Effect of sensitization on corrosion fatigue crack propagation of type 304 stainless steel in 3.5% NaCl

The present study was carried out to investigate the effect of sensitization on the kinetics of environmentally assisted fatigue crack propagation (FCP) in type 304 stainless steel exposed to chloride aqueous solution at ambient temperature. Sensitization was performed at 650°C for 10 hours. Constant ΔK fatigue crack growth rates in annealed and sensitized compact-tension specimens have been measured as a function of the loading frequency (0.1-30 Hz) in 3.5%NaCl solution. The effect of closure shielding has been monitored to separate any extrinsic contribution to the crack kinetics. FCP results reveal accelerated FCP rates in both microstructures when tested in the corrosive environment as compared to air data. For both microstructures, da/dN increase monotonically as the applied frequency is decreased with higher cracking rates associated with the sensitized microstructure. The difference in FCP rates between the annealed and sensitized microstructures increases as the applied frequency is reduced. Fatigue fracture surfaces of the annealed specimen exhibit transgranular crack path over the whole range of frequency tested. The sensitized microstructure shows similar transgranular mode of cracking at high frequency. The low-frequency cracking path in the sensitized microstructure exhibits distinct features and apparently grows near high-angle grain boundaries.