# Nondestructive Tomographic Imaging of Rust with Rapid THz Time-Domain Spectroscopy
## 3 Results and Discussion
(**c,d**) Cross-sectional B-scan images along with the \(x\)-axis and \(y\)-axis taken from (**b**). (See dashed lines in (**b**)). Inset shows A-scan plot (i.e., THz amplitude vs. time-delay) obtained along the dashed line. We monitored the spread of rust in a steel plate under the influence of two different ionic solutions. For instance, the use of NaCl solution induced rust in metal over a relatively long period of time. Conversely, acidic solutions (such as a mixture of hydrogen peroxide and acetic acid) resulted in a more rapid corrosion process. Figure 4a shows a series of photographs acquired over two weeks after the metal was submerged in 3.5% NaCl solution. The corresponding THz images (i.e., time-integrated C images) are illustrated in Figure 4b. The THz reflection images were normalized by that of gold film. We temporarily extracted the sample from the solution for the measurements and returned it to the solution for further rust formation. Again, the scan range was 20 \(\times\) 30 mm\({}^{2}\) with a pixel size of 80 \(\times\) 120 pixel\({}^{2}\), and the THz signal was averaged five times per pixel this time. It was observed that the THz reflection was suppressed significantly with increasing time in the ionic solution. In a few regions, the THz amplitude reached less than 10% of the original amplitude (i.e., below 1% in terms of intensity). Importantly, the THz reflection images deviated significantly from the photographic images in most cases. For instance, the THz images taken on Day 3 showed a uniformly distributed reflection, which is similar to that of Day 0, whereas the photograph strongly implies rust formation over the entire surface. Similarly, in the dark region (surrounded by a dashed line) found in the photograph of Day 14, we found a bright reflection in the corresponding region of the THz reflection. In other words, the photograph was obscured  
Figure 4: **(a)** Photographs of the steel plate taken on different days after the plate was submerged in the NaCl solution. (b) THz reflection amplitude images from the rusted metal plate taken on different days. Dashed line in (a) is a guide to eye for a direct comparison between the photographic image and THz image. Figure 3: **(a)** A series of C-scan images corresponding to the frequencies of 0.4, 0.7, 1.0, and 1.3 THz. Because the signal strength decreased toward the high-frequency region, the THz amplitude signal was magnified for clarity for the high-frequency images. (b) THz amplitudes as a function of position along the vertical dashed line in (a) for the different frequencies. by the shallow oxide layers (which tend to work as a protective layer to prevent further corrosion) and does not distinguish between the lightly and heavily rusted areas, which proves the usefulness of NDT with THz imaging techniques. Figure 5 shows the results of rapid imaging where we applied an acidic solution (a mixture of 24 mL hydrogen peroxide (30%), 1.1 mL acetic acid (13%), and 4.5 g NaCl) to the steel plate. As the measurement time was 16 s for each image taken at 40 \(\times\) 40 pixels, the rapid THz-TOF system was ideal for inspecting the rust spread that occurred over the course of a few minutes. Here, we dropped the acidic solution on the steel plate for fast rust formation, whereas we washed out the solution temporarily to take the THz imaging measurements each time. This is because we could not take the image in the presence of the solution and the bubbles that are produced violently in chemical reactions. We demonstrated a series of photographs and THz reflection images (i.e., time-integrated C images), respectively, in Figure 5a,b. Although the photographs did not show a noticeable change over time after the first 5 min, the THz reflection images showed a continuous change, where the THz amplitude decreased to 80% of the original value.