The inside magnets are ceramic, they're not plastic-encased or anything, and won't rust or dissolve in the tank. I dunked one side of each ceramic magnet with some plasti-dip because their edges can be rough as well.
The drawback to the ceramic magnets is that they're not nearly as strong as the neodymium magnets - but they're strong enough to hold stuff up if they're paired up with a neo magnet on the outside of the tank. I haven't personally had any corrosion problems with the nickel-plated neo magnets getting a little splash on occasion, but I imagine it happens.