Because the exploded extinguisher failed via a circumferential

tear at the dome end, this region on the used-but-undamaged extinguishers

was examined carefully. The following observations were made:

Materials - The dome was probably made of stainless steel (Paragraph 20,

Reference 5), whereas the collar and cap appeared to be of some different metal — perhaps brass or bronze, judging from surface

luster. A precise indication of the collar metal (Paragraph 30,

Reference 5) and the cap metal (Paragraph 47 of Reference 5) was

not found in the specification, although "... a material

equivalent to the material of the- shell" is specified for the

collar, and a cap "... of corrosion-resistant material equivalent

to red brass or austenitic stainless -steel " is specified.

Cap - One cap out of the three examined was missing a quadrant from the

ring section which would be gripped to tighten or loosen the cap.

The other caps appeared to be undamaged.

All three caps had indentations on the spokes, which could have

been caused by a lever inserted between the spokes to assist in

torquing operations.

Two caps out of the three had a continuous powdery-looking deposit — mostly green, but with some blue and white — on the inside.

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I

The third cap exhibited regions of such a deposit on the inside,

but this deposit was not continuous.

All three caps contained rubber gaskets. Two gaskets of the

three retained permanent impressions of the collar rim, perhaps

from tightly torquing the cap lo the collar. The gasket which

did not have a permanent impression was associated with the cap

which did not have a continuous deposit inside the cap. One of

the gaskets was cracked and somewhat deterioriated.

The threads on all three caps appeared to be in good condition.

A bluish-green deposit was evident here and there on the threads

. of all three caps.

Collar - The threads on all three collars appeared to be in good

condition. A bluish-green deposit was evident here and there

on the threads of all three caps, and in all three cases, this

deposit was continuous over the bottom one-or-two threads.

The design of these three extinguishers — as well as the one that exploded — is such that a flange on the collar conforms

closely with the curvature of the dome. The two components are joined via a welding, brazing or 'soft soldering process. In

the case of all three extinguishers, a continuous circumferential

ring of the joining metal was visible on the outside where the

dome joined the collar. The bluish-green deposit found on the

bottom threads was also evident on this ring of joining metal.

A nearly-continuous deposit showed up on one extinguisher —

the same type of deposit showed up only here and there on the

other extinguishers.

Dome - A pronounced change in surface luster was evident to the unaided

. eye on tv;o domes out of the three. This change in luster — near

the bottom inch or so — ran around the entire circumference; it

is probably due to drawing marks.

One extinguisher out of the three exhibited some black streaks —

in the circumferential direction --- in the region of the dome

just below the ring of joining metal. The cause of these streaks

is not evident.

The outside of the dome was readily examined. A technique for

easily examining the inside was developed — this technique

involved using a mirror and a light source on a flexible tube.

A system utilizing fiber optics was also employed.

Interior examination with the :^irror revealed — in all three

extinguishers — a continuous circumferential ring of joining

metal at the dome-collar interface. White deposits were observed

on the collar, on the joining metal and on the inside surface of

the aome. In the case of all three extinguishers, occasional

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reddish-orange-brown deposits were noted on the dome below the

joining metal, and on the joining metal itself. For all three

extinguishers, some regions along the ring' of joining metal

seemed to show short segments of a step or a crack in the joining

metal . .

B. Metal lographic Examination

One of the used but undamaged fire extinguishers — number

F 700745 — was cut up to provide specimens for metal 1 ographic

examination. This extinguisher was selected at random from among the

three provided.

To prepare metal lographic specimens, the dome was sawed off the

shell, Figure 5a, and a longitudinal section, Figure 5b, was cut from it.

In the longitudinal section shown in Figure 5b, the collar is labeled . (A) and the dome wall is labeled (D). Joining metal is between the dome • wall and collar — the outside fillet is at (C), and the inside fillet'

is at (B). The joint where the dome wall meets the shell wall is at (E).

Evidence of particles from the sawing operation appears in both

Figures 5a and 5b. These particles were not removed by a cleaning

operation such as brushing or ultrasonic cleaning simply to minimize

the possibility of removing corrosion products. The region where the

collar joined the dome appeared discolored and corroded, especially near

the fillet of the joining metal — which is shown at B in Figure 5b.

Figure 6 shows the inside of the dome after the longitudinal

section was removed. A full circumferential picture of the inside of

the dome — taken before the longitudinal section was cut — appears

in Figure 7. The bottom of the collar shows up as the dark circular

ring- with whitish stains. The outermost ring of white is the inside

of the dome wall. A partial view of the strainer covering the outlet

elb'ovy is at the left. The joining metal shows up as a dark circumferential

ring between the collar and the dome.

A notable feature in Figure 7a is the circumferential step or crack which shows up between the joining metal and the dome metal. The

vicinity of this crack is stained with corrosion product around the

entire circumference. Drip-shaped stains appear at the arrows in

Figure 7a and 7b. These stains have a reddish-orange-brown color, as

is evident in Figure 7c.

Figure 7b shows a close-up view of the stain at A in Figure 7a.

The longitudinal section cut from the dome. Figure 5b, was obtained by cutting

as closely as possible through this stain. Close-up views of the joining

metal, the collar and the dome wall appear in Figures 8a (as polished)

and 8b (polished and lightly etched).

The joining metal is not delineated from the collar or dome metal

in Figure 8a — however, delineation is clear in Figure 8b due to the

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etching. Rupture of the exploded fire extinguisher — number F 407396 —

seemed to follovy a circumferential path in the dome metal just about

where the inside fillet of joining metal meets the dome metal.

