Abstract: Corrosion of carbon steel costs thousands of billions of dollars every year worldwide. Understanding of corrosion rates of carbon steel is crucial to arriving at informed decisions for controlling plant operating conditions which contribute to mitigating potential dangers and reducing cost. Conventional methods for measuring corrosion rates of carbon steel include weight loss method, linear polarization method (LPR), electrochemical impedance method (EIS), potentiodynamic polarization curve, and electrochemical noise, etc. Several new techniques have been also built in the past decades, such as galvanic sensor, multi-electrode array, and electrical resistance sensor, etc. Among these methods, weight loss method is ex situ and therefore cannot accurately resolve changes in corrosion rate with respect to time (1). On the other hand, conversion of EIS data to corrosion rate data depends on electrical equivalent circuits (EECs) whereby a successful fit is described as proof that the presented physical model is correct. However, since in reality many different models within the same accuracy will represent the data equally well, correctness of EECs can only be truly confirmed by applying a dataset of infinite accuracy to the model, which was influenced by many parameters such as composition, porosity, interphases, environmental conditions, sample geometry, and the time superposition of different phenomena (2). These factors increase subjectivity during EIS date processing. In view of the limitations of the existing methods, demand for new methods for acquiring accurate corrosion data in a more direct manner is growing.
In this work, a novel ultrasonic testing method was established to real-time monitoring natural corrosion rate of carbon steel in 3.5 wt. % NaCl solution. Thickness loss of carbon steel was regarded as the indicator for corrosion evolution. The schematic diagram of ultrasonic testing setup is shown in fig. 1a. Ultrasonic measurements were continuously undertaken by a permanently attached piezoelectric transducer, and converted into thickness loss by an advanced signal processing alrgorithm. Using this setup, thickness loss at nano-to-micrometer level during corrosion evolution was accurately measured (see fig. 1b). To achieve the high accuracy, the effect of temperature on the velocity of shear wave needed to be precisely compensated for. In the meantime, the corrosion rate of carbon steel, measured by ultrasonic testing, was verified using weight loss method and traditional electrochemical tests, including EIS and LPR tests.
Reference
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Keywords: Corrosion; Carbon steel; Ultrasonic test
