A sensor system to monitor the early-stage corrosion of A36 carbon steel

An innovative sensor containing A36 carbon steel was explored for corrosion monitoring. After being soaked in an aerated 0.2 M NaCl solution, the sensor’s normalized electrical resistance (R/R0) decreased continuously with the extent of corrosion from 1 to 0.64. Meanwhile, the sensor’s normalized capacitance (C/C0) increased continuously from 1.0 to 1.42. The sensors can be attached to a structure to be monitored via a wired or wireless network connection for automatic data acquisition, processing, storage and evaluation. Introduction Corrosion is an inherent problem which affects the construction, transportation, energy and many other industries. If corrosion is not monitored and correctly fixed, it could threaten public welfares and people’s lives. Manual inspection of corrosion is costly, low efficient, subjective and sometimes dangerous. As a result, it is highly desirable to use corrosion sensors for automatic data collection, processing, and evaluation. Compared to manual inspections, automatic monitoring by corrosion sensors has significant advantages, such as promptness, comprehensiveness and efficiency. In addition, electrical signals from corrosion sensors are much easier to transmit, analyze and store than manual methods. In this study, a corrosion sensor has been explored in order to find the systematic change in electrical properties of a metal (e.g., A36 carbon steel) during the early-stage of corrosion as it is exposed to a corrosive environment. During the course of corrosion and degradation, there is a change of the surface morphology and the electric conductivity of the metal, which is directly reflected by systematic changes of the capacitance and the resistance readings from the sensor. The sensor was powered by a 1.9 V and 1 kHz AC to minimize electrolysis and electrode polarization problems brought by a DC power. In practice, multiple sensors can be connected to a wired or wireless network for automatic data acquisition, processing and storage. Experimental Methods Cylindrical Capacitor ASTM A36 steel was used in this study. As shown in Fig. 1, a cylindrical capacitor was created with the inner cylinder made from A36 carbon steel rod and the outer ring made from 316 stainless steel. Each of them had a height of 0.64 cm. The inner cylinder had a diameter of 1.27 cm while the outer ring had an outer diameter of 2.64 and inner diameter of 2.22 cm. The A36 carbon steel was polished by a 3M® 80 grit then a 3M® 600 grit sandpaper. Two wires were attached to the capacitor, one was soldered to the base of the center A36 steel rod and the other was welded to the International Conference on Material Science and Application (ICMSA 2015) © 2015. The authors Published by Atlantis Press 276 base of the outer ring of the 316 stainless steel. A bridge made of a glass substrate epoxy resin insulator from the circuit board (BM-FR4-1SS2, T-Tech Inc.) was adhered on the bottom of both the inner cylinder and the ring through a waterproof epoxy (15206 Anchor-Tite, Super Glue) to fix their relative positions. Finally, the waterproof epoxy was used to cover the connections of these wires as well as the base of the center rod to prevent corrosion of the connections and the wires. The uncovered A36 steel had 2.66 cm 2 , which was subject to corrosion. Similarly, a reference sensor was made following the same procedures and dimensions except replacing the A36 carbon steel rod with a 316 stainless steel rod of the same dimension, and then being welded to connect to the circuit-board bridge. The connections including welding points of the reference sensor were coated with waterproof epoxy (15206 Anchor-Tite, Super Glue) to prevent corrosion. The reference sensor was served to draw baseline information by addressing environmental conditions such as the temperature and the moisture level of air other than corrosion. Fig. 1. Diagram of the corrosion sensor made of a cylindrical capacitor used for corrosion monitoring [1] Corrosion Test A 500-ml 0.2 M sodium chloride solution was used for corrosion testing. An air pump (Aqua Culture®) with a flow rate of ~1.2 L/min was continuously bubbling air through a diffuser to the solution to provide oxygen for the corrosion process. The air diffuser was porous sandstone. The dissolved oxygen level of the NaCl solution was maintained at around 8.8 mg/L. The corrosion sensor was submerged in the sodium chloride solution above the air diffuser. Every day during the course of the test, the sensor was removed from the solution, rinsed with DI water, dried at room temperature for 2~3 hours, and then tested with an automatic RCL meter (PM6303A, Fluke). At each measurement, multiple readings from the RCL meter with time were recorded until stable readings obtained, indicating the senor was dried with equilibrium to air moisture. As a result, the sensor experienced periodically wet/dry cycles twice a day for total 11 days. A new 0.2 M sodium chloride solution was made daily. The accumulated corrosion time of the senor in the sodium chloride solution was 225.5 hours. As a control test, a 316 stainless steel ring and the reference sensor was soaked in an aerated 500-mL 0.2 M sodium chloride solution separately to investigate the degree of corrosion of the 316 stainless steel and the reference sensor, respectively. The electrical resistance and the capacitance of the reference sensor were measured by the RCL meter following the same procedures as the corrosion sensor. Sensor Measurements The RCL meter was used to measure the resistance and the capacitance of the corrosion sensor in series mode, which was an available function of the meter. The meter used a frequency of 1 kHz and a voltage of 1.9 V for the measurements. In addition, the mass of the dried sensor was examined by a digital balance (ESA-3000, Salter-Brecknell) with a precision of 0.05 g. The initial weight of the corrosion sensor was 25.20 g. For the corrosion sensor, the initial resistance (R0) in series was 7.2410 6 ; the initial capacitance (C0) in series was 9.2 pF before corrosion test. For the