Design and Validation of Carbon Nanotube Thin Film Wireless Sensors for Ph and Corrosion Monitoring

Corrosion damage in civil, aeronautical, and mechanical systems poses significant risks to users and occupants while simultaneously burdening owners with costly repairs and maintenance. Although many different sensing technologies are available to monitor corrosion processes, many cannot be easily implemented in field environments due to requiring expensive data acquisition systems and their destructive and intrusive measurement strategies. In this study, a novel layer-by-layer assembled carbon nanotube and poly(aniline)-based nanocomposite pH sensor is developed for monitoring corrosion of metallic and reinforced concrete structures. First, the electrochemical response of the proposed nanocomposite pH sensor is characterized using time-domain two-point resistance probing measurements to validate its resistance change to different pH buffer solutions (1 to 13). Frequencydomain electrical impedance spectroscopic studies and equivalent circuit analyses confirm changes in film resistance to pH. Upon sensor characterization, these nanocomposites are directly deposited onto printed circuit board coil antennas to realize a miniature passive wireless sensor capable of being embedded within structural materials. Preliminary wireless pH sensing results are presented to demonstrate that the wireless sensor’s bandwidth decreases at 3.9 kHz-pH with increasing pH. INTRODUCTION Structural systems, such as buildings, bridges, and aircraft often operate in harsh environmental conditions and are subjected to extreme loading events that accelerate structural deterioration. For instance, permanent deformations, corrosion, fatigue cracking, and many other types of damage can occur throughout a system’s service lifetime. However, corrosion, an electrochemical and physical process, ranks as one of the most complex, severe, and difficult to detect damage phenomenon. Furthermore, prolonged undetected corrosion can escalate to eventual catastrophic structural failure. For example, corrosion of aircraft fuselages can cause over-stressing at rivets and formation of stress-corrosion cracks; the Aloha Airlines plane that ripped apart mid-flight in 1988 is a classic example [1]. In addition to the safety risks posed to structural users, the cost of corrosion poses a significant economic burden to society. It has been estimated that the total annual direct corrosion cost to the entire U.S. industry (i.e., infrastructure, utilities, transportation, production and manufacturing, and government) in 1998 was approximately $300 billion (or more than 3% of the gross domestic product) [2]. Similarly, the U.S. Air Force Corrosion Prevention and Control Office (AFCPCO) estimates that more than $800 million is spent annually to maintain its existing aeronautical fleet [3]. As a result, new and robust sensing technologies capable of monitoring corrosion processes are required. Traditionally, various types of sensing techniques including visual inspection, core sampling, half-cell potential [4], linear polarization resistance (LPR) [5], electrical impedance spectroscopy (EIS) [6], among others [7-9], have been applied to detect corrosion in reinforced concrete (i.e., civil infrastructures) and aluminum structures (e.g., airfoils and naval vessels). The aforementioned corrosion monitoring techniques have been validated to provide accurate corrosion measurements in the laboratory. However, they all cannot be readily implemented in field environments since measurements typically require destructive removal of structural material (e.g., concrete cover to access embedded steel reinforcement) and/or expensive data acquisition systems. Alternatively, the nondestructive evaluation (NDE) and structural health monitoring (SHM) community has begun to explore the development of low-cost embeddable corrosion sensors. The main objective is to develop miniature sensors capable of being embedded during construction to measure parameters indicative of corrosion (e.g., moisture, chloride ingress, pH, dissolved ions, among many others) throughout structural service lifetimes. For example, Fuhr and Huston [10] have developed a specie-independent fiber optic sensor that can