Potential Failure Modes of Welded Plain Carbon Steel Lap Patch Plates in Outdoor and Laboratory Corrosion Environments
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2023
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University of Hawaii at Manoa
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Many industries employ the use of underground storage tanks (UST) to store regulated substances, in most instances petroleum products. Over time, steel components of the UST can corrode and release contents through holes resulting from corrosion. The Red Hill Underground Storage Tanks (RHUST) walls are comprised of plain-carbon steel that is in contact with concrete. The steel passivates if the pH of the concrete is greater than pH 12 but can begin to corrode if the pH drops below this value, usually due to the aging of the concrete. Current repairs to the RHUST involve welding lap patch plates on the tank walls in locations that have been compromised by backside pitting. The lapped patch plates could eventually be susceptible to crevice corrosion when corrosion pit penetrates the original tank wall. Various steel corrosion products can be formed, expand, and occupy approximately 2-5 times the original volume of uncorroded steel. The expansion of the corrosion products can exert great forces and even fracture concrete and bend metal. Additional stresses could be induced in the welds if corrosion products expand in the crevice between the original tank wall and patch plate. Currently, the American Petroleum Institute (API) does not have standards for lapped patch plates in underground storage tanks, therefore, these processes need to be further investigated. Patch plate samples were created through joining an A36 steel base and patch plate through shielded metal arc welding. To simulate pitting corrosion of the tank walls, holes of varying sizes were machined into the base plates exposing the back sides of the patch plates. These machined holes will allow moisture to percolate into the crevice between the base plate and welded patch plate. This research aimed to fabricate patch plate samples to emulate, simulate, and analyze the potential failure modes that would occur in the RHUST. Preliminary Finite Element Analysis (FEA) models showed that the expansion due to corrosion products will cause the patch plate to deform. Six patch plate samples were deployed to the Marine Corps Base Hawaii (MCBH) test site in early February and have been monitored for deformation after 1 month and 3 months by obtaining topographical scans using a 3D profilometer. The inside crevice area between the patch plate and base plate was coated in three plates and left bare in the other three plates. Initial findings have found deformation occurring within the bare plates in the first three months of exposure; whereas, the plates with the coated crevice did not show comparable deformation. As the patch plate samples are exposed for a longer period of time, it is expected that the patch plates will continue to deform and reveal potential failure modes.
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Mechanical engineering
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