Flangehead
Some call that deterioration of the metal Zinkpest. Not sure what causes it myself. I was given an old Mantua switcher that was infested with it, the cab ,drivers and tender trucks were the only cast metal parts that weren't affected. I don't understand why those parts weren't affected.
I saw a tender for that Rivarrossi at a show last year, Parts for the old Rivarossis are difficult to find, but they're out there.
I loved seeing that loco, it's one I don't have.