When you have metals joined with a liquid like car cooling systems, the metals produce electricity causing the corrosion
This is how a dry cell battery works
For a battery produce electricity. The + terminal of the battery is a corrosion-resistant metal while the - terminal is a less corrosion-resistant metal and therefore corrodes. This makes the power in the battery. When the least corrosion-resistant metal is corroded away, the battery is dead.
So when you think about it our engines are big batterys slowley corroding away from the inside.
The best way to stop it is to use one metal for the entire engine and cooling system.
alloy block, alloy heads, alloy radiator.