オーステナイト系ステンレス鋼の腐食疲労に関する研究 (第2報)

Abstract

In the present investigation, we have studied on the effects of acid temperature covering 25°C, 50°C, and 75°C at a cyclic speed of 1700cpm and those of cyclic speed ranging from 100cpm to 4000cpm at acid temperature of 25°C on corrosion fatigue of austenitic stainless steel exposed to air-saturated dilute sulfuric acid having transitional characteristics between passivation and activation.The effect of acid temperature on the endurance property of austenitic stainless steel exposed to air-saturated dilute sulfuric acids in which corrosion resistance was not improved by alloying molybdenum, nearly coincided with what was obtained from a treatment of pure reaction kinetics, as the steel was scarcely passivated by oxygen, dissolved in these acids. Accordingly, logarithm of number of cycles to failure at each corrosion fatigue strength, i.e., logarithm of average velocity of corrosion fatigue failure at each stress level was proportional to the reciprocal of absolute temperature of the acid. Existence of dangerous downward S-N curve obtained at 75°C could also be explained by this view of reaction kinetics.Increase of frequency of cyclic loading for these steels, as would be expected, also increased number of cycles to failure, but did not necessarily decrease time to failure.In austenitic stainless steel whose corrosion resistance to air-saturated dilute sulfuric acid solution was improved by alloying molybdenum, the effect of acid temperature was much complicated due to its delicate nature of passivation at high temperature that fluctuates depending upon the variety of steels and variation of testing conditions, and its cause could not be explained.The effect of cyclic speed on these steels, also, was more complicated, and did not follow the general concept in which increased speed increases number of cycles to failure, but decreases time to failure. On this ground it was expected that controlling factor in speed effect was competitive actions between passivation and activation in the course of progressive increase of cyclic speed.