Fig, 7. Microanalyses of the Sc-1 sample: (a and b) M photographs showing pitting on the outer surface and small particles depositing in the holes(c) EDs spectrum of the residue within the pit showing significant concentrations of chlorine. sulfur, selicon and aluminum and(d)EDs spectrum of the normal affected zone(HAZ). Examination the inner surface of the tube revealed that severe discoloration was noticed in the HAZ region(Fig. 9) ndicating that the region might be sensitized during welding. Close examination of the WD-1 sample indicated corrosion attack at the weld metal adjacent to the HAZ The outer sur in Fig. 1ca. High magnification examination o the outer surface with SEM found grain face of the perforation is shown Fig 10b probably caused by acid pickling during tube welding, The inner surface of the perforation boundaries as shown in hown in Fig. 10c, no intergranular corrosion was detected. Comparing the corrosion damage between the outer and inner surfaces of the tube around the perforation location, there was more severe corrosion on the outer wall, High magnification of area 2 in F 2. ub show micro-galvanic corrosion at the interface between the 6-ferrite and 10d) The small and black precipitates deposited on surface of the weldment were further examined as shown in F a. High mag nification of the small hole shows shrinkage defect(Fig 11 High magnification observation with SEM of the black precip- From chemical composition analysis itates deposited area indicated the presence of skeletal dendritic structure l 11 c) 11d). They could segregate of the black precipitates by EDs, contaminants including sulfur and manganese were found(Fit to shrinkage area in weld. Cross-section through the hole indicated that the pitting corrosion occurred at the interface be tween the weld metal and HAZ. and extended into the weld metal in the transverse direction(through thickness), then ex tended further into the base metal in the longitudinal direction with irregular shape like tunnel shown in a resembles the cast structure consisting of 6-ferrite inter- Fig 1 a. The microstructure of the fusion line as shown in ris structure in matrix. High magnification observation at the fusion zone revealed general corrosion, which was caused by the micro-galvanic effect between 5-ferrite and(Fis 12b). This general attack could initiate the preferred sites for growth of pits 15-ti This observation indicated that corrosion was initiated from the outside and propagated to the inner wall. As the corrosion progresses, the lithickness will decrease, and finally, leading to perforation. analyses at the welds. concluded that the perforated hole had started from outer face and propagated to the inner surface at the interface between the weld metal and HAZ. Although the openings of the pits are very large subsurface cavities are formed. G boundary corrosion on the outer surfac the HAZ and the black precipitates containing the elemental sulfur and manganese were also observed The microstructure of the base metal was normal austenitic structure ferrite and weld. No evidence of chromium carbide precipitate at the grain boundaries was observed