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 MIL-HDBK-1003/13A
Most heat transfer fluids contain some degree of toxicity. To minimize the
probability of contamination of potable water systems the following items
should be addressed in any specification or bid:
a.
Assurances to preclude the possibility of cross connection of
potable water piping with heat transfer fluid piping. The use of
tags, color coding, different pipe connections, etc, are
suggestions.
b.
Hydrostatic testing of system to find leaks.
c.
Color indicators in heat transfer fluid to find leaks.
d.
Safe designs for heat exchangers as given in Figure 2-5.
e.
Determine toxicity classification of heat transfer fluids.
Suggested categories as a minimum are:
(1)
Oral toxicity (ORAL LD50).
(2)
Carcinogenic.
(3)
Ocular irritant (eye).
(4)
Dermal irritant (skin).
Before heat transfer fluids are discussed, a review of basic corrosion theory
is in order. The two types of corrosion which cause the most damage in solar
systems are galvanic and pitting corrosion (Eyre, 1978). Galvanic corrosion
is a type of corrosion which is caused by an electrochemical reaction between
two or more different metals in contact with each other. A chemical reaction
between the metals causes a small electrical current which erodes material
from one of the metals. Solar energy systems generally contain a number of
different metals such as aluminum, copper, brass, tin, and steel. This makes
the solar system a prime candidate for galvanic corrosion. If the dissimilar
metals are physically joined or if they are contacted by a common storage or
heat-transfer fluid, the possibility of galvanic corrosion becomes much
greater.
Pitting corrosion is a highly localized form of corrosion resulting in deep
penetration at only a few spots. It is one of the most destructive forms of
corrosion because it causes equipment to fail by perforation with only a very
small weight loss. When heavy metal ions such as iron or copper plate on a
more anodic metal such as aluminum, a small local galvanic cell can be
formed. This corrosion spot or "pit" usually grows downward in the direction
of gravity. Pits can occur on vertical surfaces, although this is not as
frequent. The corrosion pits may require an extended period (months to
years) to form, but once started they may penetrate the metal quite rapidly.
Heavy metal ions can either come as a natural impurity in a water mixture
heat transfer fluid or from corrosion of other metal parts of the solar
system.
Pitting corrosion has the same mechanism
(concentration cell) as crevice
corrosion thus it can also be aggravated
by the presence of chloride or other
chemicals which can be part of the water
mixture or a contaminant from solder
fluxes. Aluminum is very susceptible to
pitting corrosion, while copper
generally is not.
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