The Kojima Lake is an artificial lake. The bank was built at the narrowest point of the Kojima Gulf and it separates the sea and the lake. There are 12 watergates and one set of lockgate. These gates were painted by vinyl paint about 3 years ago. The paint is deteriorated to some extent, and rust was found on the gate surfaces from place to place. These gates are now protected by sacrificial anodes. There are some problems to be considered in this cathodic protection design. The lockgates are opened 30-40times daily, and several watergates are opened daily for a few hours to adjust the lake water level. Therefore, polarization of gates will be interrupted quite frequently. The resitivity of lake water is affected by many factors, i.e. season, weather, water depth, duration and frequency of gate opening, leakage rate of the gates, etc. Zinc anodes are used in the sea and in the lock chamber, and Mg anodes are used in the lake. Protective current density is 40mA/m² and designed potential of the gates is -900mV referred to S.C.E. Satisfactory results are obtained in the seaside, but minimum protective potential of -770mV referred to S.C.E. is maintained in the lock chamber and in the lake.

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In recent years, several reports have been made on the experiments of the cavitation damage. But, for some of them, it is assumed that their cavitation impingements were severer than the real one. In this studies, more natural method (cf. Fig. 1, Photo. 1) was used for the erosion tests of the copper alloys (Sn bronze, Mn brass, Al bronze). The cavitation-erosion (or corrosion) occurred on specimens, and cathodic protections were applied on them. The results may be shown as follows:
1. These materials were intensely eroded on the cavitation parts of specimens.
2. Sn bronze and Mn brass were corroded even on the flowing parts, but Al bronze was not.
3. The protective potentials for the cavitation parts were, -350- -450mV (S.C.E.) at 25°C and -450- -500mV (S.C.E.) at 35°C for Sn bronze and Mn brass.
It therefore, follows that the cavitation erosion caused by this method may be prevented electrochemically.

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