Coolant, Chemistry and Corrosion

Plain Water is an excellent coolant (think swimming pool). Naturally occurring, abundantly available and mixes with almost anything. It can be pumped, filtered, heated, cooled, evaporated, condensed and stored. Essential for plant and animal life and the most widely used of all solvents, even though it is a great automotive coolant, it is not without a few draw backs:

First, it has a tendency to freeze at 0C` (32F`). Not considered a desired trait since during freezing, expansion occurs and thick metal castings can be rendered useless. If this does occur, hopefully the freeze plugs allow enough expansion relief.

Second, water will boil at 100C` (212F`). This is definitely not desired. As the water boils, interruption to coolant flow occurs, the steam vapor displaces the liquid capacity, further reducing efficiency.

Third, plain water will allow corrosion to occur on dissimilar metals, and deterioration to accelerate. The best way to insure your self of expensive cooling system repairs is to fill with plain water only!


Ethylene Glycol, a colorless syrupy alcohol, HOCH2CH2OH, used as an antifreeze in cooling and heating systems. Mixed with water at 50%, it is found in almost all automotive coolants, and is the backbone of (almost) all coolant chemistry. Ethylene Glycol (EG) has very important characteristics that tailor the coolant mix to the automotive application. The current thinking for automotive coolant is to dilute water with enough ethylene glycol and corrosion inhibitors to change the poor characteristics of water, while retaining the good ones. The primary additives are Ethylene Glycol and Di Ethylene Glycol for freeze/boil protection. The corrosion inhibitors used vary widely by manufacture, and by vehicle requirements. The reason for so many different coolants lies in the complexity of the inhibitors used.




The 3 Main Formulations of ethylene glycol coolant existing today are:

1) Silicate based also known as green coolant

2) Organic Acid Technology based commonly called OAT's, which include extended life or Dex-cool

3) Hybrid coolants, which is a combination of organic acid technology mixed with a low dose of silicates. Many so-called "universal coolants" are members of the hybrid family.


Green Coolant is almost a thing of the past on newer cars. What's wrong with green coolant? Well, in a word, nothing! Traditional green coolant is mostly ethylene glycol, silicates, and green dye. Newer coolants have basically the same ingredients, but with orange, red, blue, purple or amber coloring, and without the high level of silicates. Why is the trend moving away from green antifreeze? Longer maintenance cycles. These days we want every thing to last longer.

Silicates are a chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, borate, calcium, iron, magnesium, manganese, molybdate, nitrite, nitrate, phosphate, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids.  In a cooling system, silicates offer metal parts ( especially aluminum ) a high level of fast acting protection against corrosion and pitting that is vital. Silicates "charge up" a cooling system with protection that lasts about 2 years. After two years, the coolant mix needs to be replaced with new coolant and water. New every two.

Silicates offer instant protection to the cooling system. Heavy duty vehicles rely on silicates to give precise amounts of protection, and can be recharged without draining the entire coolant. Silicates do have a life, and it's a short one - two years. If left longer, build up inside the radiator begins, and the silicates actually contribute to cooling system problems. Silicate drop out occurs after the coolant has been "spent", and the inhibitors drop out of solution. Silicates are blamed for deposit build up in radiators and heaters, causing engine overheating and poor heater performance. Once the silicates drop out of solution, dissimilar metal corrosion is no longer suppressed, and metal components begin to deteriorate.



However, silicates are deemed to be abrasive in a cooling system, and are blamed for premature water pump seal failure. With the average miles per year per car on the rise, it does seem to be a valid concern. But is it worth the price of a dex-cool clean up? If your water pump is timing belt driven, the water pump may be replaced at 60,000 or 90,000 miles anyway.

Organic Acid Technology or OAT's are coolants that are formulated without the use of silicates. Since silicates have a short life, removing them allows a longer service life. Typically, coolants advertised as 5 year, long life , extended life or Dex-cool are examples of OAT's. Common components to OAT coolants are Carboxylate, sebacate, and 2-ethylhexanoic acid (2-EHA), which are corrosion inhibitors, and are used instead of silicates. Depending on which manufacturer is filling the jug, the inhibitors may change. Dex-cool uses 2-EHA which among other things is a plasticizer which has the effect of softening plastic. GM intake manifolds are made of plastic........... If it is found that Dex-cool is dissolving the gasket material, it will validate what some folks have been saying for a long time: Dex-cool eats gaskets. We'll see. Prestone Dex-Cool doesn't use 2-EHA in it's long life coolant, instead uses carboxylate.


As long as the cooling system is in good shape and the chemistry is balanced, these coolants have demonstrated that in fact they can go longer that their predecessors. However, when the balance is upset, either by improper filling procedures or weak concentrations or improper manufacturing, the OAT can come apart, causing thousands of dollars in repair bills. GM's Dex-cool is a huge problem on some vehicles. Class action suit pending.....

Hybrid Coolants, also known as HOAT's or G-O5 is a OAT coolant with a low silicate charge added. The idea is to reap the benefits of longer life OAT while retaining the positive characteristics of silicates. In areas where hard water deposits have been a problem, the HOAT blend holds up very well. The main inhibitor in popular HOAT's is Benzoate

Most GM cars changed from green coolant to Dex-cool in 1996, while Chrysler went from green to HOAT in 2002. Ford changed to HOAT in 2003. The European vehicles have never been a fan of traditional green, and have been using HOAT or OAT right along. The Asian vehicles switched from HOAT to OAT in 1996. The trend is clear, silicate levels are dropping, and as OAT inhibitors get better, soon silicates may be a thing of the past. Thanks to Zerex, a colorful chart is available to pinpoint coolant types for various vehicles.


Propylene Glycol is a colorless viscous hygroscopic liquid, CH3CHOHCH2OH, used in antifreeze solutions, in hydraulic fluids, and as a solvent. Although the thermal properties of PG (propylene glycol) are less desirable that EG (ethylene glycol), there is one main advantage- it is not toxic to humans and animals. EG is sweet to the taste and animals are attracted to it by curiosity and smell. Because of it's taste and toxicity, EG is responsible for many pet deaths per year. PG may be the logical choice if garage space is shared with a pet. Recent formulations of PG include a low silicate OAT with purported 7 year 70000 mile life. We'll see about that.


Waterless Coolants. Not enough is know (by me) about waterless coolants to be able to comment on them. However, the idea is this: if we can get rid of the limiting properties of water e.g.: boils at 100C`, freezes at 0C`, allows corrosion, conducts electricity & allows electrolysis to occur, while increasing the thermal absorption of the coolant, ...viola`! We'll see.


Coolant Additives are not coolants themselves, and must be used cautiously. They change the chemistry and properties of the coolants so that desired "behaviors" are achieved. The biggest sought after behavior is the reduction of surface tension. Other goals include pH reagents, electrolysis suppressors, water soluble oils, increased alkalinity reserves, the list goes on and on. While very useful in treating a specific conditions, it is not recommended that a person put one of everything in a cooling system. Not only would the coolant be diluted, but the chemistry might be quite interesting!


Which coolant is right for my car? Green? Orange? Blue? The folks a Zerex have manufactured 3 coolant to address the 3 main type s of EG coolant, and have generously supplied a colorful chart to help answer questions as to proper coolant selection.