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

AA5083 (Al-4.4Mg-0.7Mn-0.15Cr) is a non-heat 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. In this study, IGSCC response of sensitized 5083 was studied by constant extension rate testing (CERT) in aerated 3.5%NaCl solution under free corrosion conditions and under potentiostatic polarization. To aid in interpreting the CERT results, precipitation of β (Al₃Mg₂) intermetallic particles on grain boundaries was characterized by transmission electron microscopy. Additionally, the dissolution characteristics of the β phase were measured by potentiodynamic polarization of the compound synthesized in bulk form. CERT results showed that under open circuit conditions, measured ductility depended strongly on sensitization time, reaching a minimum at 189 hours. This trend correlated well with the fractional coverage of β on grain boundaries. In polarization experiments, bulk synthesized β was found to be spontaneously passive from its corrosion potential (-1.40V_sce) up to about-0.92V_sce where passivity was observed to breakdown. In CERT, sensitized samples polarized below the β phase breakdown potential showed almost no evidence of IGSCC testing indicating that a high β dissolution rate is a requirement for IGSCC.