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 anEDS 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 of1191–1290oF). 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 leakpathway 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 higher-melting-point filler metal (possibly 65Ag-20Cu-15Zn with a melting point of 1240-1315oF). 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 tube-to-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 Close-up view showing the brazed joint fracture surface after paint removal.

Note traces of red paint on the crack surface.

45xFigure 4 High-magnification view showing the presence of white paint deep inside

the brazed joint.

Figure 5 Cut in paint exposing 5 distinct paint layers.15x

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.

16xFigure 9 Low-magnification SEM image of the brazed joint OD fracture.

373xFigure 10 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 1240-1315oF).

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).

10xFigure 14 Low-magnification view of plane 0 cut surface showing pinholes

through the braze that communicate with the fractured joint seen in Figure 3.

143xFigure 15 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.