The vehicle cooling water system is very important to the
performance of any vehicle. Due to the demand to obtain more mileage or hours
per gallon of fuel in today's fleets, there have been changes in the types of
metals utilized in cooling systems. Aluminum alloys are being used for motor parts,
and aluminum in contact with the cooling water system can present performance
Aluminum is a reactive metal and reacts with dissolved oxygen and chemically
combined oxygen in water. Unlike most other reactive metals, aluminum forms a
dense, thin and highly protective oxide film that prevents the parent metal from
further reaction. Were it not for this film, aluminum would react spontaneously
with air or water. Some corrosion takes place until the oxide becomes thicker
and more protective. Aluminum corrosion is, therefore, self limiting because corrosion
may proceed rapidly at first, then slow down to an insignificant rate. Such rapid
initial corrosion is normal and contributes to improved corrosion protection in
the future. It is clear that any substance in the alloy or in the water that interferes
with the formation of the protective oxide, or dissolves the oxide, will cause
further corrosion. Any substance that serves to maintain this oxide film will
prevent corrosion. Such substances are called corrosion inhibitors.
When considering the use of aluminum with any water, bear in mind those factors
which effect oxide film formation. They are: choice of alloy, acidity or alkalinity
of the water, temperature, chloride content, dissolved heavy metals and other
dissolved solids, contact with dissimilar metals, dissolved oxygen in water, velocity
of water, hardness and whether antifreeze or other additives are used in the water.
The acidity or alkalinity of water is measured by the pH value. The pH of water
is noted on a scale of 0 to 14, a value of 7.0 meaning the water is neither acid
or alkaline, but neutral. A low pH value (below 7.0) denotes an acid water, a
high pH value (above 7.0) denotes alkaline water. Waters with pH values below
4.5 cause rapid corrosion of metals in the cooling system. Waters with a pH above
8.5 are corrosive to aluminum, however, the extent of corrosion depends upon the
nature of the impurity in the water that causes the high pH. Usually, rapid attack
of aluminum occurs above pH 9.5 unless a corrosion inhibitor has been added to
the water. The pH range 4.5 to 8.5 is called the safe range for aluminum. The
corrosion rate of aluminum at a given ph varies depending on which acid or base
is used. A range of corrosion rates that covers most of the common acids and bases
is shown in Figure 1.
The pH range over which aluminum forms a protective film becomes narrower with
increasing temperature until at 400 - 450° F no protective oxide forms.
Dissolved chlorides interfere with protective film formation and cause pitting
of aluminum alloys as well as other metals. The extent to which chlorides are
harmful depends heavily on the chloride content of the water, the alloy selected,
and the design of the equipment.
In general, aluminum should not be used in uninhibited waters containing more
than a few parts per billion of dissolved "heavy" metal salt such as
compounds of copper, tin, nickel, and especially mercury compounds. Such waters
cause pitting, particularly at pH values below 7.0. In general, dissolved heavy
metals tend to deposit on aluminum and cause galvanic corrosion. To avoid such
action, the water pH is kept on the alkaline side in the "safe" range
and inhibitors are employed. A "closed" system (one designed to limit
the amount of dissolved oxygen in the water) and the use of clad aluminum is helpful.
The effect of dissolved solids on the corrosion of aluminum depends largely
on what these solids are. Some (chlorides, heavy metals, etc) have already been
discussed. In general, the more dissolved solids in the water, the greater electrical
conductivity of the water. Highly conductive waters tend to enhance pitting corrosion
and, particularly, galvanic corrosion at dissimilar metal joints.
Increasing the amount of dissolved oxygen in water has the effect of increasing
the corrosion rate of aluminum especially where the water in itself is corrosive
or where uninsulated dissimilar metal joints are present.
Water hardness has little effect on the corrosion of aluminum alloys. Although
a small amount of calcium or magnesium hardness in the water may be of some benefit
to formation of protective aluminum oxide films, aluminum is capable of forming
a highly protective oxide by reaction with soft water alone.
Many recirculating water systems use inhibitors and other additives that make
the recirculating fluid highly alkaline (automotive cooling water systems). Such
systems operate at pH values from 9 to 11 or higher. At such pH values, aluminum
systems must be properly inhibited usually by establishing a protective film on
the aluminum (or other metal) that protects it from contact with the highly alkaline
and corrosive water. Should a high fluid velocity, particularly one associated
with turbulent flow condition, be established across the metal surface, the protective
inhibitor film may be removed as rapidly as it forms and rapid deterioration is
termed "erosion-corrosion". If high fluid velocities and/or turbulent
flow conditions are anticipated, care should be exercised to insure proper inhibitor
levels. In addition a complexing agent may be used.
The purpose of the pH buffer is to neutralize acid decomposition products and
to provide an optimum pH to minimize general corrosion rates. The film former
provides protection against pitting and the complexing agent prevents the deposition
of dissolved heavy metals. For all systems containing aluminum, a buffering type
of inhibitor would be chosen, possibly containing silicate. The purpose of this
inhibitor is simply to keep the pH of the recirculating fluid slightly alkaline
and to provide some filming capability. Multi-metallic systems such as aluminum-copper
would require a copper complexing agent as well.
In selecting an inhibitor for aluminum, it is best to consider a formulation
that will: (1) aid film formation, especially in the presence of some chlorides
and other dissolved solids; (2) provide some buffering capacity as required to
maintain the pH within safe limits; and (3) provide a complexing agent for protection
against deposition of heavy metals. Realize too, that the inhibitor level must
be maintained. Periodic checks of fluid and inhibitor levels are required. It
should be also realized that even the best inhibitors will not compensate for
poor design or poor water quality.
The control of all variables affecting corrosion and scaling in a recirculated
water system can be simplified by designing for a closed system and the use of
The Komplete Kool™ and NeverFlush™ formulations have been evaluated
in both dynamic and static systems using many types of metallic alloys (steel,
stainless, copper, bronze, muntz metal, 6061 aluminum). After 6 months the copper,
bronze, muntz metal alloys showed a weight loss of .004 mpy in the circulating
system but none in the stagnant system. All the metallic alloys had corrosion
rates below the .002 mpy detection limit.