Frequency effect on corrosion fatigue crack propagation of different microstructures of AISI 304 type stainless steel

Many stainless steels structures such as pipes and pressure vessels are subject to fatigue loading during service operation. The present study aimed to investigate the effect of loading frequency on the kinetics of fatigue crack propagation of type AISI 304 stainless steel exposed to 3.5% NaCl solution. Two microstructures are examined: annealed and sensitized 304 stainless steels. Sensitization is known to reduce the corrosion resistance of stainless steels and lead to intergranular corrosion. Sensitization was performed at 650 C for 10 hours.; Fatigue crack propagation tests were conducted on solution annealed and sensitized AISI 304 stainless steels in air and 3.5% NaCl. Tests were according to ASTM E-647 standard using a compact testing type (CT) specimen.; Sensitization enhanced the susceptibility to both uniform and localized corrosion as well as fatigue crack propagation in air and 3.5% NaCl. Solution annealed sample showed transgranular cracking during fatigue in air, whereas sensitized sample showed transgranular as well as intergranular cracking. The fracture surface morphology in air and 3.5% NaCl is essentially identical but numerous flat features are observed in 3.5% NaCl for annealed sample and are more irregular flat features with intergranular cracks in sensitized sample. Constant DeltaK tests was carried out at DeltaK 25 and DeltaK 35 MPa√m at frequencies ranging from 0.1 to 30 Hz in 3.5% NaCl pH 7 and pH 3 environments. It was found that for frequencies above 10 Hz, the crack growth rate was essentially constant in both environments, whereas it increased monotonically with decrease in frequency below 10 Hz. The frequency behavior was found to be independent of solution pH, DeltaK level and microstructure. Crack growth rates were higher in sensitized sample at low frequencies In all testing conditions the difference in the fatigue crack propagation rate between the annealed and sensitized microstructure vanish at higher frequency and higher DeltaK level where mechanical loading factor is dominated.