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Failure Analysis Case Histories
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Stress Corrosion Cracking of Unused Copper Tube from Air Conditioner Coil
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ENVIRONMENT:
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EQUIPMENT:
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Air Conditioner Unit |
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MATERIAL: |
Copper |
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SERVICE TIME: |
Never Used |
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FAILURE MODE:
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Stress Corrosion Cracking |
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Background
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The received section was approximately 5-1/2-inches long
by 2-3/4-inches wide by 4-inches in height (Figure 1). The section consisted of
U-shaped copper coils, presumably fabricated of UNS C12200 DHP copper, that
passed through a galvanized steel tube sheet and numerous thin corrugated
aluminum radiator fins. The copper tubing had an outer diameter (OD) of
approximately 0.386-inches, a wall thickness of approximately 0.011-inches, and
possessed internal rifling that ran parallel to the longitudinal axis of the
tubing.
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According to information provided by the manufacture,
this coil, from which the supplied section was taken, had been shipped to the
Middle East. The coil had never been used but was found to have leaks in the
copper tubes at the tube sheet. The coil was returned to the US, tested in a
water dip tank, and the leak locations marked. One of the separate copper tube
hairpins was similarly marked with an arrow indicating a leak at the same
location as the tubes on the larger section.
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Findings
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The coil section and two hairpins were examined in the
as-received condition at the indicated leak sites with an optical microscope at
up to 40X magnification. As previously mentioned, the marked leak sites were on
the copper tube as it emerged from the tube sheet, at the start of the U-bend.
Black and white deposits were noted on all the tubes in the general area of the
leak sites, but no obvious leak sites were observed.
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| Figure 1. As-received copper coil section. Arrows indicate
leak sites found. |
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One of the copper U-bends marked as possessing two leak
sites was carefully removed from the coil section. One end of the U-bend was
crimped and soldered shut, and a valve was clamped to the other end.
The U-bend was leak tested by pressurizing it to 10 psig with air and holding it
under water. Two very small leak sites, revealed by air bubble streams from the
tube surface, were observed approximately 180°
apart around the tube outer diameter (OD) surface on one leg of the U-bend, one
site being in one of the indicated areas. No leaks were observed at the
other marked leak location.
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The pressurized tube was again examined at up to 40X
magnification with an optical microscope. Residual water on the tube surface
continued to bubble during the examination, which aided in pinpointing the leak
sites. A very faint crack in the copper tube was observed at one leak site.
The other leak site, 180° around the
tube, appeared to be associated with the black deposit on the copper tubing
mentioned earlier, but no obvious hole or crack was observed.
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| An approximately 1-inch length containing the two leak
sites was cut from the examined tube. This length was then split
longitudinally. An iridescent blue discoloration was observed on the tube inner
diameter (ID) surface at both leak sites, as well as an obvious crack at one
leak site (Figure 2). |
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Figure 2. Blue discoloration on tube ID surface at leak
site. Yellow arrow indicates crack.
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| In order to determine if failure modes other than
cracking were producing the tube leaks; the leak site without the obvious crack
was chosen for a metallographic examination (Figure 3). The chosen tube section
was mounted in a cold-curing epoxy and metallographically ground and polished in
accordance with standard procedures. The prepared metallographic
specimen presented the leak site in the transverse cross-sectional direction.
Examination of the specimen revealed four branching cracks, indicative of stress
corrosion cracking (SCC), originating at the tube OD surface and proceeding
either nearly or completely through the tube wall. Negligible corrosion on the
OD surface was noted except for shallow pits (approximately 0.4 mils deep) at a
through wall crack. The specimen was etched, revealing that the cracking was
intergranular. The intergranular attack was severe enough to produce grain
dropping. Etching revealed an apparently normal annealed copper microstructure
(Figure 4). |
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Figure 3. As-polished metallographic specimen showing
through-wall crack. Note shallow pits at crack origin at bottom of
photomicrograph. (250X original magnification)
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Figure 4. Etched metallographic specimen showing intergranular
nature of cracking. (Potassium Dichromate Etch, 1250X original magnification)
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The black and white deposits on another tube from the
coil section were examined using a scanning electron microscope (SEM) fitted
with an energy dispersive spectroscopy (EDS) microprobe. EDS revealed that
the deposits were mostly zinc (approximately 30 weight percent; from the
galvanized tube sheet) with magnesium, aluminum, silicon, phosphorus, sulfur,
chloride, potassium, and calcium (i.e., “dirt”) each present in quantities from
approximately 1 to 3 weight percent. (Copper made up the balance.)
EDS analysis of matter present in the branching cracks indicated that it was
copper oxide.
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Discussion
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Stress corrosion cracking (SCC) refers to cracking of an
alloy that occurs under the simultaneous action of corrosion and sustained
tensile stress. Cracking can be either intergranular (as in the present case)
or transgranular, depending on the alloy and/or the corrodent. The classic
example of SCC is intergranular SCC of stressed austenitic stainless steels
(e.g. Type 304L stainless steel) exposed to hot, aqueous, chloride-containing
environments. Branching of the cracking occurs in the direction of crack
propagation.
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High-purity coppers, such as phosphorized copper
(C12200), are generally considered almost immune to SCC; however, intergranular
SCC of this alloy has been observed. The usual culprit in SCC of copper and
copper alloys is ammonia (or other amines capable of reacting with copper to
form complex ions) acting in tandem with stresses in the alloy from forming
and/or service conditions. The source of ammonia can be enormously diverse,
from the decomposition of organic matter, to flooring adhesives, to cleaning
products.
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In the present case, cracking of the copper tubing
initiated at the OD surface and propagated intergranularly through the tube wall
in a direction normal to the tube surface. Stress in the copper tubing was due
to the forming of the hairpin bends (where the failures occurred); away from the
hairpin bends, the copper tubing was in a stress-free annealed condition.
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EDS
analysis did not reveal a definitive “bad actor” chemical species in the
cracking failures; but, if ammonia was the culprit, very little or no evidence
of its presence would be expected to be found by EDS or other methods. The
presence of the blue discoloration on the tube ID surface at the failure sites
was indicative of an ammonia-copper complex forming species.
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