A degree of disc corrosion is normal but this is usually worn off when the vehicle is driven. It seems unlikely that there is a fault with the discs themselves as this has occurred with two different sets, so it seems most likely that it is a combination of the environment and the frequency of you driving the car that is causing the discs to corrode prematurely.

Unless you are able to park the car in a garage on a regular basis, it may be that a regular longer journey is required to enable the discs to be cleaned by using the brakes. If you are usually driving in town it may be that you have no need to brake hard enough for the rear discs to engage sufficiently, so braking from higher speeds and for longer periods may help.