# A Disposable Soft Magnetic Ribbon Impedance-Based Sensor for Corrosion Monitoring
## 3 Results and Discussion
It should be noticed that although the initial value of the impedance of ribbon used for each test was slightly different (Figure 4a), this did not affect the accuracy of the sensor, as we considered the change in impedance (not the absolute value of the impedance) due to chemical corrosion. The spacing between the two copper wires that supplied the current to the ribbon was also found to influence the sensitivity of the sensor. As the reaction time of the HNO\({}_{3}\) increased at lower concentrations, the acid was provided with more time to expand than at higher concentrations. Due to this occurrence, interference between the wires and acid was observed for separations of less than 20 mm, leading to 20 mm being the ideal amount of separation for the wires. It was found that the reaction time of the change in impedance also  
Figure 3: (**a**) Magnetic hysteresis (\(M\)–\(H\)) loops taken at 300 K for the untreated and acid-treated ribbons for different concentrations; (**b**) Saturation magnetization is plotted as a function of acid concentration. yielded important information on the concentrations of corrosive chemicals and showed an almost linear decrease in reaction time as the acid concentration grew (Figure 4c); therefore, this aspect of the sensor can simultaneously be used as a sensing unit as well. It is generally known that the ac impedance (\(Z\)) consists of its real (\(R\), ac resistance) and imaginary (\(X\), reactance) parts [18]. A fundamental question thus emerges as to how \(R\) and \(X\) were altered subject to the acid treatment. We found that although the saturation magnetization \(M_{5}\) (also the magnetic permeability) decreased significantly with an increase in acid concentration (Figure 3b), leading to the corresponding changes in magnetic reactance (\(X\)), the contribution of the change in \(X\) to the change in \(Z\) was insignificant (Figure 5). It can be clearly seen in Figure 5 that the impedance was mostly resistive in this sensor, and thus provided the main contribution to the sensitivity--as was observed from the impedance analyzer data. The change in \(Z\) due to the chemical corrosion nearly corresponded to the change in \(R\); this fact supports further investigation of a more sensitive outcome at higher frequencies than looked at in this study. An impedance-based corrosive chemical sensor to be used with liquid corrosive chemicals is a novel addition to the field of impedance-based sensors [18,19]. Many sensors have been constructed for use in the medical field as biosensors for drug delivery and cancer monitoring [18, 19, 20, 21]. Many more have been constructed using impedance spectroscopy for chemical detection in the atmosphere, but usually using non-conductive, non-magnetic materials. SMI effects have been studied with applications in corrosive chemical sensors [13, 14, 15, 16, 17], and while SMI techniques are known for their sensitivity, they also require an external magnetic field and often a reference probe. The design of our  
Figure 4: (**a**) Changes in the ac impedance (\(Z\)) at different acid concentrations as a function of measurement time at a fixed frequency of 0.2 MHz. As the acid concentration increased, a stronger change in impedance could be observed; (**b**) shows the sensor sensitivity, \(\eta\), as a function of acid concentration. The linear fit displayed on the graph implies reliable and efficient sensing capabilities; (**c**) shows the reaction time as a function of acid concentration, indicating another potential sensing source from the sensor. new chemical sensor is easy to use and construct and has the potential to be more cost effective and energy efficient than GMI chemical sensors due to its ability to function with no external magnetic field or chemicals, while also operating without any form of reference probe. The exploitation of GMI sensitivity in conjunction with the simplicity of many electrochemical sensors has brought about a novel impedance-based sensor that can be used to improve safety in many chemical-using industries.