Hey Gaminggod, most computer cases are made out of...
- satin-coat galvanized sheet steel - dull matte gray finish, good for painting up all pretty like
- chromed sheet steel - shiny chome color, mostly on really cheap cases
- aluminum - yup, needs no protective coating, paint optional / cosmetic except for marine environments
Unless your computer is in an unheated draughty space with high humidity, or a greenhouse, a boat, your bathroom, a swimming pool room, or a suana, you won't have any trouble no matter what you use. The zinc galvanizing is there specifically because it is far more reactive than the steel, so it corrodes first, turning a very dark dull gray like lead that has been outside a long time (think car battery terminals).
The other common types of sheet metal are...
- regular galvanized - crystalline pattern, think funace ducts, good stuff but ugly and doesn't paint well
- factory painted - metal roofing and siding, paint is not super hard so it scratches, white/brown for misc. flashing jobs is most common
If you ask at a sheet metal shop, they will likely sell you a peice cut to exactly the square dimensions you want, cut in a shear, which leaves the edges absolutely straight and clean and unwarped, like you will never get at home. Expect to pay from $1 to $2 per sq.ft..