The stress-corrosion behaviour of commercial Al-Zn-Mg-Cu alloy (AA 7075) was studied by measuring the relative fracture energy due to stress-corrosion cracking (SCC) as a function of microstructural features such as width of precipitate free zone (PFZ), size of grain boundary precipitate and size of matrix precipitate. All the specimens were underaged, one of the microstructural features was varied nearly without a concomitant change in the other variables and this microstructural change was observed by transmission electron microscope (TEM). The stress-corrosion cracking tests were carried out by constant strain-rate test method in 3.5 wt. % NaCl solution (pH=1) at 30℃ maintaining the corrosion potential of specimen by means of potentiostatic method. The experimental results suggested that the size of matrix precipitate had more influence on the resistance to SCC than the size of grain boundary precipitate, and the width of PFZ had no effect on the resistance to SCC. Ageing dependence of SCC behaviour of commercial Al-Zn-Mg-Cu alloy was discussed in terms of the slip mode change by interaction of dislocation with matrix precipitate.
