Gasem, Z.M. (Center of Research Excellence in Corrosion King Fahd University of Petroleum and Minerals) | Al-Jarallah, B.M. (Consulting Service Department Mechanical Engineering Division Saudi Aramco)
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 650C for 10 hours. Constant K 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.
Austenitic stainless steels such as type 304 are among the most commonly used engineering alloys in industrial applications requiring high corrosion resistance. The addition of >11% Cr in the alloy solid solution is responsible for this resistance. Sensitization has long been recognized to reduce the localized corrosion resistance of stainless steels and triggers intergranular corrosion. It is widely accepted that sensitization involves the formation of Cr23C6 particles at the grain boundaries in the temperature range of 600-850C and the localized corrosion susceptibility of grain boundaries is primarily due to Cr depletion. Fatigue crack growth in austenitic stainless steel alloys has long been recognized to increase in moist air1 and in hydrogen gas2 environments relative to growth rates in vacuum. McEvily and Gonzalez-Velazquez1 have reported enhanced crack growth rates in type 304 stainless steel when tested in air and compared to FCP rates in vacuum.