Jeff - usually called 'black oxide', like in the gun blueing process, rather than magnetite ... as opposed to red oxide, which is the rust
and Denny - note that nothing you do to remove the rust from the radio cases will restore the cad plating, the resulting clean steel patches will still require painting with something to prevent re-rusting .... I've used cold galvanize spray zinc to touch up cad plated things more or less inconspicuously
Bill
and Woody - as someone
who makes real vinegar, I can suggest you spend a LOT of time rinsing all the pinch weld joints well, maybe even flush with some metal prep [phosphoric acid], or you'll find vinegar not only works while you sleep, but works thru your new paint job
