# A Disposable Soft Magnetic Ribbon Impedance-Based Sensor for Corrosion Monitoring
## 3 Results and Discussion  
Figure 2a-d shows the images obtained from the SEM and XRD studies. Since in real time the nitric acid is not wiped off of the ribbon, the results shown are only of the images where the acid was also not wiped off. It can be seen that the surface of the ribbon experienced significant corrosion from the acid, as well as produced a significant amount of metal oxide in response to the acid treatment (Figure 2b,c). This in turn can change the geometry and magnetic properties of the ribbon, leading to a change in impedance. No structural change in the ribbons was observed by the XRD study (Figure 2d). These results suggested that the change in impedance of the ribbons was most likely a product of a change to the magnetic permeability brought about by the nitric acid's corrosion of the surface, along with a change in the topography of the ribbon, which effects the resistivity. According to the above equations, the frequency range chosen for the ac signal would also affect the sensitivity of the sensor as well. SEM images showed changes to the morphology of the ribbon, which decreases the area \(A\)--thus, assuming the change in \(R\) is small compared to the area, decreasing the resistivity. It would seem a reasonable assumption for the change in impedance to be dependent on the frequency, geometry, and magnetic permeability. Figure 2: SEM images of the (**a**) untreated and (**b,c**) acid-treated (7.4 Molar HNO\({}_{3}\)) ribbons. XRD patterns of the untreated and acid-treated (7.4 Molar HNO\({}_{3}\)) ribbons with reference to the sample holder; (**d**) XRD patterns of the untreated and HNO\({}_{3}\)-treated ribbons. The results from the PPMS furthered the legitimacy of this assumption by showing a decrease in saturation magnetization (\(M_{\text{S}}\)) for the ribbon as a function of acid concentration, which can be seen in Figure 3a,b. It can be seen that while the saturation magnetization decreased, no obvious effect existed on the magnetic coercivity or anisotropy of the ribbon. The decrease in \(M_{\text{S}}\) has been previously reported for acid-treated ribbons with increases in acid concentration [16,17]. The frequency dependence was also tested for the impedance-based sensor (not shown), where the acid concentration used to test the frequency dependence was 7.4 M due to its large effects on the topography of the ribbon. It was found that the data using frequencies above 2 MHz possessed high amounts of noise, making its reliability questionable. For this reason, a frequency of 0.2 MHz was chosen to conduct the rest of the studies, as it exhibited low noise and accurate data. Our previous GMI study [16] has shown 2 MHz to be the optimal frequency of measurement, and further research with improved noise correction should be conducted to investigate the legitimacy of this frequency when applied to this specific sensor. Figure 4a-c shows the results of the impedance testing performed as a function of the acid concentration, using the chosen frequency of 0.2 MHz. Figure 4a shows the change in impedance as a function of time for different acid concentrations. It is clearly seen that as the acid concentration increases, the sensitivity of the sensor increases as well--indicating efficient sensing capability. Figure 4b shows a graph of the sensor sensitivity, \(\eta\), as a function of acid concentration. In Figure 4b, a large slope is observed for the lower acid concentration, which lessens as the concentration increases--indicating a possible limit to the damage obtained on the ribbon from the acid. The linear fit (which was performed using an Origin function) on the graph indicates that the sensor in question would yield accurate and sensitive results for chemical sensing when properly calibrated, as well as functioning in a simple, easy-to-use manner for all users.