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Antifreeze works on the principle that the addition of an impurity to water lowers the freezing point and raises the boiling point. 

Many years ago antifreeze was only added to the cooling system at the onset of winter.  Protecting an engine from freezing is only one of the functions of a modern antifreeze. It also plays a major part in improving the ability of the water to cool the engine by increasing heat transfer. 

It has important anti-foaming properties. Fast-revving engines may cause the coolant to foam and excessive foaming allows the formation of air pockets – which, in turn, result in poor cooling. The bubbles generated by the rapid spinning of the water pump impeller cause cavitation. This is a condition caused by the collapsing vapour bubbles in coolant on the surface of the metal which result in high shock pressure. The shock waves cause development of the small pinholes in metal surfaces such as the water pump impeller or the walls of a wet cylinder liner – which could reach as far as the cylinder itself. This isn’t a long term problem, it happens quite quickly. Two ambulances recently inspected in Scotland running on a 23 per cent antifreeze mix had both seized the engines because the brass water pump impellers had been eaten away by cavitation. 

Another major function is corrosion prevention inside the engine and radiator as engines are full of electrochemically incompatible metals – cast iron, aluminium, copper, lead solder etc. Aluminium alloy engines are particularly vulnerable to corrosion. 

One to the first antifreeze substances used was methanol, in the 1930s and continued to be used in cheaper antifreezes for a long time, but it was too volatile and evaporated relatively quickly. From around 1937 ethylene glycol solutions were favoured. This was a good anti-freezing agent, but, on its own, was really corrosive. Hence the addition of corrosion inhibiting additives.   Much later BS 580 was introduced to make sure methanol based antifreezes and sub-standard corrosion prevention additive packages were not available. 

Over the past few years the additive packages used in antifreeze have become more critical because of smaller cooling system volumes, higher temperatures, higher pressures and greater use of alloys. Smaller systems are heavily loaded and are not as forgiving as the older, higher capacity cooling systems. On top of that vehicle manufacturers are specifying longer service intervals. 

The fundamental ingredient of antifreeze – glycol – has not changed very much, but the additive packages certainly have.  The vast majority of car manufacturers now use Organic Acid Technology (OAT) inhibitor packages which can be used in a base of MEG (Mono-Ethylene Glycol) or the less toxic MPG (Mono-Propylene Glycol). 

The terms G11, G12 and G12+ are classifications of antifreeze developed by the VW Group, made by BASF, but seem to have gained universal acceptance. 

The traditional antifreezes are generally classed as G11 – usually blue, green or yellow.  These contain phosphate, nitrate, nitrite and silicate-based additives. They are used in a base of mono-ethylene glycol which is the toxic stuff.

Mono-ethylene glycol (MEG) has a density of 1.110 and registers on a standard issue tester/hydrometer. It is also the basis for many of the OAT antifreezes which vary in colour from florescent yellow to red, and are classed as G12. BUT Toyota use a red non-OAT antifreeze and the MG Rover SV uses green/yellow OAT – so you certainly can’t rely on the colour to tell you which sort of antifreeze is being used in a particular engine. Japanese antifreeze is phosphate based with an OAT package.

Silicate levels in G11 antifreezes vary. The level used to be as high as 1000 ppm, but this has been reduced to around 250 ppm. At this level there shouldn’t be any precipitation of silicates if mixed with another type of antifreeze. You may have heard of, or seen, cases of horrible green/yellow gel/sludge in the radiator if different types of antifreeze are mixed.  BMW still favour a conventional high silicone green anti-freeze.

G12+ is another OAT based antifreeze, purple in colour, but this can be mixed with G11 or G12 without causing any problems.

Antifreezes needed to become less toxic and mono-propylene glycol (MPG) is now used by some car manufacturers, such as Fiat – and it’s compulsory in Switzerland, mixed with the G12 or G12+ additive package. MPG has a density of 1.033 and although it might register on your ordinary antifreeze tester, the figures won’t be right. Accurate measurement needs a refractometer. 

MPG and MEG are compatible and mixing won’t cause any problems – in fact they are used together for use in the Arctic.

Most vehicle manufacturers specify a 50:50 mix of water to antifreeze.  The principle of an impurity lowering the freezing point is limited. The minimum freezing point achievable by this method is around -50ºC and occurs at 60 or 70 per cent concentration. Above this concentration it will not lower the freezing point further and at 90 per cent antifreeze concentration the solution will begin to freeze at only -20ºC (it doesn’t form proper ice – it’s more like a nasty thick sludge). Corrosion and cavitation prevention need the 50:50 mix.

A combustion gas leak (resulting from head gasket failure) will deplete the corrosion inhibitors as the carbon dioxide dissolves in the coolant and forms carbonic acid.

Now the risk of silicate precipitation has been reduced there is still an issue with different corrosion inhibitor packages which can work against each other.

If you check a car which has an unclear service history, you’ll have no idea what is already in the cooling system. It could have been topped up a dozen times with different antifreezes so to avoid the possibility of production of the gel it is safest to use a G12+ antifreeze such as Comma Extreme Red. This is a very bland/non-reactive mix and shouldn’t cause any problems.

© Vanessa Guyll, November 08


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