It is of interest lo note that the outside fillet of joining metal

shows a rather irregular profile, suggesting that some corrosion took

place. Indeed, it was in this region that the three used-but-undamaged

extinguishers showed a bluish-green deposit, which was probably a corrosion

product. The outside fillet of joining metal is about 5.35 inches away from

the region where the dome metal of fire extinguisher number F 407396

ruptured — it appears unlikely that the corrosion of the outside

joining metal might lead to the type of rupture observed in the dome metal

of fire extinguisher number F 407396.

Another region of some interest is the end of the dome metal under

the outside fillet of joining metal — this region is near the label "C"

in Figure 8. A close-up of this region after etching — Figure 9 —

reveals evidence of severe plastic deformation, especially at the tip.

The explanation of this deformation is not definite. It may be due to

punching forces, assuming that the dome was annealed after forming as prescribed in Paragraph 20 of Reference 5 — then punched to accommodate

the collar. Regardless of the cause of this deformation, it seems unlikely that it — or the cause of it — might lead to the type of

rupture observed in the dome metal of fire extinguisher number F 407396.

A conspicuous feature in Figure 8 is the large pore at the end of

the dome metal near the collar. This pore is evidence of incomplete - joining of the dome and collar. However, this feature does not seem to

be contributory to the manner in which fire extinguisher number F 407396

exploded.

A less conspicuous feature in Figure 8 than the pore is the

rupture of the joining metal in the region where the inside fillet runs along the dome wall — this region is labeled A. This rupture leads

into a crevice between the joining metal and the dome metal — and

the crevice tip seems to be in the vicinity of the region labeled B in

Figure 8b. The overall length of the crevice seems to be about 0.20 inches,

as compared to an overall length of 0.35 inches between the tip of the fillet

and the end of the dome wall. This crevice is of potential importance in

understanding the malfunctioning of fire extinguisher number F 407396,

since this crevice is near the observed rupture path in the dome metal.

A microscope examination for cracking in the dome metal was carried out on the longitudinal section shown in Figure 8. However,

no cracks were detected. This examination was carried out because

stainless steel — which the dome metal might be made of (Paragraph

20 of Reference 5) — is sometimes susceptible to i ntergranul ar corrosion and cracking (Reference 6).

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A microscope examination was also carried out to search for

evidence of penetration of the grain boundaries of the dome metal by

another metal. None was found, however. This examination was carried

out because joining metals used with stainless steels may sometimes

penetrate grain boundaries and thereby cause embrittlement . (Reference 7).

Returning to the joining metal noted at A in Figure 8, it is

evident from the close-up views in Figure 10 that the joining metal at

the tip of the inside fillet has ruptured and separated' from the dome

wall. The step due to separation is probably what leads to the

appearance of the circumferential step or crack which was pointed out in Figure 7 The layer of corrosion product — which appears as a grey-black layer on both the joining metal and the dome metal — suggests that the rupture

of the joining metal occurred before the longitudinal section was cut from the dome. Indeed, the layer of corrosion product is apparently

quite adherent, since the layer did not flake away significantly during

the cutting process.

Corrosion product growing on the ruptured joining metal seems to

have almost closed up the rupture in Figure 10c — however, despite the

apparent impingement of these corrosion products, it is likely that the

crevice is still open to the interior of the fire extinguisher. In fact, one plausible explanation of the stains labeled A in Figure 7 is that

these stains arise from the gradual seepage of corrosive fluid out of

the crevice. Corrosive fluid such as the mixture of sulfuric acid and

water containing sodium bicarbonate may be driven into the crevice under

pressure when the fire extinguisher is used, then may seep out gradually

while the re-charged extinguisher is in an upright position awaiting its

next use. It is unlikely that even thorough cleaning could remove all

corrosive fluid driven into the crevice by pressurization during use because of surface tension effects as well as the near impingement of

corrosion product which is evident in Figure 10c.

Lastly, it is noted that at the higher magnification in Figure 10, as compared to Figure 8, there is still no evidence of cracks in the

dome metal. Nor is there evidence of i ntergranul ar penetration of the

dome metal by another metal

The crevice evident in Figure 8 — re'ferred to earlier — shows

a significant amount of corrosion product on the dome metal as well as

on the collar metal. Figure 11 is a close-up of the crevice at some

point between the regions labeled A and B in Figure 8. In addition to

the corrosion product evident in Figure 11, there also seems to be an occasional good matching of the profile of the joining metal and the dome

metal — two examples are indicated by the pairs of arrows. This

matching suggests the following sequence of events — first, the

interface between the dome metal and joining metal was ruptured, then

corrosive fluid entered the crevice and helped to form the tightly

adherent corrosion product.

A comparison of the surface profile of the dome metal on either

side of the region labeled B in Figure 8 is informative. The two pits

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at B in Figure 8 appear at higher magnification in Figure 12a. The

crevice narrows from right to left in this figure. Figure 12b was taken

in the region between A and B in Figure 8 where the crevice is wide. On

the other hand, Figure 12c was taken in the region between B and C in

Figure 8 where a tightly bonded interface is evident. There does not seem to be a significant difference in the surface profile of the dome

metal in Figures 12b and 12c. This observation suggests that there was little selective solution of the dome metal adjacent to the crevice,

for example, in localized regions such as grain boundaries. Indeed,

such a process would lead to conspicuous deep penetrations, and none are evident.

A few final features of the dome metal in the region betvyeen the

labels A and C in Figure 8 are noteworthy, namely, slip lines in

isolated grains and grain boundary precipitates. The grain structure

of the dome metal appears in Figure 13a — the crevice and the corrosion

product coating the dome metal are at the bottom. The narrow bands

traversing entire grains are probably annealing twins (A in Fig. 13b).

