It's fun to see others going through the same stuff I did and be able to help. He also stated that if not painted, the underside will have a uniform corrosion rate througtout the whole bottom and it is much slower for the steel to corrode this way. The tank is 1,000,000 gallons, 65' in diameter, for diesel storage. Any comments will be appreciated. Normally, the tank pad is sealed with a layer of bituminous compound which will seal against water from below and allow tank vertical movements.
Corrosion requires oxygen and moisture.
With the sealant, the general tank bottom underside would be dry and also fresh air is unlikely to penetrate beyond the periphery of the tank annular plates. If your practice is not to provide bituminous compound, then I would suggest using sacrificial anodes on the underside to protect against corrosion. I did not see such a detail in the API 650. My tank has two bottoms. Both tank contractor and the cathodic protection told me that they did not think the cathodic protection for the secondary bottom was necessary.