The Stress Corrosion Cracking (SCC) of Al-Zn-Mg welds in an aqueous solution of 3.5 wt.% NaCl (PH=1) was studied with specimens under constant load, related to heat input rate of welding and postweld heat treatment. Dendrite arm spacing determined by heat input rate mainly reduced the yield strength of welds. The failure time of welds was dependent on both heat input rate and postweld heat treatment: Heat input rate on welding process influenced electrochemical corrosion due to galvanic effect. Heat input rate dependence of failure time showed that the higher the susceptibility to galvanic corrosion of welds, the lower the susceptibility to SCC of welds in the case of weld specimens, postweld-heat-treated on the same condition (temperature and time). Weld specimens welded by autogeneous gas tungsten arc welding (GTAW), postweld-heat-treated at 120℃ for 24 hrs showed the highest susceptibility to SCC followed by the specimen at 80℃ for 48 hrs and then at 150℃for16 hrs. Weld specimens welded by gas metal arc welding (GMAW) (with AA-5356 Al-Mg alloy as a filler metal), postweld-heat-treated at 150℃ for 32 hrs showed the highest susceptibility to SCC, and then 120℃ for 26 hrs, 16 hrs, 8 hrs, the susceptibility decrease as ageing proceeds. The postweld-heat-treated effect on SCC was explained in terms of the ageing stage SCC relation. Scanning electron microscopic observation of fracture surface showed that stress corrosion crack in Al-Mg base alloys was induced by hydrogen diffusion.