On the other hand, wavy lines within individual grains are probably slip

lines (B in Fig. 13b). These lines show up especially well in certain

grains in Figure 13b. Slip lines form when metals are stressed above

their yield point. An example of a profusion of slip lines appears in

Figure 9. The slip lines of Figure 13b suggest that individual grains in

the dome metal were stressed above their yield point, whereas the bulk of

the dome metal itself was not. It may be that these slip lines formed

when the interface between the joining metal and the dome metal was

ruptured. Consistent with this suggestion is the observation that no evidence of slip lines in isolated grains was found in regions of the

dome metal remote from the collar and joining metal. Precautions were

taken to assure that these slip lines were not artifacts introduced

during polishing of the specimens.

An abundance of the grain boundary precipitates mentioned above

appears in Figure 14a. Were a Type 304 stainless steel — which the

dome metal might be made of (Reference 5, Paragraph 20) — to exhibit

such a microstructure, it would be described as "sensitized." Such a microstructure — which frequently forms in Type 304 stainless steel

during welding and brazing operations --- is highly susceptible to

intergranular corrosion. A close-up of a few grain boundaries is

shown in Figure 14b, where the corrosion product at the dome metal-crevice

interface appears at the bottom. A suggestion of a slight penetration of

grain boundaries by a corrosion process shows up in Figure 14b, but for

the most part, it appears that the tightly adherent corrosion product

serves as a protective film to shield the dome metal from further

penetration by a corrosion process.

Evidence of grain boundary precipitates such as those appearing

in Figure 14 was also found near the joint between the dome metal and

the shell metal. However, such evidence was not found elsewhere in the

dome metal, suggesting that the heat of the joining operation may have

contributed to the formation of the grain boundary precipitates.

- n -

C, Susceptibility to Corrosion

The materials at the dome end of the used-but-undamaged fire

extinguisher v/hich was cut up — number F 700745 — seemed to exhibit

tendencies toward corrosion. For example. Figure 15 shows a section of

the collar metal near the region labeled L) in Figure 8. The surface

film of corrosion product is clearly evident. Susceptibility of the

joining metal and dome metal to corrosion is evident along the crevice

in Figures 10, 11, 13a and 14b. The tendency of the joining metal at

the top outside fillet — C in Figure 8 — was already pointed out.

A close-up of a region near C in Figure 8, Figure 16, shows corrosion

product adhering to the surface of the joining metal. The region

labeled E in Figure 5b — where the dome metal overlaps the shell

metal — was examined for evidence of corrosion. Indeed, as shown in

Figure 17, some evidence of corrosion appears in the form of the

irregular surfaces. Adhering corrosion product was not conspicuous

on either the dome metal or zhe shell metal.

In order to evaluate the susceptibility of the dissimilar metals

to galvanic corrosion, the corrosion potentials of the dome metal,' the

collar metal and the joining metal — relative to one another — were

evaluated. A solution of sodium bicarbonate in water such as the

specification (Reference 5) calls for when charging the extinguisher

was prepared. Potential measurements versus a saturated calomel

electrode were made. Results appear in the first row of Table II. Then

three cubic centimeters of concentrated H2SO4 were added to bring

the solution to one-half the concentration called for in the

specification (Reference 5). This procedure was carried out to

roughly simulate the case of acid dribbling from the bottle when the - extinguisher is inverted. Potential measurements were again made,

with the results shown in the second row of Table II. This procedure

was repeated by adding three more cubic centimeters of concentrated

H2S0^ to bring the proportions of acid and sodium bicarbonate

to the full values called for in the specification (Reference 5).

Results of potential measurements appear in the third row of Table II.

These measurements show that the joining metal has the greatest

tendency to go into solution, whereas the aome metal has the least

tendency to go into solution. Thus, when-the specified solution is present,

and when the metal surfaces are bare, the joining metal and the collar

metal will tend to dissolve, whereas the dome metal will not. A

possible consequence of these tendencies is that a crevice between

joining metal and dome metal might widen at its mouth as a result of

galvanic corrosion of the joining metal. Another consequence of this specifi

process would be weakening of the bond between dome and collar, as joining metal dissolves. Of course, the tendency for galvanic corrosion

of an active metal will be altered if a film of corrosion product coats

the metal surface.

- 12 -

Because of the presence of many crevices, it seemed reasonable

to make some measurements of the susceptibility of the dome metal to

crevice corrosion. For experimental convenience, a crevice at the

dome metal was simulated by fastening a rubber band around a metal lographically-mounted and polished section of the dome metal. The

specimen was irmiersed in the solution remaining after the measurements

appearing in row 3 of Table II were made. Potential measurements versus

the standard calomel electrode were again made over a period of 8 days.

After an initial period of a few hours during which the dome metal

exhibited a more negative potential — the electrode potential was about -0.360 volts — the potential shifted toward the values appearing

in column 1 of Table II. Subsequent examination under the microscope

revealed no significant amount of crevice corrosion under the rubber

band. Thus, the susceptibility of the dome metal to crevice corrosion

seems rather limited. '

D. Pressurization Tests

One of the used-but-undamaged fire extinguishers (No. F 701D19) was tested lo simulate typical service use of a soda-acid fire

extinguisher. The extinguisher tested exhibited conspicuous evidence

of corrosion around the inside circumferenii al joint between the joining

metal and ihe dome metal. The objective of Lhis testing was to learn if

this fire extinguisher could be made to burst catastrophical ly , as did

the fire extinguisher which exploded in service (Section III). However,

a burst did not occur, even after 25 pressurization tests. Consequently,

this extinguisher was hydrotested as required during a periodic retest

(Reference 4) and was then pressurized to failure. The fire extinguisher

chosen for this test series had been in service previously, but no details of its service history were available.

