# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 1 Introduction
The inhomogeneity of the passive film on carbon steel is the main reason for the initiation of pitting corrosion. Many techniques have been used to study pitting corrosion. The American Society for Testing and Materials (ASTM) has standardized methods for the study/evaluation of pitting corrosion and pitting tests in 6% FeCl\({}_{3}\). Electrochemical measurements are most commonly used in investigating pitting corrosion behavior, e.g., polarization curves measuring the break potential of pitting, E\({}_{b}\); protective potential of pitting, E\({}_{p}\); and the free corrosion potential, E\({}_{corr}\). Current fluctuation behavior in potentiodynamic or potentiostatic polarization is extensively investigated to probe metastable pitting, which is widely regarded as an important phenomenon before stable pitting and can be used to predict stable pitting events. Electrochemical impedance spectroscopy (EIS) is also widely used in the evaluation of the properties of the passive film under pitting attack. Moreover, electrochemical noise (EN) is another useful technique in the investigation of pitting events and the monitoring of pitting corrosion. Although many approaches have been utilized in corrosion investigation, only limited techniques, like EN, linear polarization resistance probe [7; 8], electrical resistance probe [9], and the field signature technique [10], can be applied in pitting monitoring. All of them, however, have their limitations. For example, EN is quite sensitive in in-situ recording and identifying the initiation of pitting events [11; 12]. However, it is very difficult to interpret the data of the propagation process. Therefore, in-situ monitoring and the prediction of pitting corrosion events in service is still a great challenge in industrial applications. Acoustic emission (AE), an important technique that can be used in corrosion monitoring, has attracted great attention in recent years. In most occasions, the corrosion of metals and alloys is always accompanied by a rapid release of energy in the form of a transient elastic wave. The nature of corrosion monitoring by AE is firstly composed of the relationship between the corrosion attack and the transient elastic wave. Such a relationship is then used for in-situ monitoring of the corrosion attack of the material in service. AE is a non-destructive technique, which aims at in-situ monitoring of corrosion attack by detecting, recording, and analyzing the acoustic emission signal. The modern AE technique began with the work of [PERSON] in Germany in the 1950s. In recent years, AE has been widely used in many applications, such as the petroleum and natural gas industry, aerospace industry, transportation industry, and construction industry, etc. [13; 14; 15; 16]. Many types of corrosion have been studied by AE [17; 18; 19; 20; 21; 22; 23; 24; 25], such as stress corrosion cracking [18; 19; 20], abrasion or erosion corrosion [21], and pitting corrosion [22; 23; 24; 25]. [PERSON] [22] proved that AE signals can be easily detected in pitting corrosion. Moreover, a good correlation between AE activity and the pitting rate was observed. Then, the initiation and propagation of pitting corrosion on stainless steels was studied with AE [23]. It further demonstrated that the initiation step of pitting corrosion was not significantly emissive, whereas the propagation step was characterized by the emission of resonant signals. They ascribed the signals in the propagation step to the evolution of hydrogen bubbles. More recent studies by other researchers, such as [PERSON], used AE and potentiodynamic methods to investigate the mechanism of pitting corrosion of stainless steel [24]. [PERSON] and [PERSON] used AE to monitor the pitting corrosion on vertically positioned 304 stainless steel, and analyzed the acoustic emission energy parameter [25].