Slant six exhaust manifold choke bucket

All the old ones I've ever seen were either rusty as HELL or slap rusted through.
How do they rust when on the engine with the hood closed? Yes if they sit out in the weather and fill with water it’s a different story.

I have half a dozen here that are obviously used with carbon and gasket material on them and They are in great shape, solid, not rusty at all.