The chemicals you will use for treating fresh water in engine cooling systems will vary according to the composition of the metal you are trying to protect, the application of the engine, and the climate to which the engine is subjected. Because of these variations, our discussion of engine cooling system treatments will be general. Complete information on this subject can be obtained in the Naval Ships Technical Manual, chapter 233.

Our discussion will cover some of the chemicals that are authorized for the treatment of freshwater systems in engines. Remember, however, that the water treatment methods that will be discussed in this chapter are preventive treatments only. These treatments will not remove scale that has already formed in the cooling system.

A CORROSION INHIBITOR is a water-soluble chemical compound that protects the metallic surfaces of the cooling system against corrosive attack. Depletion of all types of inhibitors occurs through normal operation, Therefore, strength levels must be maintained by addition of inhibitors as required after the coolant is tested.

The importance of a properly inhibited coolant cannot be overstressed. A coolant that has insufficient inhibitors, the wrong inhibitors, or worse, no inhibitors at all invites the formation of rust and scale deposits within the cooling system. Rust, scale, and mineral deposits can wear out water pump seals and coat the walls of the cylinder block water passages and the outside walls of the cylinder liners. As these deposits build up, they insulate the metal and reduce the rate of heat transfer. For example, a 1/16-inch deposit of rust or scale on 1 inch of cast iron is equivalent to 4 1/4 inches of cast iron in heat transferability. (See fig. 7-20.)

An engine affected in this manner overheats gradually-over a period of weeks or months. Liner scuffing, scoring, piston seizure, and cylinder head cracking are the inevitable results. An improperly inhibited coolant may also become corrosive enough to eat away coolant passages, erode seal ring grooves, and cause leaks to develop. If sufficient coolant accumulates on top of a piston, a hydrostatic lock can occur while the engine is being started. This, in turn, can result in a bent connecting rod. As a precaution against these possibilities, an engine should be barred over by hand before being started.

An improperly inhibited coolant can also contribute to cavitation erosion. Cavitation erosion is caused by the collapse of bubbles (vapor pockets) that form on the coolant side of an engine component. The collapse results from a pressure differential in the liquid caused by the vibration of the engine part. As bubbles collapse,

Figure 7-20.-Reduction of heat transfer capacity from rust or scale deposits.

they form pinpoints of very high pressure. Over a period of time, the rapid succession of millions of tiny bursting bubbles can wear away (erode) internal engine surfaces.

Components such as freshwater pump im-pellers and the water sides of wetted cylinder liners are especially susceptible to cavitation erosion. In extreme cases their surfaces can become so deeply pitted that they appear to be spongy, and holes can develop completely through them.

When this training manual was written, there were five types of corrosion-inhibitor treatments authorized for use in naval engine cooling systems.

These treatments include:

1. Sodium chromate-disodium phosphate

2. Soluble oil

3. NALCOOL 2000

4. Inhibited antifreeze (MIL-A-46153)

5. MIL-A-53009

We will briefly discuss each of these treatments.