## 1 Introduction  
Corrosion of reinforcing steel, generally caused by carbonation or Cl\({}^{-}\), is the most important factor causing severe degradation of the durability of reinforced concrete (RC) structures. Especially, Cl\({}^{-}\) from the concrete itself or the environment leads to pitting corrosion which will quickly result in a remarkable reduction of the cross section of the reinforcing steel [1,2]. The durability deterioration of RC structures finally leads to very high repair costs, sometimes much greater than the initial construction cost, and in some extreme situations, can result in the collapse of the structure [3]. With the development of global warming and further deterioration of the environment, the service condition of RC structures has recently become much harsher. Over the past two decades, structural health monitoring (SHM) has gained worldwide acceptance as an affordable way to obtain real-time data on the health, and consequently, on the safety and the serviceability of civil infrastructure systems [4]. There have been many sensing technologies applied in earthquake engineering, wind engineering and life-cycle performance evaluation, such as the fiber optic sensing technique [5], the wave propagation-based piezoelectric ceramic (PZT) sensing technique [6, 7, 8, 9] and the smart cement-based sensing technique, _etc._[10,11]. Recently, future trends in the development of sensing technologies and SHM in civil engineering have been critically reviewed and put forward [12, 13, 14]. Considering the severity of the reinforcing steel's corrosion, it is very urgent to recognize the corrosion status of the steel bar, and then to provide early warning information before the RC structures reach an unsafe condition in the field. Unfortunately, although the corrosion mechanism of reinforcing steel has been investigated extensively in the past few decades, the corrosion monitoring of RC structures has still not been well addressed till now. Some significant challenges related to the corrosion monitoring methods and sensors, therefore, should be carefully considered and seriously clarified for the actual SHM systems. Firstly, the purpose of corrosion monitoring is to actively identify the corrosion status according to the electrochemical characteristics of RC structures. In contrast to the traditional works implemented by most researchers in laboratories, corrosion monitoring is the inverse process of the electrochemical characterization of RC structures. Secondly, the evolution of the reinforcing steel's corrosion will be influenced by the environmental conditions over time. Therefore, _in-situ_, real-time and on-line identification of the corrosion status is another key issue. Furthermore, the foregoing engineering practice indicates that the distribution of large amounts of conducting wires, demanding massive labor and financial resources represent annoying problems in SHM for civil engineering. The pitting corrosion, known as \"steel cancer\", significantly deteriorates the durability of RC structures and greatly degrades their serviceability. The electrochemical emission spectrum (EES) can be used as the most intrinsic feature of the pitting corrosion caused by Cl\({}^{-}\)[15]. Electrochemical emissions, generally considered as the \"fingerprints\" of pitting corrosion, reflect the information of the initial, metastable, repassivation and stable stages during the corrosion process [16,17]. Therefore, the pitting corrosion status could be identified effectively via this intrinsic information by passively listening to the reinforcing steel to determine how the corrosive media intrudes into the RC structures [18, 19, 20, 21]. On the other hand, the emergence of wireless sensors and networks (WSNs) provides an excellent opportunity to finally realize the _in-situ_, real-time and on-line corrosion monitoring [22, 23, 24, 25, 26, 27].