Stress-Corrosion Cracking of Austenitic Stainless Steel Evaporator Pans From an Unusual Cause
After about four years of service, cracks appeared on the internal or process-side surfaces of four evaporator pans in a sugar concentrator. The pans consisted of a Mo stabilized austenitic stainless steel inner vessel surrounded by a mild steel steam jacket. Corrosion of the external surface had taken place in the form of confluent pitting over a band adjacent to the fillet weld which attached the pan to the blocking ring. Numerous cracks were present in this corroded zone. Microscopical examination of several specimens cut from the sample revealed that the internal cracks in the pan itself originated from the external side of the plate, i.e. from the region covered by the shrouding ring. They were predominantly of the transgranular type. Because the cracks were not of the intergranular type as usually found with weld decay, they were considered to be indicative of stress-corrosion cracking. Stresses responsible for the cracking resulted from weld contraction. The pans had been hosed down periodically with water from local boreholes to remove sugar from the external surfaces, which introduced the corrosive medium.