A typical Alnico magnet used in the majority of guitar pickups consists of several different alloys. Aluminum, Nickel, Cobalt, Copper, Silicon, Titanium and Iron. With the presence of Iron, a magnet can rust due to moisture in a case, high humidity in the air and most likely the sweat coming from your hands. Moisture contacts the pole piece and can cause it to rust. Most companies do not plate the Alnico pole pieces in their pickups. Fender plates their Cunife magnet pole piece used in their guitar and bass humbuckers. Gibson plated the magnets on their rectangular adjustable Alnico Pickups, used on the 56 (neck position) Les Paul Custom . Nickel, Gold or Chrome plating helps reduce the oxidation (rust) unless the plating is worn off from extensive playing. Rusting of the poles is why many Vintage Pickups stop working. The rust continues into the bobbin where the coil is wound, breaks down the insulation until the copper magnet wire becomes exposed and shorts out then there's no reading in the coil To help prevent this in single coils it is best to keep your instrument's case dry especially in hot humid areas. Try to keep your guitar in cool areas (not in hot car trunks) so the case doesn't sweat. It's amazing how much moisture your case can absorb. Wipe down the strings and pickups after playing with a dry cloth. I've put my guitar case in a room with a dehumidifier to take the mosture out. Always keep your case closed when rehearsing. There are special potting processes that can help keep your pickup working longer. If you have specific questions you can write me. I'd be glad to help.