Weld-B-Gone: Interdendritic Corrosion of Stainless Steel in Sulfuric Acid Service

Type 304L stainless steel is susceptible to interdendritic corrosion (i.e., selective dissolution of weld metal ferrite) in some acidic environments, especially some sulfuric-acid-containing environments. This presentation will focus upon a case study of one such case, in which type 304L stainless steel piping in a bioethanol production facility was subjected to a corn mash product containing sulfuric acid at a pH of about 4.0 at about 90 ºF for approximately a year and a half. During that service period, seam welds within the pipes, production welds joining pipe segments together, and production welds joining pipe segments to an elbow were almost entirely consumed, resulting in severe leaks. However, the seam weld for the elbow itself exhibited no substantial corrosion. Metallurgical factors related to the seam welding process during the pipe manufacturing process and the fabrication welding during assembly of the piping system were identified as contributing factors to the interdendritic corrosion. Potential pathways to prevention of future failures included consideration of specifying the metallurgical condition of the pipes the plant operator purchased for future installations, producing ferrite-free production welds through welding process control, and further study to determine if upgrading the corrosion resistance of the piping material would be necessary to improve the system’s corrosion resistance if the metallurgical-condition-factors could not be practically mitigated.