33 C.F.R. § 183.512
Fuel tanks: Prohibited materials
(a) A fuel tank must not be constructed from terneplate.
(b) Unless it has an inorganic sacrificial galvanic coating on the inside and outside of the tank, a fuel tank must not be constructed from black iron or carbon steel.
(c) A fuel tank encased in cellular plastic or in fiber reinforced plastic must not be constructed from a ferrous alloy.