I went back to the 'Taller' (fabricator) for these tanks and he stated he'd had indeed ordered 5063-H116 (which I did see in his shop aluminum being used to make a tank for a 'local' captain
on a 80-foot mega yacht --a single
~200 gallon tank-- which was laser inscribed with 5063-H116).
For my tanks (I had asked for 5052-H32), apparently the supplier sent him unmarked aluminum sheets
. When this was questioned, the supplier insisted the sheets
were the correct grade. However, the SKU label sticker placed on the sheets by the supplier absolutely showed the sheets to be a rolled aluminum laminate of alloy 1100F (which is really just a low impurity aluminum and not an 'alloy').
As what I have learned off the net, 1100F can only be 'rolled' (mechanically) hardened (this is done by the press during the lamination process --the amount of pressure is the 'F' hardness [? I believe]). That the welds are harder than the 1100F tank itself, and on larger tanks, cracks can form between the harden (O-hardness depending on the weld rod alloy) weld and the soft malleable pure aluminum (1100).
Corrosion, however, is the main issue --as you've mentioned-- since the metal is surfaced pressed hardened, once a pit starts (like a tooth cavity in one's teeth), the corrosion goes fairly fast even through 6.045 (~1/4 inch) thick plate (not because of the hardness but because it lacks the 'alloys'); that the pitting will more than likely happen inside the bottom of the tank even if the sump is drained often. Surface painting on the outside to control the corrosion helps, but anyplace the paint
system may fail, like near a stringer due to mechanical reasons (abrasion caused by rubbing from even slight movements), the tank can quickly start to corrode, even if dry, just from being in a 'salt-air' (marine) environment
Since I have gone back to the 'Taller' --today-- and have confirmed with him the tanks are not being made from the 'requested' and 'correct' alloy of aluminum, he has stated he will reorder new aluminum sheets from the supplier and he will ensure the correct aluminum sheets will be used. At least this was caught early-on as only the first of four tanks --labor-wise-- was almost completed.
As far as using the 1100F tanks, I have, with no shadow of a doubt, no desire to accept anything less than the 5052-H32 alloy. Especially since getting these tanks in and out of the boat was no small feat. So hopefully I have an honest tank fabricator who will do right by me and get the correct aluminum 'alloy' tanks made.
I will try to post any follow-ups --if anyone is interested-- as things progress (and not digress!). But as far as the talent welding the tanks, both mine with the incorrect materials and the 'local' guy's ~200 gal tank with 5063-H116, the work the shop has done is excellent, and the shop does have a way to properly pressure test the tanks.
So I am hopeful this issue will get resolved correctly.