 It is most commonly attributed to oxidation in the aluminum layers by poor quality adhesives used to bond the disc halves together.Poor adhesives separate over time, which allows oxygen in the air to corrode the thin aluminum layer into aluminum oxide, visible as transparent patches or small dots in the disc.Corrosion is possible due to the thinness of the layer; normally aluminum doesn't corrode because it is coated in a thin oxide layer that forms on contact with oxygen.