For the tests to simulate typical service use, the fire extinguisher

was charged according to procedures specified by Underwriters Laboratory

"Standards for Safety-Soda-Acid Fire Extinguisher," UL-7-1969 (Reference 5).

The charge consisted of 4 ounces of concentrated sulfuric acid (1.83 specific

gravity), 1 1/2 pounds of sodium bicarbonate, and 2 1/2 gallons of water.

The fire extinguisher was charged, upended to discharge and then

cleaned by rinsing the inside with water and recharged. This procedure

was repeated a total of 25 times during an eight hour time period. For

the safety of the personnel conducting this test, the fire extinguisher

was located in a safety pit and discharged remotely.

In each discharge test, the pressure build up to full discharge

pressure took between 5 and 10 seconds. The water discharge continued

for 45 to 50 seconds as required by the Underwriters Laboratory

Specification (Reference 5). No indication of improper operation

was evident during the 25 testing cycles, and no failure of the

fire extinguisher occurred. It should be noted that although this

testing sequence simulated the operation of the fire extinguisher during

- 13 -

normal use, it was not practical to allow a long time period between

individual discharge cycles such as occurs in normal fire extinguisher

usage.

Following the 25 cycles simulating service use, a test was carried

out to evaluate the response when the discharge hose was blocked. The

discharge hose was sealed off, and the fire extinguisher was cleaned and

recharged. The extinguisher was then activated by upending it. No

discharge of water occurred and after approximately 10 minutes the discharge

hose was unblocked and normal discharge occurred. Although during this

test the pressure in the fire extinguisher built up higher than during

normal operation, the fire extinguisher did not fail catastrophical ly.

Furthermore, no damage was vi sable after the test.

This fire extinguisher was then hydrostatical ly tested, as required every 5 years during the life of a fire extinguisher

(Reference 4). The fire extinguisher was pressurized to 500 psig as currently required. Two pressuri zations were carried out without

failure. The fire extinguisher was then pressurized until failure

occurred. At a test pressure of 850 psig, the fire extinguisher

failed by bursting a disk out of the cap, as shown in Figure 18. The

significant bulging of the fire extinguisher evident in Figure 18

occurred without any conspicuous failure of the fire extinguisher

structure. This test indicates that no unacceptable deterioration of

this particular fire extinguisher had occurred during normal service,

or during the service-simulation tests described above.

A second used-but-undamaged fire extinguisher (No. G 319542)

was then hydrostatical ly tested. This extinguisher was first tested

to 500 psig, and the pressure was released. It was then repressuri zed.

Fai-lure occurred at 700 psig by bursting the cap in a manner similar to

the extinguisher No. 701019. However, no bulging of the extinguisher

side walls was observed at this lower pressure.

V. DISCUSSION :

A. Speculation About Failure Mechanism in Dome Metal of Fire Extinguishers

Results presented in this report indicate that the extinguishers

examined are so designed, constructed and handled that interior

deterioration may occur along a circumferential path following a joint

between dome and collar. Apparently, a long time — perhaps of the

order of years — may be required for deterioration to reach a critical

state leading to failure. Information suggesting that deterioration does

indeed occur over a long period of time in a variety of fire extinguisher

types is found in the report that 22;:. of stainless steel shell extinguishers

fail the first 5-year retest, 8',^ fail the second 5-year retest, and between

15 and 40:. fail the third 5-year retest (Reference 4). It should be

noted, however, that these figures pertain to all kinds of retest

- 14 -

failures, not just failure by a circumferential tear through the dome

metal

Concerning the fire extinguisher which exploded, it had been in

service for at least 6 1/2 years, as inferred from the report that this

extinguisher had been hydrostatical ly retested in November, 1973 — about

1 1/2 years before its catastrophic failure on May 6, 1975 (Reference 1).

Thus, for this extinguisher, the 1 1/2 years between retest and failure

can be regarded as the time during which interior deterioration reached

a critical state leading to failure via a circumferential tear. However,-

a long time in service is not the only factor on which to base a prediction of fire extinguisher reliability. For example, one of the

fire-extinguishers used in the pressuri zation tests, number 701019, had

been in service for a minimum of b 1/2 years (inferred from the retest

label — Table I), yet it performed satisfactorily in the pressuri zation

test. Other factors which might be taken into account besides time in

service for predicting fire extinguisher reliability include: design,

use of dissimilar materials, manufacturing procedures, handling in. service

and frequency of discharge.

As noted in Section III, the path of the fracture in the fire

extinguisher which exploded followed the circumferential path between

dome and collar. Also, no conspicuous bulging was noted. The circumferential

tear seemed to have occurred entirely in the dome metal, and the path

seemed to follow — at least approximately — the tip of the joining

metal interior fillet which touched the dome metal. The region of the

fracture path in the exploded fire extinguisher is almost the same as the region of interior corrosion found in all the fire extinguishers

examined. Hence, it is reasonable to speculate about a corrosion process

which might occur in the dome metal that could lead to a failure without

bulging. One corrosion process which could plausibly occur in the dome

metal is intergranular corrosion of sensitized stainless steel (Reference 6).

In fact, the failure of a stainless steel dry powder fire extinguisher via

intergranular corrosion has been reported (Reference 8). It should be

noted, however, tliat in the case of the soda-acid fire extinguishers,

the actual mechanism whereby failure of dome metal occurs probably cannot

be positively identified because failed hardware was not available to

evaluate.

Some evidence of dome metal which appeared to be sensitized —

in one of the used-but-undamaged extinguishers — was presented earlier.

