Failure Analysis of Hydrogen Leak in Copper Tube
Summary
A leaking copper tube assembly from a hydrogen supply line that caught fire was
evaluated to determine the cause of failure. The leak path was determined to result
from a fracture in the tube-to-fitting brazed joint that likely occurred during original
fabrication of the tube assembly. The leak pathway exhibited characteristics consistent
with movement of the brazed joint while the braze filler material was still partially liquid.
Several brazing and soldering alloys were found associated with the failed tube leak,
and they likely contributed to a deviation from the normal filler metal solidification
characteristics. Repeated paint applications are judged to have obstructed the leak,
allowing it to go unnoticed.
Results and Discussion
Figure 1 shows an overall view of the submitted copper tube assembly that reportedly
exhibited a hydrogen leak. Initial visual examinations identified charred and spalled
paint while locating the suspected leak site. Tube displacement and deformation was
also observed, along with fractured paint at the adjacent tee fitting. Compressed air
and high-pressure helium (150 psig) were initially used unsuccessfully to locate the
exact leak location. Figure 2 shows a higher-magnification view of the leak location
where a visual crack indication was found at the adjacent brazed joint. After cleaning
with a commercial paint remover, a crack was found that extended approximately
35% around the brazed joint circumference. Also shown are the approximate
locations for subsequent destructive metallographic serial grind planes through the
brazed joint. Figure 3 shows a close-up view of the crack mating faces that would
seem to indicate a solid state fracture mechanism. Slight traces of red paint are
present on the fracture surfaces. Figure 4 shows the presence of white paint deep
inside the brazed joint, indicating that the crack was painted over, possibly several
times, after it initially formed. Figure 5 shows five distinct paint layers on the tube,
indicating that it was painted multiple times.
Destructive evaluations consisted of split tube, metallographic, and scanning electron
microscope (SEM) evaluations in an effort to determine the leak path and cause of
failure. Figure 6 shows the split tube specimens that exhibited no evidence of
corrosion attack on any of the tube ID surfaces or at the brazed joints. ID deposits
extracted from the split tube specimens were evaluated in the SEM using energy
dispersive X-ray spectrometry (EDS). Figure 7 is an EDS spectrum from a deposit at
the inside of the brazed joint that shows the presence of organic and fluoride material
that is likely residual flux. Figure 8 is an EDS spectrum from a small localized ID
deposit showing the presence of lead and tin along with zinc and chlorine, indicating
that solder was used at one time. Figure 9 shows a low-magnification SEM image of
the Figure 3 fracture surface, where it is apparent that the fracture extends for some
distance along the joint. Figures 10 and 11 show sequentially higher magnification
SEM images of the fracture surface, along with an EDS spectrum showing the
elemental makeup of the braze material and residual debris. These images show no
brittle (cleavage) or ductile fracture features (dimples from micro-void coalescence)
expected for a solid-state fracture and suggest that the fracture is likely a hot tear that
occurred when strains were imposed during filler metal solidification. The elements
detected on the fracture surface consist primarily of elevated chlorine, copper,
phosphorous, silver, and oxygen, with typical dirt debris.
Figure 12 is an EDS spectrum of the braze filler material taken from the OD surface that
shows it is a silver–copper–zinc alloy (possibly 65Ag-20Cu-15Zn with a melting point of
1240-1315oF). This filler metal typically requires the use of a flux. Figure 13 is an
EDS spectrum of the braze filler material taken from serial grind plane 0 that shows it is
a silver–copper–phosphorous alloy (possibly 15Ag-80Cu-5P with a melting point of
1191–1290oF). This filler metal is a self-fluxing type that does not require the use of
fluxes for brazing copper. It is apparent that at least two different filler metals were
used, suggesting that there was an attempt to repair the joint.
Figure 2 shows the approximate planes for metallographic serial grinds through the joint,
where a 10-mil-thick diamond wafering saw was used to cut the first section immediately
adjacent to the fracture (Plane 0). Figures 14 and 15 show small (approximately 100
micron) pinholes present in the braze filler metal that communicated with the fractured
joint. Figure 16 shows that subsequent planes at approximately 25-mil steps reveal a
continuous leak pathway through the joint. The serial grinds were discontinued at plane
5, when the brazed joint exhibited less than 50% filler metal. All defects at the brazed
joints exhibited rounded edges consistent with formation from the liquid state. No
corrosion attack was observed at any of the planes evaluated.
