Several tubes in a tube bundle in an evaporator used to concentrate an acid nitrate solution failed by leakage. The feed to the evaporator contained about 6% nitrate, and the discharge about 60% nitrate. The tube bundle was comprised of type 309S (Nb) stainless steel drawn-and-welded tubes expanded and welded into two type 304L stainless steel tube sheets. The tubes failed by crevice corrosion. The failed tubes were defective as-received, and the establishment of concentration cells within the longitudinal cracks in the seam welds led to ultimate corrosive penetration of the wall. There was no evidence of crevice corrosion or any localized penetration of tubes that had sound welds. The leaking type 309S (Nb) welded tubes should be replaced with seamless tubes of type 304L stainless steel to minimize the areas requiring welding and to provide maximum weldability for the tube-sheet joints.

This article briefly reviews the general causes of weldment failures, which may arise from rejection after inspection or failure to pass mechanical testing as well as loss of function in service. It focuses on the general discontinuities observed in welds, and shows how some imperfections may be tolerable and how the other may be root-cause defects in service failures. The article explains the effects of joint design on weldment integrity. It outlines the origins of failure associated with the inherent discontinuity of welds and the imperfections that might be introduced from arc welding processes. The article also describes failure origins in other welding processes, such as electroslag welds, electrogas welds, flash welds, upset butt welds, flash welds, electron and laser beam weld, and high-frequency induction welds.