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Stress Corrosion Cracking of a Stainless Steel Pipeline


Steam, 1450 psi


MATERIAL: 304H Stainless Steel - 4", 6" & 10" Pipe


4 Years Active followed by 6 Years Inactive


 Stress Corrosion Cracking in Sensitized Metal


During the four years of active service, several cracks in the pipeline had been noticed. These cracks extended from the outside inward, Figure 1. A metallurgical examination during this period revealed that the failed material was in a highly sensitized condition. After six years of inactivity, it was desired to reactivate this steam line. A hydro-test was performed which revealed several leaks due to cracking along the pipeline. An analysis to determine the cause of the cracking was made.

Metallurgical examination was performed to verify the sensitized condition of the material. Also on-site replications were made of the valve bodies to determine if they were in the same sensitized condition. Photomicrographs below illustrate the ditched structure indicating a highly sensitized condition. The high carbon content of 304H makes this material susceptible to grain boundary precipitation of chromium carbides at elevated temperatures (>1000F) as indicated by the sensitized structure Figures 2 and 3. The sensitized structure is an indication of the materials susceptibility to intergranular corrosion and/or cracking.

Energy Dispersive Spectroscopy (EDS) was used to analyze the surface scale and crack surface. A high chlorine peak was observed in both locations indicating high chloride levels. A chemical analysis of the pipe insulation revealed a high chloride content also (360 ppm). Whether the chlorides were a constituent of the insulation or the result of contamination was not determined.

In this case, the presence of three detrimental factors - sensitization, high (1450 psi) pressure, presence of chlorides, - resulted in the multiple cracks in the pipe walls.

Figure 1. Nearly a throughwall crack that initiated from the OD (10X Original Magnification) Figure 2. Pipe sample with ditched grain boundaries indicates sensitized material (500X Original Magnification with Oxalic Acid Etch)
Figure 3. Ditched grain boundaries from valve body, sensitized material (100X Original Magnification with Oxalic Acid Etch)

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