The Effect of Sensitization on Stress Corrosion Cracking of AA5083

AA5083 alloy, known for its resistance to stress corrosion cracking (SCC) in aqueous environments, can become susceptible to SCC after exposure to temperatures from 50 ^oC to 200 ^oC for sufficient periods of time.  This phenomenon is generally believed to result from the formation of a continuous network of grain boundary precipitates of the ordered intermetallic Mg₂Al₃ known as β-phase.  β-phase is anodic with respect to the alloy matrix and thus if the proper environment is present, SCC will occur.  To characterize the sensitization effect, dropwise exposure SCC tests using pre-cracked double cantilever beam (DCB) specimens that had been heat treated for differing time periods at 175 ^oC were conducted under open-circuit conditions in 3.5% NaCl solution.  Both incubation time and crack growth rate depended strongly on the sensitization time: incubation time decreased with longer sensitization time while the measured crack growth rate rises sigmoidally with heat treatment time showing an inflection point between 120 and 240 hours of sensitization time.  These trends correlated with the amount and distribution of β-phase in grain boundaries as determined by metallographic characterization and are consistent with sigmoidal kinetics expected for nucleation and growth of β-phase precipitates.  Fracture surfaces, when investigated by scanning electron microscopy (SEM), demonstrated characteristics of intergranular cracking.  Additionally, the electrochemical properties of AA5083 alloy sensitized to differing lengths of time were characterized by free corrosion potential measurements. Findings indicate that anodic dissolution of grain boundary precipitates is the primary mechanism of SCC in AA5083 alloy.