Sources of corrosive fluid which can contact the dome metal exist — fluid

forced into the crevice between joining metal and dome metal, and

sulfuric acid dribbling from the bottle when a fire extinguisher is

inverted. Moreover, vapor from the sulfuric acid may be continuously

present in the dome region of a charged fire extinguisher awaiting its

next use. Finally, components of the flux used in the joining operation

may also contribute to a corrosion process. In order for these

corrosive media to act on the sensitized metal, it is essential that

- 15 -

fresh metal be exposed to the corrosive media. Such exposure could occur if hard bumps or high torques applied to the dome metal ruptured

any tightly-adherent, protective films on the dome metal -- such bumps

or high torques may occur in service.

The combination of corrosive attack of the dome metal — probably

over a long period of time — and the sudden stress application in the

presence of a corrosive fluid when a fire extinguisher is pressurized,

may be sufficient to result in fracture of the dome metal. Although

the findings in this report indicate that ordinary usage of these

soda-acid fire extinguishers might lead to conditions resulting in

corrosive attack of the dome metal, direct evidence of such a process

has not been found.

In the final analysis, it must be stressed that without the

opportunity to submit the failed hardware to metal 1 ographic examination,

nothing definitive can be said about the failure mechanism in the dome

metal

B. Classification of the Fire Extinguisher Behavior

The purpose of this evaluation of soda-acid fire extinguishers

was to determine if the cause of che fife extinguisher explosion

discussed in Section II might be related to a generic problem for

soda-acid fire extinguishers. Results presented in this report indicate

that the dome end of the extinguishers examined is so designed and

constructed that deterioration may occur over a long period of

time as a result of deformation and corrosion processes. Evidence

of such deterioration is corrosion along an interior circumferential

path following a joint between the dome and collar. All four of the

fire- extinguishers which were examined exhibited such deterioration.

From this limited sampling, it may be concluded that interior

deterioration at the dome end is likely to occur in many soda-acid

fire extinguishers, A possible result of this deterioration is

catastrophic failure by a circumferential tear through the dome metal

when the extinguisher is pressurized. Indeed, the fire extinguisher

which exploded (Section III) failed this way. Moreover, analysis of

retest results indicates that a circumferential tear is an expected

type of failure during retest (Reference 4).

However, extinguishers removed from service which exhibited

conspicuous interior deterioration performed satisfactorily in

pressurization tests. Hence, the susceptibility of soda-acid fire

extinguishers to catastrophic failure is probably related to a critical combination of factors, including design., use of dissimilar

metals, manufacturing procedures, handling in service, time in

service, and frequency of discharge. Thus, the results of this

evaluation are inconclusive as regards the question of whether or

not the susceptibility of soda-acid fire extinguishers to catastrophic

failure is a generic problem.

" 16 -

VI. CONCLUSIONS:

The principal findings of this investigation are:

1. The fire extinguisher which exploded separated into two parts. It

failed by a circumferential tear in the dome metal. The path of the

tear followed what appeared to be a brazed joint between the dome

and the collar.

2. A longitudinal section through the dome and collar revealed

a rupture of joining metal in the region where the inside

fillet of joining metal runs along the dome wall. This

rupture formed a crevice between the joining metal and the

dome metal

3. Dome metal near the crevice exhibited slip lines in isolated

grains and a profusion of grain boundary precipitates.

4. Abundant evidence of corrosion was found at the dome end of

the extinguisher. Corrosion was especially conspicuous along

an interior circumferential path following a joint between the

dome and the collar. Dome metal showed the least susceptibility

to galvanic corrosion whereas joining metal showed the greatest

susceptibility to galvanic corrosion. Dome metal exhibited

very little susceptibility to crevice corrosion.

5. No evidence of cracks or intergranular penetration by dissimilar

metal was found in the dome metal near the crevice.

6. Service-type pressurization tests of a used-but-undamaged fire

extinguisher showed that an extinguisher with conspicuous

evidence of corrosion along the inside circumferential joint

between dome metal and joining metal could operate without

malfunction for twenty-five such pressuri zations . Sealing the

discharge line of this extinguisher and activation by upending

could not promote a malfunction.

7. Pressurization tests to burst of two used-but-undamaged fire

extinguishers indicated that these fire extinguishers could

withstand pressures in excess of the specified retest pressure

of 375 psi — these extinguishers withstood 850 psi in one case and 700 psi in the other, without failure of the dome metal.

Each extinguisher failed by bursting a disk from the center of the

cap.

. The following conclusions can be drawn from results of this

investigation:

1. The dome end of the soda-acid extinguishers examined are so designed and constructed that deterioration may occur over

a long period of time via deformation and corrosion processes.

- 17 -

A possible result of this deterioriation is catastrophic

failure by a circumferential tear through the dome metal when

the extinguisher is pressurized.

2. Ordinary usage of soda-acid fire extinguishers of a particular

design and construction leads to conditions which might result

in corrosive attack of the dome metal. However, direct

evidence of such attack was not found in this investigation.

3. The susceptibility of soda-acid fire extinguishers to catastrophic

failure is probably related to a critical combination of several

factors, including design, use of dissimilar metals, manufacturing

procedures, handling in service, time in service, and frequency of

discharge. Thus, the results of this evaluation are inconclusive

as regards the question of whether or not the susceptibility of

soda-acid fire extinguishers to catastrophic failure is a generic

problem.

Acknowledgements

The help of Mr. Charles Brady in the preparation of the

metallographic specimens and the many photographs in this report is

gratefully acknowledged. Assistance by Mr. L. Smith in the preparation

of the report itself is also gratefully acknowledged. Mr. T. R. Shives

provided valuable advice and consultation. Dr. J. Kruger and

Dr. J. Ambrose carried out the potential measurements — their

assistance in this matter and in other matters related to corrosion is

gratefully acknowledged. Invaluable perspective on the day-to-day usage

of soda-acid fire extinguishers was provided by Charles 0. Baker,

Chief of the NBS Fire Department and Mr. Ernest E. Helbert, of

Rockville, Maryland.

