Effects of stress ratio and stress frequency on corrosion fatigue (CF) crack propagation behaviour of peakaged Al-Zn-Mg alloy have been investigated by using a single edge notched specimen in 3.5 wt.% NaCl solution at 30 C. CF tests were carried out over a range of load frequency 0.5 to 10 Hz and stress ratio of 0.1 and 0.5 at rest potential of -700 $mV_SHE$. Stress corrosion cracking (SCC) tests were also carried out in 3.5 wt.% NaCl solution at 30 C to distinguish various contributions to CF crack propagation rate. An electrical potential method was employed for monitoring crack propagation. Threshold stress intensity factor for SCC, $K_ISCC$ and stress corrosion crack propagation rate in region II were found to be about 30 MPa $\sqrt{m}$ and $4×10^{-9} m/sec$, respectively. Stress corrosion crack propagated in brittle intergranular manner, while CF crack propagated mainly in brittle transgranular manner. Based on the superposition model, CF crack propagation rate $(da/dN)_CF$ consisted of two components - the fatigue crack propagation rate in an inert environment, $(da/dN)_R$, which represents the contribution of pure mechanical fatigue, and a synergistic interaction component, $(da/dN)_cf$. These results suggest that the contribution of SCC to CF crack propagation rate was negligible. Pure CF crack propagation rate depends slightly on the load frequency at stress ratio of 0.1, while it depends heavily on the load frequency at stress ratio of 0.5. This was discussed in terms of crack closure effect.
