## 4 Degradation Mechanism by Definition and Status Analysis of Corrosion of Iron Cap and Iron Pin  
In Table 1, pollution grades A, B, and C were granted for the porcelain insulators that began service in 1979. In Korea, the grades of pollution are defined according to the distance from the coast. For the case of grade A, it means that the pollution rate is very low. In the case of a coastal region, the level of pollution depends on the distance of insulators from sea. According to the guideline in the case of the eastern coast, a site 3.5 to 5.7 km from the sea has the least polluted insulators graded as \"A\". However, when distance from the sea is between 2.0 to 3.3 km, the insulators are categorized as grade \"B\" polluted. When the distance lowers to 1.2 to 2.0 km the insulators can be categorized as grade \"C\". Sites with a distance lower than 1.2 km have the most polluted insulators, grade \"D\". Considering the west coast, it can be observed that a site 9.5 to 20.2 km from the sea has the least polluted insulators, grade \"A\". Whereas sites which are 4.8 to 9.5 and 2.4 to 4.8 km from the sea have been categorized as grade \"B\" and \"C\", respectively. However, the site closest to sea i.e., less than 2.4 km away has most polluted insulators, grade \"D\". The same can be observed in case of the south coast and Jeju island. In the case of the south coast, when sites are between 2.5 to 5.0 km and 1.35 to 2.5 km from the sea, the insulators are graded as \"A\" and \"B\", respectively. Whereas when the distance between a site and the sea is 0.7 to 1.35 km and less than 0.7 km, the pollution grade is considered to be \"C\" and \"D\", respectively. Now considering Jeju island, if the distance of a site from the sea is between 8.0 to 16.5 km and 4.1 to 8.0 km, the insulators are graded as \"A\" and \"B\", respectively. Moreover, when the distance between a site and the sea is 2.1 to 4.1 km and less than 2.1 km, the pollution grade is considered to be \"C\" and \"D\", respectively. In the case of the new product (Stage I), which had never been used, no corrosion is detected by the eye. The porcelain insulator pictured in Stage II is a Pollution Level A product. Zinc plating is shown at an early stage of corrosion. The insulator does not need to be replaced immediately but must be cleaned and monitored periodically because there is some damage. The porcelain insulator pictured in Stage III is a Pollution Level B product; it has begun to degrade as corrosion progresses, and its removal from the service line should be considered. The porcelain insulator pictured in Stage IV is a  
Figure 3: A schematic diagram of the progression of the alkali-silica reaction (ASR) in the cement inside the porcelain insulator (**a**) Stage I, (**b**) Stage II, (**c**) Stage III and (**d**) Stage IV. Pollution Level C product; the corrosion is very severe, the metal has rust flows down to the porcelain or cement part, and the insulation resistance characteristic is poor. Rust can be seen but is not merely surface corrosion; the degradation has affected the entire structure of the insulator, which needs to be removed from the service line. Figure 4 shows the X-ray diffraction (XRD) analysis of the iron pin. Peaks of Fe\({}_{2}\)O\({}_{3}\) were observed mainly at 36, 43, 48, 54, 65, 70, and 78 degrees, and the iron pin was also found to have the same tendency, largely due to exposure to the external environment. The more pollution, the more various peaks in the Fe\({}_{2}\)O\({}_{3}\). In the case of a Pollution Level C, the peak was doubled compared to other cases. Even though the iron surface is zinc-coated to prevent corrosion, depending on the degree of service and the degree of contamination, corrosion occurs on the iron cap and iron pin when iron and oxygen react to form Fe\({}_{2}\)O\({}_{3}\).