Corrosion Resistance of Next Generation Composite Tube Alloys for Recovery Boilers

Abstract Composite tubes made with either 304L, A825 or A625 exterior layers are commonly used to construct the lower-furnace waterwalls of black liquor recovery boilers currently operated in North America. While these composite tubes have been very successful in resisting sulphidation corrosion, they have experienced unanticipated problems with the exterior layer, namely cracking, and, to a lesser extent, preferential corrosion of port opening tubes. In an attempt to evaluate the suitability of alternative composite tube alloys, a laboratory-based testing program was conducted to evaluate the corrosion resistance of ferritic stainless steels (Types 430 and 446) and nickel-based alloys (Alloys 600 and 690) in simulated black liquor recovery boiler lower-furnace environments. An effort was made to utilize simulated environments that are believed to reproduce the key features of the various damage modes typically observed in the lower-furnace sections of high-pressure recovery boilers. This report documents the major findings of this effort.