The prevention rate (PR: the index calculated from the corrosion rate of the non-protection and protection time) is generally employed as the evaluation index of the effect of the cathodic protection of the port steel structures in Japan. When designing the cathodic protection, the prevention rate has been employed as 90% since ancient times. However, if the corrosion rate of the steel is sufficiently close to zero, it is considered that the prevention rate is theoretically close to 100%. In this paper, firstly, we investigated aging changes of the cathodic prevention effect for 4 years by using test pieces installed in Japanese 4 ports. We arranged aging changes of the evaluation index which showed as the effect of the cathodic protection based on the result. Next, based on the above survey results, by using a large number of test pieces (total of 650 pairs) installed at port facilities in the whole country, we grasped the actual condition of the effect of the cathodic protection and verified while comparing evaluation index which shows the effect of the cathodic protection. On the result, it was considered that it was desirable to evaluate the effect of the cathodic protection as "corrosion rate during the cathodic protection"which is a state at the time of the cathodic protection and includes factors of time as the index of the effect of the cathodic protection. In addition, it was suggested that the value of "corrosion rate at the cathodic protection"as an evaluation index of the effect of the cathodic protection was about 0.01mm/y.
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