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Failure Analysis Case Histories
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Nitrate
Corrosion of a Sight Glass Housing
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EQUIPMENT:
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Chiller Unit
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MATERIAL:
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Zinc |
FAILURE:
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Nitrate
Corrosion |
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Background
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Two sight glasses, one new, and
one corroded, and two water samples were sent to Corrosion Testing Laboratories,
Inc. (CTL) for analysis to determine the cause of corrosion occurring on the
used sight glass.
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The new sight glass was
provided as a reference, the corroded sight glass was removed from a finished
chiller unit, which had been hydro-tested using water from a closed loop
system. The water samples, which have been linked with the corrosion observed
on the sight glass housing immediately after the leak test, were taken from the
closed loop system.
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Analysis
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A portion of each water sample
was dried and analyzed by Energy Dispersive X-Ray and Fourier Transform Infrared
(FTIR) spectroscopy to identify corrosive species. The data collected indicated
the presence of significant quantities of both sodium nitrite and sodium
nitrate. Nitrates, according to literature, can significantly increase
corrosion rates of zinc alloys in aqueous environments.
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To verify the corrosivity of
the water, a corrosion test was performed using the ‘as-received’ solution. The
new sight glass was cut and a portion of the sight glass housing was immersed in
the ‘as-received’ solution for a 72 hour period. A control test, using a sodium
nitrite solution, was also performed.
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The ‘as-received’ solution
readily attacked the zinc plating on the surface of the sight glass housing,
while the portion immersed in the sodium nitrite showed no appreciable signs of
corrosion. Sodium nitrate was then added to the control solution, and the
specimens immersed for an additional 72 hours. As expected, at the end of the
second period, the control specimen showed visible signs of degradation of the
zinc plate indicating an increase in corrosion rate.
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Discussion
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Since these water samples
originated from a closed loop water system used for hydrotesting, it is standard
procedure to treat the water in the system with a corrosion inhibitor. We
understand the inhibitor currently used is sodium nitrite. Unfortunately it is
common for these closed systems to develop bacteria. Specifically nitrifying
bacteria, which oxidize nitrites to nitrates. The growth of nitrifying bacteria
in the system would explain the presence of both sodium nitrate and nitrite
found in the chemical analysis. This would also explain the increased corrosion
rate of the zinc plating on the sight glass housing.
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