The fatigue crack propagation behaviour of an Al-Zn-Mg-Mn alloy has been investigated, compared with that of Al-Zn-Mg alloy, in dry air, 0.5 M $Na_2SO_4$ solution and 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution by measuring the crack closure during the crack propagation with a personal computer-aided unloading compliance technique. Fatigue tests were carried out on center-cracked tension specimens machined in T-L orientation and fractured surfaces were systematically observed by scanning electron microscopy. For peak- aged and overaged Al-Zn-Mg-Mn alloys and peak-aged Al-Zn-Mg alloy, the apparent fatigue crack propagation rate in 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution is similar to that in dry air, whereas intrinsic crack propagation rate in 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution considerably exceeds that in the air. The crack closure level decreases in that order of 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution, 0.5 M $Na_2SO_4$ solution and dry air. Under each environmental condition, the apparent crack propagation rate decreases, but intrinsic crack propagation rate and the crack closure level increase in that order of overaged Al-Zn-Mg-Mn alloy, peak-aged Al-Zn-Mg-Mn alloy and peak-aged Al-Zn-Mg alloy. The increased crack closure level in the 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution for Al-Zn-Mg alloy as compared to 0.5 M $Na_2SO_4$ solution for Al-Zn-Mg-Mn alloy was confirmed by the roughness of the fractured surface. From the analysis of current-time curves, both specimens are not repassivated in the 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution and the amount of charge consumed during one cycle increases in that order of overaged Al-Zn-Mg-Mn alloy, peak-aged Al-Zn-Mg-Mn alloy and peak-aged Al-Zn-Mg alloy, which is in good agreement with the trends of intrinsic crack propagation rates. The enhanced intrinsic fatigue crack propagation rate and crack closure level in the 0.5 M $Na_2SO_4$ + 0.5 M NaCl solution resulted from the increase in the local corrosion on the slip steps produced at the crack tip.
