An elbow made from welded steel tube had become leaky along a well-defined line in the axial direction. The entire wall of the tube was corroded, and the longitudinally-welded seam stood out clearly as a result of particularly intensive corrosive attack. The appearance of the corroded surface indicates the action of water with a high oxygen content. The oxygen in the return water must have originated from the ventilation of the open expansion vessel. Because of the corrosion-favoring effect of a crevice, water with a high oxygen-content that was perhaps still warm or even hot found particularly favorable conditions for corrosion in the defective welded seam (crevice corrosion). The tube material itself is perfectly satisfactory and in no way responsible for the failure.