The Effect of Aging Conditions on the Relationship between Stress Corrosion Cracking and Intergranular Corrosion in 7N01 Alloy : Studies on Stress Corrosion Cracking in Al-Zn-Mg Alloy

The influence of aging conditions on the relationship between stress corrosion cracking (SCC), intergranular corrosion and electrochemical behaviors in a commercial Al-Zn-Mg 7N01 alloy was investigated metallurgically by means of optical and electron microscopy, and SCC mechanisms were discussed. Intergranular corrosion of different morphologies was observed respectively in cold aged (ie. low temperature-or short time-aged) alloys and in warm aged (ie. high temperature-or long time-aged) alloys. In the former alloys, intergranular corrosion occurred due to selective attack of anodic zone along grain margin, and in the latter alloys, due to selective dissolution of precipitate particles along grain boundaries, thus produced pit-type intergranular corrosion. The susceptibilities to SCC and intergranular corrosion were in a good correlation for cold aged alloys, but for warm aged alloys not so always. Observed actiavtion energy of SCC for a cold aged alloy was undoubtedly lower than a warm aged alloy. The energy value of alloys, naturally aged for 4 months and aged at 120℃ for 5 days, was 11.2kcal/mole and 22.6kcal/mole respectively. The results support that in cold aged alloys, SCC occurs by a stress-assisted intergranular corrosion and in warm aged alloys, it takes place by two-step mechanism combining an electrochemical and mechanical process.