Rebars corrosion is the main durability threat of Reinforced Concrete (RC) structures that reduce the service life, in marine environment. The chloride ions penetration to RC, can accelerate the corrosion process which increase RC structures collapsing risk. Therefore, regular health monitoring inspections is necessary to evaluate chlorides ingress, before they reach rebars. Many None Destructive Techniques (NDT), such as Electrochemical or Electromagnetic (EM) Methods, have been developed to detect rebar’s corrosion. However, most of them are not able to precisely determine the chlorides transfer mechanism or their results suffer from uncertainty and cannot distinguish chloride ions from other environmental factors changing. Under framework of ANR French project LabCom OHMIGOD, we want to propose a novel Structural Health Monitoring (SHM) sensor for detection of chlorides penetration front in cover concrete, as preventive ND method. The sensor is made of magnetic and ferromagnetic materials that are sensitive to chloride ions and could generate magnetic flux density (B), as output signature. By embedding sensor in cover concrete, exposed to chlorides penetration, corrosion could emerge on the sensor’s reactive part. Thereby, its geometrical properties and consequently, its output signature would be affected, too. By using an external interrogator, it would be possible to obtain variation of B, in function of corrosion evolution on sensor, as ND observable. In order to evaluate the sensor’s characteristics, we present a sensor’s numerical model and thereby, the related efficient geometrical parameters (surface and thickness) and also the location in concrete, are investigated. We used the Finite Element Method (FEM) for numerical modeling of the sensor. The simulation results show that the variation range of B, up to 70% is achieved for the reactive part geometry of 25 mm × 25 mm × 0.8 mm, in depth’s range of 1 cm up to 3 cm from concert’s surface.