A cast iron radiator can look absolutely beautiful for decades…. If you strip the paint from a cast iron rad, that same rad would change to a rusty-orange orange colour in a couple of days, yet never rust through. Cast iron responds differently to rust than steel does; steel actually gets eaten away.
The number one thing that destroys anything is oxygen. Another reason why hydronic heating systems last as long as they do is because we’re letting air out of the system, so there’s no oxygen. Once the water is heated and cooled, and the rads are bled, most of that oxygen has come out of the water – another reason why heating systems last as long as they do, because they’re full of water.