Contributing Causes

Effects of Rust Formation.

through the metal. Electrolytic corrosion in the cooling

system can result from a combination of different metals in

(1) Water jacket deposits. If rust deposits are

contact with each other, such as soldered seams in copper

or brass radiators, brazed joints in steel tubes, copper

allowed to build up in water passages of the block or head

gaskets in contact with iron, and imperfect plating on thin

(fig. 2-14), they may hold enough heat in the metal to

steel parts.

create local "hot spots", especially around the valve seats.

Steam pressure from local boiling at such hot spots is a

b. Contributing Causes. The rate of rust formation

hidden, although common cause of overflow loss. The

and corrosion within the cooling system is influenced by

metal may get so hot as to cause sticking, warping, or

many conditions of service and operation.

burned valves (fig. 2-15), or even a cracked block or

head.

(1) Air in the coolant (fig. 2-13). Aeration (mixing air

with water) can increase corrosion of iron as much as 30

times. The normal source of aeration in the cooling

system is the radiator top tank. At higher engine speeds,

the rush of coolant into the radiator is great enough to

drive air into the liquid and carry air bubbles down through

the water tubes (para 2-20b.). If coolant level is allowed to

drop as low as the top of the water tubes, suction of the

water pump will draw air in through the overflow pipe and

down through the water tubes.

(2) Temperature. Heat speeds up corrosion, and

unfortunately the rate of corrosion with iron appears

greatest at coolant temperatures corresponding to best

engine performance. Iron, solder, and copper will corrode

more than twice as fast at 175F than at 70F.

(3) Impurities in water. Some natural waters are less

corrosive to iron than distilled or rain water, but others,

Figure 2-13. Engine water jacket showing air mixed with

which contain dissolved mineral salt impurities, are

coolant.

particularly harmful to cooling system metals. Any acid

condition in natural waters will increase iron corrosion and

rust formation. Hard water containing large amounts of

lime and certain other minerals will deposit scale at "hot

spots" in the engine water jacket, if large quantities of the

water are added to the cooling system over a period of

time.

(4) Contamination of coolant. Coolant may become

contaminated as a result of extended service (para 2-32f),

a faulty condition within the system, or for improper

maintenance.

Excessive aeration from a neglected

suction leak at the water pump (para 2-13b) (fig. 2-10) or

at any point between pump and radiator, speeds up

corrosion and shortens the rust-free life of the coolant.

Combustion gas dissolved in coolant from a leak at

Figure 2-14. Rust deposits in engine water jacket.

cylinder heat joint has a similar effect to aeration (para 2-

14a.) (fig. 2-12). Corrosive contamination of coolant can

also result from failure to neutralize and flush out cleaning

solution (para 2-21b).

2-12