The corrosion is result of a reaction between the different metals within the system, namely copper, brass and steel. The steel gets coated with residue from the copper and brass, which starts a chemical process, similar to a battery, called electrolysis. This starts up a reaction within the system which corrodes the steel element, namely the radiators and boiler which is the sacrificial element, this in turn creates what is called iron oxide sludge, a black viscous liquid.
If this is left unchecked it not only causes radiators to leak, but reduces the efficiency of the heating system, by causing sludge to build up within the boiler and radiators, which in turn costs more in fuel bills.
This can be eliminated by adding a corrosion inhibitor and a magnetic filter to the initial installation, or by having a system that has been effected, dosed with a chemical cleanser to break down the sludge and power flushed, then install a filter and add the corrosion inhibitor. The filter should be checked and cleaned and the system water checked to see if the corrosion inhibitor needs topping up when the boilers annual service is due