## 2 Corrosion Depth Prediction of Carbon Steel
### Prediction of Corrosion Depth of Carbon Steel in Ningbo Area  
Figure 7 shows the division of the atmospheric environment in each district and county of Ningbo. The Zhenhai District and Beilun District were classified as industrial-marine (I-M) atmospheric environments due to the heavy concentration of industry and the existence of large petroleum refineries. Cixi City and Xiangshan County are close to the sea and were classified as marine (M) atmospheric environments. Ningbo's main urban area, Yinzhou District, Haishu District and Jiangbei District, were classified as urban/industrial (U/I) atmospheric environments. Yuyao City, Fenghua District and Ninghai County were classified as rural (R) atmospheric environments due to their proximity to the mountainous area and a weak industrial base. According to the specification in GB/T 24513.1-2009 [30], the first-year corrosion depth \(D_{0}\) of carbon steel can be estimated according to the following formula:  
\[D_{0}=1.77P_{d}^{0.52}\times\exp(0.02 RH+f_{St})+0.102\times S_{d}^{0.62}\times \exp(0.033 RH+0.04T) \tag{3}\]  
where \(T\) denotes the annual average temperature (\({}^{\circ}\)C), \(RH\) denotes the annual average relative humidity (%), \(P_{d}\) denotes the annual average \(SO_{2}\) deposition rate (mg/(cm\({}^{2}\cdot\)d)), \(P_{d}=0.8P_{c}\), \(P_{c}\) is the \(SO_{2}\) concentration (mg/cm\({}^{3}\)), \(S_{d}\) denotes the annual average  
Figure 5: Double logarithmic plot of the corrosion depth vs. exposure time for carbon steel at some typical sites. Figure 6: The corresponding \(n\) and \(D_{0}\) values under each exposure site. deposition rate (mg/(cm2d)) and \(f_{St}\) denotes the carbon steel correlation coefficient; when \(T\leq 10\)\({}^{\circ}\)C, \(f_{St}=0.150\cdot(T-10)\) and, when \(T>10\)\({}^{\circ}\)C, \(f_{St}=-0.054\cdot(T-10)\). The atmospheric environment data in the Ningbo area and the values of \(D_{0}\) and \(n\) are shown in Table 1. The data of the annual average temperature, annual average relative humidity and chloride ion deposition rate in each atmospheric environment were obtained from the China Meteorological Data System ([[http://data.cma.cn/](http://data.cma.cn/)]([http://data.cma.cn/](http://data.cma.cn/)), accessed on 1 July 2021) and National Materials Corrosion and Protection Data Center ([[https://www.corrdata.org.cn/](https://www.corrdata.org.cn/)]([https://www.corrdata.org.cn/](https://www.corrdata.org.cn/)), accessed on 1 July 2021). The sulfur dioxide concentration in Ningbo's districts and counties was specially commissioned at Qingdao Hengli Environmental Technology Research Institute Ltd. for testing (tested on 1 May 2021), and the testing locations were outdoor. As can be seen from the table, the sulfur dioxide concentration, chloride ion deposition rate, \(D_{0}\) value and \(n\) value are the maximum for the industrial-marine (I-M) atmosphere. The data in Table 1 was substituted into Equation (1), and the predicted curves of the long-term corrosion depth of carbon steel in each atmospheric environment in the Ningbo area were obtained (as shown in Figure 8). It can be seen the corrosion rate from the largest to the smallest: industrial-marine (I-M) atmosphere, marine (M) atmosphere, urban/industrial (U/I) atmosphere and rural (R) atmosphere. It is worth mentioning that the corrosion depth of carbon steel exceeds 1600 \(\upmu\)m under the 100-year corrosion age of the I-M atmospheric environment, which indicates that, in such an atmospheric environment, transmission towers are the most vulnerable to corrosion and should be focused on research.