Abstract: The invention relates to an improved tank structure having an outer shell of metal material with an inner surface and an outer surface, and an inner shell of a plastic resin material fused to the inner surface of the outer shell. The tank structure has at least two insert fittings mounted in corresponding openings formed in the outer shell, each insert fitting defining an opening for providing access to the interior of the tank structure. A top end of each insert fitting projects above the outer surface of the outer shell, and is formed with a peripheral flange for welding to the outer surface of the outer shell. The remaining body portion of each insert fitting extends into the interior of the tank structure and has a circumferential groove formed in the outer surface thereof which provides a gripping surface to which the inner shell is fused.