NBSIR 75-669

J= R. Ambrose and J= Kruger

Corrosion and Electrodeposition Section

M'Jtal lurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D, C. 20234

February 1975

Failure Analysis Report

Prepared for Office of Consumer Product Safety

Walter G. Leight, Chief

National Bureau of Standards

Washington, D. C. 20234

NBSIR 75-669

FAILURE OF FLEXIBLE BRASS GAS

APPLIANCE CONNECTORS

J= R= Ambrose and J, Kruger

Corrosion and Electrodeposition Section

Metallurgy Division

Institute for Materials Research

National Bureau of Standards

Washington, D= C. 20234

February 1975

Failure Analysis Report

Prepared for Office of Consumer Product Safety

Walter G. Leight, Chief

National Bureau of Standards

Washington, D.C. 20234

"•t.u «•

U. S. DEPARTMENT OF COMMERCE. Fredefick B. Dent. Seaetary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Diroctor

Failure of Flexible Brass Gas Appliance Connectors

BACKGROUND

A written request was received from the U. S. Consumer Product Safety

Commission (CPSC) by the Office of Consumer Product Safety at the National

Bureau of Standards to analyze the failure of two flexible brass tubing

samples provided by Bureau of Engineering Sciences under the 15 (b) Project

(Fig. 1). According to the CPSC report which accompanied the request, the

brass tubing was used as a flexible gas appliance connector and had failed

within 3 to 6 weeks following installation. At the time of installation,

proper sealing of the tubing couplings was verified by application of a

blue liquid leak detector to the couplings and flexible tubing at either

end of the connector. The report continued on to state that severe corrosion

and rupture of the tubing connectors .by cracks penetrating the tubing walls

occurred at those extremities of the connectors where the leak detector

would presumably have been applied. An independent analysis of the leak

detector solution using gas chromatographic analysis was obtained by CPSC

which reported the solution to contain a significant concentration of free

ammonia

.

A one gallon plastic container approximately three-fourth full of a

clear blue liquid accompanied the two sections of tubing, and it was further

requested that an opinion be given as to whether this solution could have

been responsible for the reported corrosion of the brass tubing.

VISUAL EXAMINATION

Visual examination of the specimens revealed discoloration and blue

and white residue in the area located near the extremities' of the tubing

sections, where couplings were attached (Fig. 2a). In addition to this.

a large number of cracks had penetrated the tubing, and, since the CPSC

report had pointed out the presence of ammonia in the leak detector solution,

it was suspected that subsequent failure resulted from what is commonly

called stress corrosion cracking (Fig. 2b). The residue appeared bluishgreen with rather large amounts of a crystalline white material as well

(Fig. 3).

'

INVESTIGATION AND RESULTS

Analysis of Metal Composition

A 10 cm section of tubing was forwarded to the Chicago Spectro Service

Laboratory for the metal composition analysis. Their results show the brass

to be within the specifications of what is commonly called Admiralty Metal

(see Table I) . The specifications and compositional maxima and minima for

Admiralty Metal are given in Table II (1) . The Laboratory report is

reproduced in Appendix A.

Metallographic Examination

Optical micrographs were made of a specimen which was sectioned from

the flexible tubing in an area near one of the major fracture sites.

Cracks can be observed throughout the body of the material and are highly

branched (Fig. 4). When the specimen surface was chemically etched two

important observations can be made: (1) the metal microstructure is of

rather large grain size, and (2) the cracks are largely transgranular in

nature, propagating through grains rather than through grain boundaries

(Fig. 5). •

•

Residue Identification

X-ray diffraction patterns were obtained from samples of the two types

of residue, and attempts were made to determine their chemical composition.

The blue material gave diffraction patterns which could not be

unambiguously identified. However, a large number of the diffraction

lines characteristic of tetramine cupric sulfate (Cu(NH„) . SO, ) were found.

J 4 4 This particular compound would be expected to be present as a corrosion

product when copper reacts with ammonia.