- 18 -

References

1. Epidemi logic In-Uepth Investigation Report - Case ilo. 750516 BEP 7001. Consumer Product Safety Commission,

Washington, D.C. 20207.

2. Memorandum dated July 8, 1975 for Mr. James Talentino,

Bureau of Engineering Safety, CPSC through Walter Leight,

Chief, Office of Consumer Product Safety from Bruce W. Christ,

Mechanical Properties Section, National Bureau of Standards.

3. Memorandum dated June 18, 1975 to Richard Armstrong, Director,

BES, through George Anikis, EES, from Carl 81 echschmi dt

Director, Office of Produce Defect Identification, CPSC.

4. Literature about fire extinguishers from the National Association

of Fire Equipment Distributors, Inc., Ill East Wacker Drive,

Chicago, Illinois 60601.

5. Standards for Safety, Soda-Acid Fire Extinguishers, Underwriters

Laboratories UL 7-1969.

6. Discussion on pp. 260-261 in Chapter 11, Section on "The

Weldability of Stainless Steel." Welding for Engineers , by

H. Udin, E. Funk and J. Wulff, John Wiley ^ Sons, Inc.,

New York, 1954.

7. "Brazing of Stainless Steel," p. 663 of Metals Handbook, Eighth

. Edition, volume 6, published by American Society of Metals,

Metals Park, Onio (1971).

8. . "Failure in Two Fire Extinguishers," ASME Paper No. 75-MAT-M,

- to appear in Transactions of the ASME, Journal of Engineering

Materials and Technology, by C. 0. Smith Prof, of Engineering,

Univ. of Detroit, Detroit, Michigan. Submitted, Sept., 1974.

- 19 -

FIGURE CAPTIONS

Fire Extinguisher Which Exploded - Number F 407396

1. View of the fire extinguisher which exploded. The cap and collar

assembly at lower left separated from the dome via a circumferential

tear. The acid bottle is positioned with the stopple-end up in the

wire cage attached to the cap-collar assembly when the extinguisher

is ready for service. Note the identifying number stamped on the

lower part of the label.

2a, Cap-collar assembly showing the ring of dome metal which remained

attached to the collar - arrow at left. A fin-like projection of

what is believed to be joining metal appears at the arrow - right

center.

Close up of the view in Figure 2a. The hole in the cap facilitates

venting during filling for hydrotesting.

Inside view of the cap-collar assembly, showing the collar and some of the fracture surface of the dome metal . Note

the white substance, which is probably sodium bicarbonate.

2do View of the cap-collar assembly almost opposite that shov;n in

Figure 2c, showing more of the fracture surface of the dome

metal

3. Close-up view of the cap-collar assembly showing dome metal and

collar metal. The dome m.etal disappears toward the top horizon.

Part of the fracture path through the dome metal is shown in

elevation and runs horizontally from arrow to arrow. The collar

metal is shown in elevation and runs horizontally across the

lowermost portion of the picture.

4a, Close-up view of the circumferential tear.

4b. Close-up view of the circumferential tear.

4Co Close-up view of the circumferential tear.

Fire Extinguisher Which Was Sectioned - Number F 700745

5a. Dome removed from fire extinguisher number F 700745.

5b. • Longitudinal section through dome wall showing collar (A),

joining metal (B and C), dome wall (D) and joint between dome

wall and shell wall (E).

2b.

2c o

- 20 -

6. Inside view of dome removed from fire extinguisher number F 700745.

7a. Full circumferential view of inside of dome. Note the circumferential

step or crack which shows up between the joining metal and the dome

metal

7b. Close-up of a section from Figure 7a near the strainer.

7c. Full circumferential view of inside of dome, showing the reddishorange-brown coloring of stains near the circumferential step or crack shown in Figures 7a and 7b (color-slide No. 18).

8a.- Close-up view of the region where the dome metal joins the collar

metal. The interface between the wall of dome metal and the

collar metal shows up as the horizontal line running left from A

(as-polished).

8b. Close-up view of the region where the dome metal joins the collar

metal. The joining metal shows up clearly between the dome metal

and the collar metal due to etching. The outside fillet of

joining metal is in the region labeled C, and the inside fillet

curves between A and D.

9a. End of the dome metal under the outside fillet of joining metal.

Note severely distorted grains at end indicating localized

distortion.

9b. Close-up of the region of localized deformation in Figure 9a.

Note transition from severely distorted grains at left to

apparently undistorted grains at right.

10a, Rupture of the inside fillet of joining metal noted at A in

Figure 3. Dome metal appears acorss the top (as polished).

10b, Close-up of the ruptured joining metal shown in Figure 10a.

Note corrosion product coating the surface of the dome metal

and the surfaces of the joining metal (as polished).

lOCo Rupture of the joining metal shown in Figure 10a. Note the

film of corrosion product and the incomplete closure of the

rupture (as polished).

11. Close-up of the crevice between dome metal (top) and joining

metal (bottom) at some point between A and B in Figure 8.

Note the good matching of the profiles of the dome metal and

the joining metal at the arrows.

12a, Close-up of the two pits at B in Figure 8. Dome metal is at

the top and joining metal is at the bottom.

- 21 -

12b. Appearance of the crevice between dome metal (top) and joining

metal (bottom) to the right of the pits in Figure 12a.

12c. Region to the left of the pits in Figure 12a, showing tightly

bonded interface between the dome metal (top) and joining

metal (bottom).

13a. Grain structure of the dome metal near the crevice at some

point between A and B in Figure 8.

13b. Close-up of some grains appearing just below the center of

Figure 13a. The feature at A traversing an entire grain is

probably an annealing twin, and the features at B -- which

appear in several grains -- are probably slip lines.

