Aluminium corrodes faster because it is more reactive and so corrodes in preference to the iron. The protective coating only occurs in the air because it forms a hard layer, but when it is in a constant flow of coolant, this doesn't occur.

While it is true that pure distilled water is not conductive, all it takes for it to become conductive is some ions in the water. As soon as it is put in the engine, it would pretty much become conductive because it would have some impurities in it from inside the radiator and engine.

Using water will definately cause more corrosion within an engine, especially to any aluminium parts, like alloy heads and the thermostat housing. Coolant helps to prevent corrosion and stop the water from boiling or freezing, and the effect of the coolant viscosity on reducing the cooling in the engine by the lack of convection currents is not really significant unless you are running a huge concentration of Glycol. I just run the cheap 5L bottles of Tectaloy in my car, and it has never caused any problems.

_________________
Before you judge someone, you should walk a mile in their shoes. That way, when you judge them, you are a mile away and you have their shoes.

I use the Nulon ethylene glycol concentrate, it avoids paying money to people for coloured water to carry home.
I mix it 25% with balance of tap water.
When it stops looking green due to water topups, I drain it and start again.

No problems with cooling on either the 1360 or the 1293, even sitting in traffic with 40C heat.

_________________
DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Makes it diificult for those seeking knowledge when people are so loose with teh language.

Water is best for cooling ....but has some downsides which need to be remedied by adding chemicals. You can add soluble oil for the water pump, a corrosion inhibitor, (so called) antifreeze to prevent freezing in cold climates and to extend the boiling point when hot, surface tension modifyers to improve heat flow.....

or you can just add proprietary "antifreeze" to ordinary water in the recommended dose. Do not be tempted to add more than recommended (or use it straight as i have seen) as this will reduce the effectiveness of the radiator.

Coolant is the stuff that goes in the radiator; it may be straight water or.......

Cheers, Ian