Materials Performance

MAY 2015

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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70 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 Fracture Surface Evaluation Figure 3 shows a stereoscopic view of the crack surface. The rusted areas close to the external surface represent crack origin sites. An indication of fatigue crack growth in the form of progression marks can be seen. The smooth fracture zone indicates a slow rate growth for the crack by a low- cycle fatigue load. The fracture surface was also observed by SEM and analyzed by EDS. The SEM image (Figure 4) for the fracture surface indicated the crack arrays characterized corrosion fatigue. These cracks started from pits, which represent common loca- tions for stress concentration. From there, the cracks slowly grew across the fatigue zone. The dimpled fracture morphology of the crack surface indicates a ductile over- load fracture where a crack had passed transgranularly. The SEM and EDS analyses (Figure 5) at the fracture origin showed that corro- sion products contained high carbon and oxygen , sulfur, calcium , and chlorin e, which can greatly accelerate corrosion attack. The rusted parts at the fracture surface indicated that it was the oldest zone of the fracture surface, which was nucleated from pits. There is a threshold stress intensity range below which the crack propagation velocity is insignifi- cantly low. Three stages may be consid- ered: one, near the threshold, where crack propagation velocity increases consider- ably with increasing stress intensity range; a second stage where the crack propaga- tion is steady and constant; and the third stage where crack propagation velocity increases rapidly with the stress intensity range leading to fracture at the fracture toughness value. Corrosion can cause a much greater increase in crack growth rates at low loads than it does at high loads. A pitting stage took place in a shorter time to nucleate cracking, while the low-cycle fatigue is responsible for crack propagation in the rest of the crack's lifetime. The fatigue load had two damaging roles in the failure of the line. The first was in crack nucleation by deteriorating the external coating and the second was in crack propagation by ac c el eratin g th e grow th of c orro sion attack and crack growth. FIGURE 3 Close-up view of the fracture surface. FIGURE 4 SEM images for crack surface. FIGURE 5 SEM and EDS analysis near fracture origin. MATERIALS SELECTION & DESIGN

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