I also have a number of guns with blued mags which corrode a lot here in the tropics. I have had about twenty mags either hard-chromed or electroless nickel plated at a place called Techplate in CA (www.techplate.com
). Service is fast and prices are affordable. They have a customer rep there named Marshall who is very helpful with questions. None of my plated mags corrode anymore after the plating. The cost was about $10/mag. Good luck.
Happy New Year- oldandslow