MTS Water Quality Guide - Manual (Page 12)

Heat Exchanger Care and Water Quality Guide 12 Heat Exchanger Care
• Freeze-Up: These failures can occur in a water cooled heat exchanger in
which the temperature drops below the freezing point of the cooling water.
Freeze-up results from failure to provide thermal protection, a malfunction
of the thermal protection control system, or protective heater device,
improper drainage of the unit for winter shutdown or an inadequate
concentration of antifreeze solutions.
Chemically induced
corrosion
Chemically induced corrosion failures result from the complex chemical
interaction between the materials of the heat exchanger and the fluids circulated
through it. There are several types of chemically induced corrosion failures:
general corrosion, pitting, stress corrosion, dezincification, galvanic corrosion,
and crevice corrosion.
• General corrosion: This type of corrosion is characterized by a slow,
uniform attack over the chamber material, with little or no evidence that
corrosion is taking place.
In copper, low cooling water pH (less than 7) combined with carbon dioxide
or oxygen produces corrosion. A blue or bluish-green color on the material
are indicative of carbon dioxide attack inside the chamber. Various
chemicals, such as acids, also produce this type of metal loss.
You can reduce general corrosion and maximize the life of your heat
exchanger by selecting a material with adequate corrosion resistance for the
operating environment and by using the proper treatment chemicals to clean
and protect the components of your heat exchanger.
• Pitting: Localized pitting is frequently encountered in metals. It is caused by
the electrochemical potential set up by differences in the concentration of
oxygen within and outside the pit, and is frequently referred to as a
concentration cell. The oxygen-starved pit acts as an anode and the
unattacked metal surface as a cathode. A small number of pits may be
present; however, any one can cause a heat exchanger failure.
Pitting corrosion is most likely to occur during shut-down periods when
there is no fluid flow and the environment is most suitable for the buildup of
concentration cells. Imperfections such as scratches, dirt or scale deposits,
surface defects, breaks in protective scale layers, breaks in metal surface
films and grain boundary conditions increase the susceptibility of the metal
to pitting corrosion.
• Stress corrosion: This form of corrosion attacks the grain boundaries
(changes in the crystalline structure of the metal) in stressed areas. Heat
exchangers usually have residual stresses that are the result of drawing or
forming the materials during manufacturing.
Failures from stress corrosion take the form of fine cracks, which follow the
lines of stress and material grain boundaries. All naturally occurring waters
contain the chloride ion, which is potentially present in any compound
formulated with chlorine. The frequency of chloride stress corrosion rises
with an increase in temperature and chloride ion concentration. Keeping
chamber wall temperatures below 125°F (52°C) prevents stress corrosion
cracking problems with chloride ion concentrations up to 50 ppm.
The substance that causes stress corrosion cracking on copper or copper
alloy is ammonia. Very small concentration (1 ppm or less) can create a
problem. Copper-nickel alloys have good resistance to stress corrosion
cracking and should be used in applications where low concentrations of
ammonia are expected.