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Nitrate Corrosion of a Sight Glass Housing

EQUIPMENT:

Chiller Unit

MATERIAL:

Zinc

FAILURE:

Nitrate Corrosion
 
Background

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.  

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. 

 
Analysis

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.   

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.   

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.  

 
Discussion

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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