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1報)

Abstract

In the present investigation, corrosion fatigue behaviors of AISI type 316 stainless steel subjected to repeated stress while exposed to some aqueous acid solutions were studied. Aqueous acid solutions selected as corrosive environments were nitrogen gas bubbled 5% HCl, air-saturated 5% HNO3 and 5% H2SO4 because of their different oxidizing characteristics, since corrosion is essentially a process of oxidation, and also, passivity of a stainless steel can be determined by oxidizing nature of an environment, and thus, if a tendency towards corrosion fatigue is to be predominantly controlled by corrosion property of a stainless steel, the oxidizing nature of an environment should be reflected also on its endurance property.This view was confirmed by experiments, which showed that the shape and characteristic of each S-N curve obtained in every aqueous acid solution was representative of each oxidizing nature of the solutions.In air-saturated 5% HNO3 with the highest oxidizing power, the highest endurance was obtained while the initial potential was rapidly shifted to the less noble direction corresponding to repeated stress levels in accordance with the progress of deterioration of protective film formed in the air, but its potential was almost kept within the passive potential range, and the rate of dissolution gradually increased. When approached fairly close to the corrosion fatigue life, slight fluctuation of the potential and more increased dissolution, namely, the local failure of passive film was recognized.In nitrogen gas bubbled 5% HCl with the poorest oxidizing power, the smallest endurance and a dangerous downward S-N curve was observed, steel being always dissolved in active state, and also, both the potential variation and fluctuation were not caused. In air-saturated 5% H2SO4 which has an oxidizing power just in between passivation and activation, repetition of passive and active state became more conspicuous, showing intermediate endurance.It was clearly concluded that corrosion fatigue failure of austenitic stainless steel is essentially caused by the localized failure of passive film.