If you add a salt system to an above ground or inground pool it should not void the warranty however some pool manufacturers may elect to not cover damage caused by misuse of a salt system by excluding its use in their written warranty.
If you test the water of an older pool that has been using chlorine for many years you will find a slight salt level in the water. If you add a salt system to the pool's equipment the slightly salinated water then passes through the electrolytic cell to produce chlorinated water, which prevents algae and purifies the pool. As the chlorinated water kills bacteria and burns up it converts back to salinated water and the process starts over again.
If the salt system setting is turned up to high it could produce a very high level of chlorine which could cause corrosion. If you test your pool chemicals on a weekly basis and maintain a 1.0-3.0 chlorine level and pH level of 7.4-7.6 corrosion will not be a problem for your pool.