Understanding Engine Coolant And Its Additives

Why using the recommended coolant is critical to engine life

In an ideal world, a single blend of engine coolant additives would work seamlessly in all engines. Unfortunately, we don’t live in such a world. A coolant additive blend that works perfectly in one engine might cause issues in another. Here’s why:

The reason is the materials used in engines, radiators, heater cores, heater tubing, thermostats, and plastic components. Some engines have a cast iron block, while others feature an aluminum block. Most modern radiators might contain plastic components. Additionally, impellers can be made of steel, aluminum, or plastic. Cooling system connectors can be made of steel, a variety of plastics, or rubber.

What’s in engine coolant?

Polyethylene glycol or polypropylene glycol are the two base chemicals used to make up about 95% of a gallon of automotive coolant. However, other glycols like 1,3-butylene glycol, hexylene glocol, diethylene glycol and glycerin can be used in small percentages. The rest of a gallon of coolant, or about 5%  is comprised of lubricants, surfactants, and corrosion inhibitors.

The engine coolant’s additive package must be compatible with the various materials used in that particular engine.

Only the carmaker knows which materials are used in their engine’s cooling system. So when they recommend a coolant, that recommendation is based on expected performance, expected coolant life and compatibility. As you’ll see below, some additives can damage components used in an automotive cooling system. Other additives, when added to existing coolant, can cancel out the anti-corrosion additives of each type and cause damage to the system.

That’s why there’s really no such thing as an “all makes, all models” universal coolant. It’s impossible to formulate a coolant that’s compatible with every engine.

For instance, some additives are excellent for aluminum but can soften plastic. Therefore, if you have an aluminum radiator with plastic components, you should avoid coolants that include plastic-softening additives.

Here are some of the most common coolant additives

Traditional Inorganic Green Coolant

Traditional green coolant contains silicates and triethanolamine phosphate, mercaptobenzothiazole sodium salt, sodium benzoate, sodium nitrate and borax. But silicate coolants have poor thermal stability, are unstable to pH and can gel when used with other corrosion inhibiting salts, reducing the cooling capacity of the liquid. Plus, silicates can precipitate out of the solution, further reducing corrosion protection.

Thus they have a short life of under two years. Thus, car makers have gradually switched away from silicate based coolant to organic acid formulations.

Organic Acid Coolants and Hybrid Organic Acid Coolant.

In addition to the glycol and carboxylate-based chemicals, many organic acid coolants use the following coolant additives:

Sodium Benzoate is used as the primary corrosion additive in hybrid organic acid (HOAT) coolants and G-05
Benzotriazole (BZT) Protects against corrosion for brass and copper
Colligative agents raise or lower the boiling and freezing points of the coolant.
Denatonium is a bittering agent that protects wildlife in the event the coolant is spilled.
Molybdate prevents coolant cavitation near the cylinder liner.
Nitrate is used to prevent corrosion
Nitrite is used to prevent cavitation near the cylinder liner. Plasticizer – A substance that softens certain plastics, silicones, and nylons.
Sodium Hydroxide is used to help maintain pH balance.
Triazole / Thiazoles are used to prevent corrosion on brass and copper.
Tolyltriazole (TTZ) is also used to prevent corrosion on brass and copper.
2-EHA (2-ethylhexanoic acid) is a coolant corrosion additive that is also a plasticizer that can damage nylon components and silicone materials. Those materials are used in most modern engines.
sebacate and carboxylic acids
azoles
aliphatic mono- and dicarboxylic acids

What the different additives do

Borate— Borate is a pH buffer that lowers the freezing point of the coolant while also increasing the boiling point of the coolant. It reduces friction and sludge, prevents corrosion at high temperatures, and helps prolong the life of the coolant during storage. However, if a coolant containing Borate is added to a system that contains the older inorganic green coolant, it will decrease the cooling capacity of the old coolant. Worse yet, it will create a corrosive condition in the cooling system.

Nitrite— Nitrite protects iron from corrosion and cavitation. It coats iron and protects it. Nitrite is used in coolants for older engines with cast iron components and for diesel engines. However, it’s not used in newer coolants because nitrite corrodes aluminum. Since more modern engines are using aluminum parts, nitrite is becoming obsolete.

Phosphate—Phosphate is a fast-acting acid that protects many different kinds of metals. Unfortunately, Phosphate is prone to flaking off and forming layers of scale. Scale formation is more likely when phosphate is mixed with hard water. Many European carmakers avoid coolants that contain phosphates because hard water is commonly mixed with coolant in Europe.

Some carmakers recommend Hybrid Organic Acid Technology coolants containing phosphate because phosphate reacts quickly to protect the metals in the cooling system. However, if you don’t follow the carmaker’s recommendations for coolant changes, the phosphate additive can flake off and clog your heater core and radiator, reducing cooling to the point of overheating.

Silicate— Silicate is a different type of fast-acting acid that provides protection from corrosion. It works with many different metals, especially aluminum. However, silicate is abrasive and can eat away at the water pump seal/gasket, and/or other gaskets in the cooling system. If you add a coolant containing silicates to the wrong engine, the cooling system may lose some cooling ability. This will be a problem if your vehicle is working especially hard while operating in higher temperatures, such as towing or hauling. Worse, you could see gasket failure.

Sodium benzoate—  Sodium benzoate helps prolong the life of the coolant while also providing protection against corrosion. Unfortunately, it increases the risk of corrosion on aluminum components.

For more information on IAT, OAT, HOAT coolants and how they differ, see this post

©, 2019 Rick Muscoplat