SIMULATION OF THE CRACK TIP CHEMISTRY OF STRESS-CORROSION CRACKS IN 7XXX ALUMINUM POWDER ALLOYS

An investigation into the pH changes of a simulated crack tip solution during stress corrosion of rapidly solidified powder metallurgy (PM) AlZnMgCuCo alloys has shown that a steady-state value of between 3. 7 and 4. 0 is attained. The rate of attaining this value is dependent on the Co, Zn, and Cu contents of the alloys and is controlled by the kinetics of hydrogen recombination and evolution. Cobalt exists in intermetallic Co//2Al//9 particles, which serve as the cathodic sites. It is shown that differences in stress corrosion behavior between the PM and the conventional AlZnMgCu alloys are not related to differences in the steady-state crack tip pH.