## 1 Introduction  
Reinforced concrete is widely used in various constructions due to its versatility, durable and strong performance throughout its service life [1, 2]. With the development of ocean engineering, reinforced concrete is extended into the building of ocean and coastal structures such as the sea-crossing bridge, subsea tunnel, seafloor mining operations, and marine fishery [3, 4]. The carbon steel rebar is always passive in the concrete by forming a thin layer of hydroxides in alkalinity conditions [5]. However, once the seawater permeates into the pore of the concrete, the passive film may be broken by aggressive ions such as chloride and sulfate ions which are largely contained in the seawater [4, 6, 7]. To decrease the construction period and cost reduction, various welded structures [8, 9, 10], such as the welded steel plate, welded steel tube, and welded wire fabric are applied in the dam, bridge deck, and the slab reinforcement of the ocean structures. Though the application of the welding technique can save the cost and ensure the structural strength, it may cause the increase of susceptibility to localized corrosion because the welding process can lead to the changes of the microstructures of the steel, formation of thermal oxide, and residual stresses [11, 12, 13]. The passive state of the welded carbon steel is not homogeneous in the mortar [8]. Especially in the marine environment, the breakdown of the passive film on a local area may result in a high localized corrosion rate and cause damage to the ocean structure [14, 15]. Because different metals existed in the welded zone (WZ) and the heat-affected zone (HAZ), once the passive film firstly degrades in these areas, galvanic corrosion may occur among the weld metal (WM) area and base metal (BM) area [16]. Since the area of the WZ and the HAZ is much smaller than the BM area, serious localized corrosion will happen when both areas become anodes. Various electrochemical methods have been used for the study of rebar corrosion and its inhibition in pore solution and mortar [5; 15; 17; 18]. However, traditional electrochemical techniques such as half-cell potential, linear polarization resistance (LPR), and electrochemical impedance measurement (EIS) only can qualitatively assess the corrosion status and evaluate the average corrosion rate [19]. The localized penetration rate is hard to be captured by using these traditional methods. In addition, the high resistance of the concrete gives a large limitation of these methods to be used in practice engineering [20]. In recent studies, electrochemical noise (EN) measurement has been adopted in monitoring rebar pitting corrosion and its inhibition [21; 22; 23]. The transients of the coupled potential and current can directly reflect the breakdown and recovery of the passive film [24]. The corrosion patterns and corrosion rate also can be revealed from the EN by calculating the localized corrosion index and the noise resistance [25]. Though the metastable corrosion process and pitting corrosion process can be analyzed from the EN signal, the time and spatial propagation of the pitting corrosion process cannot be obtained by EN measurement [22]. Meanwhile, it is difficult to be evaluated the highest localized corrosion rate based on EN signals, solely. Therefore, coupled multi-electrode array sensor is developed for monitoring localized corrosion in various studies [26]. Through coupling the dispensing of small electrodes by current meter, the new sensor can simulate a one-piece electrode. The current distributions can be well measured by the new sensor and the highest localized penetration rate of the metal can be calculated from the max anodic current [27; 28]. [PERSON] [29] studied the threshold chloride levels for localized carbon steel corrosion in simulated concrete pore solution by using a newly designed sensor. The threshold level of Cl\({}^{-}\)/OH\({}^{-}\) in different pH solutions can be seen from the transfers of the max anodic current.