I have a Magfire Pro that I've been carrying around on my EDC keychain for about 3 years now and it has a thin layer of corrosion/oxidation that has to be scraped off before it will spark well. This is not the Firesteel that I use on camping, hiking and skills trips though it always goes. It's my backup so it doesn't get used.

You might use yours enough and on a regular enough basis so that there's no noticeable corrosion/oxidation buildup.

There's not a lot of buildup on my regular use steel.