What would cause corrosion in engine bay?

It's the stock supercharger pulley (second to last picture in the first post). The truck doesn't live in a moisture rich environment, it doesn't get foggy and I'm not right on the coast. Car has pretty much always been parked under a carport as well...I don't like things looking like that so it annoys me.

 

Hard to guess via pictures alone, but based on my metallurgy and corrosion knowledge, I'd argue that's a poor quality metal pulley shaft and the less pure metal alloy is a leading cause of the state it's currently in.

That said, living in or even close to SF is close enough to reap the ill effects of salt water in the air, you don't have to be that close really.

If it's not that old of a setup, I'd contact the company and inquire about some quality control / warranty work. If it's out of warranty, lightly sanding and coating will help, but not fix for ever. I would be cautious about spraying too much anti-corrosion miracle sprays on that area without protecting the surrounding non-metallic parts.

Me, personally, I'm not a big advocate of the sprays that 'reverse or stop corrosion' anyways. They may help clean off the existing surface rust, but they aren't reversing crap and the aren't preventing crap. So plan A would be to sand down to white metal, clean/prep and coat with something...that's just a tight little space to do that in.