It has long been regarded that the corrosion of mild steel under the maritime environments is inevitable regardless of the so many devices and methods developed for the corrosion. To expect the economical and effective use of mild steel for harbour construction works, therefore, it is considered primarily important to establish a practical mechod to estimate the corrosion rate in sufficient accuracy at first, and then to develop a effective device for the protection.
　The purpose of this study is to review the theories on the estimation of corrosion of metal and to develop a new, practical, and accurate method for the estimation of corrosion rate of mild steel under the maritime environments.
　Generally, corrosion rate of metal is determined using the exchange current, which is equivalent to the one at the intersection of the anodic and cathodic polarization curves obtained through laboratory tests on the particular metal. The shape of the anodic polarization curve is consideded characteristic of a particular metal system and that of the cathodic is influenced primarily by the environmental factors. In addition to this, it is found through the field investigation and laboratory tests that under the marime environments the anodic polarization curve is quite flat while the cathodic is considerably steep.
　Hence, the intersection of these curves depends upon the shape of the cathodic polarization curve, as far as the metal system is maintained unchanged.
　In this study, only the mild steel for the maritime construction works, which is usually defined by SS 34, 39, 41, 49, or 50 in Japan Industrial Standards, is delt with,therfore, the polarization process of the steel is considered controlled by the cathodicpolarization.
　It is obvious, however, that the basic concept of this study is independent upon the type of metal and that only calibration works are required to apply the concept to any type of metal.
　This shudy consists of two parts, the first is the laboratory tests and the second is the field test.
　In the laboraratory test, it was aimed at first to find the correlation among the depolarized current density, which is naturally assumed to indicate the cathodic polarization process, the specific resistivity of the modefied environments, and the corrosive losses of mild steel samples subjected to the modified environments in the laboratory.
　The relation obtained is expressed as following:
　　　　 A＝K・IaKD・ρ-b
　　　　　 ≒K・IaKD
Where, A＝Corrosion Rate of mild steel (mm/yr), computed from the corrosive losses of samples.
　　　 IKD＝Depolarized Current Density (μA/cm2)
　　　　ρ＝Specific Resistivity (ohm-cm)
　　 K,a,b＝Constants depending on the material or the dimension of the electrodes.
　The depolarized current density quoted above is the circuited current density between the inserted two electrodes, magnesium and mild steel, measured by the suggested method under the corrosive environments.
　Basing on the laboratory test results, the apparatus of several types were designed and manufactured to be subjected to the actual field conditions. The field test results using the newly developed apparatus, subjected to a number of typical soils prevailing in Yokosuka, Japan, were quite satisfactory and conforming with the laboratory test.
　The author believes that it is possible to estimate the micro corrosion rate of mild steel under the maritime environments with sufficient accuracy using the newly deviced apparatus.
　The subject of this study is now developed into the macro corrosion problem which origins in the differential aeration and t he reuslts will be reported in a series before long.