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Failure Analysis Case Histories
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Formicary Corrosion of Copper Tubes in a Chiller
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ENVIRONMENT:
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Office Building |
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EQUIPMENT:
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Chiller |
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MATERIAL: |
Copper |
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FAILURE:
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Formicary Corrosion |
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The tube sections represent a condenser and an
evaporator tube from a centrifugal chiller located in an office building. The tubes are approximately 9-inches in length. They
are ¾-inch diameter refrigeration grade copper with helical enhanced outside
diameter (OD) fins and helical rifling on the inside diameter (ID).
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The chiller was fabricated in
July. The unit was shipped to the customer in September. The unit was installed
and piped up to the recirculation systems, however no service water entered the
unit, even though the piping system had been hydro-tested. In April the
following year, the unit was leak tested and the condenser bundle was found to
have 22 tubes with potential leaks. One of several tubes were pulled from both
the condenser and evaporator and the portion containing a suspected leak was
sent to Corrosion Testing Laboratories, Inc. (CTL).
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The OD surfaces are new-like copper-color, however they have
a significant number of circumferential bands of black staining. There is no
apparent metal loss or damage to the tube in association with this staining.
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The ID surfaces were peppered
with intermittent blue to black scale/deposits, similar to other tubes that
experienced failures that we have seen in the past. However, in the area
identified as the two leak sites, the black scale was very pronounced, covering
almost the entire ID surface for a length of approximately 4-inches. Careful
examination of the ID surface under optical microscopy (at 40X magnification)
revealed microscopic pits reminiscent of those associated with formicary
(a.k.a., “ant nest”) corrosion. |
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A cross-sectional mount was
made through the observed pits on both tubes. After polishing, the morphology
and extent of pitting confirmed the through-wall failures and the type of attack
as formicary corrosion, as seen in the figures below. |
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Figure 1. Branched
tunnels associated with formicary corrosion in the evaporator tube. (150X
magnification) |
Figure 2. Round pit
with spurs of branched tunnels, also associated with formicary attack.
(63X magnification) |
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On the evaporator tube, there were
indications of localized attack on the flat, non-enhanced tube associated with a
tube support. A specimen was cut from this area and a cross-section mount was
prepared. The figure below shows the extent and depth to which the formicary
damage exists.
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Figure
3. Cross-section through a flat, land area of the evaporator tube.
Note the many sites of pitting and the extent of depth. (30X
magnification) |
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