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.