Coolant & Antifreeze Explained


Approved trader
Redruth/ Sunny Cornwall
Coolant and antifreeze is a topic we are asked about on a regular basis, and often causes confusion; types, colours, service life etc.

Perhaps the most frequent cause of confusion is what the difference between anti-freeze or coolant is. Basically, they’re the same product (although the term “coolant” could just be applied to plain water; see below!)

To help clear some of the confusion up on the more technical details of antifreeze and coolants we enlisted the help of Martyn Mann – Technical Director, Millers Oils UK - who has provided the information below.

Not all antifreeze / coolant is the same!

Coolant can be plain water; water is a very effective coolant but would not protect against sub freezing temperatures or protect against corrosion inside the engine. The use of antifreeze protects against both problems.

Antifreeze not only suppresses the freezing point of your engine coolant, but provides good corrosion protection and increases the boiling point during use.

Most commercial antifreeze formulations include a glycol (to suppress the freezing point and raise the boiling point), corrosion inhibiting compounds and a coloured dye (commonly orange, green, red, or blue fluorescent) to aid in identification. A 1:1 dilution with water is usually used, resulting in a freezing point in the range of minus 37 °C to minus 42 °C, depending on the formulation.

There are two basic types of coolant available today dependent on the corrosion inhibitors used:

·inorganic additive technology (IAT)
·organic additive technology (OAT)

Inorganic Additive Technology

This is the traditional coolant based on inorganic additives and is called inorganic additive technology (IAT). It is a tried and proven chemistry that provides a fast acting protective film. The additives deplete and the coolant needs to be drained and replenished every couple of years. This type can be used on all mixed metal engines with components including steel, cast iron, copper, brass, aluminium and solder without any detrimental effect.

Organic Acid Technology

The newer OAT coolants work differently from the older silicate based IAT coolants. Aluminium and ferrous metals form a surface-layer of corrosion in the presence of moisture, even with the little bit of moisture in the air. OAT coolants prevent this metal-oxide layer that protects the surface against this corrosion. Inherent with their design, the OAT coolants last longer than the older traditional IAT coolants. This category of antifreeze cannot be used in systems containing yellow metals.

A couple of questions and answers.

Why are coolants different colours?

Coolants/antifreezes are coloured so you can visually see them; colour intensity can be an indication of over dilution. The different colours are non specific to the different types of antifreeze. The manufacturer can dye the product any colour they want. The colour is no guide to the actual type of antifreeze type and the label should be read before use.

What is best for performance use?

It is always best to use the engine manufacturer’s advice. If engine contains yellow metals [copper and brass as in older vehicles] then the long life products based on organic technology should not be used. As a general rule, most modern engines require the long life organic antifreezes.

Is there any advantage to using concentrate over pre-mixed coolants?

None other than the user may want to use the pre-mixed product due to ease of handling or cost and visa versa.

Can concentrate and pre-mixed coolants be mixed?

A simple answer is that you can, however do not mix IAT and OAT antifreeze together.

So, there we go. Hopefully this information has been useful, if you have any further questions not covered here please ask and I will try to get the answer.

With thanks to Martyn Mann and Millers Oils.


Guy and the Opie Oils team.
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This is a worthwhile article. I discovered (thankfully, not to my cost) that the colour of the concentrate has nothing to do with the formulation. For the purposes of my questions I'm going to treat anti-freeze and coolant as one and the same, as you have done.

1. What is the deal with universal antifreeze - whether pre-mixed or in concentrated form?

2. I have had my car refilled with BMW OEM coolant concentrate (correctly diluted), which is allegedly 'Free from nitrite'. Is this an OAT coolant or is it a traditional polyethylene glycol product?

BMW's 4 yr replacement schedule implies that it's a traditional formulation so I don't know what to do. Change regularly or leave well alone?

Sorry to fling questions about the place but you've dug up a couple of things which have been pestering my mind for a few months. Thanks in advance for your kind and courteous attention.

There isn't really such a thing as a universal antifreeze (I guess water could be classed as one, but that's about it). While many antifreezes can meet several specifications and be suitable for a lot of cars/bikes, none are suitable for everything. There have been legal cases over the results of using 'universal' antifreezes.

I don't know much about the BMW coolant. Looking through the info I have for the E39, it says they just use a pretty normal coolant with no particular specs that need to be met. With the lifetime of the fluid being 4 years, I would assume that it should be an OAT fluid, but I can't find a definate answer.


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