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Failure Analysis Case Histories
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Dezincification of Brass Wax Actuator
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ENVIRONMENT:
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Residential |
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EQUIPMENT:
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Water Supply System |
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MATERIAL: |
Commerical Bronze |
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SERVICE TIME: |
5 months |
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FAILURE MODE:
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Dezincification |
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Background
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The wax actuator was part of a
residential plumbing fixture. The actuator was exposed to well water at 100°F
for about 10 hours per day, 70°F for about 13.5 hours per day and 140°F for
about 0.5 hours per day for approximately 5 months.
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Findings
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Visual Examination
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The large end of the failed wax actuator was covered with a layer of green
deposits. Underneath the green layer was a layer of reddish-brown deposits.
These deposits had the appearance typically associated with dezincification
of brass, Figure 1. |
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SEM/EDS Analysis
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A small sample of each of the
two different colored deposits was removed, mounted on a carbon stub, and placed
onto the goniometer stage of a scanning electron microscope (SEM)
equipped with Energy Dispersive x-ray Spectroscopy (EDS). The EDS spectra of
each were obtained. Both types of deposits were primarily copper and oxygen.
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The new actuator was placed
into the SEM and EDS spectra was obtained of the metal surface at the large end
of the actuator. A quantitative analysis indicated the material to
be a copper-zinc alloy commonly known as commercial bronze (91% Cu, 9% Zn).
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Metallographic
Analysis
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| The failed end of the actuator
and the corresponding end of a new actuator were mounted in epoxy and polished
following the guidelines of ASTM E 3. The structures of the deposits on the
failed actuator are consistent with layer-type dezincification of brass, Figures
2 and 3. When compared to a new actuator
the failed actuator had lost from 0.5 to 3 mils of wall thickness due to
dezincification. |
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Figure 2. Polished cross-section of actuator showing wall loss due
to corrosion, as-polished, 25X magnification. |
Figure 3.
Close-up of de-alloyed surface, as-polished, 750X. |
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Water Analysis
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| The water sample was analyzed for pH,
dissolved oxygen and carbon dioxide, and by ion chromatography with the following
results (Table 1). |
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CO2 |
27.0 mg/L |
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O2 |
8.6 mg/L |
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Discussion
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The analysis indicates that the wax actuator failed due
to corrosion. The mode of corrosion is referred to as de-alloying. In the case
of copper-zinc alloys, it is dezincification. As the alloy is attacked the zinc
goes into solution and the copper is redeposited onto the metal as a spongy,
porous deposit. This deposit of pure copper usually oxidizes to copper oxide
(reddish brown deposits) and/or may further react with the corrodent to form a
layer of copper corrosion products on top (green deposits). In this case, the
green deposits are characteristic of copper carbonate.
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Dezincification of brasses is
generally limited to alloys that contain less than 85 wt% of copper. Commercial
bronze (91 wt% Cu) is considered resistant but not immune to this type of
corrosion.
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“Dezincification is the usual
form of corrosion for uninhibited brasses in prolonged contact with waters high
in oxygen and carbon dioxide” (ASM Metals Handbook Volume 13, Corrosion, pg.
614). Water that has been exposed to the atmosphere, as most city waters are,
generally contains less than 1 mg/L of carbon dioxide. However, well water may
contain substantially higher amounts of carbon dioxide, which is picked up when
rainwater percolates through carbon dioxide rich areas in the soil.
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