How corroded?
If they just have a little corrosion, you can just wipe them off.If they're ugly corroded (like, may blow apart on use kinda corrosion), dispose of them.We keep all outdated flares on-board, but seperate from current flares.Then, on a CG inspection, we easily meet code. However, in an emergency, if we run through the current supply, we have all the old ones for back-up, and hopefully they still work.Agreed with others that you should practice with a few old ones, just so you know what to expect, should you ever need to really use your flare gun for an emergency. (Make sure you do this where they won't be confusted with a real distress situation.)