The corrosion rate of coated mild steel was investigated using the technique of current interrupter method. Generally the high impedance of film had been a barrier to measure the corrosion rate of coated mild steel but this method made it easy to eliminate the impedance of film and furthermore to obtain the polarization behavior of coated mild steel. The equivalent circuit of coated mild steel immersed in a 3wt% sodium chloride solution is composed of two kinds of impedance circuit, that is, the impedance circuit of paint film and that of metal interface beneath paint film. These circuits were easily separated because of the large difference of time constants between the one circuit and another. Therefore the polarization resistance was obtained under the condition of small polarization (up to 10mV) and Tafel's coefficient was given by the relation between time and potential in decay curve of small polarization to the extent of 200mV to 300mV. And the corrosion rate of coated mild steel was calculated using Stern's equation. This method is characterised by items below. (I) Very small current was applied to paint film for a few seconds so that no damage appeared throughout measuring. (II) The impedance of film and that of metal surface under paint film were measured simultaneously. As a result, the corrosion rate of coated mild steel coincided with the appearance of underfilm corrosion throughout immersion tests.

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Concentrating behavior of metal ion, hydrogen ion and chloride ion in the anolyte on electrolyzing the stainless steel in the cell separated with a glass filter was studied quantitatively. Some corrosion tests in the prepared anolytes were also conducted on four stainless steels.
Two stages in concentrating process with slow and then fast rate were observed for chloride ion while only one stage was observed for metal ion or hydrogen ion. Incubation time of crevice corrosion calculated by measured current density of the differential aeration cell and critical quantity of electricity until active dissolution of the stainless steel occured is obtained with a few correction. The alloying elements and oxide scale of welds change the incubation time of crevice corrosion of the stainless steel.

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Using a SERT (slow extention rats technique) method, stress corrosion cracking (SCC) behaviors of Type 304 stainless steel rods with the 0.2R round notch were investigated in 42% MgCl₂ aqueous solution in the range of crosshead speed (CS) from 4×10^-5mm/min to 4mm/min. The effects of potential, temperature and alloying elements on SCC were studied by this method. Constant load tests were also carried out in the solution. After the tests the fracture mode was examined by a scanning electron microscope. In the solution at 143°C the maximum susceptibility to SCC was observed at the CS of 2×10^-4mm/min under the corrosion potential. CS at the maximum susceptibility shifted to the lower value of 1×10^-4mm/min at anodic potential and to the higher value of 8×10^-4mm/min at cathodic potential. In the solution at 100°C the effect of potential on SCC susceptibility was the same as that in the solution at 143°C. With increasing susceptibility to SCC increased the ratio of transgranular cracking in a fracture surface. Alloying elements such as Mo and Cu increased susceptibility to SCC. Mechanism of SCC in 42% MgCl₂ in SERT method was discussed from the view point of the formation rate of slip steps and their characteristics of dissolution-repassivation.

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