Influence of Cl^- Concentration on Corrosion Fatigue Crack Growth of an Austenitic Stainless Steel

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Corrosion fatigue crack growth rate of an austenitic stainless steel (SUS304) was measured in Cl^- containing water. Acceleration mechanisms of crack growth in corrosive environment were discussed, and an acceleration of da/dN in SUS3O4 was compared with that in a high-tension steel (HT55). The first step acceleration of da/dN in water environment from that in dry air was due to the reverse slip suppressing effect induced by water molecule adsorption. The second step acceleration at a low frequency (f=0.1Hz) was due to hydrogen embrittlement of plasticity-induced martensite formed at crack tips. In 3% NaCl solution at f=0.1Hz, the acceleration by stress-assisted dissolution was also observed. The crack opening ratio (U) was independent of Cl^- concentration as well as of cycle frequencies. The acceleration of da/dN in SUS304 amounted to 9.2 times that in HT55,and a water environment sensitive crack growth behavior under cyclic stresses was clear. However, there existed a threshold ΔK to commencing to accelerate da/dN from that in air, and the acceleration in HT55 exceeded that in SUS304 at f=5Hz near ΔK_th.
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