Studies of degraded smelt spout opening tubes

Co-extruded type 304L stainless steel/SA210 carbon steel tubes have been used on the floors and lower walls of many black liquor recovery boilers to address the wall thinning problem that has been an issue for boiler owners and operators. Use of these tubes greatly reduced the corrosion issue, but corrosion was still sometimes observed and cracking was discovered in some tubes, particularly those that are bent to form the openings for smelt spouts. Because cracks in the opening tubes were sometimes observed to extend a significant distance into the tube wall and because these cracks were found fairly frequently, tubes made from a number of alternate cladding materials were tried in place of the 304L clad opening tubes. This paper describes the results of examinations of spout opening tubes of the standard 304L/carbon steel and of several alternate materials. In addition to the corrosion and cracking seen in the spout opening tubes, another issue associated with these tubes has been observed: preferential corrosion of the weld cap is sometimes seen on butt welds attaching the spout opening tubes made with alternate cladding materials to the standard 304L/carbon steel co-extruded wall tubes. Some information on the observations of this corrosion is also included in this paper.