Corrosion in black liquor evaporators

This paper describes the corrosion observed during inspections of pulp mill evaporators (mainly Nordic mills). Test racks with five stainless steel grades were installed in the evaporator systems at three of the pulp mills. The steel grades for the racks were AISI 304L, AISI 316L, SAF 2205, SAF 2304, and 904L. The results of these corrosion coupons are discussed with regard to the liquor chemistry of the mills in question. Followup inspections have been under way at these mills for some time now. Duplex materials have been the solution in many problematic cases; however, there are still many problems to be solved concerning the welding of duplex steels. Inadequate welding leads to a growing number of failure cases for evaporation units and their surroundings.