Causes
In CDs, the reflective layer is immediately beneath a thin protective layer of lacquer, and is also exposed at the edge of the disc. The lacquer protecting the edge of an optical disc can usually be seen without magnification. It is rarely uniformly thick; thickness variations are usually visible. The reflective layer is typically aluminum, which reacts easily with several commonly encountered chemicals such as oxygen, sulphur, and certain ions carried by condensed water. In ordinary use a surface layer of aluminum oxide is formed very quickly when an aluminum surface is exposed to the atmosphere; it serves as passivation for the bulk aluminum with regard to many but not all contaminants. CD reflective layers are so thin that this passivation is less effective. In the case of CD-R and CD-RW media, the materials used in the reflecting layer are more complex than a simple aluminum layer, but also can present problems if contaminated. The thin 10-20 thousandths of an inch layer of protective lacquer is equivalent.
DVDs have a different structure from CDs, using a plastic disc over the reflecting layer. This means that a scratch on either surface of a DVD is not as likely to reach the reflective layer and expose it to environmental contamination and perhaps to cause corrosion, perhaps progressive corrosion. Each type of optical disc thus has different susceptibility to contamination and corrosion of its reflecting layer; furthermore, the writeable and re-writable versions of each optical disc type are somewhat different as well. Finally, discs made with gold as the reflecting layer are considerably less vulnerable to corrosion problems, though no less susceptible to physical damage to that layer. Because they are less expensive, the industry has adopted aluminum reflecting layers as the standard for factory pressed optical discs.
Read more about this topic: Disc Rot