Monitoring corrosion is a vital step to prevent catastrophic structural failure. With accurate corrosion rate data, the risk of failure can be mitigated by performing routine maintenance on components that have become compromised. When corrosion occurs in an inaccessible area such as the inside of a pipe or back side of an underground storage tank, however, conventional monitoring techniques such as visual inspection are inadequate or impossible. Non-destructive evaluation (NDE) techniques must be used to determine the condition of the structure to maintain its integrity. Ultrasonic (UT) Thickness Sensors were investigated for corrosion monitoring use. The chosen sensors had a resolution of a limit of 25.4μm. A system was designed to utilize Electrochemical Machining (ECM) to simulate a corrosion at various rates in a localized region (approximately 1 cm in diameter) while its thickness was monitored by a permanently attached UT sensor. Multiple techniques such as a 3D profilometer, and depth-gauge were used to validate the measurement of the permanently attached UT sensor. Multiple ECM experiments were conducted using NaCl electrolyte solution at 1, 10, and 100 g/L concentrations. Occasionally, variations in pit formation were observed and likely caused by the orientation of the counter electrode to the steel surface. The smallest and most consistent surface roughness was obtained using the 10 g/L concentration. Larger variations in surface roughness were observed at 1 and 100 g/L NaCl concentrations leading to the selection of 10 g/L solution for experiments. The ECM system rendered reproducible machining rates showing that the UT sensors were capable of monitoring very low corrosion rates averaged to 3μm per day. Discrepancies up to a few mm were sometimes measured between the UT sensor, 3D profilometer and depth-gage values. The discrepancies were attributed to non-uniform corrosion in the vicinity of the permanently attached UT sensor. The sensor therefore provided readings of a specific localized spot of the corroded region, or the profile of the corroded region interfered with the UT signals. The accuracy of UT sensor readings was also studied for various geometrical pit profiles. Certain defect geometries made it impossible for the UT sensor to record accurate measurements. This was also observed on naturally corroded steel plates. Therefore, UT sensors may sometimes provide inaccurate or no reading over certain corrosion profiles and should therefore be used with prudence.
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