Corrosion - Galvanic Corrosion

Galvanic corrosion is an electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte.  The same galvanic reaction is exploited in primary batteries to generate a voltage. In this case your radiator has turned into a battery and is having the same chemical reactions that you find there.  One metal becomes an anode and corrodes a lot faster than it usually would - with the other metal becoming a cathode and corroding slower.

The principle behind Galvanic Corrosion has long been used to generate electricity in lead acid batteries.  Two disimiliar metals placed in a solution use the electrochemical reaction to generate a charge.

Unfortunately, under the right circumstances the same thing can happen in the cooling system.  In the absence of an effective corrosion inhibitor the engine's cooling system can effectively become a large low output wet cell battery, but with disastrous results for its alloy components.

Metals with a high galvanic potential are eroded away and deposited in to other more stable metals within the cooling system, such as the engine block or the steel impellor of the water pump.