Corrosion was ruled out as being involved with the leak, based on the absence of any
detectable corrosion attack at the tube ID or at brazed joints. Available evidence
strongly suggests that a bending moment was applied to the brazed joint during initial
assembly when the filler metal was still molten, which resulted in a continuous leak
pathway through the joint. Two different braze alloys were identified at the leaking joint,
and residual lead–tin solder was found on the tube ID surface. These observations
indicate that he tube section in question may have been recycled from another soldered
tube assembly, and that there were problems with the initial brazing operation that may
have included an attempt at repair. It is surmised the initial braze attempt that utilized a
self-fluxing, lower-melting-point filler metal (possibly 15Ag-80Cu-5P with a melting point
of 1191–1290oF) may have remelted when a repair was attempted using a highermelting-point filler metal (possibly 65Ag-20Cu-15Zn with a melting point of 12401315oF). Also, mixing of the two filler metals when molten may have resulted in an
even lower-melting alloy that exhibited significantly different solidification characteristics
that could have gone unnoticed by the operator. Given the small tortuous leak pathway,
it is unlikely that a water pressure test (typically utilized to evaluate plumbing) would
have identified the leak.
The unsuccessful initial attempts to identify the leak pathway using compressed air and
helium are attributed to either paint or flux resealing the small leak as a result of the fire.
Paint applied to the fractured brazed joint likely initially sealed the leak, but due to the
high permeability of hydrogen coupled with the relatively small leak pathway, the leak
likely redeveloped and continued for some time unnoticed. The leak was probably only
discovered after exposure to an ignition source.
It is recommend to perform a thorough visual examination of all solder joints in the
affected line to determine if other joints exhibit deformation suggesting they were moved
(bent) while the braze was molten during fabrication. Also recommended is to use new
unused tubing and fittings when fabricating critical service assemblies such as hydrogen
lines.
Conclusions


The hydrogen leak and fire resulted from a leak pathway through a brazed tubeto-fitting joint in the copper tube assembly. Corrosion was ruled out as a cause
of the leak.
The leak pathway likely resulted from bending of the joint during the original
assembly fabrication before the filler metal fully solidified, and may have existed
undetected for an extended period of time.
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Figure 1
Overall view of the failed tube assembly (top) with a close-up view of the
leak location (bottom). Note tube deformation I displacement at braze
joints adjacent to leak location.
Plane 0 1 2 3 4 5
Figure 2
Close up of leak location before paint removal (top) and after paint
removal (bottom). Red lines represent metallographic serial grind
analysis planes through joint at approximately 0.025-inch steps.
Figure 3
Figure 4
Close-up view showing the brazed joint fracture surface after paint removal.
Note traces of red paint on the crack surface.
45x
High-magnification view showing the presence of white paint deep inside
the brazed joint.
15x
Figure 5
Cut in paint exposing 5 distinct paint layers.
Figure 6
Split tube specimens showing no evidence of either uniform or localized
corrosion attack on any of the copper tube ID surfaces.
Figure 7
EDS spectrum showing ID deposit contains fluoride + organic-based
material that is likely residual flux.
Figure 8
EDS spectrum showing the presence of lead and tin along with zinc and
chlorine on the copper tube ID surface, indicating that conventional
solder was used at one time.
Figure 9
16x
Low-magnification SEM image of the brazed joint OD fracture.
Figure 10
373x
Intermediate-magnification SEM image of the fracture surface.
1270x
Figure 11
High magnification SEM image and EDS spectrum from the fracture
surface.
Figure 12
EDS spectrum showing braze filler material at the OD surface is a silver–
copper–zinc alloy (possibly 65Ag-20Cu-15Zn with a melting point of 12401315oF).
Figure 13
EDS spectrum showing the braze filler material taken from serial grind
plane 0 is a silver–copper–phosphorous alloy (possibly Ag-80Cu-5P with a
melting point of 1191–1290oF).
Figure 14
10x
Low-magnification view of plane 0 cut surface showing pinholes through
the braze that communicate with the fractured joint seen in Figure 3.
Figure 15
143x
High-magnification SEM image showing the pinholes are approximately
100 microns (4 mils) in diameter.
Plane 1
10x
Plane 2
10x
Plane 3
10x
Plane 4
10x
Figure 16
Serial grind planes showing a continuous leak pathway through the brazed
joint. No corrosion attack was observed at any of the planes evaluated.