TM 5-3810-300-24&P-3
13.3
ENGINE COOLANT
Engine coolant is considered as any solution which is
accelerate corrosion, while hardness (percentage of
circulated through the engine to provide the means for
magnesium and calcium present) will cause deposits of
heat transfer from the various engine components. In
scale. Total dissolved solids may cause scale deposits,
general, water containing various materials in solution is
sludge deposits, corrosion or a combination of these.
used for this purpose.
Chlorides, sulfates, magnesium and calcium are among
The function of the coolant is basic in the design and the
but not necessarily all the materials which make up
successful operation of the engine and must be carefully
dissolved solids. Water, within the limits specified in
selected and properly maintained.
Tables I and 2 of Fig. 1, is satisfactory as an engine
coolant when proper inhibitors are added.
COOLANT REQUIREMENTS
CORROSION INHIBITORS
A suitable coolant solution must meet the following five
basic requirements:
A corrosion inhibitor is a water soluble chemical
compound which protects the metallic surfaces of the
1. Provide for adequate heat transfer.
cooling system against corrosive attack. Some of the
more commonly used corrosion inhibitors are chromates,
2. Provide a corrosion resistant environment within the
borates, nitrates, nitrites and soluble oil. Depletion of all
cooling system.
types of inhibitors occur through normal operation and
therefore strength levels must be maintained by the
3. Prevent formation of scale or sludge deposits in the
addition of inhibitors at prescribed intervals. Always
cooling system.
follow the supplier's recommendations on inhibitor usage
and handling.
4. Be compatible with the cooling system hose and seal
materials.
NOTE:
Methoxy propanol base permanent
antifreeze (such as Dowtherm 209, or
5. Provide adequate freeze protection during cold
equivalent) must be re-inhibited only with
weather operation.
compatible corrosion inhibitor systems.
Normally requirements 1 through 4 are satisfied by
Chromates
combining a suitable water with reliable inhibitors. When
operating conditions dictate the need for freeze
Sodium chromate and potassium dichromate are two of
protection, a solution of suitable water and an ethylene
the more commonly used water system corrosion
glycol type antifreeze containing adequate inhibitors will
inhibitors.
However, the restrictive use of these
provide a satisfactory coolant.
materials, due to ecology considerations, has de-
emphasized their use in favor of non-chromates. Care
WATER
should be exercised in handling these materials due to
their toxic nature.
Any water, whether of drinking quality or not, will produce
a corrosive environment in the cooling system. Also,
Chromate inhibitors must not be used in ethylene glycol
scale deposits may form on the internal surfaces of the
antifreeze solutions. Chromium hydroxide, commonly
cooling system due to the mineral content of the water.
called "green slime", can result from the use of chromate
Therefore, water selected as a coolant must be properly
inhibitors with- permanent type antifreeze. This material
treated with inhibitors to control corrosion and scale
deposits on the cooling system passages, reducing the
deposition.
heat transfer rate, and will result in engine overheating.
Engines which have operated with a chromate inhibited
To determine if a particular water is suitable for use as a
water must be chemically cleaned before the addition of
coolant when properly inhibited, the following
ethylene glycol type antifreeze. A commercial heavy duty
characteristics must be considered. The concentration
descaler should be used in accordance with the
manufacturer's recommendation for this purpose.
dissolved solids. These materials are objectionable for a
number of reasons: chlorides and/or sulfates will
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