The white product also gave a large number of diffraction lines which

could not be completely resolved. Certain similarities were observed

between these patterns and certain organic compounds (e.g. stearates) to

indicate the presence of an organic compound. The white product on the

tubing foamed when distilled water was sprayed on the surface and gave an

indication of alkaline pH when tested with pH indicator paper. Although

not absolutely conclusive, these tests point to the probability that the

white material may be some type of detergent.

Leak Detector Solution Analysis

The seal was broken on the plastic bottle containing the blue leak

detector solution; a strong odor characteristic of ammonia was iimnediately

noticed upon opening the bottle. Two 100 ml aliquots were withdrawn. One

was reserved for our use in an exposure test and in a pH measurement. The

pH was measured with a conventional glass electrode which gave a reading

of 10.0. The other aliquot was sent to the Sadtler Research Laboratories

for a total nitrogen and an ammonia nitrogen analysis. Results are given

in Table III and show a high total nitrogen content as well as a significant

concentration of ammonia nitrogen. The laboratory report is reproduced in

Appendix B.

"

- 3 -

* t

The analytical technique used here gave approximately a 10 time higher

concentration of ammonia than did the CPSC gas chromatography analysis.

The Sadtler Laboratory procedure as given in Appendix B would give an

estimate of the total available nitrogen as ammonia in the solution. As

the pH of the solution is increased (by MgO addition) , the equilibrium

between NH^"^ and NH^ would be shifted to the right (favoring NH^ liberation):

NH^"*" + oh" = NH^ + H^O . (1)

DISCUSSION

Most brasses, and this includes Admiralty Metal, are susceptible to

stress corrosion cracking in the presence of as little as trace amounts of

ammonia in the atmosphere, providing four ingredients are present (2):

(a) ammonia or an ammonia derivative,

(b) water or water vapor,

(c) oxygen and

(d) stress.

The analysis of the leak detector solution reveals the presence of

ammonia; water vapor and oxygen are always present in the atmosphere. The

presence of residual stress, probably arising from fabrication of the tubing

or from flexing during its installation, was verified through the mercurous

nitrate test (3) (see Fig. 6).

A comparison of the optical micrographs taken of cracks in this

material (Fig. 5) with those observed for stress corrosion cracking of 70-30

brass in non- tarnishing aqueous ammonia solution show the morphology of

cracking to be quite similar (Fig. 7, from (A)). Furthermore, the particular

heat treatment of this brass which resulted in the large grain size (Fig. 5)

would be expected to cause an increase in susceptibility to stress corrosion

cracking (5)

.

Finally, a test specimen was cut from a section of the brass tubing,

bent once to simulate treatment during installation of the coupling unit,

dipped into a sample of the leak detector solution, then placed into a

sealed glass container. The test exposure was allowed to proceed for two

weeks, after which the specimen was removed, sectioned, and mounted for

metallographic examination. Although the extent of crack propagation is

less than that in the failed specimens, the initial stages of cracking are

evident in Fig. 8. It should be remembered that service failures were

reported to have taken from 3 to 6 weeks to occur, while the short term

laboratory test was all that was possible, within the time limitations set

to prepare this report. »

CONCLUSIONS

From the results of the failure analysis performed on the brass tubing

supplied, from the chemical analysis of the leak detector solution, and from

the simulated exposure test in this laboratory, it is concluded that failure

resulted from the stress corrosion cracking of flexible Admiralty Metal

tubing due to the combined effects of residual stress and ammonia, oxygen

and water vapor.

- 5 -

BIBLIOGRAPHY

1. Metals Handbook, T. Lyman, Editor, p. 920 (American Society for Metals,

Novelty, Ohio, I960).

2. D. H. Thompson, Stress Corrosion Cracking of Metals - A State of the

Art, STP 518, p. 39 (American Society for Testing and Materials,

Philadelphia, 1972).

3. 1974 Annual Book of ASTM Standards, Part 6, Designation B 154-73,

p. 278 (American Society for Testing and Materials, Philadelphia,

1974).

A. E. N. Pugh, J. V. Craig, and A. J. Sedriks, Proc. of Conf. on Fund.

Aspects of Stress Corr. Cracking, OSU, Columbus, Ohio, p. 135

(National Assoc. of Corr. Engineers, Houston, 1969).

