High frequency corrosion fatigue crack propagation behavior of AISI 316L stainless steel was studied in 1M NaCl and 1M NaCl + 0.01M Na₂S₂O₃, under various anodically and cathodically polarized potentials, and dessicated air at 22°C and 1 atmosphere pressure. Constant load amplitude fracture mechanics techniques employing single edge notch specimens were used to assess the fatigue crack growth rate in the various environments. Unique specimen preparation procedures were developed which allowed near-threshold behaviors to be studied under gradually rising crack tip stress intensity conditions. Polarization studies showed that the presence of thiosulphate catalyzed the dissolution of stainless steel in low pH solutions(pH~1), due to reduction of thiosulphate species to H₂S, but had no effect in the near neutral solutions. Fatigue tests conducted in the neutral NaCl + Na₂S₂O₃ solution at cathodic potentials showed that the presence of thiosulphate had an insignificant effect. This indicated that high frequency fatigue produces efficient exchange of bulk solution with the crack tip environment, which prevented the lowering of pH in the crack by hydrolysis effects and prevented reduction of thiosulphate to H₂S. Fatigue crack retardation phenomena were very pronounced in the near-threshold regions in most of the fatigue tests. The cause of this retardation was attributed mainly to the surface-roughness- induced crack closure effect, which reduced the effective crack tip cyclic stress intensity ΔKth to a lower level. This closure effect only predominated in the near-threshold region where significant Mode II loading was present. The influence of various imposed anodic and cathodic potentials was found to be consistent with the surface-roughness-induced crack closure effects. The observed crack growth accelerating effect of high anodic potentials was attributed to the corresponding high removal rate of surface roughness in the wake of the crack, which kept the effective cyclic stress intensity level near the applied values. Crack fractography was studied by scanning electron microscopy. It showed that the fractography generally consisted of three regions; a crystallographic cleavage-like near-threshold region, a feathery and fibrous transition region, and a striated region. Using an etch pitting technique, it was determined that the crack plane and crack propagation directions in the near-threshold region were mainly those of {111} <110>, {110} <112>, {110} <001>, and other higher indexed planes. These crack orientations were effected by the activation of a single slip system or the alternate activation of two intersecting slip systems. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate