The reasons for the metal corrosion can be found in the layers that are in direct contact with the metal, and the metal itself. Referring to the disc construction diagram again, the polycarbonate substrate is on the bottom side of the metal. In order to not affect the metal, the plastic resin used must be as pure as possible. Any impurities in the plastic could move from the plastic and attack the metal surface. To prevent this from occurring, manufacturers should always use high quality polycarbonate (which is more expensive) that is free of damaging impurities. The metal itself must also be free from impurities, to keep the disc from corroding from within. It is this absence of potentially damaging impurities that contributes to longevity, not whether the metal is aluminum, brass, or gold. In addition, to make sure that the drive gets enough readback signal, the metal layer should be thick enough to be essentially opaque, with no pinholes to add errors. The next layer, the seal coat, is also important to the longevity of CD discs. Not only does it have to be free of impurities, it has to be formulated to not react with the metal that it is covering. And to really "seal," it also must be designed to stick to both metal and plastic. This insures that it will seal properly at the inside and outside edges, where there is no metal to stick to. This story isn't over yet, as the ink used on the disc label is important too. The ink must be chemically compatible with the seal coat or it will "eat" through the disc over time. As discussed, as long as manufacturers take care to use the right raw materials and processes, there is no reason to think that all discs will fail.

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