tank-plate

from The Century Dictionary.

• noun A form of cast-iron plate used in building up sectional tanks for holding water. The plates are thin, and are cast with stiffening ribs and with flanges, by which latter they are bolted together to form the sides and bottom. The plates may be of greater thickness and with more massive ribs where higher pressures are to be withstood, as in the bottom layers of deep tanks.
• noun An inferior grade of wrought-iron plates, intended for use in tanks for water, oil, or similar liquids which are not to be exposed to heat or to high pressure.