Most of the examination regarding corrosion of steel structures at harbors has been conducted from point of the so-called general corrosion of steel buried in a single zone, for example, in sea water or in mud zone, and so on.
However, since long steel contacts nearly with several different zones in field, for example, sea water, mud zone unsaturated with sea water and mud zone saturated with sea water, it may be supposed that another corrosion unlike general corrosion will occur along the surface of long steel passing through different zones.
Accordingly, to distinguish corrosion tendencies of steel in a single zone and in different zones, in this report the former was called micro-corrosion, and the latter macro-corrosion.
To obtain a clue to the elucidation of steel corrosion problem at harbors, the relation between macro-and micro-corrosion rate of steel was investigated at laboratory and in field, and the following results were obtained:
(1) When steel contacted with both mud zones unsaturated and saturated with sea water, steel in unsaturated mud zone served as cathode and steel in saturated mud zone as anode, and then potential difference was about 150 to 200mV. In this case, macro-corrosion rate was directly proportional to the ratio of cathodic to anodic surface areas of steel, A_c/A_a. So, steel may become more dangerous due to macro-corrosion with increasing of A_c/A_a, and macro-corrosion rate may be neglisible at reduced A_c/A_a.
(2) When steel contacted with flowing sea water and mud zone saturated with sea water, there was such tendency that macro-corrosion cell occurred, whose cathode was steel in sea water and whose anode was steel in saturated mud zone. Then, potential difference was about 50mV, and contrary to the case (1), macro-corrosion occurred only when A_c/A_a was less than 0.3 to 0.6.
(3) When steel contacted with flowing fresh water and saturated mud zone, tendency of macrocorrosion was similar to that in the case (1).
(4) When steel contacted with flowing sea water, unsaturated and saturated mud zones, steel in sea water and unsaturated mud zone acted as cathode and steel in saturated mud zone as anode.
Therefore, it seems that such structures constructed with steel sheet pile as quay must be the using example of steel at harbors which is effective to diminish the danger resulting from corrosion because A_c/A_a in quay becomes about 0.3 in general.