With the change in heavy-duty diesel engine technology, the call for sustainability has been the driving force behind the evolution of this equipment. One of these directions is linked to reducing the weight of the equipment, with the aim of reducing fuel consumption, which is linked to the reduction in energy needed to create the movement that transports the loads.

Manufacturers of heavy-duty diesel engines, which proportionally consume a greater amount of materials to manufacture this equipment, decided to use aluminum components in the cooling systems in order to contribute to this reduction in vehicle weight. For this reason, aluminum has become one of the most widely used metals in engine cooling systems.

This change in material has brought about a new need to improve cooling fluids, since aluminum components are more sensitive to chemical reactions that cause corrosion.

Chemical components present in various cooling fluids, when not properly balanced, accelerate corrosion in cooling system components made of aluminum. Coolants with a pH above 9 can be found on the market, which in general is not a problem for copper alloys, but leads to corrosion and consequently the formation of aluminum oxide (alumina) in cooling systems. The metallic particles generated, if not properly treated, can circulate in the cooling system causing erosion damage to other parts and can obstruct the passageways, starting a catastrophic chain reaction throughout the system. In addition, aluminum also reacts in acidic environments, forming aluminum hydroxide, particles of which will potentially start a chain reaction in cooling systems.

Therefore, a fluid designed for aluminum components should always be used when this metal is present and it is very important that the pH of the circulating fluid is kept in the alkaline range, but below 9.

Corrosion x Brazing

Brazing is the process in which two or more pieces of metal are joined by pouring a filler metal into the joint. The filler metal fills the gap between the tightly fitting parts by means of capillary action. The filler is brought to just above its melting temperature and is usually protected by a flux, such as borax. This flows over the base metal in a process called wetting and is then cooled to join the parts.

Variables such as temperature, vacuum level and cycle time are strictly controlled during the brazing process, based on the specific materials being used, and are one of the most critical points in the manufacture of aluminum radiators. The cleanliness of the parts is very important in brazing because as less heat is used during this process, contaminants are less likely to burn. Failures generated in this process can create non-passivated aluminum spots, which are more susceptible to corrosion.

In these cases of failure during the brazing process, the nitrite present in some cooling fluids and the brazed aluminum react to form ammonia and hydrated aluminum oxide (bayerite), which in excess in the system for long periods of time will cause excessive corrosion.excess in the system for long periods will cause excessive corrosion of the aluminum components due to the rise in pH caused by the formation of ammonia in this reaction. For this reason, the use of cooling fluids containing nitrite in their formulations is not recommended in cooling systems where aluminum may be present.

With regard to aluminum oxide, as mentioned earlier, particles precipitated in the fluid are also a cause of corrosion and erosion of cooling system components, especially in water pump housings, where the flow is generally turbulent and constant.

Electrolytic corrosion

Another characteristic of aluminum is that it is a softer and extremely conductive metal. For this reason, aluminum components are also subject to failure due to electrolysis. Since engine cooling systems are made of different metals, this can be a challenge when maintaining this type of system. The contacts of different metals in couplings, for example copper with aluminum, are the areas where it is most difficult to prevent this type of failure.

Electrolysis can also cause metal corrosion due to eddy currents, as the coolant can conduct electricity to metal parts and the stray voltage will travel through the cooling system until it finds a ground. As aluminum is a soft metal, it will be more susceptible to damage from electrolysis corrosion, but different metals used in engine cooling systems can also contribute to electrolysis.

Another problem that can occur due to electrolysis is the acidification of the fluid, which increases the chemical imbalance of the fluid and ends up increasing the conductivity within a cooling system.

For this reason, and several others, maintaining the integrity of cooling systems requires care when choosing the type of fluid to use. The simple addition of a component that is beneficial in some cases can cause failures in other types of cooling systems. In addition, and as a basic condition, the use of distilled or purified water is mandatory, since the use of water of uncontrolled origin increases the potential for corrosion and electrolysis of the components of the aluminum cooling system.

Authors:
Paulo Felipe Silva Berto (Product Researcher grease, coolants and specialties.)
Paulo B erto (Product Researcher for the Fluid and Grease Line).