Steam from under the bonnet!
"Stop and get the cooling system looked at now"
Electrolysis is stray electrical current produced when the engine is operating. This stray current produces a devastating corrosive effect on alloys, like a cancer slowly eating away at unprotected parts of the vehicle, such as the alloy cylinder head, water pump, radiator and many water fittings that are vital components of your cars cooling system.
To be kept in peak running condition a cooling system is certainly about maintaining the correct operating temperature and reducing corrosion. It also involves controlling these stray currents (the unseen danger to modern cooling systems).