# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 3 Results
### Surface Morphology Monitoring of Pitting Corrosion  
Figure 3 shows the surface morphologies from in-situ monitoring of the propagation of pitting corrosion in different Cl\({}^{-}\) concentrations at different time intervals of 0 s, 40,000 s, 60,000 s, and 80,000 s, respectively. The surface morphology monitoring experiments were conducted simultaneously with the OCP and AE measurements. After the experiments, the surface morphologies of pits after the removal of loosened corrosion products were observed by SEM, as shown in Figure 4.  
In the presence of 500 mg/L NaCl, several pits can be observed on the surface after 40,000 s of immersion. It can be seen from the images that the pits did not grow as the immersion time increased, indicating the pits re-passivated and would not propagate any more. SEM images also proved, as shown in Figure 4a,b, that the pits were open and shallow, without the cover of corrosion products. This type of pitting emitted weak AE signals because it was caused by localized anodic dissolution, which is not an effective generation source of AE. Additionally, some of them may have been too weak to pass the threshold of 27 dB in the acquisition setup. No apparent pits can be observed on the carbon steel surface by an optical microscope before 40,000 s in the presence of 800 mg/L NaCl. However, the occluded cell was formed on the specimen surface. This was the reason for the increase of cumulative hits at the beginning of the experiment. After 40,000 s, the two pits gradually initiated and propagated on the surface. The pits continued to grow as the immersion time increased and the corrosion damage area at 80,000 s increased. This behavior was in good accordance with the variation of cumulative hits. In addition, many corrosion products can be observed on the damage area. The occluded environment beneath the corrosion products formed, which accelerated the propagation of the pitting corrosion. SEM images showed some corrosion products that were covered on the pit, as shown in Figure 4c,d. The carbon steel beneath the cover continued to dissolve in the occluded area, which corresponded to the continuous increase of the cumulative hits in Figure 2.  
The corrosion morphology evolution and SEM images of the specimen in 1000 mg/L of neutral NaCl solution are shown in Figure 3c1-c4 and Figure 4e-f. Two apparent pits can be observed on the specimen surface at 40,000 s, which continued to grow with immersion time. Many corrosion products can be observed on the damaged area at 80,000 s. The SEM images show that there were some  
Figure 3: Surface morphologies from in-situ monitoring of the propagation of pitting corrosion of carbon steel with an optical microscope in the solutions with different concentrations of chloride ions: **(a1)**, **(a2)**, **(a3)**, **(a4)** 500 mg/L NaCl, **(b1)**, **(b2)**, **(b3)**, **(b4)** 800 mg/L NaCl, **(c1)**, **(c2)**, **(c3)**, **(c4)** 1000 mg/L NaCl, **(d1)**, **(d2)**, **(d3)**, **(d4)** 1200 mg/L NaCl. shallow and small pits on the sample surface. However, because the solution became more corrosive, the corrosion started earlier and its rate increased. As a result, higher AE activity was recorded at the beginning of the monitoring and the AE hits over a short duration near 20,000 s showed a high absolute energy. As implied by the OCP, the sample of Q235 carbon steel was corroded as soon as it was put in the 1200 mg/L NaCl solution, as shown in Figure 3d1-d4. Combined with the cumulative hits curve in Figure 2, the corrosion initiation and propagation were very fast from the first second. Correspondingly, the highest AE activities were in the beginning period, but these did not have a high energy. This can be attributed to the shallow and open shapes of the pits, as shown in Figure 4g-h. The corrosion mechanism of the carbon steel is due to the general corrosion at high Cl\({}^{-}\) concentrations while pitting corrosion occurs at low Cl\({}^{-}\) concentrations.