14a, Grain structure of the dome metal at some point between A and B in Figure 8. An abundance of grain boundary precipitates lead

to the darkened and dotted appearance of grain boundaries.

14b. Close-up of grain boundaries in the dome metal near- the crevice,

showing evidence of grain boundary precipitates. Note that

the film of corrosion product exhibits a slight tendency to

penetrate some of the grain boundaries.

15. A section of the collar metal near D in Figure 8, showing

surface film of corrosion product.

16. A section of the joining metal near C in Figure 8, showing

film of corrosion product.

17.- Upper region of lap joint between dome metal and shell metal

shown at E in Figure 5b. Irregular surfaces suggest corrosive

attack, possibly from corrosive fluid retained in the crevice

formed by the lap joint.

Pressurization Tests On Used-But-Undamaged Fire Extinguishers

18, Fire extinguisher number F 701019 pressurized to burst. The

extinguisher bulged si.gnificantly, but remained structurally

intact. Failure occurred at 850 psig when the center section

of the cap blew out.

- 22 -

TABLE I

FIRE EXTINGUISHER RETEST DATA

OBTAINED FROM RETEST LABELS

Number

F 700745

F 701019

G 319542

Date of

Last Retest

May, 1973

November, 1973

No Retest Label was

found

Retest

Pressure (psi)

375

375

- 23 -

TABLE II

ELECTRODE POTENTIALS FOR DOME METAL,

JOINING METAL AND COLLAR METAL

(POTENTIAL AFTER 5 MINUTES VERSUS SATURATED CALOMEL ELECTRODE)

Metal

Solution Dome Collar Joining

Sodium Bicarbonate -0.120 volts -0.180 volts -0.440 volts

. Sodium Bicarbonate,

plus 3 cc Cone.

H2SO4

-0.145 -0.295

———

-0.470

Sodium Bicarbonate,

pi us • 6 cc Cone

H2SO4

-0.150 -0.334 1 -0.514

1

i .

i

1

- 24 -

Figure 1. View of the fire extinguisher which exploded. The cap

and collar assembly at lower left separated from the

dome via a circumferential tear. The acid bottle is

positioned with the stopple-end up in the wire cage

attached to the cap-collar assembly when the extinguisher

is ready for service. Note the identifying number

stamped on the lower part of the label. X 1/3

Cap-collar assembly showing the ring of dome metal

which remained attached to the collar - arrow at

left. A fin-like projection of what is believed

to be joining metal appears at the arrow - right

center. X 1

Figure 2b. Close up of the view in Figure 2a. The hole in

the cap facilitates venting during filling for

hydrotesting . X 1 1/4
I
Figure 2c. Inside view of the cap-collar assembly, showing

the collar and some of the fracture surface of

the dome metal. Note the white substance, which

is probably sodium bicarbonate. X 1 1/4

Figure 2d. View of the cap-collar assembly almost opposite

that shown in Figure 2c, showing more of the

fracture of the dome metal. X 1 1/4

1—1 Qj

fO 0 1—

jj 1) frt (-1 P fl) U/ ni rn (X' UJ

t , _C P >> S-H r-( r-> J 1 U/ h h u

^ w C H •H M ^ 10 'Cj U-i

frt rr-t

f)\ • UJ c>-» nw fii Vll H JJ Li •4-'

1 1 1 /I \ r! t-i r" K,. 1 \ f~l u Cc

vj lu ' to •H C >—

*^ O . Pi rr( (1^ M-) 'u U/* U —

rH Cll ^-1 4-1 rH •H -H M e

e w +j 0 }H 0) -rH N C 0 0) w -H -H Pa C

0

fC iH to 0 C -p

CO w

^4 -M 0 0 rH

fd <D 0) w x: 6 fd -H g M c W u P

rH p ;3 0 (D -M ^^ w ^ (U O g U -H 0

1 0 fd ^ H C

0

fd m n3 fd Q) -H

x: (u c 0) +J -H

CU Eh 4:: 0 g >

+J -H W -H

-t-1 +J >^ w

• M-i fd (d o

4-1 M o > H Sh q; 0 fd (D rH u H

-p -P tH o fd fd ^ Q) U d) O O

0) g fd >i to •H fXi C 0) iH > 5h -H 4:! H iH

fd Eh fd rH

^3 M G ^ eg 1 0 0 0 • o w

CU O N ;G ^ N

W -H W O -H (U O ^ M >H C

rH C O W >H O O

u fd ^ -H fd ^ ^

u

p

tn •H

Cm

Figure 4a. Close-up view of the circumferential tear. X 1 1/4

Figure 4b. Close-up view of the circumferential tear. X 1 1/4
I

I

I

I

I


gure 5a. Dome removed from fire extinguisher

number F 700745. X 3/4

CN \ < c rH —' 0)

(U rH

i 1 M >

fO -P X

iH Q) .H ^

0

O -Pc

C 0

•H -r-i 0 "T! ^ C

CO nj

iH ^

rH Q

fd —

rH g fd 0 15

Q) ^ g

tJ« 0 w

0

>-( - H

x: H

-P u fd

0 c H

•H fd H

-p CD u PQ

0) ^ CO w

iH T3

-H fd

fd 4-) fd

c: (u •H e nH

iH P Cn fd -P C 15 •H -H

<D C -H g

0 0 0

h:1 -r-i

•

LD

u

tJi H

P4

Figure 6. Inside view of dome removed from fire

extinguisher number F 700745. X 3/4

Figure 7a. Full circumferential view of inside of dome.

