Conclusion. The pitting in the austenitic stainless steel pipe is believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low flow and stagnant conditions present in the piping are primary contributors to the pit progression. This type of material will perform significantly better when there is more flow of water Retesting of the water indicated similar high levels of aerobic bacteria, so these must be considered a part of th design basis Recommendations. Due to the extensive amount of piping, the pinhole size of the leaks, and environmental consequences of leaks, repair of the existing pitted pipe was impractical. Replacement of the pipe was recommended Several alloys, nonmetallic materials, and lined materials were proposed for coupon testing to determine which is the best in this particular environment Example 2: Analysis of a Corrosion Failure of an Aboveground Storage Tank. A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures(approximately 177C, or 350F)and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection On-Site Examination. The underside of the floor plates showed extensive localized wall thinning. Figure 3 shows a pit in one of the tank floor plates. While it was expected that the ground under the tank was near operational temperatures, it was determined that the temperatures were below 104C(220F)at most areas evaluated in the ground. Thus, it was possible for moisture to accumulate at the tank floor, producing intermittent wetting and drying conditions. This provided an explanation of the circumferential corrosion attern observed several feet in from the periphery of the tank. Heating was provided near the center of the tank and water in the backfill below the tank floor should be minimal. Perforations of the tank were large, and thinning was apparent in the adjacent areas PIT Fig3 View of an isolated pit on the outside of the floor tank of a liquid asphalt tank. the asphalt had been leaking for some time as the surrounding area was covered with deposits and the wall thinning was significant. Courtesy of s.R. Freeman, Millennium Metallurgy, Ltd On-Site Sampling Soil samples were taken and tested, but did not indicate high corrosivity. Sample of the scale deposits from the outer surface of the tank floor were taken Laboratory Examination. Scale samples analyzed by EDS showed the deposits consisted of primarily Fe203 (iron oxide). The rapid exfoliation, scale buildup, and pitting of the tank floor from the external (underside) surface also prompted MIC testing of the soil and of the deposits in the area of wall thinning. Although the temperatures were somewhat high for MIC to thrive, the results of the samples confirmed high levels of various Thefileisdownloadedfromwww.bzfxw.comConclusion. The pitting in the austenitic stainless steel pipe is believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low￾flow and stagnant conditions present in the piping are primary contributors to the pit progression. This type of material will perform significantly better when there is more flow of water. Retesting of the water indicated similar high levels of aerobic bacteria, so these must be considered a part of the design basis. Recommendations. Due to the extensive amount of piping, the pinhole size of the leaks, and environmental consequences of leaks, repair of the existing pitted pipe was impractical. Replacement of the pipe was recommended. Several alloys, nonmetallic materials, and lined materials were proposed for coupon testing to determine which is the best in this particular environment. Example 2: Analysis of a Corrosion Failure of an Aboveground Storage Tank. A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures (approximately 177 °C, or 350 °F) and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection. On-Site Examination. The underside of the floor plates showed extensive localized wall thinning. Figure 3 shows a pit in one of the tank floor plates. While it was expected that the ground under the tank was near operational temperatures, it was determined that the temperatures were below 104 °C (220 °F) at most areas evaluated in the ground. Thus, it was possible for moisture to accumulate at the tank floor, producing intermittent wetting and drying conditions. This provided an explanation of the circumferential corrosion pattern observed several feet in from the periphery of the tank. Heating was provided near the center of the tank and water in the backfill below the tank floor should be minimal. Perforations of the tank were large, and thinning was apparent in the adjacent areas. Fig. 3 View of an isolated pit on the outside of the floor tank of a liquid asphalt tank. The asphalt had been leaking for some time as the surrounding area was covered with deposits and the wall thinning was significant. Courtesy of S.R. Freeman, Millennium Metallurgy, Ltd. On-Site Sampling. Soil samples were taken and tested, but did not indicate high corrosivity. Sample of the scale deposits from the outer surface of the tank floor were taken. Laboratory Examination. Scale samples analyzed by EDS showed the deposits consisted of primarily Fe2O3 (iron oxide). The rapid exfoliation, scale buildup, and pitting of the tank floor from the external (underside) surface also prompted MIC testing of the soil and of the deposits in the area of wall thinning. Although the temperatures were somewhat high for MIC to thrive, the results of the samples confirmed high levels of various The file is downloaded from www.bzfxw.com