I just finished visiting the C-Products site. In the evaluation document they post, the light coat of oxidation is visible through the catch hole - http://www.cproductsllc.com/eval.pdf
They also are quite up front about it in their FAQ's http://www.cproductsllc.com/faq.html
Q: Will a chrome silicon spring rust?
A: Yes, initially it will develop a slight film which will not affect the function of the spring.

I have both the AR Stoner and C-Products mags and they have performed flawlessly. The stainless steel mags are unbelievable, virtually indestructible. They see a lot of very wet use and long storage in humid conditions. I do a lot of shooting and varmint hunting in very sandy areas here near the lake. I have not seen any rust or had any failures from sand contamination. I do always very lightly lube my springs with Slick 50 or ATF thinned with mineral oil. I must note that the sand here in NW Ohio is much coarser that our boys in the sandbox are experiencing. Probably best to leave them completely dry in that environment.
My only complaint is the floor plates are extremely hard to remove for cleaning, but I can live with that. They have never popped when dropped on on the rocks.