Note the circumferential step or crack which

shows up between the joining metal and the

dome metal. X 1 1/4

Figure 7b. Close-up of a section from Figure 7a

near the strainer. X 2 1/2

\ 1

Figure 7c. Full circumferential view of inside of dome,

showing the reddish-orange-brown coloring of

stains near the circumferential step or crack

shown in Figures 7a and 7b. X 3/4

As Polished X 10

Figure 8a. Close-up view of the region where the dome metal joins

the collar metal. The interface between the wall of

dome metal and the collar metal shows up as the

horizontal line running left from A.

Etchant: Glyceregia X 10

Figure 8b. Close-up view of the region where the dome metal joins

the collar metal. The joining metal shows up clearly

between the dome metal and the collar metal due to

etching. The outside fillet of joining metal is in the

region labeled C, and the inside fillet curves between

A and D.
I
Etchant: Chromic Acid X 50

Figure 9a. End of the dome metal under the outside fillet of

joining metal. Note severely distorted grains at

end indicating localized distortion.

Etchant: Chromic Acid X 100

Figure 9b. Close-up of the region of localized deformation in

Figure 9a. Note transition from severely distorted

grains at left to apparently undistorted grains at

right.

As Polished

Figure 10. a. Rupture of the inside fillet of joining metal

noted at A in Figure 8. Dome metal appears

across the top. X 100

b. Close-up of the ruptured joining metal shown in

Figure 10a. Note corrosion product coating the

surface of the dome metal and the surfaces of

the joining metal. X 200

c. Rupture of the joining metal shown in Figure 10a.

Note the film of corrosion product and the

incomplete closure of the rupture. X 500



r n. . : 1

As Polished X 500

Figure 11. Close-up of the crevice between dome metal (top)

and joining metal (bottom) at some point between

A and B in Figure 8. Note the good matching of

the profiles of the dome metal and the joining

metal at the arrows.

Close-up of the two pits at B in Figure 8.

Dome metal is at the top and joining metal

is at the bottom. X 100

Appearance of the crevice between dome metal

(top) and joining metal (bottom) to the

right of the pits in Figure 12a. X 200

Region to the left of the pits in Figure 12a,

showing tightly bonded interface between the

dome metal (top) and joining metal (bottom)

X200



/

10% Oxalic Acid-Electrolytic X 500

Figure 13a. Grain structure of the dome metal near the crevice

at some point between A and B in Figure 8.

10% Oxalic Acid-Electrolytic X 1200

Figure 13b. Close-up of some grains appearing just below the center

of Figure 13a. The feature at A traversing an entire

grain is probably an annealing twin, and the features

at B which appear in several grains are

probably slip lines.

10% Oxalic Acid-Electrolytic X 500

Figure 14a. Grain structure of the dome metal at some point between

A and B in Figure 8. An abundance of grain boundaryprecipitates lead to the darkened and dotted appearance

of grain boundaries.

10% Oxalic Acid-Electrolytic X 1000

Figure 14b. Close-up of grain boundaries in the dome metal near the

crevice, showing evidence of grain boundary precipitates.

Note that the film of corrosion product exhibits a slight

tendency to penetrate some of the grain boundaries.

As Polished X 100

Figure 15. A section of the collar metal near D in Figure 8,

showing surface film of corrosion product.

As Polished X 100

Figure 16. A section of the joining metal near C in Figure 8,

showing film of corrosion product.

As Polished X 500

Figure 17. Upper region of lap joint between dome metal and shell

metal shown at E in Figure 5b. Irregular surfaces

suggest corrosive attack, possibly from corrosive fluid

retained in the crevice formed by the lap joint.

Figure 18. Fire extinguisher number F 701019 pressurized to

burst. The extinguisher bulged significantly, but

remained structurally intact. Failure occurred at

850 psig when the center section of the cap blew

out. X 1/3

NBS-IMA (REV. 7-73)

U.S. DEPT. OF COMM.

RIRI mnPAPHIf DATA

SHEET

1. PlJUl.K ATION OR REPORT NO.

NBSIR 75-922

2. Gov't Accession

LN O.

3. Recipient's Accession No.

4. TITLE AND SUHTITLH

SAFETY EVALUATION OF SODA-ACID FIRE EXTINGUISHERS

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Bruce W. Christ and John H. Smith

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 4401411

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Bureau of Engineering Safety

Consumer Product Safety Commission

Washington, D.C. 20207

13. Type of Report & Period Covered p^il^^e

Analysis Report

1 4 • Sponsoring Agency Clocic

15. SUPPLEMENTARY NOTES

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

bibliography or literature survey, mention it here.)

A safety evaluation of soda-acid fire extinguishers was carried

out. One extinguisher which exploded was photographed at low magnification, and three extinguishers rem.oved from service were evaluated by

metallographic examination and by pressurization tests. The objective

of these evaluations was to determine if the susceptibility of soda-acid

fire extinguishers to catastrophic failure during use were a generic

problem. Results indicated that deterioriation at the dome end is

likely to occur in many soda-acid fire extinguishers. Susceptibility to

such deterioriation seems attributable to a combination of several

factors, including design, use of dissimilar metals, manufacturing

practice, handling in service, time in service and frequency of discharge.

However, even extinguishers removed from service which exhibited

conspicuous interior deterioriation performed satisfactorily in

pressurization tests. Hence, the results of this evaluation are

inconclusive as regards the question of whether or not the susceptibility

of soda-acid fire extinguishers to catastrophic failure is a generic

problem.

17. KEY WORDS fsix to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Circumferential tear; fire extinguisher; metallographic examination;

pressurization tests; safety evaluation; soda-acid

18. AVAILABILITY Unlimited

1 X For Official Distribution. Do Not Release to NTIS

1

^ Order From Sup. of Doc, U.S. Government Printing Office WashinEton. D.C, 20402. SD Cat. No. CH

1

' Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

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UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74