Lame excuse... Iron reacts with the oxygen of the air and forms oxides of iron or rust. It happens readily but you do not see it readily with naked eyes. It takes some time for the rust to be thick enough. (like the brown formation we know of as rust) Rust formation slows down because the initial layer prevents the oxygen and iron from getting in contact and reacting. With a good painted case (and most of them are galvanized) however it would take several years before any signs of 'rust' would be noticeable and even ages before your case collapses because of rust, unless you have your case outside in the monsoon 24/7 (I would worry about the hardware inside it more) the case would be fine.