Kinetics of Stress-Corrosion (SC) crack propagation behaviour of commercial Al-Zn-Mg alloy has been studied as a function of temperature and $Cl^-$ concentration in distilled water using a DCB specimen. Stress-Corrosion Cracking (SCC) tests were carried out over a range of temperature from 298 to 328 K and over a range of $Cl^-$ concentration 0 to 0.6 M. Stress-Corrosion crack propagation rate with stress intensity factor (K) was measured. Intergranular brittle fracture mode was observed, irrespective of temperature and $Cl^-$ concentration. Activation energy for the SCC process in region I was found to be 97-99 KJ/mole in 2.0 wt.% NaCl and 3.5 wt.% NaCl solution, and in region II 32 KJ/mole in distilled water and 54 KJ/mole in NaCl solution. The role of $Cl^-$ ion enhanced SC crack propagation rate due to the formation of $AlCl_3$ in crack tip. Rate controlling step for SC crack propagation process was thought to be Al dissolution reaction or hydrogen transport in material in region I and to be diffusion of $Al^{+3}$ from crack tip to bulk solution in distilled water and hydrogen adsorption reaction in NaCl solution in region II.
