Apparently the ethanol content has 2 negative qualities. It allows the fuel to hold more water and it also makes the mixture conductive so the alloy can also experience "galvanic corrosion". As long as the ethanol content is below 10% then the oxide layer already present on the surface of the alloy should be enough to protect it.
The issue with tanks that have been sealed already, maybe several years ago, is that the sealer used may not be ethanol resistant.