1530-437X (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2015.2476997, IEEE Sensors Journal IEEE SENSORS JOURNAL 1 A RFID Sensor for Corrosion Monitoring in Concrete Walter D. Leon-Salas, Member, IEEE, and Ceki Halmen Abstract—A radio frequency identification (RFID) sensor for corrosion monitoring in concrete is presented. The sensor can perform linear polarization, open circuit potential and tem- perature measurements. The sensor obtains its power from an external RFID reader, which also functions as a datalogger. The sensor’s electronic circuit comprises a RFID modem, a low-power microcontroller and a three-electrode low-power potentionstat. The electronic circuit and the three electrodes are housed in a 3D- printed case measuring 11.8 cm×4 cm×5.6 cm. An analysis of the inductive coupling between the reader’s and the sensor’s antennas is carried out to guide the optimization of the RFID commu- nication link. Tests with a wet and a concrete electrochemical corrosion cells show that the developed sensor has a performance comparable to costly and bulkier benchtop potentiostats. An accelerated corrosion test was conducted by embedding the electrodes in concrete for 24 days. Linear polarization resistance measurements obtained from the developed sensor show the initiation and progression of corrosion. An uncertainty evaluation is carried out showing that the developed RFID sensor has an accuracy compatible with precision benchtop instruments. Index Terms—RFID, corrosion, concrete, monitoring. I. I NTRODUCTION C ORROSION of concrete reinforcement is one of the main factors leading to premature deterioration of con- crete structures worldwide [1]. According to the U.S. Federal Highway Administration (FHWA), the cost of corrosion in the United States is estimated to be around $276 billion per year [2]. The direct cost due to corrosion of highway bridges is estimated to be $8.29 billion, while the indirect costs due to traffic delays and productivity loss is estimated to be much higher [3]. Detecting corrosion in its early stages can inform the scheduling of preventive maintenance measures in order to prolong the service life of the concrete structure and reduce the likelihood of catastrophic failures. Corrosion monitoring with non-destructive methods are preferable. In particular, the use of wireless concrete- embedded corrosion sensors is appealing since it avoids ex- posed wires that can themselves corrode or break. Given that most reinforced concrete structures are designed to have a service life of 50 to 100 years, corrosion-monitoring sensors cannot rely on batteries for their power. Instead, passive sensors that harvest their energy from their environment, or from an external reader through inductive coupling, are the most suitable solution. Several efforts to develop passive embeddable corrosion sensors have been reported in the literature. In [4]–[6] a W. D. Leon-Salas is with the School of Engineering Technology, Purdue University, West Lafayette, IN, 47907 USA, e-mail: wleonsal@purdue.edu. Ceki Halmen is with the Department of Civil and Mechanical Engineering, University of Missouri-Kansas City, Kansas City, MO. binary passive sensor based on radio frequency resonance is reported. The sensor consists of an LC tank whose resonant frequency is monitored by an external reader. The LC tank has a steel wire that is exposed to corrosion. As the exposed wire corrodes, it eventually breaks, changing the capacitance of the LC tank and its resonant frequency, which is detected by the external reader. Once the exposed wire breaks, the sensor stops providing new information about the corrosion process. In [7] a binary passive corrosion sensor dubbed Smart Pebble is reported. The Smart Pebble monitors the potential difference between two electrodes of an electrolytic cell: an ion-selective electrode and a reference electrode. When the po- tential difference between the electrodes exceeds a threshold, a bit of information is conveyed to an external RFID reader via a RFID tag (MCRF202 from Microchip) by inverting the tag’s identification code. Resonant LC tanks embedded in concrete have been used in [8,9] to monitor the corrosion potential of a steel electrode. The capacitance of the LC tank is set by varactor. The varac- tor’s capacitance varies in response to changes in the corrosion potential, thus changing the resonant frequency of the LC tank. The change in resonant frequency is detected by an exter- nal reader. A drawback of this technique is that the corrosion potential is affected by other factors besides corrosion, such as limited diffusion of oxygen, concrete porosity and the presence of highly resistive layers [1]. Hence, monitoring the steel’s corrosion potential only provides the likelihood or tendency of corrosion, but not the actual corrosion rate. Another drawback of resonant LC tanks is that the transmission of corrosion information is inherently analog, hence, susceptible to noise. The effect of eddy currents in the response of RFID tags mounted on corroding steel plates has been investigated in [10]. It was found that the peak amplitude of the RFID tag response is correlated with the atmospheric exposure time of the steel plates. A related work by the same research group presented in [11] showed that the permittivity of corroding steel can be detected using a vector network analyzer and a waveguide. A well-established technique for determining actual corro- sion rates is the linear polarization resistance measurement [12]. Commercially-available sensors such as the Embedded Corrosion instrument (ECi-2) from Virginia Technologies can perform linear polarization resistance readings as well as temperature, resistivity and chloride concentration measure- ments [13]. The ECi-2 is a digital peripheral instrument on a local area network embedded inside the concrete structure. The network communicates with an external datalogger for corrosion data read-out. Due to the large scale of the embedded local area network, this solution is most suitable for new