Stress corrosion cracking of sensitized AA5083 (Al-4.5Mg-1.0Mn)

The AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) alloy is a nonheat-treatable aluminum alloy known for its excellent corrosion resistance. However, it can become susceptible to intergranular stress corrosion cracking (IGSCC) when exposed to temperatures ranging from 50 to 200 C for sufficient lengths of time. This IGSCC is widely believed to be associated with dissolution of the electrochemically active beta phase, Al3Mg2, which is precipitated on grain boundaries. Recently, alternative mechanisms have been invoked related to hydrogen effects and/or free Mg segregation or depletion in the grain-boundary regions. To establish a baseline for the sensitization effect, constant-extension-rate tests (CERTs) were conducted under open-circuit conditions and under potential control in 3.5 pct NaCl on samples isothermally treated at 150 C. To aid in interpreting the CERT results, grain-boundary precipitation and solute depletion were characterized by TEM. Additionally, the electrochemical behavior of the beta phase was characterized by anodic polarization of the intermetallic compound synthesized in bulk form. In CERTs under open-circuit conditions, the measured ductility depended strongly on sensitization time, reaching a minimum at 189 h, followed by a slight increase at longer times. This trend correlated well with the fractional coverage of beta phase on grain boundaries, which increased up to 189 h, where it existed with nearly continuous coverage. (Author)