Metallurgical Technologies Incorporated

Several concentrating still condenser tubes exhibiting pitting were received to determine the cause of pitting. Results indicate the tubes pitted initially from the inner diameter (ID) surface due to microbiological corrosion (MIC). MIC pitting initiated preferentially at the tube seam weld. Additional ID surface pits and significant pit enlargement of MIC pits due to chloride pitting were also observed. The chlorine is likely remains from the hydrochloric cleaning process. The acid residue becomes trapped in the complex network of inter-dendritic penetrations resulting from preferential MIC attack of ferritic dendrites in the seam weld. Additional pitting occurred at what appears to be witness marks from contact with a cleaning tool. Chloride pitting initiated on tube outer diameter (OD) surfaces was also observed, but to a lesser severity. It is theorized the pitting initiated from the ID results in leakage and introduction of chlorides to the shell side of the tubes. Chemical analysis of a tube sample identified the alloy as Type 304L stainless steel. Microhardness testing revealed the material is at the high end of hardness for this grade in the proper annealed condition. No sensitization (carbide precipitation at the grain boundaries) was observed in the microstructure. The composition, microstructure and hardness are in compliance with ASTM A249-03 (equivalent to ASME SA249) for Grade TP304L welded stainless steel condenser tubing. A biocide water treatment is recommended. Mechanical or water jet cleaning and elimination of the hydrochloric acid cleaning are recommended.