The corrosion behavior of mild steel was investigated in a culture where hydrogen gas was bubbled. A dual cell system was employed. That is, in the first cell the bacteria produced hydrogen sulfide, which was introduced into the second cell by hydrogen gas. The bacteria were cultured in autotrophic or heterotrophic conditions. The electrode potential and polarization resistance of the specimens in both of the cells were measured. The variations of electrode potential and corrosion rate of the steel in both of the cell could be explained, corresponding to the variation of the concentration of hydrogen sulfide. The weight loss of the steel in the first cell was smaller than that in the second cell. There was no evidence that hydrogen depolarization by the bacteria occured. It seems to be more resonable that the anodic and catholic reactions were stimulated catalytically by hydrogen sulfide, not by the bacteria. On the other hand, in autotrophic condition, the bacteria could not grow so much, and the amount of biogenic hydrogen sulfide was much smaller than that in heterotrophic condition. Organic substances is necessary for the biosynthesis of the bacteria and for the great evolution of hydrogen sulfide. Therefore, in order to prevent the corrosion by the bacteria, in practice, a desalting plant ought to be designed to eliminate organic substances as much as possible.

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The present paper deals with the cavitation erosion of comparatively low intensity by vibrating a disc oscillator close to the test specimen in saline water. Results obtained under potentiostatic catholic polarization are as follows. The cathodic polarization for the cavitation erosion of a tough material can be effective in the early incubation period of the cavitation damage, but not so effective for the erosion in the steady state of the damages. A fruitful availability of cathodic polarization can hardly be expected for the erosion of a high strength steel sensitive to hydrogen embrittlement due to the occuluded hydrogen produced by cathodic current. The application of cathodic protection to the cavitation erosion of Al-alloy whose corrosion rate is much increased in the solution of higher pH-values is rather dangerous because alkaline solution would be produced by the protection current.

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Corrosion rates of zinc have been measured in cyanide solutions under rectangular A. C. polarization. Plots of zinc dissolution rates vs. A. C. frequency consisted of three parts, namely, high frequency region, low frequency region, and middle transitional region. In the high frequency region the corrosion rates of zinc were not affected by A. C. polarization. In the low frequency region the corrosion rates were almost equal to the value obtained under D. C. polarization. In the middle transitional region the corrosion rates increased with the decrease of A. C. frequency. Critical A. C. frequencies at which the middle transitional region was separated from the high frequency region were obtained from the plots of corrosion rates vs. A. C, frequency. The critical A. C. frequencies were also obtained from cell potentials measured with oscillographic technique. The values of the critical frequencies obtained from the above two ways were identical. Under various A. C. frequencies faradaic currents calculated from the corrosion rates were equal to those obtained from plots of E vs. A. C. frequency. It has been concluded that rate of A. C. corrosion is equal to the remainder which is subtracted non-faradaic currents from total currents.

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Modulation spectroscopy is seemed to be one of the most promising methods for studying electrode surface phenomena in aqueous solution because it provides sensitive and in situ detection of the phenomena. The theory and experimental techniques of this method were explained and some interesting results concerning to passivation films and adsorption layers on metal electrodes were reviewed.

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