Materials Performance

JUN 2019

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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43 MATERIALS PERFORMANCE: VOL. 58, NO. 6 JUNE 2019 circumferential direction. The internal sur- face of the sample was covered with a layer of dark gray/brown deposit. A section was cut from the sample to examine the inter- nal surface more closely. The photo on the right shows the appearance of the internal surface after removal of the deposits. Microstructure Analysis Figure 2 shows a macroscopic image of the longitudinal section. Many cracks were growing from the external surface of the pipe. Note that the largest crack has a depth that is >50% of the wall thickness. Figure 3 shows one of the cracks growing from pitting damage at the external sur- face—crack branching is apparent toward the tip of this crack. Clearly, the cracking is transgranular in nature. Scanning Electron Microscopy Study Some areas of relatively bare metal showed the presence of micro-pits (~80 to 100 µ m in diameter). Figure 4 shows loca- tions where energy-dispersive x-ray spec- trometry (EDS) analysis of bare metal was performed . Note Figure 5 (Spectrum 1) showed enhanced oxygen levels, and the whole surface has some level of oxidation. Random analyses of surface deposits at dif- ferent locations showed enhanced levels of multiple elements in varying proportions, including oxygen, chlorine, sulfur, calcium, aluminum, silicon, titanium, and magne- sium. No significant differences were found in the elements identified in each of the areas examined, though relative propor- tions varied between individual analysis spots. Hardness Testing Hardness measurements were carried out on the metallographic sections and their average HV5 value was 208.1. Discussion As previously mentioned, the external surface was generally covered with small "slugs" of corrosion product primarily ori- entated in the pipe circumferential direc- tion. It seems likely that the circumferential FIGURE 1 External (left) and internal (right) surfaces of the damaged sample. FIGURE 2 Numerous cracks growing from the external surface. FIGURE 3 A crack growing from pitting damage at the external surface. orientation of the "slugs" is due to moisture running in this direction over the surface of the pipe (which was installed horizontally). Much of the internal surface was covered with a layer of dark gray/brown deposit, but the underlying metal at the internal surface showed no obvious indications of corrosion or other damage. Metallography showed that numerous cracks were growing from the external sur- face, and the cracks were observed in both longitudinal and transverse metallographic sections. The largest crack had a depth >50% of the wall thickness. The cracks were growing from pitting damage at the exter- nal surface, crack branching was apparent, and the cracking was transgranular in nature. Results of chemical analysis were consistent with requirements for Type 304L SS (UNS S30403). Obser vations of multiple, branching cracks, cracking in both longitudinal and transverse orientations, crack initiation from corrosion pits, and transgranular crack propagation are characteristics of Cl-SCC in austenitic SS. For S C C to o ccur, th ere must b e a combination of a tensile stress, a corrosive

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