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TB 55-1900-207-24
SECTION II. GENERAL INSTRUCTIONS
1-3. Heat Exchangers.
a. This bulletin is primarily concerned with the chemical treatment of the recirculating fresh water which passes
through the heat exchanger.
b. Retarding corrosion of the saltwater side of a heat exchange is best accomplished by the use of corrosion-
resistant alloys or coating and sacrificial anodes (zinc plugs).
c. Precautions must be taken to prevent the entrance of marine organisms which may adhere to the heat exchanger
surfaces and thus impair the rate of heat transfer and accelerate corrosion. Screens are normally installed to prevent
this.
1-4. Need for Treatment. Fresh water used for cooling marine diesel engines contains varying concentrations of
impurities depending upon its source. It contains gases which are corrosive, such as carbon dioxide and oxygen. It also
contains harmful scale-forming elements, such as calcium, magnesium, chlorides, sulfates, silica and organic matter.
The mineral content makes the water 'hard." Watercraft are apt to take on make-up water from different localities.
Therefore, this bulletin will prescribe the treatment of cooling water of average hardness and mineral content.
1-5. Scale Formation. Some of the dissolved salts in the circulating water have a tendency to form either hard scale or
soft sludge. Scale formation and sludge deposits may cause the failure of cylinder liners, heads, and the engine block.
Scale formed on heat transfer surfaces slows up heat exchange and can cause local overheating or "hot spots."
Overheating causes breakdown of lubricating oil, sticky piston rings and increased engine wear. The capillary spaces
that exist under scale and sludge deposits are breeding places for rust and corrosion.
1-6. Corrosion.
a. Corrosion is a natural process affecting all metals, but it can be considerably retarded by chemical and physical
means. Dissolved carbon dioxide will form carbonic acid. Water with high acid content will dissolve iron and steel.
Water with high alkalinity will dissolve many of the non-ferrous metals.
b. Elevated engine operating temperatures will also accelerate corrosion. (For example, iron, solder, and copper
will corrode twice as fast at 175 than at lower temperatures.)
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c. Dissimilar metals not insulated from each other will corrode at a much faster rate than if they are insulated from
each other.
d. Severe pitting on the water side of cylinder liners is caused by mechanical vibration (cavitation) at high speeds.
The localized stress produced is great enough to rapidly erode the metal surface. It has been found that chromate
chemicals are effective in reducing this type of damage.
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