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 problems.
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 suitable inhibitors.
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.