# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 3 Results
### AE and OCP Monitoring
On the other hand, the OCP stabilized (even a slightly increasing trend) at a certain time in 1000 mg/L (65,000 s) and 1200 mg/L (30,000 s), which indicated the stage of the stable propagation. The high emissive period of AE was before this stage in these two solutions. The results of the AE monitoring and OCP monitoring were in agreement. It was noticed that the significant increase in the AE hits was earlier than the sharp drop of the OCP in these two experiments. It revealed that the AE monitoring is more sensitive to the corrosion evolution compared with the OCP monitoring. It is interesting that the cumulative hits kept increasing after the stabilization of the OCP curve. The nature of the pitting corrosion is randomly sporadic and stochastic [34]. Therefore, the initiation, accelerated the propagation, and the stable growth/ re-passivation (the corrosion in the pit stopped) of the pitting corrosion is hard to predict. In most circumstances, pitting corrosion happens and propagates at some localized sites independently. The OCP behavior represents the general thermodynamic property of the whole interface, which depends on whether the pitting is metastable or stable, in which stage--initiation, accelerated propagation, stable growth, or re-passivation--and even the number of active pits. While the increase of cumulative hits stands for the new corrosion events at any local area. These corrosion events refer to any event during corrosion initiation, propagation, and stable growth. While for the whole interface, such corrosion events may have no apparent influence on the OCP in the stable growth stage. However, only the AE signal amplitude of these events exceeds the threshold of the AE acquisition setup, and the recording of AE data can be triggered. That is, the cumulative AE hits in Figure 2 only increased for such corrosion events that can generate an AE signal with an amplitude equal or higher than 27 db. Figure 2b shows the cumulative absolute energy changes over the time of the AE monitoring. Basically, the higher the AE hits' activity, the faster the increase of the absolute energy than the other stages in the experiments, as shown in Figure 2b, except in the 1200 mg/L NaCl solution. Not only did the acoustic emission of the corrosion event depend on the corrosion stages, and also on the corrosion morphology and other factors, such as the propagation route of pits (i.e., go through the crystalline grain or boundary), but so did the energy of the AE. Although energetic AE signals in pitting normally correspond to the propagation [23; 24] and the bursting of the hydrogen bubble, the formation and peeling of thick corrosion products may also generate energetic AE signals if they exist in the 1200 mg/L NaCl solution. As indicated from the above discussion, the AE events are directly associated to the corrosion phenomena, such as passive film breakdown, hydrogen evolution, pit propagation, etc., compared with OCP monitoring, which only reflects the general electrochemical thermodynamic information. Thus, all the stages of corrosion can be detected by AE monitoring. However, the OCP is only sensitive to the corrosion initiation stage because only the corrosion initiation experiences a significant change in the corrosion potential.