I've repaired a few steel tanks that have rotted though and they have all had the most serious corrosion on the side stand side at the lowest point which I took to be from the presence of water either though ingress or from aqueous ethanol coming out of solution.
Quite often what looks like just a small pinhole or two turns out to be the tip of the iceberg with whole areas around the holes left tissue paper thin.
I don't know the history of one particular tank that was holed and thin but had no visible rust on the area inside the tank. It may have had the rust removed/treated before it came to me but from the description above I guess it could have been acidic corrosion.
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