# A Disposable Soft Magnetic Ribbon Impedance-Based Sensor for Corrosion Monitoring
## 1 Introduction  
The development of a quick, cost-effective, and reliable technique to estimate the concentration of corrosive chemicals has been of technological interest for safety in industry and for the environment for many years [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. Numerous different sensors have been established to achieve this goal, such as a carbon electrode-based sensor for the detection of perfluorooctanoic acid [4], titanium oxide-based semiconductor sensors used to detect formaldehyde in the air [6], and enzymatic conductometric sensors commonly used to detect the amount of pollution in water samples [3; 7]. Each different type of sensor carries with it advantages and disadvantages that are useful for different situations. Capacitive sensors are praised for their easy fabrication and cost-effective construction, yet can cause issues as their sensitivity depends on obtaining the proper thickness for the original sensing layer [7]. Metal oxide semiconductor gas sensors are highly sensitive, simple, and have the possibility to be portable and low-cost, yet are only applicable for chemicals in the gas phase [1; 6; 11]. Among the variety of chemical sensors available today, there is an evident lack of chemical sensors for distinguishing between different concentrations and types of corrosive chemicals in liquid form, which are often used in industries such as the steel industry on a daily basis. Here, a novel sensor is proposed for this issue which utilizes the impedimetric sensing benefits of electrochemical sensors [2; 8; 9; 10; 12], whilealso taking advantage of the inductive impedance benefits of the recently popular giant magneto-impedance (GMI)-based chemical sensors [13; 14; 15; 16; 17]. Electrochemical impedimetric sensors exhibit many advantages that have contributed to their importance in today's society [2; 8; 9; 10; 12]. The main asset of these sensors is their ability to provide information on different aspects of electrochemical cells such as the ohmic resistance, capacitive information at the electrode/solution interface, and the rate constant of the reactions involved [2; 8; 11]. Through this information it is possible to determine many characteristics about a specific analyte in question, such as the presence of bacteria in water samples [12], the concentration of humidity in the air [2], and the presence of certain proteins within the body [10; 11; 12]. Unfortunately, many impedimetric electrochemical sensors require external chemicals aside from those being detected in order to function, such as enzymatic conductometric sensors, which utilize enzymatic reactions that produce conductivity-modifying ionic species from the reaction solution [3]. These extra chemicals can increase the complexity of use and the cost of the sensors. Recently, another form of impedance-based sensor referred to as GMI sensors has come into importance for their utilization of inductive impedance using soft ferromagnetic ribbons and external magnetic fields [13; 14; 15; 16; 17]. GMI sensors are praised for their low power consumption, high sensitivity, and thermal stability--yielding accurate detections for different concentrations of corrosive chemicals such as HNO\({}_{3}\)[16]. However--similar to electrochemical impedimetric sensors--GMI sensors require an external magnetic field, which adds to the complexity of their development and use. The novel sensor created here utilizes the advantages of both impedimetric electrochemical sensors and GMI sensors by exploring the benefits of GMI sensors without an added external magnetic field. Such a sensor that utilizes the ac impedance \(Z\) of a magnetic ribbon of half thickness \(t\), resistivity \(\rho\), and magnetic permeability \(\mu_{T}\) at a frequency\(f\) and skin depth \(\delta\) has the potential to improve safety in chemical-using industries.