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Failure Analysis Case Histories
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Formicary Corrosion of Unused Copper Tubes
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EQUIPMENT:
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Chiller |
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MATERIAL: |
Copper |
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SERVICE TIME:
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Never Used |
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FAILURE:
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Formicary Corrosion |
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Five chiller units were
installed two years ago after fabrication. The units were never put
into operation. The mechanics installing the units found 3 of the 5 units to
have leaking condenser and evaporator tubes. Forty (40) evaporator tubes were
found to be leaking. Suspect tubes were pulled and pressurized to 150 psi with
nitrogen to identify specific through-wall penetrations.
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The five samples received by
Corrosion Testing Laboratories were sealed and air underwater tested to 120
pounds of pressure to confirm the marked areas of leaks. All tubes experienced
leaks as verified by bubbles present on the OD surface during air testing. The
sites of air leakage corresponded with markings painted on the OD.
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The OD surfaces of all five
tubes were observed to have a dull, brownish copper patina, typical of air
oxidized copper. There was no apparent metal loss or damage to the tubes in
association with this appearance.
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All five samples were then
split longitudinally to expose the ID surface. The ID surfaces were covered
with black scale/deposits, with some tubes having substantial black product
build-up.
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The ID rifled surfaces were
examined under an optical microscope (at 40X magnification). The areas of
heaviest deposits correlated with the areas marked as leak sites on the OD. In
these areas, deposits of red-to-purple were seen, and within this area at least
one pit was observed, as typified by the figure below.
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The scale was chemically removed with inhibited
hydrochloric acid, revealing microscopic pits in the black deposited areas;
reminiscent of those associated with formicary (a.k.a., “ant nest”) corrosion.
A cross-sectional mount was made through one of the observed pits. After
polishing, the morphology and extent of pitting confirmed the through-wall
failure and the type of attack as formicary corrosion, as seen below. |
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