As a result of corrosion survey of steel structures in the marine environments, it has found that in some cases corrosion concentrated at the portion just below M.L.W.L. Since occurrence of the corrosion is dangerous for steel structures, especially, steel pipe pile structures, applicability and effectiveness of cathodic
protection to the corrosion were investigated in the field water tank. Besides, a field test of cathodic protection was done by using a steel pipe pile.
　Consequently, the following results were obtained:
(1) Protective efficiency of the protection to the corrosion was more than 97 percents.
(2) Current density being required to protect steel portion at tidal zone was greater than a typica lvalue of the initial protective current density in sea water, 100mA/m2. Accordingly, it is desired that protective current requirement is calculated regarding steel surface at tidal zone too.
(3) It is effective to cover steel surface at tidal zone for reducing the requirement.
(4) When applying cathodic protection to existing steel structures, we have to supply extra protective current temporarily for reducing rust layers on steel surface.