# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 3 Results
### Clustering Analysis  
A function was established to input waveform information, time and system information including the maximum duration, hit definition time (HDT), hit lockout time (HLT), peek definition time (PDT), sampling frequency, and the threshold, to Matlab. Some AE parameters were used for the clustering analysis, for instance, the \"count\", the \"duration\", and the \"average frequency\". The count of the AE event is the number of times the signal rises and crosses the threshold of the AE setup. The duration of the AE event is the time between the rising edge of the first count and the falling edge of the last count. The average frequency (Hz) was calculated from N\({}_{\text{AE}}\)/t\({}_{\text{AE}}\) to estimate the characteristic frequency of an AE event. These parameters were recorded by the AE acquisition system in-situ and extracted in Matlab after waveform processing, respectively. After the extracting feature, these new parameters were plotted in additional correlation diagrams. The comparison between the original parameters and the new parameters under different corrosion conditions are shown in Figures 8 and 9. These results showed that the performing of the developed signal processing can effectively classify AE signals in the correlation diagram. Based on the analysis of the relationship between the duration and average frequency of the AE waveforms, the AE signals of Q235 carbon steel in different corrosive media (2000 mg/L NaHCO\({}_{3}\) + 800 mg/L NaCl and 2000 mg/L NaHCO\({}_{3}\) + 1000 mg/L NaCl solution) during the pitting process were classified into three types: High duration with high frequency, high duration with low frequency, and low duration with low frequency. In addition, the three groups of signals were identified according to the duration versus counts correlation diagram: Low duration with low counts, high duration with low counts, and high duration with high counts. The representative waveforms of the three types of signal are shown in Figure 10. The low duration (<2000 us) with low counts (<50) signals and high duration (>2000 us) with low counts (<50) signals appeared most frequently at the beginning of the experiment, indicating that they belong  
Figure 8: Analysis of the parameters of the signals of Q235 carbon steel in 2000 mg/L NaHCO\({}_{3}\) + 800 mg/L NaCl: (**a**) and (**c**) original relationship of the duration-average frequency and duration-counts, (**b**) and (**d**) relationship of the duration-average frequency and duration-counts after the extracting feature. to the corrosion event of the surface breakdown. They were the burst signal (Figure 10a, duration 492 \(\upmu\)s, counts 23) and asymmetric signal (Figure 10b, duration 2732 \(\upmu\)s, counts 44). Most of the high duration (>2000 \(\upmu\)s) and high counts (50-100) signals occurred during the accelerated propagation of pitting corrosion. They were resonant signals (Figure 10c, duration 2336 \(\upmu\)s, counts 98). In the stable growth process of the pitting corrosion after the accelerated propagation, the AE waveforms were also resonant signals. However, the signal feature was more complex than that in the accelerated propagation process (Figure 10d, duration 1878 \(\upmu\)s, counts 77), which may be due to the combination of the breakdown of corrosion products and the growth of the pits at the same time. The above result is consistent with other studies on the acoustic emissions of pitting corrosion [23; 35; 36]. Figure 10: _Cont._  
Figure 9: Analysis of the parameters of the signals of Q235 carbon steel in 2000 mg/L NaHCO\({}_{3}\) + 1000 mg/L NaCl: (**a**) and (**c**) original relationship of the duration-average frequency and duration-counts, (**b**) and (**d**) the relationship of duration-average frequency and duration-counts after the extracting feature. Many researchers have tried to quantitatively correlate AE parameters to corrosion based on parameter recognition or clusters.