Well, cut a 20 cm piece out of the bottom of one tank and found more or less what I was expecting; many isolated spots on the inside of the tank where the metal was eaten away and one pin hole where it was eaten all the way through.
What I didn't expect was that most of the metal looked perfect, the corroded spots were small (largest 3-4 mm across) isolated and randomly distributed. I was expecting to see the corrosion
over most of the bottom and not so spotty.
In a piece 10 X 20 cm there were 4 large spots and about a dozen down to about 0.5 mm. With a strong magnifier I could see a lot more down to the limits of the lens, but still in isolated areas. Lots of areas were completely clear of any visible corrosion
. Next step, see how far it extends up the bottom of the tank.