The answer here is in two parts:
#1: you have to understand that all metals corrode. aluminium however, will protect itself, with its own corrosion. Steel and Copper will not. So... the next thing to understand is that if two metals are electrically connected with water, the corrosion process is accelerated, because of the nature of the metals. Obviously, a poly top does not create this accelerated corrosion.
#2: you need to understand that the anodized aluminium offers some protection, but still leaves potential for the same corrosion, so you have to use a corrosion inhibitor. Further, the anodized finish can easily be scratched, especially if metal barbs are installed. Once exposed, not only will the corrosion be accelerated, it will be accelerated at the exposed point of the aluminium, which can lead to an early failure, and leak.
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