At the time of writing, we offer S15 mags in three different coating options: black oxide, electroless nickel, and black electroless nickel. Black oxide offers the least amount of corrosion resistance and electroless nickel offers the most, so if you live in a humid/wet environment, or if you sweat a lot, we recommend one of the two electroless nickel options.
If you have black oxide mags and you're having issues with rust, we recommend that you immediately clean the rust off the magazine, and then coat the magazine in dry lubricant like EEZOX. So far in our testing, EEZOX has worked extremely well as a CLP, and best of all, EEZOX is inexpensive and can be used on all of your gun parts.
-Brandon