The effectiveness of two type corrosion inhibitors, such as chromate-polyphosphate type (inhibitor A) and zinc-polyphosphate type (inhibitor B) in cooling water has been studied. The inhibitor A is more effective than B for mild steel in water. Threshold concentrations of A and B under test condition were 30-50ppm and 70-100ppm respectively. Suitable pH-ranges for the inhibitors were 4-9 and 5-7 respectively. The inhibitor A shows good corrosion inhibition for mild steel in water containing 0-500ppm of SO^2-₄ or Cl^-, but the efficiency of the inhibitor B decreases with the increase of concentrations of these anions.

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Several types of macro-cells in soil, mainly in black soil, were studied in a laboratory regarding the cause of the intense corrosion which occurred on the pipeline in a volcanic soil area. Results obtained were summarized as follows: (1) In the case of a macro-cell composed of polished steel and steel covered with oxide film formed by alternate drying and wetting in soil at 75°C, the polished surface is anodic and suffers from accelerated corrosion. (2) If a differential aeration cell due to water content is superposed to the macro-cell, the corrosion rate is highly increased. (3) Effect of a macro-cell caused by the heterogeneity of soils tested is considerable but the cell is less effective than the differential aeration cell and a macro-cell caused by the oxide film.

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A new electrochemical method was proposed for an evaluation of the protective ability of the inner paint film of doubly coated metal specimen with two different types of paint. The method consisted of the measurement of A. C. impedance of the doubly coated specimen and that of singly coated specimens with the constituent paint film. The impedance of the imagined doubly coated specimen was calculated from the measurements of the singly coated specimens. A comparison was made between the calculated and the observed impedances of doubly coated specimen in order to elucidate the influence of direct contact with the salt solution upon the impedance of inner paint film.
The method was applied to three typical paint systems immersed in 0.5M NaCl solution, that is, the primer-top coat paint system, the wash primer-primer system and the zinc rich paint-primer system. No actual difference was observed in the first system. Moreover, in the second system, the inner layer of wash primer was found to exist in a highly conductive state even when it was covered with a primer paint film.
The most remarkable change was observed in the last system, since the zinc rich paint was highly conductive when it was directly in contact with salt solution and, in contrast, highly resistive when indirectly in contact. From this observation, it is expected that, from the initial period of immersion, zinc rich paint acts in accordance with the so-called resistant control mechanism for a reasonably long while until the beginning of the breakdown of primer or top coat paint film.

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