Inner Coil Pole Corrosion happens when the magnet rod polepiece on your single coil pickups starts to rust causing the insulation to break down inside the coil. If the insulation (Formvar or Plain Enamel) breaks down, the exposed copper wire can oxidize and eventually break inside the coil. This usually happens from excessive moisture inside your guitar case and/or moisture from sweat or environment. Moisture can seep into the inner edges of the flatwork and retain higher amounts of moisture within and around the magnet, eventually causing them to rust.

As the oxidation and rust that travel down the diameter of the magnet it can put extreme pressure on the already tightly wound coil. Leo Fender used wax potting to reduce unwanted moisture form penetrating the coils and thus reduced insulation breakdown for a longer period of time. Older insulations can be quite brittle and vulnerable to outside elements, especially moisture and even extreme dryness or heat. I’ve seen this happen to many older instruments when the coil is wound directly onto and around the magnet. Coils wound on molded bobbins (humbuckers) usually never have a problem with ICPC. The problem usually exists when the poles are subjected to excessive environments.

Moisture is the biggest problem that can cause ICPC. If the coil has no protection from moisture (wax potting) the insulation can shrink, leaving fine cracks in the insulation.