5. G. Edmunds, Symposium on Stress Corr. Cracking of Metals, p. 67

(ASTM-AIME, 1945).

TABLE I

Copper

Tin

Zinc

Iron

Others Total

71.03%

0.90

28.00

0.01

0.10

TABLE II

MiwMfttafli Minimum

* Copper • 70.00% 73.00%

Tin 0.75 1.20

Iron 0.06

Lead ' 0.075

Others Total 0.10

Zinc remainder

Ammonia N

Total N

TABLE III

1.175% by weight

7.26% by weight

Appendix A

Research Laboratories, Inc.

Subsidiary ot Block Engineering. Inc.

3316 Spring Garden Street Philadelphia. Pa. 19W4

Area Code: 215/382-7800

TWX 710-670-1186 Cable SADTLABS

February 18, 1975

REPORT OF ANALYSIS

SAMPLE:

RECEIVED:

LABORATORY NO:

SUBJECT:

REQUEST:

results:

COMMENT:

One ( 1 )

February 3, 1975

80792

FROM;

ATTN:

U. S. Department of Commerce

National Bureau of Standards

-.Washington, D.C. 20234

Dr. J, R. Ambrose

B243 BIdg. 223

One ( 1 ) sample of "Leak Test Solution"

Determination of ammonia and total nitrogen content

Ammonia nitrogen as N, % by wt ' 1.19 (1.16)

Total nitrogen as N, % by vA. 7.31 (7.21

)

The above reported results were corrected for reagent blank

determinations. The analyses were conducted in duplicate.

Ammonia nitrogen was determined by distillation of weighed

portions of sample in the presence of magnesium oxide. The

distillates were recovered in boric acid and titrated to methyl

red end point with standardized hydrochloric acid.

Total nitrogen was determined by the Kjeldahl digestion

method using the distillation procedure described above.

Guenter Niessen

Research Chemist

GN:bs

SERVING SCIENCE SINCE 1874

C^I€AGO SPisCraO SERVKE l^BOnmO^Y^ m€.

ylf' Spectrographic and Chemical Analysts

Metallurgists

4848 S. KEDZIE AVE. • CHICAGO, ILL. 60632

ALYSIS REPORT FOR: AREA CODE 312 • 523-7088

U.S. Department of Commerce

National Bureau of Standards purchase order no. Washington, D. C. 20 2 34

DATE

Attention: Mr. John R. Ambrose, Corrosion and Feb. 5, 1975

Electrodeposition Section

Report No. 6363

Sample: Flexible Brass Tubing

Copper 71..03%

Tin 0..90

Zinc 28, 00

Iron 0. 01

Others Total <. 10

Fig. 1 Photograph o£ the two flexible brass samples.

Close-up view of an end of one tubing, showing

where the corrosion attack was located.

Close-up photograph of an end of a tubing

sample, showing the severe cracking of the

brass

.

Fig. 3 Close-up photograph o£ the corrosion product

on the brass tubing, showing both the white

and the bluish- green materials.

Fig. 4 Optical Micrograph (x200) o£ a cross section

of the flexible, brass tubing, showing the

branched nature of the cracks.

Fig. 5 Optical Micrograph (x200) o£ a cross section

of the flexible brass tubing, etched in 1 part

water (H^O) , 1 part hydrogen peroxide (}^2'^2^ ' and 1 part concentrated ammonium hydroxide

(NH^OH) , showing the metal micros tructure. Cracks

are predominantly transgranular.

Scanning Electron Micrograph (x90) showing

cracks in a flexible brass tubing specimen

after exposure in the mercurous nitrate test.

Cracks indicate presence of residual stress.

Fig. 7 Optical Micrograpli showing stress corrosion

cracks in 70-30 brass exposed to aqueous

ammonia solution (from (3)).

Fig. 8 Optical Micrograph (x500) of the cross section

o£ a 5 cm specimen of brass tubing exposed

for t\,<o weeks to the leak detector solution,

then etched as in Fig. 5, showing initiation

of cracking at the metal surface.