One of five underground drain lines intended to carry a highly acidic effluent from a chemical-processing plant to distant holding tanks failed in just a few months. Each line was made of 304L stainless steel pipe 73 mm (2 in.) in diam with a 5 mm (0.203 in.) wall thickness. Lengths of pipe were joined by shielded metal arc welding. Soundness of the welded joints was determined by water back-pressure testing after several lengths of pipe had been installed and joined. Before completion of the pipeline, a pressure drop was observed during back-pressure testing. An extreme depression in the...