# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 3 Results  
### AE and OCP Monitoring  
Figure 2 shows the OCP and AE monitoring of the corrosion process of Q235 carbon steel in 2000 mg/L NaHCO\({}_{3}\) in the presence of different concentrations of NaCl. In Figure 2a, OCP behaved differently with time in various NaCl concentrations. The OCP shifted slightly towards the positive direction and gradually stabilized after 50,000 s of immersion in the presence of 500 mg/L NaCl, indicating that the interface condition was influenced by the presence of NaCl. With the increase of the NaCl concentration, the OCP behavior changed. In 800 mg/L NaCl, the OCP shifted negatively and could not reach a stable state in approximately 90,000 s immersion time. At the NaCl concentration of 1000 mg/L NaCl and 1200 mg/L, the OCP moved sharply towards the negative direction and reached a relative stable state at 65,000 s and 30,000 s, respectively. The above results revealed the different \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Threshold** & **PDT** & **HDT** & **HLT** & **Analog Filter** & **Sample Rate** & **Pre-Trigger** & **Length** \\ \hline
27 dB & 200 μs & 400 μs & 200 μs & 100 K–400 KHz & 2 MPS & 40 μs & 2 K \\ \hline \hline \end{tabular}
\end{table} 
Table 2: AE acquisition options. Figure 1: Schematic diagram of the acoustic emission (AE) monitoring of pitting corrosion: (**a**) Q235 specimen (dimension in mm), (**b**) pitting assembly, and the AE system and potential measurement. interface conditions of carbon steel in NaHCO\({}_{3}\) solution in the presence of different concentrations of NaCl. It was commonly believed that the iron oxide and ferrous bicarbonate form as the passive film of carbon steel in bicarbonate solutions [5,6]. The addition of chloride iron accelerated the breakdown of the passive film and triggered the initiation of pitting. Thus, the higher the concentration of Cl\({}^{-}\), the faster the passive film is broken and the OCP is stabilized. Figure 2a also presents the influence of the NaCl concentration on cumulative hits of acoustic emission and the correlation between the OCP and cumulative hits. In the concentration of 500 mg/L, the cumulative hits remained at very low level during the first 5000 s and then started to increase slightly, indicating the corrosion events occurring such Cl\({}^{-}\) concentrations were not very noticeable. The few cumulative hits in the AE monitoring is in accordance with the relatively small variation range of the OCP. The cumulative hits continuously increased with time in the presence of 2000 mg/L NaHCO\({}_{3}\) + 800 mg/L NaCl, which represented the occurrence of corrosion events all the time. Repeating metal dissolution beneath the cover of the pits (occluded cell) and the initiation of new pits resulted in the continuous [6] increasing of cumulative hits. Most likely, the breakdown of the corrosion product film upon the occlude cell induced the sudden ascent of the cumulative hits at about 80,000 s because this process could be highly emissive of the acoustic emission. The cumulative hits behaved differently in 1000 mg/L and 1200 mg/L of NaCl. In the 1000 mg/L NaCl solution, the cumulative hits increased quickly with time and suddenly vised at about 20,000 s, indicating a breakdown of the passive film in a localized area. Then, the cumulative hits increased slowly with time. However, in 1200 mg/L NaCl, the cumulative hits increased rapidly at the beginning  
Figure 2: The open circuit potential (OCP) and AE monitoring of the pitting corrosion of carbon steel in NaCl + NaHCO\({}_{3}\) solutions: (**a**) OCP and cumulative hits versus time, (**b**) cumulative absolute energy versus time. then gradually and slowly with time, indicating a relatively stable development of the corrosion attack on the surface after 5000 s. Before that time, the pitting corrosion started from the first second and propagated very fast.