# Corrosion-Fatigue Evaluation of Uncoated Weathering Steel Bridges
## 3 Corrosion Pitting Evaluation  
### Mechanism of Pitting Corrosion  
In UWS bridges, corrosion is always a big challenge to durability [27]. Generally, the corrosion process can be classified into two types, i.e., uniform corrosion and pitting corrosion. The two types of corrosion usually occur simultaneously in steel bridges, both of which can degrade the performance of the bridge. Under the uniform corrosion, the thickness of structural members will be reduced with time, which raises the stress level in the members. However, the effect of uniform corrosion in UWS bridges can be almost ignored due to the corrodent-proof rust film.  
Thus, the pitting corrosion is of particular concerns in UWS bridges. The pitting corrosion is an autocatalytic process inducing local corrosion interspersing on the material surface. The interspersing could be regarded as a kind of local irregularity, which in turns escalates the local stress concentration. Generally, the pitting corrosion is believed as the most detrimental form of corrosion, which could even cause structural failure [28]. In a C-F analysis, pitting flaw can be regarded as an equivalent initial flaw, from which the final critical fatigue crack can be formed under cyclic stress [29; 30].  
Based on the mechanism of corrosion, [PERSON] [31] divided the post-rust pitting corrosion into four different stages as the following: (1) The rupture of the stable rust layer; (2) the achievement of the critical condition that the pit begins to grow; (3) the dissolution of the substrate and (4) the formation of visible pitting irregularity.  
Referring to the above classification, the pitting process in the UWS bridges can be defined in a similar way, as shown in Figure 2. It is shown that the rupture of the protective film occurs at first, followed by the initiation of the pitting flaw with weakened protective film. Finally, the pitting flaw grows without a protective film. However, some researchers [32; 33; 34] indicated that the rupture of the protective film occurred very rapidly. Besides, [PERSON] reported that the initiation of the pitting process only costs as short as several microseconds [32]. On this end, it is assumed in the following analysis that the C-F life is roughly equal to the time cost by the latter two stages, i.e., the initiation of pitting flaws and the propagation of the flaw.  
The pitting corrosion process can be accelerated by both the mechanical (e.g., cyclic loads) and chemical factors (e.g., corrosive environment), as suggested by studies [23; 35]. In the presence of cyclic loads, the protective film is prone to rupture because of the reduced strength. At the same time, the pitting corrosion rate could be notably increased under the high-level stress range induced by the cyclic loads [23; 35]. Besides the mechanical factor, the pitting process can also escalate the corrosive environment, especially the concentration of chlorine (Cl). [PERSON] et al. [36] claimed that pitting flaw could appear to the surface of the steel in high Cl ions concentration. For instance, the pitting corrosion rate of bridges in the marine environment, where the Cl ions concentration is high, is significantly higher than those in the non-marine environment. Meanwhile, the structural member will experience serious pitting corrosion when located in the sheltered position of UWS bridges, since the Cl ions attached on the surface cannot be washed directly by the rain [37].