Modeling the effect of attachment welds on stress-assisted corrosion in recovery boiler tubes

Stress-assisted corrosion (SAC) is a term given to waterside corrosion of steel recovery boiler tubes that occurs as a result of a combination of stress and corrosion, most commonly in the immediate vicinity of external attachment welds. A finite element model has been developed to perform thermal and mechanical analyses of tube wall panels containing attachment welds in order to examine the effect of the attachment weld on the stress distribution in the tubes. The results of the analyses show that effects of the attachment weld are confined to a short section of the tube close to the weld, and that the constraint from the weld on the free expansion of the tube alters the stress distribution near the weld. The variation in hoop stress at the waterside surface during a normal operating cycle is different for tubes with and without an attachment weld, and provides a possible explanation for the observation of SAC in tubes only near the attachment welds. Modeling of tube wall panels made of co-extruded composite tubes shows that co-extruded tubing may be less susceptible to SAC than its plain carbon steel counterpart.