In order to study the monitoring of corrosivity of metals in fresh water, atmospheric corrosion monitor （ACM） sensors were immersed in various aqueous solutions to measure their output currents. The results showed that the ACM sensors showed instantaneous responsiveness to water temperature differences and minute amounts of water treatment additives, demonstrating that they can detect and output changes in corrosivity within very short times. The sensors produced different outputs in response to four types of fresh waters. These outputs were found to have high correlation with the amounts of corrosion measured by resistmetric corrosion monitor （RCM） sensors. Thus, it was suggested that ACM sensors may provide a useful means of monitoring corrosivity in fresh water systems.
In the present study, the iron rust layer formed on the low ally steel in air-solution alternating condition was investigated by cross-sectional observation and analysis, and the mechanism of accelerated corrosion of the steel in the alternating condition was clarified. Observation and analysis showed that the multi-layered iron rust layer composed of red rust layer （γ-FeOOH）, rust crust layer （Fe3O4）, inner crystal （Fe3O4）, and inner rust layer was formed on the low alloy steel. It can be considered that the multi-layered iron rust layer accelerated the cathodic reaction rate of dissolved oxygen in the alternating condition. This acceleration would be the reason why the corrosion rate of the low alloy steel in the alternating condition was accelerated.
We fabricated the ACM sensor consisted of a Fe/C-galvanic couple and investigated the sensor output behaviors in outdoor exposure test or in constant humidity test. In the exposure tests of Fukui and Okinawa, the FeC sensor output is almost the same as the FeAg sensor output, but decreases when several days after the start of exposure and during rain period. The FeC sensor output in the I-RH calibration curve increases with increasing an amount of deposited sea salt and a humidity, and is the same as FeAg sensor output when Ws is in the range of 0.01 to 1 g/m2 and RH is 30 to 90％. The Igal of the FeC sensor in an artificial seawater drops to 90μA, which is less than 1/3 of that of the FeAg sensor. The cathode current density of the C electrode in an artificial seawater is lower than that of the Ag electrode. By analysis impedance spectrum of the C electrode, the change transfer resistance, Rct increased and the electric double layer capacitance, Cdl reduced than that of the Ag electrode. It is considered that the C electrode/liquid interface has reactive resistance to electron transfer in the reduction reaction of dissolved oxygen.
The prevention rate（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 harbor steel structures. 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 protection effect for 4 years by using test pieces installed in Japanese 4 harbors. 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 harbor 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 0.01 mm/y.