# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 5 Corrosion Monitoring Techniques
### Localized Electrochemical Impedance Spectroscopy  
LEIS, a scanning probe approach, is a nondestructive electrochemical testing technique. The sample can also be directly observed in a solution of a suitable electrolyte in this technique [147]. The most commonly used method for examining the surface of metals or semiconductors is EIS. It cannot, however, be utilized to analyze local changes, such those brought on by corrosion. Localized electrochemical impedance spectroscopy (LEIS), which was created by merging the direct current (DC) and alternating current (AC) approaches, is a much better instrument for analyzing the local changes on the electrode surface [148].  
Similar to the previously mentioned global EIS approach, a global voltage is applied in LEIS; however, a local potential difference is recorded at a mobile probe or dual-probe setup. The local current can be calculated using this potential difference. The majority of contemporary LEIS instruments have a sensitivity of about 1 nV. The LEIS technique's resolution is claimed to be between 30 and 40 m, with this resolution depending on the probe's size and/or spacing (if twin probes are employed) [149]. Figure 17 demonstrates the basic setup for LEIS technique [150].  
There are two primary types of this method. The first method involves moving the probe tip over the sample in a grid-like pattern while immersed in an electrolyte and applying a fixed frequency potential disturbance. Usually, to identify any intriguing occurrences, such as a low impedance defect, this method is used. The second variation of the experiment can be carried out once a region of interest has been located. Here, the probe is positioned above the region of interest, and all of the impedance spectra are obtained over a certain frequency range. While the processing and graphing of the data after it has been acquired are similar to those of EIS, 3D graphs are frequently used to depict location-dependent information gained using the LEIS technique [151].  
For use in regenerative medicine and personalized medicine, [PERSON] and colleagues investigated novel methods to change the surface of the biomedical Ti-13 Zr-13 Nb alloy. They anodized the alloy in 0.5% HF electrolyte at 20 V for 120 min to produce first-generation oxide nanotubes (ONTs) layers on a Ti-13 Zr-13 Nb alloy. The 1G CNTs' in vitro corrosion resistance was tested using an open-circuit voltage in a saline solution at 37 \({}^{\circ}\)C compared to a Ti-13 Zr-13 Nb substrate. The distributions of the local impedance modulus \(|Z|\) values over the surface of the Ti-13 Zr-Nb alloy before and after electrochemical oxidation in 0.9% NaCl solution are shown in Figure 18 (before and after). For Ti-13 Zr-Nb that has not been anodized and after electrochemical oxidation, the average value of impedance irregularities is around 1.6 k and 13, respectively. This leads to the conclusion that, compared to the Ti-13 Zr-Nb alloy with a 1G CNTs layer, the surface of the non  
Figure 17: Block diagram of the LEIS setup [150].  
-nodized Ti-13 Zr-Nb alloy exhibits a significantly heterogeneous distribution of impedance values. It was discovered that under the suggested circumstances, anodizing the surface of the Ti-13 Zr-13 Nb alloy's surface made porous ONTs significantly more resistant to pitting corrosion [152].  
LEIS frequently calls for specific hardware and setup, such as scanning probe microscopy in conjunction with impedance measurement methods. The size of the measurement electrode or probe has a big impact on LEIS's spatial resolution. It may be difficult to analyze extremely small or highly localized regions of interest due to this constraint. The LEIS experimental setup can be very intricate and necessitates exact alignment of the probing electrode or tip. In particular, in time-constrained research contexts, this complexity might result in higher experimental difficulties and lengthier setup periods, thus, restricting the efficiency of measurements [153].