Old coolant can become corrosive itself also, pH becomes acidic. More of a problem with diesels than gas I think, and more of a problem with aluminum radiators than copper. In general the more aluminum parts in the engine, the more vulnerable also I'd say. Stay up on your coolant changes and use the grounds mentioned. Would be nice if someone made a radiator cap with a zinc anode hanging from it...

At some point, corrosion becomes rampant and takes on a life of its own in the entire system or vehicle. Once the corrosion has become that entrenched, there is almost nothing you can do to stop the process more than briefly. You put new parts on and they corrode within months. When it gets to that point, time to look for another ride.