The purpose of this study is to investigate the effect of oxygen concentration in the air on the corrosion rate of carbon steel in an air/solution alternating environment in the low oxygen concentration range and to clarify the corrosion rate and corrosion mechanism of carbon steel depending on the oxygen concentration in air by the mass change of specimens before and after the corrosion test and observing the iron rust layer formed on the surface of carbon steel. The corrosion rate increases with increasing oxygen concentration in the air, and the gradient of the corrosion rate decreases gradually. The maximum erosion depth increased with increasing oxygen concentration except for the case of 1％ oxygen concentration, however, the maximum erosion depth for 1％ oxygen concentration was larger than that for 5％ air oxygen concentration. It was clarified from the cross-sectional observation that the reason for the specific increase in the maximum erosion depth only when the oxygen concentration was 1％ due to the localized corrosion of carbon steel.

All nonmetallic inclusions on the surface of a stainless steel sample were analyzed with a scanning electron microscope with a function of automatically observing and analyzing nonmetallic inclusions. The effect of the inclusion type and size on the tendency of pit initiation was then quantitatively evaluated by observing the pits generated on the sample surface. It was found that the probability of stable pit growth increased as the major axis of the inclusions increased, but no effect of the inclusion type was observed. However, an effect of the inclusion type on the tendency of repassivation pit initiation was observed.