# In-Situ Monitoring and Analysis of the Fitting Corrosion of Carbon Steel by Acoustic Emission
## 1 Introduction  
In the modern construction industry, carbon steel is widely used because of its low price and good mechanical properties. However, its corrosion resistance in an aqueous environment is very low. Especially in the presence of a saline medium, the probability of corrosion increases greatly, mostly including localized corrosion. Structural damage and mechanical performance degradation can easily happen during long term service with localized corrosion. As one of the most destructive forms of localized corrosion, pitting corrosion of carbon steel intensively occurs in many sites, such as steel in reinforced concrete, and steel with degraded coatings of a bridge or water containment structure. Many investigations [1; 2; 3] have been done for the detection of structural failure of concrete with the acoustic emission (AE) technique. As the initiation stage of many structural degradation cases, the pitting corrosion of carbon steel needs to be monitored. It is commonly believed that pitting corrosion happens on passivated metals and alloys in a solution of halide ions [4]. The properties of the passive film plays an important role in the initiation, growth, and re-passivation of pits. In most occasions, the pitting process is regarded as containing several stages: (1) Local breakdown of the passive film, represented as the nucleation process, (2) propagation (accelerated corrosion), (3) stable growth, and (possibly) (4) re-passivation. Stainless steels are typically passive initially. The initiation of pitting is caused by the adsorption of halide ions through the passive film. Additionally, the growth of the pit occurs mainly at the bottom as the anodic dissolution inside the pit. The cathodic reaction is nevertheless outside the pit,on inclusions or other defects. Stainless steels are relatively highly resistant to pit initiation, which can be ascribed to the excellent passivity of the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) plus Cr\({}_{2}\)O\({}_{3}\) oxide film at the surface. Hence, only a limited amount of pits can be formed on the stainless steel surface. However, when the pit forms, it often grows rapidly and develops in depth. The aggregation of corrosion products at the pit mouth accelerates the corrosion attack beneath the occluded area. The rupture or breakdown of the cover at the pit mouth, including the corrosion products and passive film, may result in re-passivation, which implies the end of the pitting corrosion. As for other materials with a less pronounced passive film, such as carbon steels, pitting corrosion may start at inconsistent pores of the passive film or corrosion products [5]. Different from stainless steels, the growth of pitting undergoes at a relatively slow rate. Pits may repeatedly experience the initiation--growth--re-passivation process [6]. In many occasions, the corrosion attack consists of many shallow pits in the nearby area. The re-passivation of the pit is most probably caused by the passivator in the solution, not by the metal itself. The rupture or breakdown of the passive film and product film is not as notable as stainless steel. It is generally assumed that the iron oxide and ferrous bicarbonate can form as the passive film of carbon steel in a carbonate solution [5; 6]. The origin of the passive film is from the oxidation of iron by dissolved oxygen and the deposition of insoluble ferrous bicarbonate. Many studies have focused on the structure and composition of the passive film. Although there are two models of the passive film (the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) layer and the \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) plus Fe\({}_{3}\)O\({}_{4}\) double layer), it is not surprising that the passive film on iron mainly consists of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\). It is also more acceptable that the passive film of carbon steel in a bicarbonate solution consists of an inner Fe\({}_{3}\)O\({}_{4}\) layer and an outer \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) layer, both combined with insoluble ferrous bicarbonate [5].