Abstract
The large amounts of localized corrosion were often observed at the part under low water level (L.W.L) of the steel structures built at the marine environment. But the reason why such corrosion maximum was observed at this part has not been well understood. Therefore, the corrosion phenomena at this part were investigated using the model samples to measure both amounts of corrosion and macro-cell currents. It was confirmed that the corrosion at the part under L.W.L. was influenced by not only the tidal zone but also the immerged part in sea water. When the tidal zone was painted, the maximum corrosion at this part was not observed. The large amount of corrosion at this part was observed in the case of the longer immersed part in the sea water than the shorter one.
Macro-cell current measurement shows that the large current was observed at the time to exchange from low tide to high tide. This current flows from tidal zone as a cathode to the part under L.W.L. as an anode, and the deeper part of immersed zone acts as anode, too. And the time changes to low tide, the part under L.W.L. also stayed as an anode but the deeper part of immerged zone changed to cathode. The reason of the phenomenon is estimated that the strongly anodic polarized rust layer at the part under L.W.L. during the time of high tide is gradually return to the rest potential taking long time. Then, the part under L.W.L. stayed as anode and created macro-cell with deeper part of immerged part as cathode till the next coming high tide continuously.
