A coupling assembly having an annular coupling with internal threads having a continuous plastic, corrosion-resistant coating. Opposed tubes are provided with a similar internal coating. The tubes have external threads inserted into the coupling with the internal and external threads being in threaded engagement. An annular gasket is provided of rigid material and is mounted between and in sealing engagement with opposed ends of the tubes within the coupling. The gasket has a continuous radial portion extending between the opposed ends of the tubes and in spaced apart relation thereto. O-rings are in sealing engagement with the tubing ends and the continuous radial portion of the gasket. A pair of opposed flanges extend from opposite sides of the continuous radial portion of the gasket and are in sealing contact with an interior surface portion of the tubes.