Coarse intermetallic particles (larger than 1 mu m in size) in Al-Zn-Mg-Cu-Zr (7010) alloy were found to significantly influence the crack initiation of the over aged alloy while not affecting the more susceptible peak aged alloy, when subjected to slow strain rate testing (SSRT) in 3.5% NaCl solution. A detailed study was undertaken to examine the causes of such an observation. The study shows that the galvanic action and/or dealloying of the coarse intermetallic particles are responsible for the crack initiation in the over aged alloy. However, this phenomenon is not seen in the peak aged alloy due to its inherent environmentally assisted cracking (EAC) susceptibility and the consequent failure in shorter duration, before the coarse particles can exert an influence.