A method and apparatus for lining process tanks. The method comprises lining a tank with a plurality of sheets, wherein the tank has walls and a bottom that intersect at a corner of the tank. The method comprises bonding a bottom sheet of the plurality of sheets to the bottom of the tank. Next, a pair of sheets of the plurality of sheets are bonded to adjacent walls of the tank and above the bottom sheet, each sheet of the pair of sheets having a first edge and a second edge such that the first edges of the pair of sheets are positioned at the intersection of adjacent walls of the tank to position an interface between the pair of sheets. The method also comprises extrusion welding the pair of sheets together by infusing a molten thermoplastic material along the pair of sheets and within the interface of the pair of sheets. Additionally, the method comprises extrusion welding the pair of sheets to the bottom sheet by infusing the molten thermoplastic material along and between the pair of sheets and the bottom sheet wherein the infused thermoplastic material seals the pair of sheets and bottom sheet to isolate the tank from contents that contact the pair of sheets and bottom sheet.