I was referring normally to replacement (aftermarket) caps.
Most factory molded plugs are made of what seems to be nickel-plated brass (it definitely isn't steel because it's not magnetic). On a side note, I have a small GE alarm clock (probably 1960s or 70s vintage) where the blades on the plug are aluminium!!
The rusted prongs on cord caps I've run across are American.
Joe Tedesco once posted a picture of a stump of lamp cord with one of those round rubber replacement caps and the pins were visibly rusted.
I haven't been able to find the picture (perhaps it was deleted). Maybe Joe can repost it?
Don't have a digital camera, or I would take a pic of one or two of such old ones I've removed that I still have lying around at home.