I've heard the chrome thing too, but its supposedly safe on all surfaces.
I've just been erring on the side of caution cause I've been too lazy to
ask someone important.
Its a pretty cool product though. Especially useful for new cars with
rail dust and cars with deeply embedded contaminates.
As for exactly how it works, I don't think I could provide a better
description of the actual chemical reaction that takes place than
CarPro does here:
Iron X
It sucks as a dedicated wheel cleaner though. Sonax, which IMHO is
an overly hyped product, is better at cleaning than it is.
Use it straight, hold your breath (it smells like a stink bomb) and if its
windy close your eyes (ask me how I know).
One thing I'm interested to try in the future is Wolf's Chemicals Brake Duster.
Supposedly it works like Iron-X does on paint, but is also good on wheels too.
Gonna try to put an order in for that this week.