One needs to understand how alum works. It actually creates a sorta of protective skin on itself. BUT.... It must be exposed to dry air to do so.
When a tank is foamed in to stabilize it in place, where the foam is ... is not exposed to dry air.
Tanks are in bilges and by the very nature of a bilge is a Dark, Damp place.
Now add a water leak..... The leak you never know of, until it's kinda late.
The foam used is a Urethane 4lb closed cell foam to lock the tank in. There are a few lag bolts that hold it also... But those are there also to hold the tank from floating up during the foam pouring process.
There are tons of articals out there that say Don't Foam a tank in.
I contend they have not really thought the WHOLE process thru.
In THAT situation.. They are relying on 4] 1/4 or 5/16in lag bolts to hold the tank.
One must remember and we will just use a 100gal tank in our talk here.
100gal X 6.5lbs per gal [what gas weighs] = 650lbs
Those 4 fastners would work just fine for a object that just sits there as a static display.... A boat is Anything but a static display. The forces put on a boat are tremendous. Especially 1 of my young customers.... He likes to Fly the Hull.
I'm on his ass all the time about that. But he's Young and Dumb and has plenty of $$$.
Back on Subject....... The answer is Keep moisture OFF the alum.
The surface gets prepped and then Coal tar epoxied. Then Lag bolted in....Then 4lb density foam poured or cut even with the top of the tank... Then fiberglass gets installed bridging from coffin wall across the exposed foam and onto the top of the tank.
This protects the alum from the elements and also protects any water intrusion ever to get into the foam.
Thats How you make one Last.
